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THE FUNGAL DISEASES
OF THE
COMMON LARCH
BY
W. E. HILEY, M.A.
SCHOOL OF FORESTRY, OXFORD
OXFORD
AT THE CLARENDON PRESS
3 1919
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PREFACE
Tuts book arose from an investigation which Sir William
Schlich asked me to make into the larch canker, a disease
on which many articles and papers have been published,
though no full and connected account of it has ever been
- written in English. While studying the canker in the field,
I very frequently encountered certain other diseases on the
larch, especially the heart-rot and the honey fungus, and it
seemed better to include these in the scope of the investi-
gation. For the sake of completeness, all the known diseases
on the common larch have also been described, and it is
hoped that a book of more general usefulness has been
thereby produced. Certain fungi, such as Lophodermium
laricis and Botrytis cinerea, which have occasionally been
reported as parasitic on the larch, have been omitted, as no
reliable evidence of their causing disease has been forth-
coming.
The arrangement of the diseases bears no relation to the
systematic position of the fungi. They are mostly given in
the order in which they were studied, which approximates
closely to the order of their importance.
-The work was made possible by a grant from the De-
velopment Commissioners, received through the Board of
_ Agriculture and Fisheries, and latterly through the Interim
Forest Authority. Especial acknowledgement is due to
these departments, particularly to the last named, which
has assisted me in a variety of ways. I also desire to thank
Sir William Schlich for constant help and encouragement,
and Messrs. A. W. Borthwick and Malcolm Wilson for
permission to reproduce Fig. 72 from the Transactions of
the Royal Scottish Arboricultural Society.
OxForD, August 1919.
CONTENTS
CHAP,
I. INTRODUCTION.
The relation of a fungus to its host. Life-history of a fungus.
The morphology of the larch ; long and dwarf shoots ; internal
structure of the stem : ; : ;
Il. THE LARCH CANKER.
General. Historical. The mycelium of Dasyscypha calycina
and its effect on the tissues. The canker as a pathological
structure ,
lil. THE LARCH CANKER (continued).
The reproductive organs of the fungus. Germination of spores.
Pure cultures on nutrient media. Artificial infection with canker
IV. THE LARCH CANKER, (concluded).
On the mode of infection in nature. Importance of wounds as a
source of canker. Contributory causes of canker. Methods of
prevention. The synonymy of Dasyscypha calycina
V. -HEART-ROT. Fomes annosus.
Various fungi which cause heart-rot. Fomes annosus: general ;
historical. Secretions induced by Fomes annosus: turpentine
and resin ; soluble gum ; insoluble gum, Decomposition of the
wood .
Vi. HEART-ROT. Fomes annosus (concluded).
Reproductive organs: fructifications; conidiophores. Pure
- cultures on artificial media. Cultures on natural media. Infec-
tion experiments. Mode of attack in nature. The frequency
of heart-rot in plantations which form the first rotation on culti-
vated soil. Methods of prevention ;
Vil. HEART-ROT CAUSED BY OTHER FUNGI.
Polyporus Schweinitzii, Poria vaporaria, Polyporus sulphu-
reus, Trametes Pini. : ‘ ° ; ,
PAGE
16
37
52
80
100
126
Viii CONTENTS
CHAP,
Vill, ARMILLARIA MELLEA, THE HONEY
FUNGUS.
General. Microscopic details of the fructification. Rhizo-
morphs. Effect on the host. The black line and resin flow.
The method of infection. Means of prevention *
IX. LEAF AND SEEDLING DISEASES.
The larch needle-cast (Sphaerella laricina). Meria laricis.
Hypodermella laricis, The larch needle-rusts: Melampsoridium
betulinum, Melampsora tremulae, &c. Damping-off diseases :
' Phytophthora omnivora ; Fusoma parasiticum ces
X. GENERAL SUMMARY .
’ BIBLIOGRAPHY
INDEX .
PAGE
. 144
LIST OF ILLUSTRATIONS
- FIG.
1, Section through canker ‘ : , frontispiece
2. Transverse section of one-year-old stem of larch ‘ . page 8
3. % x », three-year-old stem oflarch . facing page 8
4, is ts ,, abnormal wood ph eer zane 8
5. Method of formation of periderm — ' : ‘ . page 10
6. Longitudinal section of phloem | tan +5
Soe? . Frnotification showing pores in
longitudinal section . veal gy
Bo os bs Under surface of fructification
showing mouths of pores
40. Upper surface of fructification showing concentric
furrows and white margin . -- rs
41. Fomesannosus . es . page
42, Diagram showing ehuate for pores to be vertical =
43. Fomes annosus. Conidiophore and conidia ~ . 5 Sone
44,45, Cultures of Fomes annosus : . , . facing page
46. Polyporus Schweinitzii. Stipitate fructification a es
47. mrs os Bracket-shaped fructification ,, ‘i
48, Hymenial layer of Polyporus Schweinitzii : . page
49, 50. Sections of larch-tree showing rot caused By
Polyporus Schweinitzii : ‘ : . facing page
51. Piece of larch wood rotted by Poiyppotsis Schweinitzii—,,
52. Section of Scots pine stem with rot caused by Poly-
porus Schweinitzit : ;
53. Mycelium of Polyporus Schweiniteit in sve of bisok - page
54. Section of larch wood after attack by Polyporus Schweinitzii ,,
55. Poria vaporaria ..
99
99
56, 57. Sections of larch wood rotted ea Ticoiites Pini . facing page
58,59. Armillaria mellea. Group of toadstools growing
from base of deodar . See :
60, 61. a » Toadstools growing from rhi-
zomorphs in soil P 8 9
& &
92
92
94
96
97
99
99
10]
101
101
102
103
105
108
127
127
128
128
130
130
131
132
136
143
145
146
LIST OF ILLUSTRATIONS
IG.
“ 62. Hymenial layer of Armillaria mellea : ; . page
63. Rhizomorphs of Armillaria mellea between bark a
wood of larch stem . ; : ; ; - facing page
64. Small portion of Fig. 63 enlarged r
65. Section of larch stem attacked by Armillaria mellea .
66. Hollow larch stem rotted by Polyporus sulphureus
67. Armillaria mellea. Radial longitudinal section of
_- larch wood showing ‘blackline’ . page
c x , Transverse section of larch wood |
_ showing ‘ black line ’ P
f 69, 70. Living larch-trees with honey fungus growing from _
their roots : , ; d 3 . facing page
71. Diagram showing life-cycle of heteipecrate rust . - : «page
72. Melampsoridium betulinum . ‘ ‘ : . facing page
73. Sporangiophore of Phytophthora omnivora and coni-
diophore of Fusoma parasiticum . é ; . ioe “page
-
xl
150
150
150
155
155
155
157
(166
174
175
180
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CHAPTER I
INTRODUCTION
The relation of a fungus to its host. Life-history of a fungus. The
morphology of the larch; long and dwarf shoots; internal structure of
_ the stem.
Historical. The history of larch-growing in Britain pre-
sents features of peculiar interest. In no other tree have
such high hopes been placed, hopes which too often have
led to disappointment, and with no other tree has it been so
necessary to modify the method of sylvicultural treatment
on account of liability to disease.
The story is as follows. The larch, which is a native of
the Alps, Carpathians, and a part of southern Russia, was
introduced in the early part of the seventeenth century. It
was then only grown for decorative purposes, and a hundred
years elapsed before the larch was employed as a timber-
producing species. From 1730 onwards, however, it was
planted extensively in Scotland, particularly by three
successive Dukes of Atholl on their estates of Atholl and
Dunkeld, and the faith of its ducal sponsors proved to be
so well founded that the tree waxed great in popularity,
and before the close of the century large plantations might
have been found on the south as well as on the north side
of the Tweed.
In 1820 a frigate was built for the navy of Scotch larch
to test the durability of the wood under conditions of
exposure. She was appropriately called the Athol, after
the duke who grew the timber, and for the sake of com-
_ parison the Nieman, also a ship of war, was built at about
_ the same time of Baltic fir (i. e. Scots pine). According to
Laslett, who wrote in 1875, when he was Inspector of
Timber to the Admiralty, the former lasted for a very
1888 B |
2 INTRODUCTION
long time without any considerable repairs, whilst the other
decayed very rapidly, and from this comparison the superio-.
rity of Scotch larch over Scots pine, for durability, was
considered to be pretty well established. If Scotland could
grow larch timber which was more durable than Baltic fir,
there was no reason to despair of British forestry, and the
reputation of larch, already great, was still further enhanced. °
So foresters continued to plant larch wherever the site
was suitable, and in a great many places where it was not.
It was used for railway sleepers and other outdoor purposes,
and further experience confirmed the belief in its capacity
for bearing exposure ; and it must have been at this time,
about 1840 to 1850, that its popularity reached the height
from which it subsequently and continuously declined. It —
was fast-growing, straight, and easy to handle, but it had
one great weakness: it became extremely liable to disease.
A book on the larch, written by Mackintosh in 1860, is
a sufficient index of the change that had come over the
opinion of foresters, and nearly all writings since this date
have partaken of the nature of apologetics for a tree that
had failed to fulfil its early promise.
The disease that Mackintosh was most concerned with
was variously known as heart-rot, dry-rot, piping, pumping,
and internal decay, and was believed to be caused by a
fungus which they called Polyporus destructor. Exactly what
disease this was is not clear, but it was no doubt one of the
heart-rots as we know them to-day, and very likely Fomes
annosus was alone responsible for it. In addition to this,
larch canker was becoming frequent, and in time proved to
be an even more serious trouble than heart-rot, or, at any
rate, one that was more visible to the eye and consequently
more feared. At the present day there is scarcely a young
plantation anywhere in the country which is entirely free
from canker, and felling all too frequently discloses the
presence of heart-rot as well. So larch is no longer the
favourite tree that it was, and the tendency is to use it less
often in pure plantations, though it is still very frequently
employed in admixture with other trees since canker is
ee eee
INTRODUCTION 3
less to be feared in mixtures than in pure stands. So far
is this true that in Germany larch now scarcely appears at
all, except in mixed forest.
The relation of fungus to host. What is the cause of
disease ? Why, when the disease has once appeared, does
it spread and become more and more general? It is the
object of this book to answer these questions ; but before
going into the details of various forms of disease, it will add
to the clearness of subsequent descriptions to explain a few
of the fundamental ideas on which all our knowledge of
tree diseases is based ; and as all the diseases described in
this book are caused by fungi, the peculiar nature of fungal
nutrition and the parasitism which it often involves should
be made clear at the outset.
The normal green plant is self-supporting in a sense which
is not applicable to other organisms, even to the higher
animals. That is to say, it can obtain all its food without
the intervention of any other living being. Water and
mineral salts it obtains from the soil through its roots,
and carbon-compounds it manufactures from the carbon
dioxide of the air which diffuses in through the pores of
the leaves. Combining the carbon dioxide with water and
giving out part of the oxygen, the plant forms carbohydrates,
and subsequently more complex organic compounds. But |
this process of carbon assimilation can only be performed
by those parts of plants which contain the green colouring
~matter, chlorophyll, and, even so, only in the light. Such
plants are called autotrophic, or self-feeding. |
In the course of evolution some plants have taken to
living on carbon compounds which have already been
elaborated ; that is to say, these plants have become either
parasitic on other living plants or animals, or else sapro-
phytic on the remains of such plants or animals. Such
plants always tend to lose their chlorophyll, and are called
heterotrophic. Some of the higher plants are heterotrophic,
such as the bird’s-nest orchis, but the phenomenon is most
commonly seen in the large group of lower plants known
as the Fungi. In no member of this group has chlorophyll
B2
4 | INTRODUCTION
been found, and the green colour seen in some fungi is due
to another pigment. Thus all fungi have to live either
parasitically or saprophytically, and their mode of life has
become entirely specialized to assist them in this kind of
existence. Though the distinction between parasites and
saprophytes is not always easy to draw, there is a very
marked difference between the two modes of life, at any
rate in the extreme cases, and in pathology, which is the
science of disease, we shall naturally be chiefly interested
in parasites. But, as will be shown later, saprophytes must
also claim our attention for the reason that they can some-
times live on the dead parts of a living tree, such as the
heart-wood of the trunk, and thereby not only destroy the
timber but weaken the tree and render it more easily blown
by the wind.
Parasites, again, are not all of the same kind. Some can
thrive only on living organisms, and are called obligate
parasites, and others can grow either on living organisms
or on dead ones, and are therefore called facultative parasites.
An example of the former is any one of the rusts such as
Peridermium and Coeoma on larch needles, and of the latter
the canker fungus or the heart-rot fungus which are to be
described. The common mushroom will serve as an instance
of a saprophyte, as it lives on the organic remains in the
soil.
Life-history of a fungus. The part of the fungus which
absorbs food from whatever it is growing on is called the
mycelium, and is composed of a more or less felted mass of
fine threads or hyphae. Each hypha grows at its extremity
and may also branch, giving rise to numerous other hyphae
as shown in fig. 21. It contains protoplasm and nuclei, and
certain spaces in the protoplasm, known as vacuoles, filled
with an aqueous solution, besides drops of oil and othe?
food reserves. The mycelium is formed of these hyphae
growing and becoming intermingled, and, though a single
hypha is too fine to be seen with the naked eye, the mycelium
as a whole may be very conspicuous, and is often seen as
a white felt-like mass on pulling away the bark of a rotten
eee eee ee
a ———
INTRODUCTION 5
stump. The mycelium may itself become reproductive in
various ways, but, in principle, it is the vegetative part of
the fungus. The true reproductive part is generally very
specialized, and in nearly all the fungi considered in this
book it has a marked form peculiar to each species of fungus,
and is (somewhat loosely) called the fructification. This
fructification bears a large number of spores or single, -
generally very small, cells which are distributed by wind
or other agencies ; and each spore, if it falls on a suitable
feeding ground, is capable of producing a new mycelium,
which in its turn produces a fresh fructification. A familiar
fructification is a mushroom or any toadstool, which is
solely a reproducing organ, and is nourished entirely by
the mycelium, which is out of sight and buried in the soil.
The large bracket fungi found on trees are also fructifica-
tions, only these fungi differ from a mushroom in the fact
that their mycelium lives and grows in the trees instead of
in the ground.
The fructifications of the fungi which grow on trees are
often very large; and when it is remembered that all the
food necessary to produce them is derived from the tree, it
will be clear that the tree must suffer accordingly. Frequently
the tree is killed, but if the fungus is a facultative and not
an obligate parasite, its growth will not immediately be
checked by the death of the tree, and not uncommonly
dead trees are a dangerous source of infection to living
ones. The tree or other plant on which a parasite grows
is somewhat unkindly called its host.
When investigating the life-history of a fungus it is often
found that there is another form of reproduction besides
the fructification. In some species almost any part of the
mycelium may give rise to specialized hyphae, which bear
cells which are not unlike spores and have the same faculty
of reproducing the plant. These cells are given the dis-
tinguishing name of conidia, and the hypha that bears
them is called a conidiophore. The distinction between
spores and conidia has given rise to much controversy, and.
is now based chiefly on certain nuclear phenomena which
6 INTRODUCTION
precede their formation ; but as nuclear phenomena will not
be dealt with in this book, the application of the terms will
have to be taken on trust. At the same time the word
conidium is very useful as a term for an extra reproductive
cell which is generally not an essential part of the life-cycle
of the fungus: the qualification is used advisedly, for in
a large number of fungi no typical fructifications have ever
been found, so that they must reproduce themselves solely
by conidia. The discovery of the formation of conidia in
the life-history of any fungus is manifestly of the first
importance to the pathologist, for infection may be caused
_ by conidia just as well as by spores. For instance, a whole
plantation of larch may be heart-rotted by Fomes annosus
without a single fructification being produced, and this is
probably due solely to infection by conidia. (For conidia
of this fungus, see fig. 43.)
The morphology of the lareh. The study of disease
thus resolves itself into a study of the relationship of the
parasite and host. The complete life-history of the parasite
must be known as well as the structure and mode of growth
of the host. In this book many different parasites are
dealt with, but the host is always the same; so a descrip-
tion of those parts of the host with which we shall be chiefly
concerned will be given at the outset. The account will be
made as simple as is consistent with accuracy, and only
those parts will be described which are necessary to an
understanding of the more important diseases.
Long and dwarf shoots. When growing under forest con-
ditions the leading shoot of a larch grows rapidly and
maintains its vertical direction until maturity, while the
lateral branches are mainly horizontal. The apical bud of
each shoot, and those other buds which develop into branches,
elongate in the spring and become what are called long
shoots, i.e. shoots in which the internodes, or portions «of
the stem between the leaf insertions, are lengthened. The
young stems are at first green, but soon put on a layer of
bark and then appear yellowish brown. The leaf bases are
decurrent, so that any transverse section shows five swellings
——
INTRODUCTION. ;
which reach down from the five leaves next above. The
leaves are deciduous and bear buds in their axils which
develop in the following year. The majority of these buds,
however, do not develop into long shoots, but remain
stunted, and are called dwarf shoots. In these shoots the
internodes do not lengthen, so that all the leaves borne
by them are inserted very close together, and appear in
a rosette. These dwarf shoots generally bear leaves for
a number of years, and their age can be determined by
counting the whorls of dark-brown bud-scales which remain
attached at their bases. The dwarf shoots may, under
favourable circumstances, grow out into long shoots, but
they more commonly get left behind by the growth of the
tree, and when they become too much shaded by other
branches the shoots die and remain on the branches as
tufts of brown bud-scales, until they are ultimately cast.
off the tree.
Internal structure of the stem. ‘This will be described
in detail, as the stem forms the feeding ground for the
mycelia of all the most important disease-causing fungi.
As it will be necessary to introduce a number of botanical —
terms which could not be explained except at great length,
readers who are unacquainted with the general features
of a plant’s anatomy are recommended to omit this section
and pass on to the general ‘account of larch canker in the
next chapter. cle
If a first-year shoot, about 1°5 mm: in diameter, be cut
in winter and a transverse section examined under the low
power of a microscope, the tissues will be found to be
arranged as in fig. 2. On the outside are five protrusions,
the decurrent leaf-bases or ‘ leaf-cushions ’, between which
‘are narrow furrows. The cushions are nearly 1 mm. in
breadth and about 0:2 mm. in thickness. These cushions
contain rather large resin ducts, one near each edge, which
pass down from:the leaves and end blindly in the cortex.
Their length is one to four times that of an internode, so
that any section may disclose their presence in one to four
cushions. Not infrequently one resin duct in a cushion is
8 - INTRODUCTION
longer than the other, so that only one of a pair is cut
across in a transverse section of some cushions.
Immediately inside the cushions is a continuous cork
layer, which kills the outer cortex and epidermis, and
inside this again is the inner cortex composed of live cells,
with intercellular spaces, somewhat thick cellulose walls,
and living contents with chloroplasts. A narrow continuous
layer of phloem surrounds the wood cylinder, and imme-
diately outside the
phloem are a few nearly
spherical resin cysts,
which look like resin
ducts in section, but are
not extended longitudi-
nally. The wood cylin-
breadth, and contains
a few resin ducts; and
in the centre is a star-
shaped medulla. ,
Sections of older
Fie. 2.—Diagram of a transverse section Stems show progressive
of a one-year-old stem of larch. ca., cam- stages of thickening.
bium; co., cork; i.c., inner cortex; /-r.,
foliar resin duct ; m., medulla or pith ; m.r.,
medullary ray; 0.c., outer cortex; p., allydisintegrate and fall
hl : #0. Tesi ; rd., resin duct i
phloem; 7.c., resin cyst; r.d., is off, and are usually in-
in wood ; w., wood.
distinguishable after the
fifth year. New cork layers are formed immediately under
the first one. These layers are not always complete, and,
in the second year especially, partial layers are frequently
formed stretching under the furrows. In general a new
cork layer is formed each year. The inner cortex remains
living for an indefinite time, but loses its chlorophyll after
a while. The phloem becomes thicker, and the outer first-
made ‘elements are so stretched tangentially as to leave
numerous large intercellular spaces. A fresh ring of wood
is put on each year.
Detailed structure of tissues : epidermis. The cells measure
der may be 40r5mm.in |
The leaf-cushionsgradu--
F1q. 3.—Microphotograph of transverse section of
three-year-old stem of larch.
~
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TS = .
‘fee Keng
:
a*
Fre. 4.—Microphotograph of transverse section of
abnormal wood.
INTRODUCTION 9
10-20 » tangentially, 17 radially, and are longitudinally
extended to about 400. They are not only covered by
a thick cuticle, but the outer and radial walls are strongly
cuticularized, and a cellulose wall is superposed on this
cuticularized wall, leaving a lumen with a diameter about
half that of the original cell. Owing to the formation of
the cork layer, this and the outer cortical layers are killed
before the lapse of the first year.
Outer cortex. Next the epidermis are one to three layers
of collenchyma, the walls of which give both the chlor-zinc-
iodine reaction for cellulose and the phloroglucol and
hydrochloric acid. reaction for lignified walls. The walls
are thick, with well-marked secondary and tertiary layers,
and there are small intercellular spaces at the corners.
The parenchyma inside this is composed of large cells
- 30-100, in diameter, and nearly isodiametric in all three
planes. The resin ducts of the cushions are formed in
this tissue, and may either touch the collenchyma or be
separated from it by one or two layers of parenchyma. _
They are up to 170» in breadth, and are lined by an epi-
thelium protected by small parenchymatous cells on the
outside. There is no further mechanical tissue to prevent
their being crushed. These, with all the parenchyma in
the cushions, are cut off by the primary cork layer, and are
thus functional for less than one year.
Periderm. The phellogen is first formed in the layer of
_ parenchyma immediately inside the collenchyma at the
furrows, and forms a circle by cutting across the parenchyma
of the leaf cushions (see fig. 2). The phellogen forms 1-3
layers of cork cells on the outside, and as many layers of
phelloderm cells on the inside.
A new cork layer is formed each year in the following
way. The innermost phelloderm layer becomes meri-
stematic, and forms a new phellogen. The outer phello-
derm cells and the phellogen of the previous year develop
thick walls, become lignified, and take on the appearance of
- stone cells. In this way layers of stone cells,. laterally
welded together, are formed, and the walls become so thick
* of formation of periderm: c., cortex;
10 | INTRODUCTION
that it is often nearly impossible to find the lumina or to
distinguish one cell from another; the layer may be two
cells in thickness, or it may be only half a cell in thickness,
only one side of the cell having taken on a thick wall. The
new phellogen then forms a few layers of cork cells and
a fresh phelloderm.
This process is repeated year by year, so that a lari
number of cork layers are formed, all in the same radial
rows with the rest, except where phellogen cells have
divided tangentially, and
without encroaching on the
= © inner cortex, which remains
kL intact.
ah :
te In special cases, how-
=Se-=-pn =
hile —=epm ever, e. g. where the cortex
has become injured, a fresh
phellogen may be formed
through the cortex or outer
phloem, as in ordinary scale
c,, first-year cork; ¢,, second-year bark trees. /
eke th ous corks Lililod 4” longitudinal section
shows at the top of each leaf
cushion a leaf scar where the abscission layer has cut off
the leaf. A poorly developed cork layer is formed under the
abscission layer, and the cells beneath this become suberized.
The cork layer is joined on to the primary cork layer of the
stem on all sides. These leaf scars would appear to be weak
points in the defensive armour of the stem. At the same
time they may serve for the interchange of gases, since no
lenticels are present in the cork until the second year.
Inner cortex. The parenchyma inside the cork layer
remains alive, and the cells contain chloroplasts. The cells
are at first nearly circular in all sections, but later become
extended tangentially to keep pace with the increasing
girth of the stem. These cells usually divide by radial
walls, and each elongated cell is replaced by two or three
circular ones. ‘This continued vitality of the cortex is
rendered necessary by the peculiar form of periderm
Fic. 5.—Diagram showing method
INTRODUCTION 11
formation, which leaves the inner cortex intact even in
old stems.
Two kinds of specialized elements occur in this layer.
‘The first are small cells set aside for the purpose of con-
taining crystals of calcium oxalate. These cells are only
found in older stems, and are very frequent in the neighbour-
hood of cankers. The second are irregularly branched
sclerenchymatous cells with :
_very thick walls (fig. 6, s.e.).
These elements grow longi-
tudinally, forcing their way
between the parenchymatous
cells, and often braneh. The
branching occurs when a .
sclerenchymatous cell = en-
counters a parenchymatous
cell broadside, and one branch
grows down on each side of
the obstructing cell. The
function of these cells is
obscure. :
There is no distinction be-
tween the cortex and pericycle,
but in the parenchyma im-
mediately abutting on the
phloem (and thus presumably Bi
pericycle) there are here and ,,%, °—Lovsivedina sti of
there large intercellular spaces cell; m.r., medullary ray; p.p.,
which contain resin. When phloem parenchyma; .¢., scleren-
. chymatous element; s./., sieve
- young they are small and tube (420).
spherical, but with the in-
creased girth of the stem they become laterally extended
and are conspicuous objects in all sections ; they also acquire
a poorly developed epithelium. They appear to be associated
with some of the larger medullary rays, which, when followed
into the xylem, are found to be in contact with resin ducts
in the first-year wood.
The phloem is composed of sieve tubes, crystal-containing
=
Z
=
—
=
zB
a
—
—
Zz
—
Z
2
ZZ
Z
FA
A
\
12 INTRODUCTION
cells, parenchyma, branched sclerenchymatous elements,
medullary rays, resin sacs, and horizontal resin ducts.
When young these elements are found td be in regular
radial rows, but in the older phloem the increased girth
ruptures the rows and they become still more irregular
through the tangential expansion of the medullary rays.
(i) The sieve tubes (figs. 6 and 7, s.t.) are small in section,
tangential and radial dimensions being 8-20» and 5-84
respectively. Their length is difficult to determine, but
has in certain cases been estimated at 0°8 to 1mm. The
walls are rather thick, with sieve areas of the normal E
Abietineae type (vide Hill, 1901); these sieve areas are
especially numerous on the radial walls, where they are
uniseriate, and less numerous on the tangential walls, where
they are uni- or bi-seriate. In winter the young sieve
tubes contain a small amount of protein, but the older ones
appear to be empty.
(ii) The crystal-containing cells (fig. 7, c.c.) are of the
same size and shape as the sieve tubes, but have thinner
walls. They contain crystals of monohydrated calcium
oxalate (CaC,0,.H,O). These crystals are rhombs of the
monoclinic (klinorhombic) system ; the angle of the rhomb
is 78-80°, and in polarized light the extinction is straight
and the double refraction strong. They are very small and
numerous, and occur in two or three series lying in a tan-
gential plane. The growth of the crystals often causes
radial swellings in the walls. Besides crystals the cells
contain an emulsion which responds to stains for protein,
tannin, and resin. No nucleus has been observed.
(iii) The cells of phloem parenchyma are large, 20-40
20-35 1 in transverse section, but only 60-90, in length.
They are formed in longitudinal series and have thin
cellulose walls. Each cell has a lining layer of protoplasm
and a rather large nucleus flattened against one side. Some
cells in each series contain tannin (ferric chloride and
potassium bichromate tests), whilst intervening elements
are entirely without this substance. The osmic acid test
also discloses the presence of fat, especially in the older
i
OO ae
E
i
i
i
;
%
| re
}
3
|
ie often extended longitudinally.
_ years that the stem has
sufficiently definite to
_ also be found in the outer
INTRODUCTION 13
parts of the phloem. Tetroctohedric crystals of trihydrated
calcium oxalate (CaC,0,.3H,O) are occasionally present in
the vacuoles of these cells (fig. 6, p.p.). - These crystals are
| _ larger than those in the small crystal-containing cells.
The phloem parenchymatous elements occur more or less
_ regularly in tangential rows, giving the effect of annual
rings. Also the number
) ofrings is approximately
equal to the number of
grown, but they are not
determine an exact cor-
respondence.
(iv) Branched scleren-
chymatous elements, like
those in the cortex, may
phloem. They seem to
replace elements of
phloem parenchyma.
(v) Medullary shh aga Fie. 7.—Transverse section of phloem
very humerous, and may (x 420): c.c., crystal-containing cell; 7.s.,
be from one to ten cells intercellular space ; m.r., medullary ray ;
i -p., phi h 3; &t., sieve
high. They are at first a ee phloem parenchyma; s |
only one cell in thickness,
but through cell divisions they may become two_or more
cells thick. The cells remain thin walled, and contain
protoplasm and nuclei; among the contents may also
_be seen numerous resin drops, especially in the vicinity of
the cambium, and these drops are apparently conducted
outwards to the resin sacs. The dimensions are about
5-20 » in tangential, 20 in longitudinal, and 40, in radial
direction, but the cells at the top and bottom of a ray are
(vi) Resin cysts may also be found in the outer phloem.
s They have the same form as those in the pericycle, and are
_ made by the expansion of a two-cell-thick medullary ray.
14 INTRODUCTION
(vii) Horizontal resin ducts are formed in some of the
medullary rays. According to Willkomm these may be
made as early as the second year, but I have not observed
any ducts in such young stems.
Cambium. This is composed of very narrow thin-walled
elongated cells. They are mostly about 10-20 » in tangential
direction, but those which form medullary rays are narrower.
Xylem. This has the characteristically simple form of
the Abietineae, and is composed of tracheides, tannin-
containing cells, resin ducts, and medullary rays.
(i) The tracheides compose the greater part of the wood.
In transverse section they are 10-20» square in the spring
wood, but in summer wood they may be as little as 5 in
radial measurement. They have lignified walls with a single,
or occasionally double, row of bordered pits on the radial
walls, and the last two or three rows of summer wood have
them also on the tangential walls.
(ii) Tannin-containing cells have the same form in trans-
verse section as the tracheides. They have, however,
cellulose walls with simple pits, and accurately transverse
septa occur at distances of about 1004. The simple pits
connect with the bordered pits of adjoining tracheides or
with the simple pits of medullary ray parenchyma cells.
Their contents give reactions for tannin and protein, but not
for resin, at any rate in March. They probably correspond
to resin cells in allied types.
(iii) Resin ducts occur scattered irregularly in the summer
wood. They are always small, and are formed in the
following way. Four cells, which correspond to tracheides,
form an intercellular space between them, and, remaining
thin walled and alive, secrete resin into this space. The
intercellular space then becomes the duct, and the four cells
correspond to the epithelium. The walls of the latter may
become weakly lignified, though remaining thin, and the
cells may divide so that there are ultimately six or eight
epithelial cells surrounding a duct. About nine or ten ducts,
but generally less, may be formed in the wood of the first
year, and corresponding numbers in succeeding years.
INTRODUCTION - TED
(iv) The medullary rays (vide fig. 67) are composed of
_ two kinds of cells. Firstly, the medullary ray parenchyma
cells with thickened cellulose, or slightly lignified, walls
with 2-5 (generally 4) slit-shaped simple pits into each
_tracheide they cross. These cells contain protoplasm,
nuclei, starch, and resin. And, secondly, medullary ray
tracheides which are not present in all rays, but are present
on the upper and lower sides of many of them, and occur
as intermediate layers of some. These cells have thin
lignified walls with small bordered pits, and contain only
water. ‘
_ Medulla. This is the star-shaped central portion, having
usually five unequal points which are left by the tracks of
leaf traces, and thus correspond to the five rows of leaves.
The medullary cells have at first cellulose walls, but when
secondary wood is formed they become lignified, though
the cells may still contain starch.
CHAPTER II
THE LARCH CANKER
General. Historical. The mycelium of Dasyscypha calycina and its ;
effect on the tissues. The canker as a pathological structure.
General. The canker or blister of the larch is by far the
best known of the diseases of this tree. It is exceedingly
common and very destructive, and, since none of the methods
which have been adopted with a view to preventing attack
have met with success, the pest bids fair to become even
- more disastrous in the future than it has been in the past.
In Germany it has already made larch growing so un-
profitable that the tree has almost ceased to be planted |
except where sparingly mixed with other species, and a like
fate must follow it in many parts of Britain unless a system
of growing can be adopted which will to some extent obviate
the evil.
The disease is due to a fungus which has been called by
a variety of names, but is now generally known as Dasyscypha
calycina' in Britain and D. Willkommii on the Continent.
This fungus belongs to the class Ascomycetes, since it bears —
its spores in eights inside an enlarged hypha or ascus
(fig. 18, A, p. 40), and it is placed in the sub-class Dis-
comycetes since its fructification is in the form of an open
cup or apothecium (fig. 17, p. 38), which is lined on the
upper concave side by the asci arranged at right angles to
the surface. .
D. calycina is almost universal on recently dead branches
of larch trees. Its fructifications are very small, being
seldom more than one-eighth of an inch in diameter, of bright
orange or yellow colour above and white below, and each —
+ The synonymy of Dasyscypha calycina is discussed in a note at the
end of Chapter IV.
THE LARCH CANKER 17
apothecium has a short white stalk. When growing in this
way (i.e. saprophytically) it does no damage to the tree.
It is only when it begins to live parasitically and to prey
on the living tissues that a canker is formed.
When it has penetrated the cork protection of a tree, the
mycelium of the fungus kills the cortex and phloem (the
soft tissues which surround the wood) and causes them to
turn brown. Next it attacks the cambium, and since this
is the tissue which gives rise to new layers of wood and
phloem, its death renders the tree incapable of further
_ growth in thickness-at this point. The fungus spreads very
_ slowly into the surrounding living tissues, and gradually
kills a wider and wider area of cambium, so that a larger
_ patch of the tree each year fails to keep up with the growth
of the rest. In this way a flattened cavity is formed at the
point where the cambium has been killed, the dead bark
becomes blackened, resin oozes out and streams down the
tree, and the whole presents the ugly appearance of a black
blister or running sore. This is the canker.
On the blister may often be fgund the fungus fructifica-
tions, which are generally larger than the similar apothecia
on the dead branches. When fresh these are orange-red
in colour, but later they become bleached to a dirty white
or yellow, and in this state may be preserved for many
years in a stream of resin.
The effect which canker has on a tree depends on the age
at which the attack occurs. To young trees it is often
fatal, for the trunks grow but slowly during their first five
_ or six years, and may consequently be entirely girdled by
_ the fungus. This very frequently happens when grass and
weeds are allowed to grow long among recently planted-out
trees, producing humid conditions which encourage canker
formation. When a tree is girdled in this way the upper
_ part continues to grow for some time, for it still obtains
_ water and mineral food from the soil and carbon from the
_ air; but organic compounds which it sends down towards
_ the roots are checked at the point of girdling, causing
increased growth and bulging just above it, and the roots
1888 a
sn i SMe ' THE LARCH CANKER
_are starved. Thus the roots die before the upper part of —
the tree, and the tree subsequently has all the appearance
of having died from a root disease. Similar girdling may
be seen in the side-branches of older trees, since these also
grow very slowly in thickness. But when a main trunk is
attacked in a portion which is more than four or five years
old, the annual growth.in girth is usually sufficient to
confine the canker to one side. Also the healthy cambium
at the back of a canker is more active than it is above or
below, and the annual rings are consequently especially 4
broad, so that a swelling is formed at the back of a canker
which prevents the water-current of the tree being appre-
ciably interrupted at this point (see figs. 1 and 9). There is
thus no reason why the top of such a tree should not go on
growing just as vigorously as one which has no canker ;
and such is the case, for often in a twenty-year-old plantation
the tallest and strongest-growing trees are found to be
cankered near the base. :
One canker in an otherwise sound tree may not prove
a very serious blemish; but when, as is not infrequent, we
find as many as six or eight cankers on the main trunks of
nearly every tree in a young plantation, then the value of
each tree is reduced to a very small-figure, and the wood is
certain to prove a financial failure. It is against these
attacks in which canker becomes epidemic that we have to
protect our forests. The disease would not be so notorious
were it not so extraordinarily hard to prevent. Only the
forester, who, over acres of otherwise healthy larch planta-
tions, sees canker after canker appearing, on his best trees
as well as on his worst, can know what a curse this pest has
become to European forestry.
Sometimes a tree apparently recovers from a canker
(fig. 8 and fig. 25). This only happens when the cork layers,
by which the tree always tries to prevent the spread of the
fungus, have been successful, and failing to find new feeding
grounds the fungus has died of starvation. The surrounding
tissues then grow over and occlude the canker just as they
occlude the wound formed by the fall of a branch. It may
Re aoc ee 2 Be a De,
‘(+ x) wniquivo Sutureurer Aq
Opel SSULI [enUUv JO YIpPLeIq pojViedsexe SuLMOYS
‘u1a4s 94} polpaLo Ajrvou svy YOIyA AoyuULgQ—'G “O17
.
med
3 a]
x ) sapIs 4JOG UO pa]vay JayUuYQ—'g “DIA
CE ———
mr Py a ow
" ; 7 e PPT Sey eee eeu.
ae A 7 tf j Leh co x : At x Be ] | af ep t ate ae oy ! . ‘ Dutan tou satel a\? Ve 2
shiaeiian! Ss 6&- 8 eee | + AT! y ar ee ae .% Ls : *
’ ; Pt , = —_ ’ + ‘
nfinrt ieee fe pone coighye tee! SEG : :
as < ep eh A ; . 5 ait
Ps aera Sik Ay are STOR tae oe Oe ‘ we '
* * / »
hag
? bya
THE LARCH CANKER 19
| eventually be impossible to detect from the outside where
| the canker has been, but though the tree is to all appear-
fae ances sound, there remains inside the blemish which the
| __ canker has left. | |
|} At present larch canker is prevalent only in Europe. It
has been recorded from Britain (until twenty years ago it
| was rarely found in Ireland, but has now become common),
ce tees
_ France, Holland and Belgium, Scandinavia, Russia, Ger-
many, Austria, Hungary, Italy and the Balkans, and
__ probably its boundaries here are coterminous with those of
larch cultivation. Australian and American text-books do
not include the disease among their own pests, but care will
_ be needed to prevent.its introduction to these countries
_ with seedlings from Europe. Indeed it has already been
_ reported from Newfoundland, so that the danger to America
is imminent. , 3
Historical. It would be hard to say when or how the
- canker fungus was first introduced to Britain. According
to Booth (1904) it was known to the Duke of Atholl at the
beginning of the nineteenth century,! and Loudon (1838,
|p. 2384) quotes as follows from de Candolle: ‘Sometimes,
also, we see the larches having a wound of resinous cancer ;
but this seems to proceed from some accidental cause, such
- as a blow or a knock, which the tree may have received
Es 4 when it was in full sap. All these observations incline me
' to think that the cause of the diseases which attack the
' British larches must be sought for in some difference exist-
_ ing in the physical nature or in the culture of your trees
| and ours.’ From this quotation we learn not only that the
- canker was fairly common in Britain by 1838, but also
that it was less frequent in France than here.
‘The suggestions offered by de Candolle on the etiology
_ of the canker are not happy, and Loudon’s volume had
_ reached the conventional age of manhood before the true
)
TPS Ey
pew
: % + T have been unable to find a copy of the Duke of Atholl’s book (1832)
| to confirm this reference. Schotte’s view (1917) that ‘ canker, as a rule,
has always been found where the larch occurs ’, may probably be accepted
2 for Britain as well as Sweden. .
= co?
20 THE LARCH CANKER
cause of canker was discovered by M. J. Berkeley, who
published a short article on the diseases of the larch in the
Gardener's Chronicle for 1859. The canker which he described
was found in a specimen ‘forwarded by Sir Walter C.
Trevelyan in which mycelium has penetrated through the
bark and produced its proper Fungus, under the form of .. .
Peziza calycina. In a small plantation, most of the trees
of which are young, nearly all are more or less attacked on
stem or branch with the Peziza.’
Berkeley’s observations were characteristically direct and
accurate, and he demonstrated the following features of
the disease. (i) In affected portions the cambium is first
killed in winter, since inside the canker the year’s wood is
always complete. The cambium must thus have been
destroyed after the formation of the summer wood and
before that of the spring wood. (ii) The growth of the
cambium in the spring and summer is sufficient to counteract
that of the fungus, but each winter more and more of the
cambium is destroyed, so that a section of the cankered
portion shows a step-like arrangement of the wood, one
step corresponding to each year; or, as Berkeley described
it, the wood is like an amphitheatre with its seats raised
one above another on each side of a central depression
(see fig. 12). (iii) The actual wood inside is attacked, but
‘it should seem that the disease does not originate from
the wood, and that the fungus is introduced into the wood
1 Miles Joseph Berkeley was born April 1, 1803, near Oundle in Northants.
He was educated at Rugby and Christ’s College, Cambridge, and took
Orders in 1826. In 1829 he went as curate to Margate, and found time to
study the anatomy of molluscs, and later, seaweeds. From 1833 to 1868
he was perpetual curate at Apethorpe and Wood Newton and lived at
King’s Cliffe, Northants, and from 1868 till his death in 1889 he was vicar
of Sibbertoft, near Market Harboro’. Berkeley wrote the volume on
fungi in Smith’s English Flora (1836), and described (with Broome) the
fungi collected by Darwin during the voyage of the Beagle, as well as other
collections. He also published a number of books, chiefly on fungi, and
many articles from his pen appeared in the Gardener’s Chronicle and
elsewhere. His writings are characterized by extreme care and lucidity
(vide Dictionary of Naticnal Biography, vol. xxii, 1901).
j
‘.
THE LARCH CANKER 21
irom the bark’. (iv) The disease occurred most frequently
in damp situations. (v) Berkeley observed that the Peziza
also occurred on many dead branches and on branches that
had been left on the ground after thinning.
Recent authors have commonly lost sight of the fact that
canker was first attributed to its true cause by an English-
man, and Berkeley’s article has often been ignored by
writers who have taken their descriptions from the more
detailed papers of the German professors Willkomm and
Robert Hartig. Willkomm’s! treatise, published in 1867,
is, for the time at which it was written, a remarkably full
account of the parasitology and pathology of the disease.
But it is marred at the outset by an inaccuracy in nomen-
clature, which he would have avoided had he been acquainted
with Berkeley’s article. His description and figures leave
no doubt that he was studying the fungus Dasyscypha
calycina, but he confused this species with another fungus,
Corticium amorphum,? a Basidiomycete, belonging to an
entirely different group of fungi, which has a superficial
resemblance to Dasyscypha and may sometimes be found
growing with it. If this error in nomenclature be corrected
throughout the paper, the reader will find an accurate
record of much that was not previously known about the
1 Moritz W. Willkomm was born June 29, 1821, at Hewigsdorf near
Zittau. He studied medicine and science at Leipzig. He travelled over
a great part of Europe, taking a special interest in field botany, and wrote
extensively on the flora of Spain and Portugal. He took his Ph.D. at
Leipzig in 1850 and remained there as a Privatdozent. In 1855 he was
created extraordinary professor and custodian of the herbarium. Soon
after he was appointed professor of biology in the Forstakademie at
Tharandt, where he remained till 1868, when he proceeded to Dorpat as
director of the botanic garden. In 1874 he went to the German university
at Prague and stayed there till 1893, when he retired. He died in Bohemia
in 1898. Willkomm is chiefly known for his writings on the flora and
ecology of Spain and Portugal, but he published papers on many other
~ botanical subjects. . His Wicroscopischen Feinde is, as far as I am aware,
his only contribution to pathology (vide Allg. Deutsche Biographie, Bd. 43,
1898).
2 Hoffman (1868) was the first to call attention to this error. He
correctly named the fungus Peziza calycina
22 THE LARCH CANKER
disease. Willkomm described the formation of a canker in | :
- great detail, and even grew cultures of the fungus and
observed the germination of the spores. But he assumed,
without experimental evidence, that spores of the fungus
could germinate on larch stems and could give rise to
cankers by piercing the unwounded bark with their germ
tubes, a supposition which was subsequently shown by
Hartig to be inaccurate. Also he never proved that the
disease could be caused by the fungus acting alone, or
indeed that the fungus was a cause at all. All he showed
was that in every canker he examined mycelium, and
generally apothecia, were present ;. but whether the fungus
caused the disease or the diseased spots formed a suitable
breeding-ground for the fungus was left undecided. Patho-
logy, as we know it, was not then born.
Thirteen years later, Robert Hartig 1 (1880), after working
out numerous other tree-diseases, published his paper on
larch canker, which has remained since then as the standard
account of the disease. He states that the first complaints
of the disease were made in Germany about 1850, and by
1870 it had become a source of danger throughout the
whole of Germany and Scotland. It was especially pre-
valent in damp and foggy regions, and he thought it was
largely encouraged by inhibition of transpiration. It
occurred, however, in the Tyrol up to nearly 2,000 m., and
in that region cankers nearly one hundred years old were
1 Robert Hartig was born May 30, 1839, at Brunswick. Both his father
and grandfather were distinguished foresters, and he had an early training
in scientific forestry, both under his father at Brunswick (1861-3) and at
Berlin (1863-4). He served his time in the forests, and after taking his
doctor's degree at the university of Marburg he filled various positions in
the forest service. In 1869 he was appointed professor of botany at
Eberswalde, and from here published some of his most important patho-
logical papers, including * Wichtige Krankheiten der Waldbaume ’(1874)
and the ‘Zersetzungserscheinungen ’ (1878). In 1878 he was elected to
the new chair of forest botany at Munich, and remained there till his death
in 1901. His extraordinary energy is shown by the number and impor-
tance of his published works, which comprised 16 books and 130 other
papers (vide Biographisches Jahrbuch u. Deutscher Nekrolog, Bd. vi, 8. 93,
and Zentralblatt fiir das gesamte Forstwesen, 1902, pp. 37-46).
a ee ee ee ee ee ee a re
>:
THE LARCH CANKER _ 23
found. Hartig noticed certain microdimensional differences
_ between the canker fungus and the type Peziza calycina,
and named the former Peziza Willkommii, but as these
differences are not constant this name may be dropped.
The synonomy of the fungus is difficult, and is paneuscd) in
a note at the end of Chapter IV.
The most important contribution which Hartig made to
our knowledge of the disease was the result of his experi-
ments on artificial infection. Previous experiments of this
sort had been carried out by Fischer, a practical forester,
in 1877; he cut pieces of bark and phloem out of a canker
area and fitted them into suitably-shaped holes made in
healthy trees; the latter became infected and ultimately
showed all the features- of canker. This proved that the
canker was due to some transmissible cause, and was not
entirely the result of unsuitable growth-conditions, as seems
to have been generally supposed by English foresters even
as late as 1895! But it did not establish a connexion between
the disease and any one fungus, since a portion of infected
tissue cannot be regarded as a pure culture of any particular
parasite, and, as far as Fischer’s experiments could testify,
the Peziza might be a more or less constant concomitant of
the canker without being its cause.
Hartig’s experiments were of a more exact nature. He
grew small larches one metre high in pots and infected
them on September 29, 1879, with ascospores through small
- wounds. The pots were left in the open till the beginning
of January, when they were placed in a room, and by the
middle of January disease, accompanied by death of the
bark, was noticed within a centimetre of the points of infec
tion. By the middle of February normal fructifications
were formed. Investigations showed that the mycelium
had grown in October, but rested in November and Decem-
ber. All attempts at infection with uninjured trees failed,
and it was only-found possible to inoculate trees through
wounds. On this rather slender evidence Hartig based his
theory that the fungus could not gain admission to trees
which were entirely uninjured.
24 THE LARCH CANKER
In addition to this experimental work*Hartig added to
Willkomm’s description of the mycelium in the bark,
phloem, and wood of the larch. In the wood it was dis- |
tinguishable in the medullary rays, the resin ducts, and
tracheides. In the sieve tubes he noticed that the hyphae
bore short extensions, with or without a few branches.
He added little to our knowledge of macroscopic features
of the disease, with the exception of some observations on
the resin flow from the surface of the bark, but he provided
some interesting measurements of the growth of canker in
the longitudinal direction of the stem. They were taken
from three isolated cankers from different altitudes, and
though they were insufficient for a just comparison of the
rates of growth at these levels, they nevertheless give some
indication of the growth of a canker generally. The cankers
were measured at—
1. Steinach (Tyrol), 1,300 m. (4,300 ft.) above sea-level.
2. Grafrath (near Munich), 700 m. (2,300 ft.) above sea-—
level. :
3. Brunswick Forest Garden, 170. m. (550 ft.) above sea-
level. |
The results were as follows :
After Steinach. Grafrath. Brunswick.
years cm. cm. cm.
1 1-0 1-2 1-0
2 2-2 3°9 3°3
3 7:0 6-5 8-1
4 9-4 9-2 10-5
5 11-6 13-1 13-0
6. 16-8 17-7 14-7
7 20-2 20-6 16-0
8 21-8 248 16-9
9 24-6 30°5 19-3
10 27:0 — 23-0
The importance of*Hartig’s work on tree pathology will
be best appreciated when it is observed that reference has
been made to contributions by this author under the head
of nearly every disease mentioned in this book. The vast
field of research covered by Hartig appears still more
remarkable if we remembér that when he started on his
work the subject was in an entirely chaotic state, and it is
THE LARCH CANKER 25
chiefly through his untiring zeal that chaos has given place
to order. The authority of Hartig’s name had become so
great that it has been thought scarcely worth while to
reinvestigate diseases on which he had written, and con-
sequently less is often known about parasites which he
investigated than about others, of smaller importance,
which were unknown in his time. The ground has been left
as Hartig tilled it, and pathologists have sought new fields
where the spirit of the pioneer has attracted them. Hartig’s
views were presented in English by Marshall Ward (1889),.
but without addition or correction.
Among subsequent papers the following may here be
noticed :
Carruthers (1891) noted the blackening of the bark in the
neighbourhood of tlie canker, which he attributed to the
fungus Antennaria pithyophila, Fr., which looks like a covering
of soot. As the result of observations on young cankered trees
which showed no sign of having been wounded, he expressed
the opinion that young trees might be attacked by the Dasy-
scypha while still unwounded, if the bark and air were damp.
Somerville (1895) published the results of an inquiry
among foresters, instituted by the English Arboricultural
Society, in pursuance of which a number of questions were
put in relation to the causes and nature of larch canker.
Forty answers were received, the chief value of which was
to elicit evidence as to the kinds of soil and climate in
which the disease was most to be feared. The majority
*
agreed that damp situations are favourable to the canker,
and many regarded larch aphis as a predisposing factor.
Somerville suggests that this is due to the partial inhibition
of transpiration caused by the aphis, and also that the
aphis makes holes in the twigs and spurs, through which
the spores of Dasyscypha may infect. A similar inquiry
was held by the Scottish Society in 1905 (vide Richardson,
Borthwick, and Mackenzie).
Massee (1902) performed experiments to Ue tipnatpates the
connexion between the aphis, Chermes abietis, and the entry
of the Dasyscypha, and found that canker spots resulted
26 THE LARCH CANKER
from placing spores in contact with the mucilaginous
excreta of the Chermes in the spring.
A. P. Anderson (1902) gave an account of canker on
' Abies balsamea, which is caused by an allied species, Dasy-
scypha resinaria. This canker resembles, in most respects,
that of the larch, and the further details which he provides,
especially in connexion with resin flow, are for the most
part equally true for the larch canker. :
Miinch (1909) carried out some interesting experiments
on the relation between the rate of growth of a canker and
(i) the air-content of the tissues, (ii) the temperature. He
cut young shoots (5 cm. broad), which, though cankered,
were still living, into strips 20-30 cm. long, each with
a canker. These he dried to varying extents, and found
that the canker spread much more rapidly in the drier than
the moister stems. He attributes this to greater air-content,
but it may also be accounted for by the lessened vitality
of the tissues induced by drying. He also found that the ©
minimum temperature for the growth of the fungus is
above 0° C., so that it must be incapable of spreading during
an Alpine winter. rire
The mycelium of Dasyseypha calyecina in the larch stem
and its effect on the tissues. The hyphae of D yscypha
calycina have been found in the outer cortex of the leaf
cushions, the inner cortex, the phloem aud xylem, and the
medullary ray portions of the cambium.
Its presence in the leaf cushions of uninjured larch stems
is of no practical importance. No great development of
Dasyscypha hyphae has been observed in this region ; but
that there is sufficient nutriment for fungal sustenance in
these cushions, even when they have been cut off by a cork
layer, is demonstrated by the fact that fungal pycnidia may
not infrequently be found growing in them. If the hyphae
of Dasyscypha obtained a firm hold on the cushion, any
weak points in the cork armour would be liable to discovery,
which would put the plant in danger of infection without
previous wounding ; but there is no direct evidence for this
ever having taken place.
A
im
?
7)
t,
{
Pa
\
4
4
small oil drops. As the hyphae use
‘ a ae A,
t
‘close masses of hyphae that the
THE LARCH CANKER Ges
The most marked development of the mycelium is locatedin
the inner cortex and outer phloem. The hyphae grow freely
in the intercellular spaces (which, as shown in Chapter I,
are especially large and frequent in the outer phloem) and
the resin cysts, and send branches through the walls into
the cells, and these branches ramify and pass from cell to
cell. The hyphae are usually rather small in the inter-
cellular spaces, and the branches
which enter cells are broader.
These larger hyphae have often a _
wavy outline, and contain numerous
up the food contained in the cells of
the cortex and phloem, these cells
contract and leave large spaces which
become tightly packed with the
mycelium of the fungus, and it is
from the more superficial of these
fructifications arise.
Except in cases of advanced
disease, the hyphae do not grow ||;->-™
luxuriantly in the inner phloem, N
cambium, or wood; this is probably — fy¢. 10.—Mycelium of
due to there being few intercellular Dasyscypha in the wood
of the larch. Radial
©)
—S
a] spaces, and consequently insufficient ection (x 630).
aeration. When they do enter the ° ,
wood they are at first nearly confined to the medullary
ray parenchyma ; only a few branches enter the tracheides,
and these do not grow freely. But as the disease advances
the air-content of the wood becomes relatively much higher,
--and the mycelium enters the tracheides and changes the
- wood from the normal yellow to a reddish-brown colour.
Gradually in this way the mycelium may penetrate to the
centre of the stem:and attack the heart-wood, which, having
a higher air-content, provides a more suitable substratum
than the sap-wood ; and, as the heart-wood increases, the
fungus can flourish in it and fill many of the tracheides with
28 THE LARCH CANKER
an anastomosing mass of hyphae. In this case a red-brown
stain is formed in the centre of the stem, irregularly star-
shaped in section and continuous on one side with the
canker, but spreading for a short distance above and below
it in the heart-wood.
The hyphae are colourless and of markedly varying
width ; they pass through the tracheide walls by very fine
bore-holes, which are usually accurately transverse, but
may be somewhat oblique or may even be transverse on
one side of the middle lamella and oblique on the other.
The whole mycelium in this region is evanescent and does
little damage to the wood.
An interesting phenomenon in connexion with this attack
is the formation of a gum by the infected wood. This gum
has been worked out more in detail in connexion with the
larch heart-rot which also stimulates its formation.
Under the influence of fungal attack the tissues show
certain changes which must be described in detail.
(i) The walls of all the cells in the neighbourhood of the
mycelium turn brownish yellow in colour ; frequently their
contents are modified ; the protoplasm becomes markedly
vacuolated, and the nuclei lose their rounded outline. —
Large accumulations of crystals of calcium oxalate, both of
the mono- and tri-hydrated form, appear in the cortex,
and crystals of the trihydrated form become much more
frequent in the phloem parenchyma, and may also occur
in the medullary ray cells. Large quantities of resin are
formed in all cells, much more than can be contained in
the resin cysts, so that all the tissues become saturated
with it. The tannin content is also increased. Tannin and
resin are probably products of excessive katabolism, show-
ing that the protoplasm of the cells has respired to an 4
abnormal extent, only ending in death. The calcium
oxalate may arise partly in this way, though as shown
below it is also excreted from the fungus.
These phenomena are not confined to the cells which
either contain hyphae or have hyphae touching them ; but
all the cortical and phloem tissue, for a thickness of about
wv
~
perc!
en ®
THE LARCH CANKER 29
three cells round the penetrated area, becomes yellow, and
cells at a considerably greater distance contain more crystals
than normally. The contents of the medullary ray paren-
chyma become brown, and the nuclei appear irregular,
through two or three annual rings of xylem: before the
hyphae can be observed to have reached the cambium.
There is thus evidence that some poisonous substance is
transmitted from the hyphae to surrounding cells (especially
along the medullary rays), which first stimulates excessive:
respiration in the cells, and ultimately causes death. It is
not clear whether this substance is actually secreted by
the hyphae, or whether it is some product of decomposition
of the attacked cells, but there are certain reasons for
favouring the former view. A phenomenon which is of
interest in this connexion was observed in artificial cultures
of Dasyscypha calycina on nutrient agar.’ Streak cultures
were grown in Petri dishes, and all round the mycelium
a kind of halo was noticed, which proved to be due to
numerous crystals of monohydrated calcium oxalate extend-
ing to a distance of 1 cm. from the mycelium. Presumably
oxalic acid was secreted by the hyphae, and was converted
into calcium oxalate by the calcium in the substratum.
This phenomenon is not confined to cultures of Dasyscypha
calycina ; but it has a particular interest in this case, as it
suggests that oxalic acid may be the actual substance
secreted by the fungus which kills the cells of the host ;
_ for probably the same process takes place in the tissues of
the larch. At first the cells in the neighbourhood of the
hyphae are able to render the acid harmless by means of
the calcium at their disposal, which accounts for the accumu-
lation of calcium oxalate in the affected tissues. But when
this is used up the concentration of acid gradually kills the
cells. By this means the fungus is able to spread through
living tissues without being a true parasite ; that is to say,
it kills the cells by secretions and enters them when they
__aredead. This kind of false parasitism has been investigated
1 Agar-agar 10 grm., malt extract 30 grm., meat extract 3 grm., and
citric acid 0-3 grm. in 1,000 c.c. distilled water.
30 THE LARCH CANKER
by de Bary (1886), and later by Kissling (1889), Smith
(1902), and Brooks (1908), in the case of Botrytis cinerea.
Various views have been entertained as to the substance
which Botrytis secretes, but recent work by Blackman and
Welsford (1916) and W. Brown (1915, 1917) has shown
that oxalic acid plays no significant part in the parasitism
of this fungus. These authors have further demonstrated _
that the only active substance secreted by the germ tubes
of the fungus is an enzyme which destroys the cellulose
walls of the host. Whether or no oxalic acid plays a part
in the parasitism of Dasyscypha, there seems to be little
doubt that the fungus should be regarded rather as a plant
poisoner than a true parasite. It first kills the cells in its
neighbourhood and then grows into them.
(ii) The tissues in the proximity of the fungus grow
faster than usual, causing a swelling which is noticeable in
young cankers. A specialized form of this activity is the
formation of new cork layers. These may surround an
infected area and inhibit further advance of the fungus.
Cases have been observed where the mycelium has been
successfully prevented from reaching the phloem by such
a cork layer, and probably this frequently happens, though
it easily escapes observation, as the general healthy appear-
ance of the stem is so little disturbed. The tree always tries
to isolate the fungus, and when the mycelium has penetrated
the phloem and killed the cambium at any point, phellogen
layers are instituted, which cross the cortex radially and
then traverse the phloem in an oblique direction, and this
phellogen forms a ring of cork which tends to prevent the
canker spreading in a lateral direction. The cork layer
formed by such a phellogen for a time completely prevents
the further growth of the mycelium, and in some sections
I have seen the tissues on one side of a cork layer healthy
and apparently normal, whereas on the other side the cells
were brown and had contracted, leaving large intercellular
spaces filled with hyphae. Often, however, such cork
layers may be found embedded in the brown attacked
tissues, showing that the fungus has succeeded in ffassing
Se oe ee ee ee ee ee Se
——
THE LARCH CANKER 31
them. The hyphal development in the wood never seems
to be of a nature that can cause reinfection of the phloem,
so that probably the hyphae get round the cork layer in
the cambium just outside the wood, and spread from that
point. Cork formed in the vicinity of the fungus is apt to
become red. This is due to infiltration with resin, and cork
cut fresh from a healthy tree may be made to assume this
red colour by boiling in resin. u
(iii) The character of the secondary Sand which is made
in the neighbourhood of the canker is also profoundly
affected.
Where the cambium is killed, of course no more wood
can be formed ; but the cambium which is still living on the
- flanks of this dead patch cuts off wood elements which are
essentially different from normal xylem (fig. 4 and fig. 11).
The cells which are first differentiated in the spring wood
remain, thin walled and show no tendency towards sliding
growth. Their walls become lignified and have numerous
simple pits. Sometimes the living contents remain, but not
infrequently they become disorganized and are replaced by
water. Thus in many ways they resemble medullary ray
parenchyma, except that they are extended longitudinally
instead of radially. Irregular intercellular spaces are formed
between them, and these often become filled’ with resin.
_ Normal resin. ducts are also formed, besides such irregular
_ abnormal ones (fig. 4, p. 8).
As the summer advances tracheides are formed. These,
however, have an irregular wavy outline, which gives
_ a truly longitudinal section the appearance of being oblique,
} although entire tracheides may be included in it. Subse-
quently formed tracheides are normal except for the fre-
quency of a tertiary spiral thickening of the walls. Tannin
cells are frequent throughout this portion.
Kor
The significance of this abnormal wood is difficult to
define. In many respects it resembles the wound-wood
described by Véchting (1892) and Kuster (1903); but
- Hartig (1892) figures a very similar development in spruce
_ after defoliation by the nun moth, and Harper (1913) has
32 THE LARCH CANKER
found a similar formation of resin ducts in larch which has
been attacked by the sawfly. These comparisons do not
support the contention of de Vries (1876) that such wood
is the result of reduced tangential pressure produced by
a wound, since in two.cases the tree was not wounded
_———....
{1
Fia. 11.—Radial longitudinal section of abnormal wood: a, summer
wood of previous year, abnormal—one cell shows tertiary thickening ;
b, short sclerenchymatous elements ; c, long sclerenchymatous elements ;
d, nearly normal xylem.
except in its crown. We have rather to look for the cause
in the reduction of food supply which may be brought about,
either by defoliation, or by the death of the phloem in the
neighbourhood of the developing wood.
In the canker the formation of normal wood is resumed
only when new phloem has been made outside, which forms
THE LARCH CANKER 33
efficient channels for the conduction of organized food
material sent down from the leaves. _
The canker as a pathological structure. The type of
structure known as a canker is the outcome of the action
_ of a fungus on a stem. The same general result may be
_ produced in different trees by different fungi (cf. canker
_ of apple and ash caused by Nectria ditissima). But fungi
_which are capable of producing cankers must have four
properties which, together, distinguish them from most other
_ parasites. These features may be summed up as follows:
(i) They must be capable of killing living tissues, i.e. either
¢ they must be parasites in the ordinary significance of the
word, or they must be capable of secreting some substance
_ which kills the cells in their vicinity so that the fungus can
penetrate them.
(ii) They must be essentially ‘rind fungi’, i.e. fungi
whose mycelia flourish in the cortex and phloem of the stem
~ and kill the cambium ; and if the hyphae enter the wood
i ate asl
ape i ea
_ at all, they must be incapable of spreading extensively in
_ this part of the stem, or reinfecting the phloem ; otherwise
the hyphae would soon spread right through the tree and
» k kill an entire section of the stem.
(iii) The mycelium must be perennial in the tissues of the
cortex and phloem.
(iv) The mycelium must spread from cell to cell extremely
slowly, so that its rate of tangential extension is approxi-
_ mately equal to the rate of increase in girth of the stem.
_ If the mycelium spread too fast, it would soon extend the
whole way round the stem, whereas a canker can only be
produced as the result of continued growth of the stem
_ whilst being attacked. And if the mycelium spread too
_ slowly it would never get through the phloem and reach
the cambium. When a fungus with these four characteris-
_ tics attacks a host, a canker is the necessary result.
The canker of the larch usually obtains its hold on the
2 stem within the first six years of its growth. The fungus
gains admission—in what way will be shown later—and
- flourishes in the cortex and outer phloem, where there are
1888 D
384 THE LARCH CANKER
large intercellular spaces. All the cells in the neighbourhood —
of the fungus are killed by secretions, and the fungus follows
these secretions and feeds on the cells which have been
killed bythem. The further development of the canker can
best be followed by reference to the accompanying diagrams.
A
Peat ae ee “ee
>
hug
Cr al oe W
a ee
if a
Fic, 12.,—Diagrams showing stages in the development of a canker:
A, first spring; B, first autumn; ©, second spring; pb, third spring; 48,
fourth spring. For description, see text.
Fig. 12, A, shows a transverse section of a three-year-
old stem in which'a portion ‘x’ has been attacked and
killed by Dasyscypha. In.the spring of the first year the
fungus has not reached the cambium, but is near enough to
the section AaB of the cambium to have affected it and
caused it to make the abnormal wood AB of fig. 12, B.
This latter figure shows the state of affairs at the end of
THE LARCH CANKER 35
the first year ; the formation of the phloem outside AB has
kept the mycelium away from the cambium, though the
mycelium has spread, especially in a tangential direction.
Fig. 12, c, shows the position in the following spring. The
section cD of the cambium has been killed by the inward
growth of the fungus, which has also come sufficiently near
the segments EC, DF to affect them in the same way as AB
had been affected a year earlier. By the next spring the
canker has grown to the stage seen in fig. 12, D; op has
_ made no wood, EC, DF have made abnormal wood similar
_ to AB, and during the winter the mycelium has killed the
further stretches of cambium Go and DH. Fig. 12, 2, shows
_ the canker after developing one more year on the same
_ system. It is thus seen how the amphitheatre-like canker’
is formed, and also how the abnormal wood is related to it.
The step-like configuration of the sides of the amphi- ©
theatre clearly shows that the fungus makes greater inroads
on the host in the winter than in the summer months. This
can be accounted for most simply by the winter cessation
__ of growth on the part of the tree, and it is quite unnecessary
_ to assume with Hartig that the fungus grows more actively
in winter than in summer. Hartig suggested that in the
summer the tree tissues contain a larger percentage of air
than during the winter (since in summer the soil and air
_ are drier and the transpiration pull is greater), and that ©
ED the reduction in water content inhibited the mycelial
_ development of the fungus. But Miinch (see p. 26) has
__ shown that a high air-content stimulates, rather than retards,
_ the growth of the fungus, so that Hartig’s explanation .
_ is not in keeping with the experimental evidence. yi ae
_ Miinch’s discovery that the lower limit of temperature for
¢ fungal growth is above freezing-point precludes the possi-
bility of fungal development during an Alpine winter. It
is sufficient for our purpose to observe that during the
é winter the dormant cambium is more exposed to attack
_ by the hyphal secretions, whereas during the summer it is
_ protected by a constantly. thickening layer of new and
active phloem,
D 2
36 THE LARCH CANKER |
When part of the cambium has been destroyed, the rest
makes up for the loss by greater activity, and opposite
a canker the wood is abnormally thick, and the whole
stem may bulge in a curious fashion. The same may happen ~
at the sides as well, so that when one looks the canker full
in the face it appears to be set in an especially broadened
part of the stem’ (figs. 13 and 14). yh
All the cells in the neighbourhood of a canker form large
quantities of resin, which permeates all the tissues. This
gradually oozes out through the shrivelled cortex and
phloem, and losing its more volatile constituents on exposure
to the air, hardens into large whitish drops. Such large
quantities are often formed that it runs down a stem in
streams, covering the bark for several feet.
22% he ll
aoeg
‘SOpIs JV SUT[[IMS SUIMOYS IOYULQ—'F ‘DIY ye SUT[[AMS SUTMOYS YOIV~T SunoOd UO JOyULO—'ET “OI
af
Fie. 15.—Apothecia of Dasyscypha calycina
on dead larch stem (x #). Closed in dry
weather.
ye
Fria. 16. -Same apothecia as in Fig, 15,
open in wet weather.
‘<4
CHAPTER III
THE LARCH CANKER (continued)
The reproductive organs of the fungus. Germination of spores. Pure
cultures on nutrient media. Artificial infection with canker.
The reproductive organs of the fungus. The reproductive
organs are of two kinds:
1. Apothecia.
2. Spermogonia or pycnidia, the name depending on the
interpretation of their morphological nature.
When fructifications are about to be formed there first
appear on the surface of the bark a number of white or pale
yellow felt-like mycelial outgrowths. These may give rise
to both apothecia and spermogonia, the latter being generally
mature just at the time when the former are initiated.
Apothecia. The youngest stage which I have traced in
the formation of apothecia consists in a group of hyphae
_ standing out from a small portion of one of the mycelial
masses on the surface of the stem. These hyphae are easily
distinguished by numerous very fine wart-like outgrowths
on their walls. The portion bearing them is circular in
outline, and towards the centre the hyphae are bent inwards.
- Underneath these hyphae there next appears the first
indication of a layer of hyphae directed at right angles to
the surface. This layer, which is the young hymenium or
ascus-bearing layer, becomes broadened by the interpolation
of fresh hyphae, and the apex of the apothecium opens,
leaving the warted hyphae as a broad fringe of hairs round
the hymenium, which is orange in colour.
The apothecium grows for a long time, reaching a size of
_ 2-4mm., orin extreme cases 5mm. The hymenium becomes
_ paler with age, and the margin and under-surface are white
throughout. The whole is raised on a short stalk about
_ 1 mm. in height and rather less in thickness (fig. 17).
38 “THE LARCH CANKER
A section through a ripe apothecium shows the lower
portion, or excipulum, differentiated into two parts. The
outer (lower) portion, or cortex (fig. 17, c), is composed of
a mass of closely interwoven and rather swollen hyphae of
firm texture. The middle portion is made up of hyphae
which are much less closely interwoven. The hard cortex
not only prevents. loss of water from the apothecium, but
also assists in closing it in dry weather. For on drying the
central portion contracts more than the cortex, and this
WN ant NE
Me oe nf AU ei anit
NS ST WW a ne eee
aS = SSNS NY WAL
Ze ne, nh
Fie. 17.—Section through apothecium of Dasyscypha calycina: c,
cortex ; h, hymenial layer; s./., subhymenial layer.
causes the margin to fold over the hymenial layer (figs.
15 and 16). |
The hymenium (fig. 17, 2) is a layer composed of asci
and paraphyses, and is borne by a dense subhymenial
layer, s.l. The details of the hymenial layer are more clearly
shown in fig. 18, A. The ascus, a, is the organ which pro-
duces and ejects the spores. The eight spores occur in
a single row and fill the upper part of the ascus, whilst the
lower part contains hyaline protoplasm. The length of the
ascus is 150-200, and the greatest breadth is 10-l4y. It
is nearly cylindrical, but somewhat swollen towards the
upper end, and tapering at the lower end. The walls are
sufficiently thick to show a double outline under a D objective,
except at the extreme apex, and are quite smooth. The
- spores (fig. 20, A) are ellipsoidal, the long axis being 20-23,
a. a a
.
y
ee ee! Se
am = A Air
on
+
OO Se Gr heey LF SE PO Gy A ay t
a ae
we
fe
¥ be
i.
me
z
‘Md
Lae?
a
i
Lae.
re
i -
he
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) org
ka
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i
,
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es
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ae
oa
THE LARCH CANKER 39
and the short axis 9-10. Each contains, when ripe, a single
central nucleus and a large hyaline vacuole, containing
glycogen, on each side of it, lying with it in the long axis
of the spore. The wall is thin, but appears with a double
contour under a D objective. The spores are ejected by
a sudden squirt action to a height of about half an inch,
and after ejection the top portion of the ascus can be seen
to be bent back as in fig. 19.
The apex of the ascus, at any rate in my specimens, does
not turn blue with iodine. As some authors have described
this as a feature of the fungus, it is apparently variable in
the species. |
The paraphyses (fig. 18, p) are thin hyphae which are
found between the asci.. They are unseptate and very fine,
1-5-2 in diameter, and about 30, longer than the asci.
The apices are scarcely perceptibly swollen, but they tend
to bend over the asci and protect them. When kept in
a saturated chamber the whole apothecium often becomes
filled with a drop of water. This is probably secreted by
the paraphyses, and as the secretion presumably continues
in a dry atmosphere also, it may serve by evaporation to
moisten the air round the asci and save them from drying up.
Fresh asci continue to be formed for a long time. They —
appear as short swollen hyphae growing up from the sub-
hymenial layer, but ascospores cannot be traced in them
until they are fully grown.
The margin of the apothecium is covered by stiff bristle-
like hyphae, which, as described on p. 35, have numerous
very fine warts on their walls. Anderson (1902) describes
these warts as crystals of calcium oxalate. But I find that
they do not dissolve in acetic acid or in dilute sulphuric
or nitric acid. I am inclined to regard them, therefore, as
outgrowths of the wall substance.
The same general description applies to the apothecia
found on dead branches. These are, however, smaller, and
ior their dimensions the reader is referred to p. 79.
Spermogonia (fig. 18, B, c). The spermogonia are also
formed on the mycelial cushions, and may be found on the
40) THE LARCH CANKER.
same cushions as very young apothecia. They consist of
a ramifying series of cavities nearly filled with hyphae
Fic. 18,—Dasyscypha calycina, reproductive organs: A, asci and
paraphyses (x 430); a, ascus; p, paraphysis; B, spermogonia (x 430); ©,
spermogonia, showing escape of spermatia (430); D, spermatia (x 1,000).
radiating towards the centre. These fine hyphae abstract
from their apices numerous spermatia, small slightly elon- _
gated cells (about 1°5 X Ly), which remain for a consider-
THE LARCH CANKER 41
able period in the cavities. The cavities usually, though
so far as I can determine not invariably, connect with the
outside air by irregular mouths, whieh are mere gaps in the
mycelial cortex, and are not lined by specialized hyphae.
Through these mouths the spermatia are forced out by the
formation of numerous younger ones behind, and often
remain in masses stuck together by a CURE ONS liquid
which is exuded with them.
These spermatia are quite functionless, andl as the name
implies, are regarded as vestigial male cells. Many botanists |
' now consider them to be functionless
conidia. The reasons against this view are
shortly as follows.
Firstly, they will not germinate. Massee
(1902) describes and figures cells, which he
considers to be conidia, germinating by
normal germ tubes in the presence of
sections of larch bark. But the cells
which he describes are round and larger
than spermatia, and bear an _ intimate
resemblance to conidia of Penicillium sp., jae igs: "6
which grows commonly on the apothecia charged its spores.
of Dasyscypha. Brefeld (1891), who also
regarded these cells as conidia, states that Dr. Moller, working
in his laboratory, found. that they became slightly swollen
when placed in a nutrient solution, but did not germinate.
_ This incapacity for germination is normal in spermatia,
whose only function is fertilization; but it is not easy to
understand why conidia should ever become functionless in
this way.
Secondly, we know from the researches of Baur (1898),
Darbishire (1900), Thaxter, &c., that in some Ascomycetes
fertilization is still effected by means of spermatia. In
other Ascomycetes fertilization by spermatia has been
replaced by fecundation by a hypha in proximity with the
female organ, or by the fusion in pairs of the nuclei in this
organ. Probably one of these methods obtains in Dasy-
scypha, but the investigation of this point is outside the
42 THE LARCH CANKER
scope of the present inquiry. That spermogonia and
spermatia can continue to be formed after they have
become functionless is shown by the rusts, where first
Blackman (1904) and later Christman (1905) and others
have shown that fertilization occurs through the fusion of
neighbouring hyphae just at the time when the spermatia
are shed. The spermatia of Dasyscypha recall those of the
rusts in many respects. They are small cells which do not
germinate ; they are set free just at the time when the
young apothecia are appearing, and they are forced out in
a mass of mucilaginous liquid. We are thus justified in
calling these organs ‘ spermogonia’ and ‘ spermatia’ with
de Bary and Willkomm, rather than ‘pycnidia’ and
‘conidia ’ with Brefeld and Massee. In either case they
appear to be entirely functionless, and only the vestigial
remains of what was once part of the reproductive
system.
Germination of the spores. If ascospores are required for
germination, it is essential that they should be ripe and
naturally ejected. They may be collected by placing
apothecia on damp filter-paper on the bottom of a drop-
culture chamber. The spores are ejected in eights, and
usually cling to the overlying cover-slip as a cluster; but ,
not infrequently they are somewhat scattered, five or six
appearing in one spot, whilst two or three separate spores
hit neighbouring parts of the cover-slip at the same time.
Sometimes it is impossible to trace more than six or seven
of the eight spores of an ascus, probably because the last
one or two spores to leave the ascus are not expelled with
sufficient velocity to reach the cover-slip. In support of
this view it has been observed that when the cover-slip is
raised by another culture-ring placed on the top of the
first one, it frequently happens that not more than four of
the spores reach the cover-slip. If two rings are placed
above the first, making three in all, none of the spores
reach the cover-slip, though the apothecia be subsequently
proved to be ejecting spores. The height of a culture-ring
is 5 mm., so that the maximum distance of ejection of
tinh:
THE LARCH CANKER 2 eS
spores from the ascus of this species is between 10 and
15 mm. In a damp chamber such as that described, the
spore-ejection may take place very rapidly, and from two
small apothecia, measuring 1-25 mm. and 1:5 mm. respec-
tively in diameter, 104 spores were found to have been
ejected in2 minutes. Ina dry atmosphere the asci cease to
shed their spores, a fact which is no doubt correlated with
the closing of the apothecid in dry weather.
The spores may germinate in either of the two following
ways: (i) The spores first divide by one, or less commonly
hike:
ee
ee
Fie. 20.—Spores and germination: a, spore, showing nucleus and
vacuoles ; B, germinating at both ends; co, two germ tubes at one end ;
D, segmentation ; E, F, G, methods of germination.
more septa which are put in at right angles to the long
axis of the spore (fig. 20,D, £). The first of these may be
completed twenty hours after the ejection of the spore.
When the spore becomes divided into three segments
(fig. 20, F) these are usually unequal, one being nearly half
the size of the whole spore ; a third septum may then be
formed in the larger cell. The germ tubes may arise in
_ many different ways. Most commonly one grows out from
_ each end of the spore, or one of them may originate near
the middle septum, but cases in which three or four germ
tubes arise from a single spore are not infrequent. (ii) The
spore may give rise to germ tubes without first becoming
44 THE LARCH CANKER
septate, and in this case the germ tube is developed rather
earlier than when the spore becomes segmented, and it
may appear as soon as twenty hours after “the ejection of
the spore. This method of germination is less common
than the last, but very many cases were observed, and
the spores did not afterwards become septate. Either
one germ tube grows out terminally, or one from each end;
in one case two germ tubes ‘appeared at the same end
(fig. 20, c).
The germ tubes vary in size (the larger ones are about
4. broad), and may grow to a considerable length without
branching. One unbranched hypha attained a length of
140 » in less than twenty-six hours. But generally the germ
tubes branch when they are much shorter than this and
form a fairly densely interwoven mass, even in tap-water
without any added nutriment. The walls of these hyphae
are thin and colourless. | |
As the hyphae grow and use up the food at their dis-
posal, they frequently fuse with each other. These fusions
were observed by Willkomm, though his figures are mis-
leading as to their method of formation. I have noticed
three kinds of fusions. Firstly, when one hypha grows so
as to meet another broadside, the apical wall of the one
may be digested, and at the same time a hole be formed
_in the side of the other. In this case the former hypha does
not continue its growth. Secondly, one hypha may grow
over another and lie across it ; a clamp connexion may then
be made, in the form of a branch, which grows out from one
hypha and fuses with the other. I have not observed from
which hypha this branch usually arises. It does not generally
connect points which have been in contact, but is usually
bent, and connects the side of the upper hypha with the
top of the lower hypha. Thirdly, two hyphae growing
parallel with each other 5-20 apart may each send out
a branch hypha, and these two branches fuse by their
apices. Possibly their meeting in this way is merely acci-
dental, though the large number of fusions of this kind
suggests that some stimulus attracts the two growing apices
eee ta ae ae” Ne, ee ee ee ee ne ae
a) My) fore
———— ee
ee
THE LARCH CANKER 45
to each other. Further evidence, however, would be neces-
sary to substantiate: this view.
For a fusion to take place it is of no consequence whether
the two hyphae are branches of the same mycelium or not,
and a number of spores germinating together form a close
network in which it is impossible to determine from which
Br Geses ie
Fie. 21.—The growth of mycelium in drop cultures: sp., spore.
_ spore many parts of the mycelium have arisen. Such a net-
work is best seen in cultures in distilled water, as until the
_ hyphae become starved the fusions are not so frequently
formed.
___Insuch cultures the primary germ tubes become markedly
septate (fig. 21, B), the septa being usually 12-20 apart.
But the branch hyphae which arise from these are often quite
_ unseptate, and septa, when present, are far apart. As the
available food supply is used up, drops of some liquid
(probably an oily substance) appear, which become more
numerous as the segments die. I have often noticed that
some segments die before others and lose their turgidity,
and then the more turgid ones are markedly refringent and
46 THE LARCH CANKER
show no such drops, or only a few very small ones, whereas
the dead segments are not noticeably more refractive than
the surrounding water, and contain numerous large drops.
These drops often escape fromthe dead cells into the surround-
ing water, and may then cling to the outside of the hyphal
walls. It was probably these drops which Willkomm (1867,
p. 206) described as ‘ micrococcusschwarmer ’. Occasionally
movement can be traced in them, but this is not a necessary
sign of life. The great variability in the size of the so-called
“micrococcusschwarmer’ figured by Willkomm (Taf. xxiii,
24, E) is against their being bacterial cells (cf. Hartig’s
criticism on p. 73).
Willkomm states that the hyphae in caus may bear
lateral conidia, but though I have observed hyphae with
laterally borne bodies, like those figured in Taf. xxii, 24 ¢, ,
these bodies never became separated from the hyphae, and
they showed no inclination to germinate.
Hyphae of a Penicillium bearing conidiophores and
conidia often turn up in drop cultures of the Dasyscypha.
This Penicillium grows on old apothecia of the Dasyscypha,
and it is often difficult to avoid collecting some of these
conidia when the ascospores are obtained.
Pure cultures on nutrient media. The method of obtaining
pure cultures of Dasyscypha calycina which I have found
most successful is this. Portions of bark with vigorous
apothecia of the fungus are placed on wet filter-paper in
the bottom of a drop-culture chamber, as described on
p. 42. A sterilized cover-slip is then placed over the top
and fixed with vaseline. Almost immediately the under-
side of the cover-slip becomes cloudy with condensed
vapour, and in the film of water so formed the spores ejected
from the apothecium remain attached to the cover-slip. If
the apdthecia are giving off spores at the time when they
are put in the culture-chamber, these may be gathered in
an hour or two, but generally the chambers have to be
left till the next day, when sufficient spores will have
accumulated.
Two culture-chamber rings are then placed on a slide
SS
Fhe «pg. On, Re
ce a ee oe ee ee
THE LARCH CANKER 47
and a few drops of water are allowed to lie between them.
A cover-slip is then very carefully removed from one of
the culture-chambers, and placed on the two rings, so that,
resting above the drops of water, the moisture on its under-
side does not dry up. The whole can be put under the
microscope and the spores removed by a sterilized needle
or glass filament and transferred to culture media in Petri
dishes or Cogit’s flat glass bottles, the usual precautions
being taken against the introduction of foreign spores.
A suitable medium was found to be either gelatine or
agar-agar containing malt extract (3 per cent.), meat
extract (0°3 per cent.), and citric acid (0-03 per cent.).
There is at first some difference between the cultures
obtained from the large spores of the parasitic form and
the smaller spores of the saprophytic form. The germ
_ tubes of the latter are smaller, and growth is at first rather
_ slower, but after a day or two very little difference can be
- found, and older streak cultures are indistinguishable.
The mycelium is characterized by forming a very close
felt-like mass, and it grows so evenly on all sides that
a culture from a single spore becomes a circle (fig. 22) and
_ a streak culture has the form of an even band. When
_ growing on gelatine the mycelium puckers up the sub-
stratum into folds, but agar-agar remains flat.
3 Temperature has a considerable modifying influence on
_ the growth of cultures, and the following table is instructive
in this respect. Twelve streak cultures were grown on agar-
_ agar in Petri dishes. The first four, A;—-Aa, were grown at
_ the temperature of the room (13°-14°C.), the next four,
_ A;—Ag,in an incubator at.22°-23°C.,and the last four, Ay—A1s,
~ in an incubator at 26°-27° C.
____ Itcan be seen from the following table that a temperature
of 12°-13° C. is below the optimum for growth, and 26°-27° C.
is above the optimum. It is interesting to note that the
_ higher temperature encourages germination, which takes
_ place very slowly at the lower temperature, but for sub-
sequent growth the latter is more favourable. Also the
high temperature encourages excessive branching.
|
THE LARCH CANKER
48
TABLE SHOWING RATES OF GROWTH OF
After Eighteen hours, i Forty-three
Average
length
Number | of those Number
Longest. | measured.| measured. | Remarks. | Longest. | measured.
Cultures A,—A, 35 wu — — | Very few 238 p 19
Temp. 13°-14° C. germina-
ting
Cultures A,—A, 165 p ll 138» =| Very 490 p 7
Temp. 22°-23° C. many
germi-
nating
Cultures A,-A,, 132 pu 22 91y | Very 194 p 17
Temp. 26°-27° C. many
germi-
nating
In agar-agar cultures numerous spermogonia were formed -
after a few months. These spermogonia were somewhat
larger than those found on larch trees and, but for their
apices, were sunk in the substratum. They produced
normal spermatia which resisted all my efforts to germinate
them. No such spermogonia were formed in gelatine
cultures. | |
Cultures on other media may be obtained directly from
spores, but on the whole it is easier to start from the my-
celium growing on gelatine or agar-agar. In this way
cultures have been grown on moistened sterilized bread
cubes in Erlenmeyer flasks and on pieces of branches of
European and Japanese larch which have been sterilized in
steam. On these larch stems apothecia were developed
completing the life-cycle from spore to spore (fig. 23) ; there
is no difference between such cultures obtained from the
parasitic or saprophytic form of the fungus.
Mycelial cultures were also grown successfully on sterilized
Fie, 22.—Pure culture of Dasyscypha Frq@. 23.—Pure culture on sterilized
ealycina on nutrient gelatine. larch stems showing apothecium.
oT, CTT YE Re
ee eaten aeieteaed
Adie
Op ae ene
ea 3
aS ee
THE LARCH CANKER 49
MYCELIUM AT DIFFERENT TEMPERATURES
hours. Three days. - Fifteen days.
Average Average
length length
of those Number | of those
measured. | Remarks. | Longest. | measured. | measured. | Remarks. Remarks.
175 p Very 650 pu 9 440 wu — United into band
many 1-5-2 mm.
germi- across. Very
nated thin growth.
346 p Very —- — — Large United into band —
many branch- | 4-5-7 mm.
germi- ing across. Very
nated masses close dense
700- growth.
1,000 p
across
161y | Very 240 pu 10 206 =| Very Temp. of incu-
many much bator not con-
germi- branched| stant, so statis-
nated. tics valueless,
Very
much
branched
twigs of other conifers in damp chambers. The conifers
which I experimented with were spruce, silver fir, Scots
pine, and Corsican pine. In all these the mycelium grew
freely, and on silver fir apothecia were produced.
Cultures were also obtained on sterilized leaves of larch,
spruce, and Scots pine. These showed no tendency to grow
for several weeks; but ultimately masses of dead leaves
became covered with mycelium, and on sectioning the needles
hyphae were found to have ramified the mesophyll, though
they had not penetrated into the vascular bundle region.
This shows that the mat of dead larch needles lying on
the floor of larch plantations forms a suitable substratuin
for the mycelium of Dasyscypha, and the mycelium which
is commonly found growing on these needles may belong to
this fungus. It is not suggested that the fungus can reinfect
. the trees by this means, since no fructifications are formed.
But it may assist in the decomposition of the foliar débris.
Artificial infection with the canker fungus. The first
1888 E
50 THE LARCH CANKER
recorded artificial infection with larch canker was _ per-
formed by a practical forester named Fischer, who caused
canker in trees by inserting into a wound made in healthy
bark a suitably shaped piece of bark and phloem from
a canker on another tree. This experiment proved that
canker was due to a transmissible cause and was not merely
the result of unsuitable growth conditions. That the fungus.
Dasyscypha calycina is actually responsible for canker was
first proved by Hartig when he induced the disease by
infection with ascospores of the fungus. An account of his
experiments has already been given on p. 23, and need
not be repeated here.
Massee (1902) also produced canker by means of artificial
infection, and obtained results which are important in two
respects. Firstly he found that canker resulted when
ascospores were placed on the mucilaginous excretion of
Chermes abietis in the spring. This he attributed to the
small holes which the insect makes in forcing its proboscis
through the cork to the living tissues beneath; and he
supposed that the fungal germ tubes grew down sites holes
and thus gained admission to the cortex and ‘phloem below.
Massee attached great importance to this method of
infection as a source of canker in nature, and.considered
that if the larch aphis could be destroyed, canker would
cease to be a serious epidemic disease. As I shall show,
however, the aphis can only assist in the infection of com-
paratively young stems, whereas the cankers which are
important to foresters are mostly those which have been
induced after the stems are three years old.
Secondly, Massee found that Dasyscypha calycina is
capable of infecting other trees besides the European
larch. From the spores of the fungus he produced canker
on the Scots pine and on two other species of larch—the
Siberian (Larix siberica) and the Japanese (Larix leptolepis).
In my own infection experiments, inoculation was
obtained by inserting a small piece of agar-agar, with °
fungus mycelium in pure culture, in a slit made with a knife
in the bark of young trees. Infections made in Munich in
~~
THE LARCH CANKER 51
1914 with cultures grown from spores of the saprophytic
form of Dasyscypha calycina developed so far as to produce
the earlier apparent signs of canker, but no fructifications
of the fungus had been formed before my departure in June.
Subsequent infections, made in the same way with cultures
grown from the parasitic form, at Oxford, produced normal
cankers bearing fructifications, but no new features were
demonstrated by these experiments.
CHAPTER IV
THE LARCH CANKER (concluded)
On the mode of infection in nature.'_ Importance of wounds as a source
of canker. Contributory causes of canker. Methods of prevention. The
synonymy of Dasyscypha calycina.
On the mode of infection in nature. After the examination
of a large number of cankers I have arrived at the conclusion |
that the ordinarily accepted methods of infection by the
disease are insufficient to account for the numerous cankers
which occur on larch stems. I shall consequently give
a summary of the theories that have been advanced, and
then proceed to the consideration of a new one which may
explain the occurrence of canker on main axes.
For the sake of clearness it will be well to divide infec-
tions into two classes: (i) those on young stems and small
lateral shoots; (ii) those on main trunks which occur when
the latter are more than two years old.
The first class of canker is comparatively unimportant,
since side branches die quickly whether they are attacked
or not ; and when a main shoot is affected early, it is gener-
ally killed and its place taken by a lateral.
(i) It is a matter of observation that a shoot does. not
become cankered till the end of its first year’s growth.
This is a necessary corollary of the view propounded by
Berkeley and accepted by all subsequent writers that the
annual ring is always complete at the base of a canker, so
that the cambium must be killed in the winter. This does
not, however, preclude the possibility of infection having
occurred in the outer tissues during the summer, and it is
only the failure to find any sign of fungal growth in shoots
1 I have already published a preliminary note on this subject in the
Quarterly Journal of Forestry, 1915.
Oe
during their first growing season that justifies the conclusion
that shoots do not become infected until the first year’s
growth is complete. This is in keeping with the theory
already enunciated that Dasyscypha requires dead tissue
to grow on while it is secreting those substances by which
it kills the living cells in its vicinity.
By the end of the first year of growth a larch shoot has
made a continuous cork layer, and as long as this remains
unbroken it is quite impervious to the hyphae outside it.
The only weak points in this armour are at the leaf bases
(see p. 10), and though no artificial infections have proved
successful at such points without wounding, yet it is just
possible that in an extremely small percentage of cases
infection does actually occur here.
In addition, wounds of various kinds occur on young
branches, which may all help in furthering infection. These
wounds may be caused by :
1. Frost. The late frosts often cause ruptures in the cork
protection of young shoots, presumably through the swelling
of the saturated cortical cells inside. Such wounds are
usually seen as whitish specks, the white colour being due
to resin which is secreted through the wound. The mycelium
will tolerate a considerable percentage of resin in its sub-
stratum, but an almost pure resin layer, such as is here
formed, is impermeable to it. It is thus unlikely that
infection will take place through these wounds, except
_ -immediately after their formation. |
2. Hail. . Hail-stones often make wounds on young
stems and may even break off young shoots. These wounds
also are quickly covered by resin, and after a very short
interval they are protected against the canker fungus.
3. Chermes. Massee’s infection experiments, which showed
the possible connexion between canker and attacks of
Chermes abietis, have been cited above, and the punctures
made by the Chermes should be included among the list of
wounds. Further evidence on this point may be looked for
as a result of the experiments initiated by Burdon (1908),
in which larches infected with Chermes were to be compared,
/ THE LARCH CANKER 53
54 | THE LARCH CANKER
as regards canker, with others which were sprayed with an
insecticide.
Somerville’s suggestion that Chermes may partially
inhibit transpiration and so increase the growth of the
fungus after infection is based on Hartig’s theory of the
connexion between mycelial growth and transpiration,
which has a very insecure foundation. At present the
evidence is not conclusive that Chermes does in any way
encourage canker (vide Gardener’s Chronicle, 1896, p. 435).
4. Other wounds may be caused by men or animals
breaking off twigs, and perhaps by one twig rubbing against
another.
We thus see that, if the fungus were able to enter the
tree through any sort of wound, it would have many oppor-
tunities of infecting young twigs, and it is remarkable that
cankers are not more frequent than they are on side branches.
But branch cankers are of small practical importance to the
forester. What he must try to prevent are cankers on the
main stems of the trees.
(ii) Now the factors which bring about these important
cankers are of a different nature, because such cankers are
usually found to have been initiated when the portion of the
stem is three or more years old. This is seen by observing
the number of annual rings which are left intact in a trans-
verse section of a canker. Cankers dating from the first
or second year of growth are extremely rare on trunks, and
probably when infections occur at this early age the leader
is killed by the fungus and its place is taken by a side shoot.
But the predisposing factors that we have considered
are not applicable to the main stems after three years of
growth. The periderm layer is then sufficiently strong to
prevent damage by frost (except in extreme cases), hail, or
Chermes, and the central axis is not likely to be affected by
abrasions caused by the rubbing of other branches. We have
thus to seek fresh causes for the infection of the main axis.
Two sources of wounds have been suggested. The first
is that in planting out the young trees the labourer may
graze the bark of the trees when pressing down the earth
THE LARCH CANKER | 55
with his foot. If this were so cankers should be especially
frequent near the ground, which I have not found to be
the case. It is true that in some woods cankers are apt to
occur at rather definite levels ; but this level may be near
the ground, or 3 ft., 6 ft., or even 30 ft. above it. This
probably means that in certain years the meteorological
conditions have favoured the formation of cankers, which
have then occurred at. the level most susceptible to attack.
On examination of a plantation in which attack had com-
monly occurred near the base one might be tempted to
adopt such a theory as that suggested. But my own
experience has not led me.to attach much importance to
this source of infection.
The second suggestion as to the source of wounds is
based on the observation that on such main stems cankers
nearly invariably occur at the base of branches which have
died. On this account it has been thought that the swaying
of branches in the wind, and their depression under snow,
may cause cracks in the bark at the branch bases, through
which infection may take place. This would further be
encouraged by the fact that spores settling on the trunk
would be washed down by the rain and often get lodged in
the axils of the branches. Now if this were really a serious
cause of canker we should expect to find cankers particularly
frequent on the edge of plantations, where the side branches
grow bigger and thus offer more surface to the wind and
snow, and where also the wind is more pronounced and better
able to sway the branches. But, as a matter of fact, the
reverse is the case, and cankers are much more frequent
inside a larch plantation than on its edges. Also it will
not be found easy, by any ordinary swaying of a branch, to
cause such ruptures as this theory requires.
In order to test the various theories that have been
advanced to account for infection, | have examined a large
number of cankers on main stems of larch. Most of the
specimens were obtained from Bagley and Tubney woods
near Oxford, and in all cases the cankers were taken from
otherwise healthy trees which had full crowns and gave no
56 : THE LARCH CANKER
indication of being suppressed. The trees were cut down
and the parts containing the cankers were brought into
the laboratory. Here the cankers were sawn through
transversely and the surfaces planed or smoothed with
a sharp knife until the section was reached where the
cambium had first been killed.
The inspection of these cankers confirmed the two
important observations that in the large bulk of cases :
(i) The cankers had been initiated at the bases of Jateral
branches.
(ii) The cankers were initiated when the sections of stem
were from three (occasionally two) to eight (occasionally
nine) years old.
We must thus consider in detail the condition of affairs
which exists at the bases of lateral branches on those parts
of the tree which are from two to nine years old.
In an ordinary ten-year-old larch plot planted at 3 x
3 ft., the weaker shoots die off when two or three years old,
the stronger shoots when five, six, or even eight years old.
In younger plantations, before the branches have begun
to shade each other, all of them may remain alive, but
generally some small dead branches will be found at the
base of the trunk, especially when they have been smothered
by rank grass and weeds. Thus the part of the tree which
is from two to eight years old, in which cankers are generally
initiated, is also that part where some or all of the lateral
branches have lately died. Further, in nearly all woods
the lateral branches become infected with Dasyscypha
calycina, which grows saprophytically on them as soon as
they are dead, and continues to flourish on them for three or
more years, filling all the dead bark with its mycelium.
When a dead branch is seen projecting from a canker it
has generally been supposed that the canker has killed the
branch, but it may equally be true that the fungus spreads
from the dead branch and causes the canker. In order to
determine this it was necessary to notice the relative times
at which the branch died and the canker was initiated.
The date of the death of the branch may readily be deter
a
Fig. 25.—Canker showing origin from branch.
Subsequently healed on one side. (x 3).
THE LARCH CANKER 57
mined by observing the last annual ring of the trunk which
is continuous with the wood of the branch.
In 30 cankers taken at random, in which this point could
be precisely determined, it was found that in 5 cases the
canker was initiated the same winter as the branch died,
in 12 cases it began the year after the branch died, in 7 cases
two years after, in 1 case three years after, in 3 cases four
years after, and in 2 cases six years after.
In the following table the observations are set out at
length :
Age of section
of tree in Number of years of
which canker Agewhen Agewhen initiation of canker
Specimen. occurred. branchdied. canker began. after death of branch.
20
11 6 7 1
21 19 7 8 1
Be Pa 6 7 1
4 13 2 8 6
5 13 3 5 2
6 12 6 7 1
7 12 3 7 4
8 12 2 6 4
9 12 4 7 3
10 11 4 5 1
ll il 3 4 1
12 il 8 9 1
13 il 2 3 1
14 il 5 7 2
15 il 2 3 1
16 10 2 3 1
17 10 6 ~ 2
18 10 2 8 6
19 9 3 5 2
20 9 3 7 4
2) 9 3 3 0
22 9 6 6 0
23 8 4 4 0
24 8 5 6 1
25 8 6 8 2
26 7% 2 2 0
27 6 2 2 0
28 6 2 3 1
29 6 — 3 5 2
30 5 2 4 2
This shows that in the large majority of cases the canker
begins at least a year after the death of the branch. In
1 These specimens were kindly sent me by Mr. Alex. Luttrell from
Dunster Castle, Somerset.
58 THE LARCH CANKER
the other cases the branch probably died in the autumn and
the canker was initiated in the spring. This cannot be
proved from my specimens, but since the branches may
become infected with Dasyscypha immediately after death,
this seems the easiest explanation of the facts.
It is thus clear that the conception of a canker being
initiated at the base of a living branch and killing the
branch is erroneous, and in the majority of instances it can
be definitely proved that infection takes place a year or
more after the branch has died. Thus at the base of a
branch, just at the time when it regularly contains mycelium
of Dasyscypha calycina, the main trunk becomes infected
by the fungus. It is scarcely possible to avoid the con-
clusion that the mycelium passes from the dead branch into
the living trunk, and that this is commonly the source of
infection of those cankers which are seen on the older
trunks of the larch.
There is, however, one important obstruction which the
fungus has to pass before it can grow from the branch into
the main stem. This is a cork layer which is always made
across the cortex and phloem of a branch just before death,
and which in the majority of cases is a sufficient obstacle
to prevent the passage of the fungus. But in many instances,
as in that shown in fig. 27, the mycelium occurs on both sides
of the cork layer, and must presumably have passed it. There
are three possible ways in which this might occur. Hither
‘ (i) The mycelium obtains entrance to the trunk before
the cork layer is completed, or
1 A similar conclusion has been reached independently by Mr. P. V.
Laidlaw on an estate in Northumberland (vide Quart. Journ. Forestry,
1914, p. 216). Also, since the publication of my paper in 1915, Mr. A. C.
Forbes has pointed out in the Gardener’s Chronicle for February 6, 1915,
that he suggested as early as November 15, 1902, in the same paper, that
the canker fungus might attack a living twig, and, having killed it, grow
down to the main stem and cause a canker there. As this article is not
referred to in the index for the last six months of 1902, I may be forgiven
for not having seen it earlier. The fact that two foresters have reached
a similar conclusion by another route lends support to the correctness of
the theory.
- Te ee? Oe ee Se ST CS Leh
a eke
THE LARCH CANKER ae Ye
(ii) It grows through the cork layer, or
(iii) It grows round the cork layer.
(i) This is impossible, for the formation of the cork layer
is the last act of the living cortex at the base of the branch.
The mycelium, however, does not spread till the branch is
quite dead. So this alternative must be rejected, except
in the case of trees whose vitality has become very much
reduced before the death of the branch.
(ii) This also is apparently impossible. I have examined
hundreds of cankers but have never found any mycelium
growing through a cork layer—which in fact forms an
impassable barrier to the hyphae, except when they attack
it from the inside.
(iii) We are thus reduced to the third alternative. And
since the cork layer is continuous with the peripheral cork
layer of the tree, it is impossible for the mycelium to get
round it on the outside. It can thus only get round it on
the inner or wood side. Here again there are two possibili-
ties—either
(a) It grows through the cambium, just outside the
wood, the cork layer not having been welded
sufficiently perfectly on to the wood, or
(6) It grows through the. wood.
To discuss these alternatives the anatomy of the cork
_ layer must be described. It will best be understood by
following the accompanying diagrams (fig. 26). Fig. 26, 4,
shows the general arrangement of the tissues, at the point
where a branch joins the main stem. A is the main axis,
B the branch. Everywhere to the outside is a layer of cork
cc’, which becomes wrinkled at the point of junction by the
continuous thickening of each member. Between c and d
_ are the tissues known as the cortex and phloem; at d is
the cambium, inside which is the wood w. On the death
of the branch the dead tissue of the latter would come into
direct contact with the living tissues of the main stem, were
it not for the interpolation of a cork layer c.l. This layer
is put in usually, not at the base of the branch, but about
1 cm. above it.
60 THE LARCH CANKER
Fig. 26, B, which is an enlarged drawing of the top right-
hand corner of fig. 26, A, shows in greater detail the forma-
tion of this‘new cork layer ; PH is here the normal phellogen
or cork cambium which makes cork on its outside. The
A c’
~— eer oe “ Cc
Fria. 26, Aand p.—Cork layer across base of branch. For description,
see text.
new cork is formed by a new phellogen ph, which is .
developed right across the cortex and phloem, and makes
a downward turn as it approaches the cambium. It is in
direct continuation with the outer phellogen at 2, and
forms, as it were, a side branch of this phellogen. It passes
usually transversely across the cortex and phloem (it may
be oblique) until it reaches the layer outside the cambium,
Fie, 27.—Canker fungus spreading down from a branch.
This was photographed froma living specimen. The dark-
coloured cortex near the branch is dead. The lighter
coloured, above and below this, is alive.
Fig. 28.—Spread of fungus down the branches. This
stem was taken from a young tree which had been felled
and allowed to lie in the wood for a few months.
THE LARCH CANKER 61
where it turns sharply down the stem at F and ends blindly.
The cork made from this layer becomes welded perfectly
with the outside cork layer, and is peter continuous
across the cortex and phloem.
Thus the two alternatives (a) and (b) of p. 59 resolve
themselves into these: either (a) the mycelium grows
through the gap between the cork layer and the wood at
F, or (b) it grows round through the wood by the route GH.
I think the course by F is the more probable. The space
left here, though narrow, is not infrequently just large
enough. And, though I have never been able to trace
hyphae right through it, I have seen hyphae at each end of
it. The mycelium of the fungus can under certain circum-
stances be found in the wood, but I have never observed it
growing to any extent up or down the stem except when
the wood is quite dead ; when it occurs it is usually either
in the wood just inside a canker, where fungal secretions
have killed all the living cells, or else in the heart-wood.
It is conceivable, however, that in some circumstances it
might grow down through the wood from the branch to
the main stem, and there attack the phloem.
For the present the question must be left open whether
the mycelium passes from a dead branch to the main axis
through the wood or just outside it. That it can pass such
a cork layer is shown by an ordinary canker. For this is
surrounded each year by a cork layer in every way com-
parable with that at the base of a branch, yet in the
majority of cases the mycelium successfully penetrates to
the other side.
It is difficult to obtain experimental proof of this method
of infection. But it may be somewhat strikingly demon-
strated by cutting down a larch tree, 15-18 years old,
which has on it numerous dead branches, bearing the canker
1 As the trunk grows in thickness, the cortex and phloem at the base
of a side branch generally gets torn away from the wood, and frequently
the bark of the trunk becomes corrugated round the branch bases. It is
likely that the canker fungus may sometimes reach the trunk through
ruptures caused in this way.
62 THE LARCH CANKER
fungus. After a time it will be found that at the base of
nearly all these dead branches the fungus. has spread on
to the main trunk, and fructifications are duly made there
(fig. 28). This shows that, when the resistance of the living
tissues is reduced, the fungus finds no difficulty in growing
down from the branches; and, under these conditions,
there regularly occurs what only occasionally happens
when the trees are alive and vigorous. It may be that
when the tree is alive it can usually neutralize the secretion
which the fungus pours into it through the gap, and thus
prevent the mycelium from obtaining any foothold.
There is another way in which Dasyscypha may gain
admission to living stems without previous wounding ; this
is through dormant buds which have died. There is always
a large number of these dormant buds on a larch stem ;
they are surrounded by a number of dead brown scales, but
in the middle is a growing apex with the living leaf primordia.
Given light and food these may at any time develop into
side shoots. As the tree gets older such buds on the lower
part die, and they immediately become attacked by germ
tubes of Dasyscypha spores. But in a healthy tree a cork
layer is formed beneath the buds at the time of their death,
so that there is very slight danger of the stem bearing them
becoming infected. In trees, however, which from lack of
light, or other causes, are growing poorly, such buds are
responsible for a large number of cankers.
This source of canker, though of frequent occurrence, is
of comparatively little importance to the forester, since it
only affects the poorer trees in a plantation, which would
be ordinarily removed in the first thinning.
The importance of wounds as a source of canker. In the
foregoing section it has been shown that there are two
ways in which the canker fungus can gain admission to
trees without their previously being wounded, and it is
probable that the importance of wounds as a source of
infection has been greatly exaggerated. The dogma that
trees can only become cankered through wounds is due to
Hartig’s infection experiments, for he was unable to infect
or
i
:
;
:
4
|
THE LARCH CANKER 63
trees artificially unless they were first wounded in some way.
But infection experiments may be incapable of demonstrating
a phenomenon which is almost ubiquitous in nature, and
the case of larch canker is one in which it is exceedingly
difficult to repeat in the culture house a process which we
can observe in the forest. For every time the mycelium
successfully grows from a dead branch to the trunk, it
must fail at least a dozen times. But experiments which
at the luckiest were only successful in 8 per cent. of the
inoculations would not by: themselves be conclusive, espe-
cially when we know that minute punctures, too small to
be seen with a simple lens, may be sufficient to admit the
fungus, and these might have been overlooked in the experi-
ment. It is thus by no means easy to get satisfactory
results with experiments set up to show that Dasyscypha
can infect living trunks from dead branches, and if such
experiments did apparently succeed, it is difficult to be
quite certain that the fungus has not entered in some other
way. It is for these reasons that the arguments in favour
of this means of infection have been drawn, not from culture
experiments, but from observation.
Conversely, wounds which have been found necessary for
artificial inoculation are probably of much less importance
in the forest. I have often seen billhook wounds which
have healed naturally even in plantations where canker was
epidemic. And where shooting rides have been cut in larch
woods there is not generally any increase in the frequency
of the fungus. Again, when larches are grown as nurses
for other trees, the branches have regularly to be cut back
to make room for the main crop, and yet, in such cases,
- cankers are often less numerous than in other plantations
_ in which there has been no pruning; indeed, in a plot
_ treated in this way in Bagley Wood near Oxford, where
_ larch was grown as nurses for deodar, there was no canker
_ at all on these nurse trees, whereas a larch plot less than
_ 200 yds. away was attacked with moderate severity.
_ The reason why laboratory experiments give a false
impression of the importance of wounds is by no means
64 THE LARCH CANKER
obscure. When a wound occurs in nature, it is covered in
a very short time with a layer of resin, through which
infection cannot take place, so that it is only a source of
danger for a very brief period. But in an experiment,
immediately a wound is made, spores or pieces of mycelium
are thrust into it, and the resin which is secreted is poured
outside the fungus, which can then attack the tissues inside.
Contributory causes of canker. As has been known ever
since Berkeley first investigated larch canker, the primary
cause of the disease is Dasyscypha calycina. But there are
many secondary or contributory causes, which while unable
by themselves to bring about a canker, encourage the
fungus, or in some way make the tree more susceptible to
its attack. These factors are therefore of great importance,
and the means adopted by foresters in their attempts to get
rid of canker have been based much more on the mastery
of secondary causes than of the pleas cause, of which
they are often ignorant.
One class of contributory causes has already been dealt
with in the section on ‘the mode of infection in nature ’.
This included all the factors which may cause wounds.
A great deal of attention has formerly been paid to this
class, and probably its importance has been over-estimated.
Other contributory causes may be grouped under headings -
of climate (including meee soil, and mixtures of trees
in plantations.
We have here chiefly to rely on the published opinions
of practical foresters, which may be found in articles and
letters from time to time in the Gardener’s Chronicle and
in answers to the English Arboricultural Society’s inquiry
into the causes of larch canker (Somerville, 1895), and to
the similar Scottish inquiry (Richardson, Borthwick, and
Mackenzie, 1905). The best and most critical survey of the
whole question is given by Forbes (1904), and as there is
much diversity of opinion among foresters as to the causes
in question, Forbes’s account is most valuable.
Altitude and climate. From the time of Hartig (1880)
onwards it has always been a matter of speculation why
THE LARCH CANKER 65
canker should be more frequent in lowlands than in the
larch’s natural home in the Tyrol. Hartig adduced three
reasons to account for this difference. The first is that
when the larch is growing in its native home, where natural
regeneration takes place freely, young trees are always
springing up among the older ones, and forests are com-
posed of trees of all ages. But canker, as an epidemic, is
nearly confined to plantations under twenty years of age,
and trees which are older than this are generally free from
attack. So in the Tyrol the canker fungus can only pick
out here and there the younger trees from among a much
larger number of older ones. (This leaves out of account
the frequency of canker on side branches of older trees
which I have particularly noticed in the Tyrol.) The second
reason is that in the mountains the trees are not often
surrounded by damp stagnant air, which is conducive to
the formation of fructifications of the fungus and to the
germination of the spores. Lastly, the early spring in the
lowlands causes a premature renewal of vigour. The twigs
become full of sap and the needles begin to appear. But
too often this is followed by May frosts, which cut back the
shoots and make a number of frost-wounds on the young
twigs, which reduces the vitality of the trees. In the Alps .
summer development does not begin so early, so that the
trees are less likely to be cut back by frosts.
All these causes are no doubt operative to a limited
extent, but their importance must not be over-estimated.
Indeed, larch in its native habitat is by no means free from
canker, and as high as 5,000 ft. in the south Tyrol 1 have
found parts of trees almost riddled with the disease, though
not so much on the main trunks as onthe side branches.
Boden (1904) also comments on the fact that trees may be
cankered as high as 7,200 ft. in the Alps, whereas the
disease is often absent from plantations in the plains. In
parts of the Tyrol, which I have seen, the larches do not grow
in dense formation, but are sufficiently scattered to allow
the full development of the side branches. Of course many
branches must die, but not, as a rule, till they have grown
1888 RF
66 THE LARCH CANKER
for many years, and probably snow accounts for the fall of
most of them. At any rate the dense screen of small lateral
dry branches so characteristic of our own woods is generally
absent in the Alps; and since, as has been shown above,
dead branches are one of the most important sources of
canker, the absence of such branches in the Alps is a note-
worthy factor in the suppression of the fungus.
In the case of British plantations it is questionable
whether the altitude at which larch is planted is ever great
enough materially to lessen the attack of the canker fungus.
Trees may be as badly attacked at a height of 1,000 ft. as
at sea-level, and where a mountain-side is immune from the
disease this may be accounted for by the favourable soil
conditions that such a slope provides.
Though altitude has little effect in restricting canker, low
hollows seem to encourage it very markedly. Such hollows
are peculiarly liable to late frosts, which have an adverse
effect on the general health of the trees and render them
more liable to infection, and frost-wounds may admit the
fungus in some cases. Also the atmosphere is there more
humid, and conditions are favourable for the formation of
fructifications of the fungus..
No direct relation can be traced between the frequency
of canker and the rainfall or the site exposure.
Soil. Though it is very difficult to determine exactly the
soil conditions which favour canker and the reverse, never-
theless the edaphic factor seems to be of more importance
than the climatic. Even in flat country neighbouring plots
of larch may be very variously affected, and since this cannot ©
be attributed to climatic differences, it seems a just inference
that soil is responsible. On the whole we may say that
soil conditions which favour larch-growing in other respects
are generally least conducive to canker ; a good, deep, well-
aerated soil is better than one which is peaty or rendered
shallow by a pan near the surface. Forbes (1904) lays
great stress on the fact that young trees are often notched ~
or slitted into the upper six inches of soil quite unsuited to
their requirements, If it is composed of dry peat or débris —
THE LARCH CANKER 67
of recently cleared coniferous woods, the superficial layer
is poor in nutriment ; and if the site is one where grass or
heather has recently been growing, the roots of these plants
will prevent a free interchange of gases between the soil
and the air. Larch is peculiarly sensitive to the condition
of the surface, for being in its youth a surface-rooter it
cannot range through the deeper layers in search of the
nutriment which it requires. For the same reason the sub-
soil is of secondary importance, i. e. for the normal growth
of the tree. (The nature of the subsoil is an all-important
factor in susceptibility to heart-rot.) So long as this is |
well drained a good thick surface-soil will grow satisfactory
larch, and if the trees are growing vigorously there is a good
chance of their remaining free from canker. Perhaps the
safest situation is a mountain-side, where the porous, well-
drained, gravelly soil derived from the rocks above provides
all the factors favourable to larch culture. —
Only in comparatively few cases can we say that a soil
will or will not grow healthy larch until it has been tried.
I have seen healthy woods grown on a clay subsoil and
plantations riddled with canker on sand. Mitchie (1885)
says that soil which is suitable for barley will generally
prove successful with larch. :
_ Conditions of culture. Pure larch-plantations are more
liable to attack than trees which are mixed sparingly with
hardwoods, such as hornbeam or beech. This is usually
explained by supposing that the hardwoods act as a kind
of screen, protecting the larch from fungal spores which are
blown through the woods. Such a theory, however, cannot
be entertained when, as is often the case even in mixed
woods, the dead lateral branches of the larch become
’ covered with the fructifications of Dasyscypha. For this
‘shows that the spores have found out the larch trees, and
the number of fructifications made on the dead branches
would be sufficient to ensure infection of the trees, were
they susceptible. Probably Forbes is right in accounting
for the comparative immunity in mixed woods in quite
another way.
F 2
68 ‘THE LARCH CANKER
As he points out, finer larch trees are found in mixed
woods than generally occur in pure plantations, even when
these are free from canker. This is partly due to the fact
that ‘ when mixed sparingly with deep-rooting or compact-
rooting trees, the larch-roots can spread without meeting
much opposition, while their more rapid stem-growth gives
them a lead over other species from the first ’. Also the’soil.
under mixed woods is maintained in a state more generally
suitable to tree-growth than in pure coniferous plantations.
One result which we gain from the considerations of
contributory causes is that the conditions which are favour-
able to the restriction of canker are in general identical
with those that encourage the active growth of the tree.
Keep the trees growing vigorously and canker is not. so
likely to become epidemic. The explanation of this may be
sought in the nature of the struggle between the parasite
and the host. Given an uninterrupted tissue of living cells,
the fungus can push forward, killing the cells in advance of
it by its secretions and itself growing into them. But the
tree resists this progress by cork layers, which we may
liken to a series of trench systems, each holding up the
enemy for a time, and perhaps entirely preventing his
advance. When the fungus grows down from a branch,
the first trench it has to capture is the cork layer normally
made at the base of a dying branch. If this is passed, fresh
cork layers are made in the tissues of the tree, and it often
happens that the first of these prevents the canker from
spreading farther. If this fails, more and more cork layers
are formed, and so the struggle progresses—purely offensive
on the one hand, and purely defensive on the other. For-
tunately the advance of the fungus through living tissue is
very slow, especially in a tree which is healthy and fast-
growing, so that such a tree has time to make a fresh cork
layer (or, to revert to the simile of war, it can complete new
earthworks before they are reached by the advancing
enemy), and the better and more thoroughly this layer is
made the less chance the fungus has of passing it. Not
infrequently trees which are growing strongly show a number
THE LARCH CANKER 69
of incipient cankers at the bases of their branches. Blackened
bark and slight exudation of resin show that the fungus has
penetrated to the cortex of the trunk, but, though they may
develop into normal cankers, these spots frequently dis-
appear as the tree grows older, and the fungus, held from
further penetration by a cork layer, has died from lack of
nutriment.
The greater susceptibility of weakly-growing trees may
thus be associated with their enfeebled power of forming
cork layers. The surprising rarity of canker on other
conifers, such as spruce or Scots pine, which have been
shown experimentally to be susceptible to infection, may
be due to their possessing this power in a higher degree.
In particular, isolated cankers have not infrequently been
found on the Japanese larch, but in general this tree is
surprisingly free from the disease, even when grown in close
association with cankered European larch. It has yet to
be demonstrated that Japanese larch has a more pro-
nounced faculty for making cork layers than the European
larch, but it is probably along these lines that the explana-
tion for the comparative immunity of the former tree should
be sought. And as it is now considered that nearly every
species is composed of a number of ‘races’ which differ
only in small particulars, it may be expected that some
races of European larch are better able to make such cork
layers than others, and the selection of such races may be
- the ultimate means of growing larch without canker. Selec-
tion of tree races is a slow affair, but it is clearly advisable,
when collecting seed, to choose those trees which have
grown free from canker, in the hope that their progeny
may share their immunity.
Methods of prevention. The treatment of larch which
has been recommended at various times for the prevention
of canker has always been based on the current views as
to the causes which lead to the disease. Thus Hartig
insisted that every kind of wound must be, as far as possible,
avoided, and he is particularly emphatic that branches
must never be removed until they are dead. Somerville
70 THE LARCH CANKER
laid stress on preventing wounds when planting young
trees ; and Massee, on finding that infection could take
place through the punctures made by Chermes abietis,
declared that, if this insect could be suppressed, canker
would cease to be an epidemic disease.
All these suggestions have proved useless in obviating
the disease, and foresters have been forced to adopt other
means for reducing the pecuniary loss for which canker is
responsible. One doctrine is that larch-growing should be
confined to situations and soils which have proved capable
of bearing healthy plantations. But since this rules out the
majority of districts in which larch is at present grown, it
is a confession of failure except in a few favoured localities.
Sir Donald Munro Ferguson has adopted, at Novar, a method
of treating larch which was described by Somerville (1906)
in the Journal of the Board of Agriculture. He plants pure
larch at 3,500 to the acre, the vast majority of which become
cankered. At the_age of 16-20 years he cuts out all but
300-500 of the best trees, and is able to sell the 3,000 or so
thinnings for £20 to £25. The lateral dead branches are
removed and the remaining larch are underplanted with
beech, or preferably such conifers as Picea sitchensis, Pseudo-
tsuga Douglasii, T'suga albertiana, Thuja plicata, Cupressus
lawsoniana, and Abies grandis.
In most plantations it will be possible to select 10 per cent.
of healthy trees, and the 300-500 trees that are left may
be exempt from canker during the remainder of their lives.
But obviously the treatment will be impossible in woods
where, as sometimes happens, even the best trees have three
or four cankers apiece. Nisbet (1907) criticizes the system
on the ground that the pure larch woods make an excellent
breeding-ground for the fungus, and become a source of
danger to all the intermixed larch woods in the district.
We need therefore some method by which canker can be.
prevented, and the method of infection which I have described
as accounting for the greater number of dangerous cankers
suggests a treatment which may possibly prove helpful in
preventing the disease. Until successful experiments have
THE LARCH CANKER 71
proved its adequacy, it must be put forward somewhat
tentatively.
If dead branches are the cause of infection, then dead
branches must be removed, and, since branches are attacked
by Dasyscypha almost immediately they die, they should
-be removed before they have any chance of being infected.
To be on the safe side they might be pruned a year before
they would naturally have died if allowed to remain on
the tree. In a wood planted with pure larch at 3x4 ft.
lateral branches generally die when they are 5-6 years old,
so that this is the age at which they should be cut. But
cutting off branches will leave wounds, which Hartig has
so strongly recommended us to prevent, and though, as has
_ been shown on p. 62, the importance of wounds has been
over-estimated, we may as well be on the safe side and cut
the branches in dry weather, preferably in winter or early
spring, when the spores of the fungus are not being liberated
in large quantities. It may be objected that such a treat-
ment is expensive, and that forestry is not sufficiently
lucrative to allow of individual attention to the trees. But
the expense of cutting off the small lateral branches would
not be excessive so long as the woodman confined himself
to plantations from 6 to 18 years old, since up to that age he
can probably reach the still living branches without a ladder.
Beyond this age the trees are much less liable to serious
attacks of canker. |
In districts where the canker is comparatively infrequent
the treatment would be unprofitable; but where it has
ravaged in the past, it may still prove to be possible to
grow healthy plantations by this means, though it can
only be determined empirically whether the increased value
of the trees will repay the outlay.
An experimental plot in Bagley Wood, near Oxford, was
pruned in the manner suggested during the winter of
1915-16. A large number of dead branches had to be
removed from the lower 5 ft. of the stems, and living
branches were cut for about a foot above this. Unfor-
tunately, owing to the war, this experiment could not be
72 THE LARCH CANKER
continued, and the trees have now grown too tall to be
suitable for further work of this kind. But it is observable
that those parts of the stems from which living branches
have been removed are much less attacked by canker than
the remainder of the trunks, and none of the wounds that
were made while the work was in progress have become
a source of infection. |
Since the cut branches form a suitable breeding-ground |
for the fungus, they should not be allowed to lie on the
ground, but should be collected and burnt. This will
reduce the number of fructifications in the neighbourhood
of the trees, and will probably assist in lessening the chance
of infection.
Special treatment of individual trees must nevertheless —
be avoided as far as possible. It involves labour which is
now a serious expense, and this is incurred early in the
rotation, so that, with compound interest, it must deplete
the already slender profits of forests. The chief of pro-
phylactic measures .will always be the correct sylvicultural
treatment of the tree. Maintain the general health of the
trees and canker will not be a serious pest. The point of
first importance is to plant larch in mixtures, which main-
tain the soil in better condition than pure larch woods and
stimulate better development of roots. Forbes’s views on
this subject have already been quoted, and Schotte (1917),
after a careful survey of the larch woods of Sweden, adopts
a very similar standpoint. The most suitable tree for
mixture with larch is probably the beech, but where beech-
growing is unprofitable other trees, such as hornbeam and
chestnut, may be tried. Among conifers Scots pine can,
according to Schotte, ‘be unreservedly recommended’; but
woods of larch and Scots pine in this country are apt to
open out in later life and need to be underplanted either
with beech or with that most promising of shade-bearers,
the western hemlock. Spruce should be avoided in mixture
with larch on account of Chermes abietis, which has alternate
generations on the two species, and Schotte states that in
Sweden spruce is apt to shade the larch overmuch, and
THE LARCH CANKER 73
thereby reduce its vitality and power of resistance to
canker. Douglas fir grows well with larch up to twenty or
thirty years, but on suitable soil for Douglas the larch is
then suppressed. This mixture may be recommended
where pure Douglas stands are ultimately required, and
it is probably preferable to Douglas planted pure in the
first instance, but mature larch can seldom be grown in this
association.
The question of site is of less importance. Healthy trees
can be grown at any reasonable altitude in Bgjtain, and
woods in low-lying localities may be just as healthy as those
on hills. Damp hollows, however, where there is danger of
cold stagnant air collecting, should be avoided, as larch
is here especially liable to damage by spring frosts. Very -
poor sandy soil is no doubt unsuitable for growing larch,
and on heavy clay other trees are likely to be more profit-
able, but between these limits there is a wide range of soils
in which larch can be satisfactorily grown, when mixed
with other trees. A fuller account of the sylvicultural
requirements of the larch will be found in Chapter X.
SUMMARY OF CHAPTERS ON LARCH CANKER
Larch canker, which is one of the most virulent diseases |
of forest trees, has been known in this country since the
early part of the nineteenth century, and has been described
in detail by Berkeley, Willkomm, and Hartig. Subsequent
writers have not:added anything of first importance to our
knowledge of the disease.
_ The canker is due to the mycelium of the fungus Dasy-
scypha calycina, which can live either as a saprophyte or as
a parasite on the larch tree. Dead branches are usually
filled with the mycelium of the fungus, which makes small
cup-shaped fructifications on the surface of the bark.
Under suitable conditions the mycelium gains admission
to the cortex and: phloem of living parts of the tree, and
can then cause canker. By killing the cambium at any
spot it prevents further growth in thickness at that point,
and each year it kills a larger and larger area of cambium,
74 THE LARCH CANKER
thus producing an amphitheatre-like appearance in the
wood of a transverse section. The mycelium flourishes
chiefly in the cortex and the phloem, but can also attack the
wood to a limited extent. .
It has been shown that this fungus is not a parasite in
the sense in which the term is generally used. The mycelium
does not directly attack the living cells of the host, but first
kills them by secretions. The mycelium grows both in the
cells of the host and in the intercellular spaces. Fructifica-
tions are borne on the dead bark in the canker region. These
are larger than those on the dead branches in all their parts,
but it has been shown that this difference is due to growth
conditions, and the dimensional differences. cannot be
made the basis of specific or varietal separation. |
Large quantities of resin are made by all attacked parts
of the tree. This oozes out on to the surface of the canker,
and often forms long streams down the trunk.
Pure cultures of the fungus have been grown on various
media, including sterilized twigs of different conifers and
dead needles of larch, spruce, and Scots pine. In a damp
chamber larch needles may become matted together by the
mycelium which ramifies the mesophyll. It is suggested
that the matting of needles on the floor of larch plantations
may be largely due to the mycelium of this fungus, a process
which may help in the decomposition of the foliar débris.
Pure cultures on sterilized larch stems were grown up to
the stage of bearing apothecia,.thus completing the cycle
from spore to spore.
The fungus may gain admission to the living parts of the
tree in three ways: |
1. Through wounds, which may be caused by a variety
of agents.
2. By growirig down from the dead branches into the
living tissues of the trunk. |
3. By means of buds which have been killed by the —
shading of the upper branches. :
It has been shown that the second of these methods gives
THE LARCH CANKER 75
rise to most of the serious cankers on main trunks. The
third is less important, since it only affects trees which are
being killed out through lack of light. The importance of
wounds has been over-estimated by most previous writers.
Contributory causes, rendering the tree more liable to
canker, are any agencies which cause wounds, damp stagnant
atmosphere, and poor badly-drained soil. Pure larch
plantations are more liable-to the disease than those where
the trees are intermixed with hardwoods. In general it
may be said that any conditions which are prejudicial to
the vigorous growth of larch are favourable to the spread
_ of the disease.
Healthy plantations can best be grown by selecting sites
where all the conditions are favourable to larch growth,
but a treatment has been suggested whereby it may become
possible to grow sound trees where canker has usually been
epidemic. By the pruning of branches it is hoped that
infection from these members may be prevented.
ON THE SyNoNYMY OF Dasyscypha calycina, (Schum.)
Fuck.
The Marsinire of larch canker has become much confused
by the variety of names which have been applied to the
' fungus causing it.
Berkeley (1859) called it Peziza calycina; Willkomm
(1867) confused it with a somewhat similar member of the
Basidiomycetes, Corticiwm amorphum, and called it by that
name; Hoffmann (1868) corrected this error, and adopted
_ the same name.as Berkeley ; and Hartig (1880) said that
the fungus showed microdimensional differences from
P. calycina, and instituted a new name, Peziza Willkommic.
Most English mycologists, however, have refused to adopt.
this new name, maintaining that the conventions of nomen- _
clature demand that it should be called Dasyscypha calycina.
This confusion has arisen from the fact that there are
three or four different species of Dasyscypha which are
indistinguishable with the naked eye or a pocket lens, and
= .
76 THE LARCH CANKER
these were consequently regarded as one species by earlier
investigators. For clearness we will at present adopt the
names given by Massee (1895) for four of these. They are :
1. Dasyscypha calycina, Fuck.
Spores 18-25 x 6-8 p.
On larch and Scots pine.
2. D. subtilisima, Sacc.
Spores 8-10 x 2p.
On firs (larch, silver fir, and spruce ?).
3. D. abietis, Sacc.
Spores 12-14 3y.
Paraphyses longer than the asci.
On silver fir.
4. D. resinaria, Rehm.
Spores 3X 1°5-2 p.
On spruce, pine, larch, and Pinus excelsa. (Massee,
1902.)
Though there is to-day a considerable difference of opinion
as to the value of all these ea they will serve for pure
poses of argument.
Types which might sini any or all of these were
described by
1. Batsch (1786), p. 195. Desesiniiol and facie of |
a species not more than 1 mm. across, and rather longer
than broad, under the name of Elvella calyciformis. The
under-side of this fungus was grey-brown or flesh-coloured, —
so that Batsch was probably not describing the fungus
which causes canker.
2. Hedwig (1789), ii, p. 64, Tab. xxii, described the same
form under the name of Peziza calyciformis or Octospora —
calyciformis. He was the first to find the eight spores in
the ascus.
3. Willdenow (1787), p. 404, gave a similar account of
the fungus.
4. Schumacher (1803) first occur? the name Peziza
calycina, He found his specimens ‘ in strobylo Pini abietis ’,
THE LARCH CANKER 77
but since no spore measurements were given, it is impossible
to decide which species he was describing.
5. Fries (1822), p. 91 (and 1828) described three forms of
Peziza calycina, viz.:
a. Pint sylvestris = P. calyciformis of Willd., Batsch,
and Hedwig.
B. Abietis.
y. Laricis—‘ albido-testacea ’. ‘In ramis pini Laricis,
Chaillet. P. balsameae. Weinmann. (v.s.) ’
It is a convention among mycologists! to accept Fries’s
names for Ascomycetes where these are sufficiently dis-
tinctive ; but where a species is omitted, or insufficiently
described by Fries, to take the name given by the first
investigator after him who gave a description on which
a species can be based. What may be called the Continental
contention is that y Laricis is the equivalent of the canker-
producing fungus, and, if this is going to be raised to specific
rank, a new name [P. (Dasyscypha) Willkommii, Hart.] must
be adopted for it, since ‘ P. calycina’ is retained for a Pini
silvestris. |
It must be objected, however, that since each of the
species of Dasyscypha under consideration grows on a variety
of trees, and since D. calycina, D. subtilissima, and D.
resinaria, as described on p. 76, can each grow on the
larch, it is very doubtful to which of these species y Laricis
of Fries belongs, and it is also doubtful which form he
intended by a Pini sylvestris. Certainly ‘ albido-testacea ’
_ is not peculiarly applicable to the canker-producing fungus.
Fries introduced ‘ Dasyscyphae’ (Sacvs, hairy ; oxvdos,
a cup) as tribe vi of series ii (Lachnea) of the large genus
Peziza.
Passing over Corda (1837), v. 78, and Hornemann (1839),
plate 1917, 1, whose species given under the name of Peziza
calycina are both doubtful, we come to
6. Fuckel (1869), who is the first to give spore measure-
ments. Raising Fries’s name Dasyscyphae to generic rank, he
1 International Rules of Botanical Nomenclature, Brussels, art. 19, f.
78 THE LARCH CANKER
calls the fungus Dasyscypha calycina (Schum.), and gives the
dimensions of the spores as 20X 8p. This, with the habitat
of the fungus ‘an diirren berindeten Aesten von Larix
europaea nicht selten im Herbst ’, leaves no doubt that he
was describing the canker Fania:
It is on this description that the name Dasysouahe
calycina must be founded, since it is the first about which
no doubt can be entertained. Fuckel does not mention
any other species of Dasyscypha which grows on conifers.
Subsequent authors may be quoted in the usual way.
Dasyscypha calycina, (Schum.) Fuck.
Synon.: Peziza calycina (Schum.). Rehm (1876).
Peziza calycina (Schum.). Cooke (1876).
? Lachnea calycina (Schum.). Gillet (1879, p. 71).
Peziza Willkommit. Hartig (1880).
Lachnella calycina (Schum.). Phillips (1887, p. 241),
Helotium Willkommii (Hartig). Wettstein (1887).
Dasyscypha calycina (Fuck). Massee (1895). *
Trichoscypha Willkommii (Hartig). Boudier (1907).
Dasyscypha Willkommii, (Hartig) Rehm. Lind (1913).
It is probably not the same as
Erinella calycina (Hedwig). Patouillard (1883).
Erinella calycina (Hedwig). Quélet (1886).
Dasyscypha calycina, (Fries) Fuck. Lind (1913).
Saccardo describes the species as follows :
Dasyscypha calycina, (Schum.) Fuck. Symb., p. 305. Pez.
- calycina, Schum. Sael., p. 424, saltem p.p. nec Nyl Karst.
P. calycina var. Laricis Cooke Handb., p. 685. Peziza Will-
kommiu, Hartig Lehrb. Gregaria v. sparsa, saepissime
caespitosa, stipitata, albo-villulosa ; cupula expansa, sicca —
concava, disco aurantiaco-luteo v. aurantiaco, concaviusculo
v. planiusculo, 1-2 mm. lat. ; stipite brevi crassiusculo, in
cupulam dilitato ; ascis subcylindraceis 120 9, octosporis ;
sporidiis elongatis, obtusis, continuis, monostichis, 16-22 x
6-7 hyalinis ; paraphysibus filiformibus, hyalinis, sursum
incrassatulis, pilis hyalinis 2-5 crassis. Hab. in cortice
Laricis et Pini sylvestris in Germania, Britannia, Italia,
Gallia,
THE LARCH CANKER 79
It should here be noted that when growing saprophytically
the fungus is generally smaller in all its parts than when
taken from a canker spot. The saprophytic form is seldom
more than 2 mm. in diameter, whereas the parasitic one
may be as much as 3 mm., and the asci of the former are
about 140-170 X 9-12, and of the latter 160-200 x 10-15 p ;
and the spores 17-20 X 8-9,, in the one case and 20-23 x
9-10 in the other.
The spores are here measured in each case when ripe and
ejected and placed in water. When measured inside the
asci they are always somewhat smaller.
At first I was-inclined to regard these as two distinct
varieties of the fungus, but that they are only different
growth forms is shown by the following considerations :
1. A canker may be produced by inoculation with
mycelium grown from the spores of the saprophytic form.
2. Mycelium grown from the spores of the parasitic form
will grow readily on dead larch twigs, and produce apothecia.
3. When a young tree with canker dies the fungus spreads
i centrifugally from the canker and produces numerous
fructifications of the saprophytic type.
4, All intermediate forms between the two types can be
found.
The large apothecia of the parasitic form are probably
the result of differences in the substratum, the most obvious —
of which is the presence of a vastly greater amount of resin
in the canker than in the bark of a dead branch.
CHAPTER V
HEART-ROT. FOMES ANNOSUS
~ Various fungi which cause heart-rot. Fomes annosus: general ;_his-
torical. Secretions induced by Fomes annosus: turpentine and resin ;
soluble gum ; insoluble gum. Decomposition of the wood.
H&ART-ROT of trees is caused by fungi which grow sapro-
phytically on the dead wood, but are either incapable of
growing parasitically, or attack living tissues very feebly.
The heart-wood of trees is dead, has lost all its protoplasm
and with it much of its power of resisting fungi. Though
it is more resistant than dead sap-wood, it is by far the
most susceptible part of the trunk of a living tree, and
is liable to become entirely decayed, leaving only a ring of
healthy sap-wood which maintains the life of the tree.
With the destruction of the heart-wood the tree may lose
none of its vital activity, but it is nevertheless weakened
in two respects. The central column of tough wood is lost
and the tree is much more liable to be wind-blown, especially
as the roots are often weakened in the same manner as the
trunk, and if the central hollow has any communication
with the outside air, a ready means of infection is provided
for truly parasitic fungi.
One of the most familiar instances of heart-rot caused
by a purely saprophytic fungus is afforded by the elm.
This tree is usually grown in avenues or hedgerows where
large lateral branches are allowed to develop which would
be suppressed*in closely-timbered woods. These branches
are brittle, and frequent breakages result in numerous large
scars which expose both sap-wood and heart-wood to fungal
attack. In summer spores are nearly everywhere present
to take advantage of these scars, and the trees become |
infected with such fungi as Fomes ulmarius, which is the
chief, if not the only, cause of heart-rot and hollowness in
HEART-ROT 81
elms. When attacked by this fungus the heart-wood
assumes a reddish-brown colour, and ultimately falls to
dust, leaving the centre of the tree hollow. Large woody
fructifications are borne inside the hollow trunk,. white or
ashy grey above and salmon coloured beneath, which provide
spores for further infection by the fungus. Often by the
subsequent attack of other fungi the tree is killed, and then
the mycelium of Fomes ulmarius spreads through to the
bark and more fructifications are produced on the outside.
I have cited the instance of the elm as it is familiar to
every one. The hollowness of the trunk is commonly
discernible from the outside, and the fructifications. can
frequently be found. With conifers this is not so. The
destructive work of the fungus is the more insidious by its |
very secrecy. The heart of the tree is eaten away, and the
evil is betrayed by scarcely an external sign which would
warn the forester to cut the trees before the damage has
progressed so far that the base of the tree has become
worthless, and most foresters must have experienced the
disappointment of finding some of their finest specimens of
larch or Scots pine rotted up to 10 or 20 ft. from the ground,
when they had expected to obtain sound healthy butts.
It must be admitted at the outset that the cure of trees
which are attacked by any of the heart-rotting species is
impossible. It is thus of supreme importance that woods
should be grown under conditions in which disease is not
_ likely to occur, and the most essential part of our investiga-
tion of the pathology of the fungi will be that concerned.
with the mode of infection of the pests. No detail which
has any bearing on the problem of infection can be ignored,
and since a fungus may become reproductive in any stage
of its existence, we must familiarize ourselves with every
part of its life-history. To prevent loss it is also necessary
to keep a careful watch on all plantations and to adopt
every means at our disposal of tracing the earlier symptoms
of the rots. It may then be possible to thin out those trees
which would afterwards have become reduced in value, and
if whole woods are attacked it may be more profitable to
1888 : cd
82 HEART-ROT
cut them down before they are mature than to leave them
to be ravaged by disease.
It is possible to detect a tree which has become actually
hollow by tapping it sharply with a stick. The peculiar
sound emitted by a hollow tree can generally be recognized.
But this only discloses the fact that damage has already
been done, and to trace disease in an earlier stage a Pressler
borer must be employed. By boring young trees with this
instrument the presence of rot can be discovered before
the tree has seriously depreciated in value. It is recom-
mended that foresters should test their woods every few
years by means of such.a borer, picking out every tenth —
tree and boring it a foot or less from the ground to test its
soundness. Where diseased trees are found all the trees in
the immediate vicinity should also be tested, and those
which are attacked should be removed, if possible, root and
all. Holes made by the borer must be pegged by pieces of
stick, which should be cut off at the level of the bark.
Heart-rot of the larch is caused by several different
fungi, each of which has its own method of attack and causes
its peculiar form of destruction in the wood. The most
important of these fungi is Fomes annosus, which is de-
structive to nearly all speeies of conifers, and attacks both
young and old trees, producing strikingly different results
in the two cases. Wood destroyed by this fungus shows
small black specks later associated with white patches
which ultimately become holes. In this way the wood
may become honeycombed and feel soft and spongy, and
finally crumble into dust, or, if wet, it may become slimy.
In either case it leaves the tree hollow.
Another very important cause of heart-rot is Polyporus
Schweinitzii, which, though it does not make the tree hollow,
converts the wood into a darkish cork-coloured substance —
which is light in weight, smells strongly of turpentine, and
when dry crumbles to dust on being lightly rubbed between
the fingers. The red-brown fructifications of this fungus —
may frequently be found growing either on the lower
part of trunks themselves, or else on the roots, often at some
—
HEART-ROT eee
distance from the trunk. The fungus is especially frequent
in the south of England, and grows on the Scots pine and
maritime pine as well as the larch.
Heart-rot of the larch is also caused by Polyporus sul-
phureus, a fungus which is easily recognized by its bright
yellow, bracket-shaped fructifications which appear in July
and August. It is most common on oak, in which tree it
also produces heart-rot, but I have also a specimen of larch
which has been rotted by this species. The rot is similar
to that caused by P. Schweinitzii, but fractures occur
especially in the first-formed elements of the spring wood,
. so that the annual rings become separated from each other.
Also the turpentine smell is absent.
In Germany Poria vaporaria has also been frequently
recorded as a cause of heart-rot in larch and other conifers.
It is very doubtful whether the species which is alluded
to in these records ever occurs in Britain. The name
has been made to cover a large number of closely associated
species, and though I have found a Portia growing on dead
larch trunks and apparently causing a rot closely resembling
that described by Hartig, I have never been able to deter-
mine its capacity to grow in living trees. This fungus also
causes a rot which resembles that of Polyporus Schweinitzit.
Trametes Pini is another wound parasite which attacks
the larch through branch stubs near the crown of large trees.
It causes a very distinctive rot, which is described on p. 140.
There are thus four, or perhaps five, distinct species of
fungi which attack the heart-wood of the larch, and each
of these will be more fully described in the ensuing chapters.
But before closing this general introduction to the heart-
_ rots, it should be noted that the first two differ from the
remainder in one all-important respect. omes annosus
and Polyporus Schweinitzii are essentially root fungi, that
is to say, they attack the roots first, and always advance
from the roots to the trunk. Polyporus sulphureus and
Trametes Pini, on the other hand, gain admission in the
same way as Fomes ulmarius on the elm. They attack the
heart-wood which is exposed when branches fall off, and
G2
84 : HEART-ROT |
when the rot reaches the trunk it spreads both upwards
and downwards. There is some doubt as to the method of
infection adopted by Poria vaporaria, but probably, like
Polyporus sulphureus, it can only reach the heart-wood
through wounds. 7
Fomes annosus, (Fr.) Cooke: General. In Britain Fomes —
annosus attacks nearly all kinds of conifers. It has, how-
ever, two different modes of attack, and as the symptoms
are very distinct in the two cases, they might easily be
considered the work of two different fungi. In one case the
fungus attacks the trees when young, usually about four —
to ten years, and kills them in a year or two. Frequently .
in young plantations individuals or groups may turn a brown —
or reddish-brown colour, lose their leaves and die, and on
pulling these trees up it is generally found that the roots |
are attacked either by Armillaria mellea or Fomes annosus.
The identity of Armillaria mellea can generally be detected —
by the presence of rhizomorphs and the large flaky masses —
of mycelium between the wood and the phloem and between
the scales of the bark of the rootstock, as well as by the —
resin flow at the base of the trunk. Jomes annosus is not
so easy to identify unless some of the perennial fructifications
are present, but small pustules of mycelium on the roots
and the very thin layers of mycelium in the bark, no thicker
than the thinnest tissue paper, are usually sufficient evidence
of its presence. The species that are most commonly
attacked in this way are Douglas fir, Lawson’s cypress,
Weymouth pine, and sometimes the monkey-puzzle tree,
but probably many other conifers are liable to this kind of
attack. In such instances the fungus is a fairly rapid —
parasite, and death follows quickly on attack. Other —
species, such as the larch,). spruce, and Scots pine, are not —
generally attacked by the Fomes until they are comparatively
old and have made heart-wood. On these trees the fungus
is a rather feeble parasite and kills living tissues very slowly.
It flourishes, however, in the heart-wood which is dead, and |
1 The death of some young larch trees in the manner described above”
is reported by Somerville (1898). |
os
ae, BS
BS eee aE
NAY
ae
+
t
Fig. 29.—Single fructification of Fomes annosus found on a dead spruce.
Upper surface.
Fig. 30.— Lower surface of above (x 4),
HEART-ROT | 85
rots the centres of the trees without destroying the vital
tissues, so that the trees continue their growth and appear
_to be healthy until they are cut down, when the basal part
of the tree is found to be valueless. The disease is some-
times accompanied by a swelling of the base of the trunk,
known to foresters as ‘ goutiness ’, but it is by no means
safe to diagnose the disease on the basis of this single
feature. |
Conifers are not so often killed by the fungus when
attacked at this later period, but when the trees are weakened
by the overshadowing of other trees, death may ensue.
And larch is killed in this way less often than spruce or
Scots pine. At the same time larch is more frequently
_heart-rotted than either spruce or pine, at any rate in the
south of England. I have never seen silver fir rotted by this
fungus, though instances are recorded on the Continent.
Since in pumped trees the fungus is confined to the heart-
wood, fructifications are not generally formed on living
trees. But when the trees die the fungus penetrates to the
bark, and often bears large fructifications such as those
shown in figs. 29 and 30. This accounts for the comparative
infrequency of fructifications of Fomes annosus on larch
trees, and consequently for the doubt that has often been
expressed as to the connexion between this fungus and the
rot. But I have twice found unmistakable fructificatiorts
of the fungus growing in direct connexion with the rot in
living trees, once on the roots of a wind-blown larch near
‘Tintern, and the other time on a tree which I had dug up
at Terringham Wood in the Forest of Dean. Probably
fructifications are frequently borne in this way on the roots,
but being subterranean they remain unseen.
If further evidence is needed to confirm the connexion
between Fomes annosus and the larch heart-rot, it is pro-
vided by series of cultures taken on the one hand from
pieces of rotted wood, and on the other from spores of the
fungus. ‘These are identical in all respects, and agree in
the possession of a peculiar type of conidiophore which is
at present unknown in any other fungus.
86 HEART-ROT
Historical. The history of our knowledge of the disease
is briefly as follows. In 1878 Hartig published a detailed
account of his investigations on the fungus (which he
called Trametes radiciperda). His paper is chiefly concerned
with details of timber rotted by the fungus and its mode
of attack. He recognized two methods of infection, (i) by
spores of the fungus and (ii) by the contact of diseased roots
with living roots, and proposes combative measures which are
suggested by the results of his researches. Apparently he
never found Fomes annosus growing on larch, though he
suspected that this tree might also be attacked. But Hartig
did not clearly distinguish betweén the two forms of disease
for which the fungus is responsible, and since larch trees
are seldom killed by it, he may have overlooked the heart-
rot. He states that in Germany Pinus sylvestris, P. strobus,
Picea excelsa, Abies pectinata, and Juniperus communis are
the species which are most frequently attacked. It is
commonest in young plantations, but trees as old as one
hundred years may succumb to it. In 1889 Brefeld * carried
out an exhaustive investigation of the fructifications, the
germination of the spores, and artificial cultures. He was
the first to discover the conidia, which are produced in great
profusion in cultures but have rarely been found under
natural conditions. On account of the similarity between
the conidia and basidiospores, and between the organs
which bear them, he renamed the fungus Heterobasidion
annosum, thereby creating a new genus which has not been
perpetuated. On questions of prophylaxis he was funda-
mentally opposed to Hartig’s doctrines, and the divergence
of opinion was not softened by the caustic style of Brefeld’s
1 Oskar Brefeld was born in 1839 at Telgte, in Westphalia. He succeeded
R. Hartig as professor of botany at Eberswalde in 1878, and proceeded in
1884 to Miinster, and in 1898 to Breslau. His researches on fungi have
included the cultivation in pure cultures of an immense number of different
species, and he has thereby discovered conidial forms of reproduction in
many species in which they were previously unknown. He has also
brought methods of cultivation to a high degree of excellence, and was
the first to introduce gelatine as a medium for cultures (New International
Encyclopedia, iii, 1910).
HEART-ROT 87
criticism. Certainly the discovery of conidia profoundly
affected the value of the measures by means of which Hartig
sought to combat the disease. A more general account of
the fungus and its effects on trees may be found in the
translation (1894) by Somerville and Marshall Ward of
Hartig’s Diseases of T'rees.
Heart-rot is the cause of very serious financial loss to
foresters. It occurs with alarming frequency in woods
which are planted as a first rotation on land that has pre-
viously been arable or waste, and acres of plantations may
be reduced through it to two-thirds their value or less.
And since in Britain new land is frequently being converted
into forest, a conversion which is likely to be much accele-
rated in the near future, the disease must be regarded as
one of our most serious enemies. Plantations on old forest
land, however, remain comparatively free from heart-rot,
so that we may look forward with more confidence to the
yield of later rotations on land which is at present being
afforested.
Though Fomes annosus is found in North America, it
does not appear to do so much damage there as other heart-
rotting fungi. Butler (1903) records its occurrence on
deodar in India, but his description and figures suggest that,
if this fungus was present, it was working in collaboration
with Armillaria mellea.
In the following sections I shall describe in detail the
various characteristics of the disease as it appears in the
larch. Afterwards I shall deal with the methods by which
the fungus attacks the trees, and with the means of com-
bating it and minimizing its depredations.
Secretions induced by Fomes annosus. (i) Z’urpentine and
resin. In the early stages of attack by the heart-rot fungus
the wood assumes a reddish-brown appearance. This red
region usually surrounds all parts which are actually rotted
by the fungus, and advances in front of the heart-rot as it
ascends the tree. At first it resembles a red duramen, but
its tone is generally redder and deeper than that of normal
duramen, and the outside limit of red coloration may cut
88 HEART-ROT
across a number of annual rings and even reach the cambium.
when the attack is more marked on one side. This may
specially be seen in a rootstock when the fungus has entered
the tree through a lateral root.
Owing to this red colour the earlier stages of the disease
are sometimes known by foresters under the name of ‘ red
rot’. |
The red colour is ascribed by Hartig to infiltration of
the wood by turpentine and resin, which are secreted by the
resin ducts and medullary rays. If shavings of the red
wood are soaked for a considerable time in methylated
spirit, the turpentine and resin are dissolved and the colour
disappears, except in the medullary rays which retain
a reddish-brown tinge. Osmic acid stains part of the con-
tents of these medullary rays a deep black, and it is probably
the proteins which both give this reaction and cause the
red-brown colour in these cells. In the specimens that
I have examined the medullary ray cells have been all dead
and no nuclei could be traced. Stray hyphae were often
found in the red rot region, and where hyphae could not be
seen the occurrence of the empty bore-holes proved that they
had formerly been present. Hyphae were even seen in the
normal coloured wood outside the turpentine region.
(ii) Soluble gum. During the red rot and earlier stages of
attack a large amount of soluble gum is present in the
tracheides and medullary rays. It can easily be seen in
microscopic sections mounted in alcohol, in which it takes
the form of an emulsion in the tissues. On addition of
water this vanishes. It may be extracted by soaking in
water shavings of wood, which has reached the red rot
stage. When the water is poured off and filtered, it is
found to be slightly opalescent, and on adding an excess of
alcohol the gum is thrown down as a flocculent precipitate.
This may be collected by filtering, and on drying the filter-
papers it remains as a yellowish mass. This is readily
soluble in water, so that a strong and moderately pure
solution may be obtained in this way. From 500 grm. of
red rot shavings I obtained 22 grm. of gum. When dry it
_ HEART-ROT 89
is hard, but readily becomes soft on the addition of water,
though the solution is not adhesive.
The gummy nature of this substance, its solubility in
water and insolubility in alcohol, showed it to be either
a gum or a dextrin. On hydrolysing for half an hour by
boiling with dilute H2SO,, it gave large quantities of sugar
which was capable of reducing Fehling’s solution. This
led me to suppose that the substance was a dextrin, but
Dr. D. H. Vernon kindly tested it for me and identified it
asagum. The distinction was based on the readiness with
which an aqueous solution is precipitated by alcohol, and
on the detection of pentose sugars as a result of its hydro-
lysis. If alcohol to 51 per cent. be added to a 20 per cent.
solution of the gum, a dark-brown gummy niass is thrown
down and a moderate amount of white precipitate is retained
in solution. If alcohol is added to the filtrate up to 64 per
cent., a considerable further white precipitate is thrown
_down, while alcohol to 70 per cent. brings down a very
copious white precipitate. No further precipitate results
from adding more alcohol, so that we may conclude that
an excess of alcohol to 70 per cent. precipitates all the gum.
Dextrins, on the other hand, do not begin to be precipitated
till the alcohol exceeds 70 per cent. If the solution be
kept for some time the presence of pentose sugars may be
detected by the phloroglucol and orsinol tests, which give
very marked results. And since the gum was derived by
precipitation by alcohol from a watery extract of the wood, ©
it cannot be maintained that the pentose sugars were present
from the first, as such impurities would not be precipitated
by the alcohol.
Now it is known that in the process of delignification
a number of pentose derivatives are removed from the cell
walls, and we may here seek the source of the gum found
in the early stages of rot. Very little is at present known
about the chemical constitution of gums, and the term is
_ applied rather vaguely by biochemists ; but as far as I know
this is the first occasion in which a member of this class of
substance has been isolated from a conifer. In the trunks
90 HEART-ROT
of broad-leaved trees, especially in the heart-wood, wood
gum is not infrequent ; this, however, is insoluble in water.
A general account of the presence of gum in heart-wood,
and as a result of wounding in Dicotyledons, is given by
Temme (1885), and the reader is referred to this and to
.a short account in Beilstein (1893).
A few solubility tests were carried out on the larch gum,
and it was found to be soluble in water and nitric acid, but
insoluble in alcohol, ether, chloroform, sulphuric acid, and
caustic potash, and is doubtfully soluble in hydrochloric
acid. It has a high power of attraction for water, even
absorbing it from the air. When heated to 120°C. in an
oven it becomes somewhat changed, and does not dissolve
so freely in ‘water, though its adhesive power is much
increased. In this respect,as also in its solubility, it resembles
gum arabic. . |
In pure cultures of Fomes annosus in sterilized blocks of
larch wood, the gum is formed in a few days after infection,
and may be found at a distance of an inch from the nearest
fungal hyphae, though none is present in similar blocks
which have not been infected. We must suppose that
enzymes, presumably including a lignase, are secreted by
the fungus, and that these, or the products of their digestion
or both, become diffused through the tissues of the wood
blocks. It is curious that gum should appear at a time
when the only delignification that is apparent is in the
bore-holes, to be described hereafter, through which the
hyphae grow from one tracheide to another. But it is
possible that some slight delignification takes place through
the whole region surrounding the hyphae.
As the rot advances this gum disappears. As shown in
the next section it is probable that some of it becomes con-
verted into an insoluble gum, but what happens to the rest
is obscure. It does not seem in any way to impede the growth
of the fungus, and it is possible that the fungus even feeds
on it, either in its original state or after hydrolysis. 1 have
examined aheart-rotted spruce tree for the presence of
soluble gum and was unable to obtain any trace of it.
HEART-ROT 91
(iii) Insoluble gum. As can be seen in figs. 32 and 36,
the larch is generally rotted in a very irregular manner.
A transverse section of the trunk has often a curious figured
appearance, the truly rotted wood being in more or less
isolated patches, or the rot may be annular as in fig. 31,
with a peg of sound wood in the middle. In every case the
rotted wood is surrounded by a layer of very dark hard
wood which appears to prevent the further progress of the
rot. The colour and hardness of this layer are due to the
precipitation of an insoluble gum which fills all the tracheides
and medullary rays and forms a prominent object in all
sections of the wood.
Though: it imparts to the wood a red-brown colour, in
microscopic sections it is yellowish, so that the red tinge is
supplied by the wood itself. It is only found in close
proximity with the fungus, and apparently only where air
is present, for it occurs especially near the outside of infected
blocks, and often appears surrounding a bubble in a tracheide.
The nature of this substance has not been ascertained.
It is insoluble, but swells slightly in water, is insoluble in
alcohol, ether, chloroform, acetic acid, HNO;; in 10 per
cent. HCl or HeSO, it is not dissolved after five days, though
it then appears slightly corroded. It contracts markedly
in a 5 per cent. oxalic acid solution in spirit, and appears
somewhat discoloured. It dissolves entirely in 10 per cent.
KOH solution after three days’ immersion.
It thus differs from the ‘wound gum’ of Dicotyledons,
which, according to Temme (1885), is insoluble in KOH,
but is soluble in warm HNO;. Also Temme states that
if thin sections containing ‘wound gum’ be placed for
a quarter of an hour in dilute HCl and potassium chloride,
the gum, though still insoluble in water and ether, becomes
soluble in alcohol. With the gummy substance in the
larch this was not the case. This gummy substance has
considerable pathological importance, for it is impermeable
to hyphae of the fungus and acts as a screen, preventing
the unlimited growth of the mycelium. It is deposited as
a layer up to twenty tracheides thick, which entirely sur-
92 HEART-ROT
rounds the rotted region in advanced stages of disease and
definitely limits the scope of the fungus. In cases of annular
attack in which a central pillar of wood is left unrotted, it
is found that this is surrounded by a layer containing the
gummy substance, so that here also it secures the central
portion from attack. And when, as so often happens, the
rot is confined to certain patches (as seen in a transverse
cut), each patch is surrounded by a layer of this gum.
How this layer is formed is obscure, but its protecting
efficacy is clearly shown by the fact that on one side of it ©
the wood may be entirely rotted, though on the other it is
quite sound. This layer may thus help to account for the
fact that larch trees are so seldom killed by Fomes annosus,
for the young wood, being protected by this gummy layer,
continues to function. In completely rotted, dead trunks,
portions may often still be found which have been saved
from rot by the preservative gummy layer, though in such
cases they are usually not entirely free from fungi, and one
must suppose that the gum is eventually decomposed.
Woodmen have told me that they consider the part immedi-
ately surrounding the rot as stronger and more resistant to
damp than ordinary larch wood. This is readily intelligible
in the light of the above observations.
In spruce the heart-rot is sometimes surrounded by a dark-
brown or grey layer. In this layer some of the tracheides
are partially filled with a gum, but it is duller in colour and
has a different appearance from the insoluble gum of larch.
Decomposition of the wood. Themycelium in the tracheides
consists, in the first place, of septate hyphae which branch
freely and have hyaline contents. These hyphae vary
greatly in thickness, and although all intermediate sizes
may be found, they are mostly either large (3°5-5, in
diameter) or small (1-2). The large ones grow somewhat
irregularly, are often wavy in outline, and have thin walls.
The fine ones usually grow very straight and arise from
thick ones without themselves giving off many branches.
In early stages of attack they usually grow from one
tracheide to another through the bordered pits, but later
Fic. 31.—Fomes annosus. Annular rot with a central
unrotted core (x 3).
Fie. 32.—Fomes annosus. Figured rot; the central portion shows advanced
rot with white specks. Farther out the rot has not reached this stage, but
clearly shows layers of insoluble gum (x 4).
HEART-ROT 93
they make themselves bore-holes which are nearly circular
and of the same size as the hyphae. These bore-holes are
always exactly perpendicular to the surface of the walls,
and thus follow the line of the shortest distance from the
lumen of one tracheide to that of the next. To make these
bore-holes the hyphal tips must excrete the necessary
enzymes (presumably a lignase and a cytase). When once
a hypha has gained a passage from one tracheide to the
next, the hole which it occupies ceases to enlarge, and it
may reasonably be deduced from this that the enzymes are
only secreted by the apices of the hyphae, since, if they
were also secreted by the sides of the hyphae, the holes
would continue to enlarge when the hyphae had grown
through. Bore-holes may be seen cut through the curved
side-walls of a bordered pit while the actual pore of the pit
is left intact. These bore-holes are often extremely numerous,
and I have counted as many as thirty-five in 0°002 sq. mm.
of tracheide wall. |
The fungal hyphae may be seen in all kinds of elements
of the wood—tracheides, resin ducts, resin cells, medullary
ray tracheides and parenchyma. The hyphae in the
tracheides are of all sizes, and generally run along the
elements in a longitudinal direction and often clinging to
the sides. Hither large or small hyphae may bore through
the tracheide walls; thus the bore-holes vary from 1 to 5p
in diameter (fig. 35, 6). |
The resin ducts often become entirely filled with a mass
of hyphae which are nearly always fine: the broad hyphae
do not commonly appear in the resin ducts. In the medullary
ray parenchyma it is otherwise, for here only the coarser
hyphae are found, and these run generally along the medul-
lary rays, passing through the end walls of the ray cells by
holes which they bore through the simple pits.
The decomposition of the wood by the hyphae in this
stage is very slight, and, apart from the bore-holes, there is
no apparent delignification. But in certain places an
entirely different development of the hyphae occurs, which
causes black specks in the wood. These black specks are
94 HEART-ROT
always small and are more or less circular in cross-section,
though they extend to greater length in the longitudinal
direction. The cross-section has a diameter of 0°1-0°4 mm.,
and covers one to a dozen or more tracheides. The length
is fairly regular, being 0°5-1 mm.
These black specks may occur in the spring or summer
wood, and do not bear any definite relation to each other,
except that they some-
times appear in longi-
tudinal series, with a dis-
tance of 1 mm. or so
between them. Under
the high power of the
microscope they are seen
to be composed of dense
masses of hyphae, all of
the broader kind, whose
walls have become more
‘than usually thick, and
dyed with a deep brown
or black colour similar to
, Brunswick black. The
rd He CH ava hyphae at these points
Bes Be Eee eee «=obranch very freely, and
on the outskirts of the
Fie. 33.—Black specks in the wood of patch they are-seen with
the larch caused by Fomes annosus. rounded ends and have
Transverse section ( x 85). ;
a tendency to grow in
fascicles. Sometimes, however, the brown hyphae may be
seen continuing as colourless hyphae, and in such cases
the coloration ceases somewhat abruptly at the limit of
the black patch. These hyphae are presumably the original
hyphae which were present before the black patch was
formed, and which gave rise by branching to the rest of
the hyphae forming that patch ; but, in addition, hyaline
branches are also sent to the neighbouring tracheides, and
these may give rise to new black patches after the parent
ones have disappeared. ,
sauiteeceae CHAE
Baraat Ch, WH Mt aide,
Ae aneweeuluua(ieeseaa a
Org Madea testes
HEART-ROT | 95
The centre of a fully formed patch is so deeply coloured
that even in a thin section no details can be made out. But,
as will be shown later, treatment with dilute HCl bleaches
‘the whole patch, so that subsequent to this reaction the
structure can be investigated.
The mode of development of these patches has been
observed in detail in artificial infections on sterilized larch
wood blocks. They originate a few weeks after infection
at points where a large amount of soluble gum has collected,
and the first sign of their formation is seen in one or two
broad hyphae becoming light brown in colour. These
hyphae are sometimes situated in the tracheides, but more
generally in the medullary ray parenchyma. Thus Hartig’s
statement that the black specks originate in the medullary
rays is in general right, though his contention that the
blackness is due to substance diffusing from the medullary
rays into the tracheides is certainly wrong. From the original
brown hyphae other hyphae arise by branching, and they
all become much darker in colour. The branch hyphae
enter neighbouring tracheides and medullary rays so that
the black patch increases in size.
The hyphae penetrate everywhere, making vel'y numerous
bore-holes and entering and filling the cavities of the bordered
pits. Where three or four tracheides abut on to each other
the middle lamella is sometimes digested, and an inter-
cellular space is formed which becomes filled with hyphae.
In the black state the hyphae delignify all the walls they
touch, so that these walls cease to stain red with phloroglucol
_and hydrochloric acid, but give a blue reaction with chlor-
zinc-iodine. This shows that the lignone is absorbed and
the cellulose substance left. Further, the fungus may destroy
this cellulose substratum and whole walls may disappear, or
half a wall may be absorbed, i.e. the part on one side of
the middle lamella, and the other half remain. Thus it will
be seen that the digestion of the cellulose is somewhat
irregular. Probably some of the cellulose is digested from
every wall, for the walls are always thinner after this kind.
of attack than before.
96 HEART-ROT
The dense blackness of the patches makes it impossible
to observe the details of development in this stage until
Fie. 34.—Black speck in the wood of the larch caused by Fomes annosus.
Transverse section ( x 600).
the black pigment has been removed. But the fact that
these dark-walled hyphae are bleached by the action of
almost any acid makes investigation possible. Convenient.
eee
HEART-ROT 97
preparations for making observations on these patches were
obtained by first soaking sections in dilute HCl until
bleached, then staining with safranin and mounting in
Canada balsam. The
hyphal walls, which were
previously dark brown,
readily absorb the saf-
ranin, whilst those which
were formerly colourless
remain so. In sections
mounted in this way the
following details may be
observed.
As shown in fig. 35, the
hyphae at the edge of
the dark patch are easily
distinguishable, though
frequently two or three
may be seen growing side
by side in fascicles and
touching for their whole
length. They grow in
this way up and down
the tracheides and along
the. medullary rays.
Nearer the middle of the
‘black patch the hyphae
are very irregular, with
swellings here and there ;
and where two hyphae
are touching, the inter-
mediate walls have often
been partially or wholly
Fra. 35.—Longitudinal section near
the edge of a black speck, after bleach-
ing in dilute hydrochloric acid: 6b, bore-
hole; h, two hyphae joined by the disso-
lution of their adjoining walls ( x 800).
digested, so that several adjoining hyphae come to have
the appearance of one very large hypha (fig. 35, h).
In the centre of the patch still further disorganization is
found. The walls of some hyphae break down entirely,
setting free a jet black liquid which stains all the contents
1888 :
if
98 | HEART-ROT
of the tracheide. In this way many tracheides become
plugged by a more or less consistent black mass, composed
of the disorganized walls and contents of the hyphae. In
slightly older patches the viscid black liquid in the centre
is absorbed by the surrounding hyphae, leaving only. the
thin white delignified walls of the tracheides, pierced and
torn by innumerable bore-holes, and with scarcely sufficient
strength to maintain their structure. A touch is often
enough to tear them away, and then nothing is left but
a small hole in the wood.
Sometimes it is found that by the side of a white patch
of delignified tracheides left by a black speck as described,
another black patch is formed, and in rare instances a black
patch has been seen entirely surrounding a white one. By
this means holes which were originally small become large,
and eventually all the wood in an attacked region becomes
honeycombed. Black patches are also found in the middle
of white ones. How this occurs has not been determined,
but Hartig, who describes this state as typical, says that the
hyphae in a black patch delignify the tracheide walls
around them, though the black patch remains intact.
Wood that has become rotted in this way is dry, very light,
and has the colour of cork. It contains many holes and
white delignified patches, and almost always some black —
specks as well.
The ultimate state of rotted wood is very variable. When
a trunk, or more commonly a single root, is killed and
rotted right through to the cambium, the affected wood
dries as it rots, and becomes pale yellow, and at the same
time spongy and fibrous. On rubbing between the two
hands, it crumbles into a mass of fibres, which remain
more or less stringy and are too weak to stand much strain.
Inside a trunk which has not been killed the gummy layer
prevents drying, so that the rot advances a stage farther.
The wood then resembles coarse sawdust of the consistency
of coco-nut fibre, such as is used for growing bulbs. Fairly
large pieces of wood may still remain intact, though these —
are always partly rotted and riddled with bore-holes.
re on
{ ;
-
Fs a
r “ * ihn +
’ 7 4
iy ;
‘ ‘ 1
. . ¥ \
i x“ j
'
‘
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Hrteet 28 awels +
Fie. 36.—Fomes annosus. Base of a rotted trunk showing the figured
arrangement of the rot. The white delignified specks can be seen in
the rotted portions.
‘
Fie, 37.—Fomes annosus. Base of a larch trunk which has been excavated
underneath. ‘The lateral roots show figured rot, whilst the tap-root and an
anchor-root are completely rotted.
HEART-ROT 99
If the rotted wood remains wet (and it is often saturated
with water), it may become sodden and still more decom-
_ posed. It is very probable that bacteria assist in this final
stage, and the large amount of cellulose present’ would be
suited to bacterial action if enough moisture were present.
The mycelium grows up the trunks much more rapidly
than it spreads transversely, since the gum layer limits
lateral expansion. The affected part of the wood may be less
than half an inch in diameter, but up to 4 or 5 ft. in
length. If the rotted portion could be dissected out it would
then appear as a long spike. Later the fungus grows up
from other basal. points, making fresh spikes, and when
a number of these are cut across in a transverse section,
a beautiful pattern is often produced. Fig. 36 is a photo-
graph of such a section, cut somewhat obliquely with an
axe. Jn the rotted portions the white cellulose patches can
be seen, and the dark gum layers round the individual
‘ spikes ’ of rotted wood are also visible. Another common
type of rot is shown in fig. 31. Here an annular portion
is rotted and a central peg of sound wood remains, which
is of course protected by a layer of insoluble gum. This
central peg is often present, and may be no thicker than
a pencil, though as much as 5 ft. long.
_CHAPTER VI
HEART-ROT. FOMES ANNOSUS (Concluded)
Reproductive organs: fructifications ; conidiophores. Pure cultures
on artificial media. Cultures on natural media. Infection experiments.
Mode of attack in nature. The frequency of heart-rot in plantations
which form the first rotation on cultivated soil. Methods of prevention.
Reproductive organs. Fomes annosus has two kinds of
reproductive organs. These are (1) fructifications, usually |
large, of Polyporus form, which may occasionally be found
associated with larch heart-rot, but which are much com-
moner on young trees of other coniferous species when
they have been killed by the fungus; and (2) conidia on
somewhat specialized conidiophores, which occur regularly
in all cultures, but have been found, so far, only very rarely
in a truly wild state. They are apparently formed only in
a saturated atmosphere, such as is provided under usual
cultural conditions, but which cannot be relied upon in
nature.
The morphology and life-history of the fungus have bac
carefully worked out by Brefeld (1889) under the name of
Heterobasidion annosum, (Fries) Brefeld, and I have made
free use of his description in the following account.
1. Typical fructifications are shown in figs. 29, 30, 39, 40.
They are of two kinds—bracket-shaped, borne usually on
the sides of trunks and above ground, and ‘ resupinate ’,
which grow on the under-sides of roots and have the whole
or nearly the whole of the upper side attached to the root.
The latter form is generally subterranean, and has its lower,
spore-bearing side exposed in some hole in the ground,
such as those made by rabbits and mice.
The fructifications first arise as small white masses of
hyphae, often no bigger than a pin’s head. These break
through the bark of the roots and broaden on the surface
Fria. 38.—Fomes annosus. Fructification cut across
showing pores in longitudinal section (x 4).
Fig. 39.
Fomes annosus. Under surface of small fructification
showing mouths of pores (x #4).
Fie. 40.—Upper surface of fructification showing concentric furrows and l
white margin. The white at the bottom edge of the photograph is a flint
included in the fungus (x +).
HEART-ROT 101
without at first becoming much thicker. If they are growing
on the under-surface of a root they cling to the root until
their width becomes greater than that of the root, when
they grow beyond it in the shape of a bracket. If they are
growing on the side of the trunk (always near the base)
they develop the bracket form quite early. When these
fructifications reach the size of a penny, and sometimes
much earlier, pores or slight depressions appear on the
lower surface. These are the beginnings of the tubes inside
which develops the hymenial surface. The fructification is
then composed of a continuous layer, of hyphae on the
upper side, about 1-2 mm. thick, and a layer of vertical
tubes on the lower side, which may be more than double
this thickness. The upper surface is at first pale brown ;
in resupinate fructifications it remains pale, but in bracket
fructifications it becomes darker, until it attains a dark
reddish-brown colour. As long as the fructification is
growing laterally the margin is white (fig. 40), but when
lateral extension ceases the margin generally becomes
dark brown like the rest (fig. 29). This surface is concen-
trically furrowed (at any rate in bracket forms), and the
furrows are striated radially. The lower surface is at first
white, and may also be pure white in older fructifications,
but it often becomes biscuit coloured and even slightly
reddish, especially at times when it is not growing actively.
The fructifications grow in size (i) by the tubes becoming
longer, thus increasing the thickness, and (ii) by marginal
growth, which makes the fructification broader. The new
parts bear pores very nearly up to the margin, and the
youngest portion is‘always the whitest. Fructifications
may reach a considerable size, and the largest I have seen
is 17-5 by 8-5 in. and 5 in. thick. This specimen, which is
shown in figs. 29 and 30, was found by my wife on a dead
spruce. Growth is most active in summer, but I have
found fructifications growing vigorously and bearing spores
in March. When growing, the marginal extension keeps
somewhat ahead of the pore-formation, so that the margin
is sterile, but as soon as marginal growth ceases, pores are
102 HEART-ROT
made very nearly up to the margin, so that in the resting
stage the fungus has the appearance shown in fig. 39.
A very constant feature of the fructifications of this
species is the holes in the sporophores. When the latter
encounter a small root or stick, or even a blade of grass,
they grow round it, leaving a hole such as those seen in
fig. 39. These holes differ from those made under similar
circumstances by most other fungi, by being surrounded by
‘apart a reddish-brown ridge of sterile
tissue. This ridge forms one
of the most useful features for
identification. Fig. 39 shows
the appearance of the lower
surface of the pores. It will
be seen that they vary from
circular to irregularly oval or
elliptical. Their width is
usually about 0-25 mm.
(= zovin.), but may be any-
Fie, 41.—Fomes annosus: A, thing from 0:15 to 0-6 mm.
hymenial layer inside @ pore; @8 The pores are not fertile
young basidium with two spores ; p
b., older —— se hee! encres ; through their whole length at
p., paraphysis (x ; B, single . Ye
spore showing attachment to basi- the seine time. As a fructi
dium ; b., basidium ; st.,sterigma; fication grows older the upper
se., septum; sp., basidiospore (x
1000) ; ©, ripe spores after shedding. pad of each pore , becomes
3 filled with hyphae, and only
the lowermost 1 to 2 cm., i.e. the part nearest the openings,
bears spores. During the summer the pores lengthen and
the fertile zone moves downwards. Fig. 38 shows the pores
in longitudinal section ; two pieces of the fungus are here
cut through, in each of which two years’ growth can be
distinguished.
In the fertile region each pore is lined with the hymenium,
a layer made up of basidia and paraphyses. Fig. 41, A,
shows this layer as seen in a longitudinal section of the
pores. The basidia are 30-40» long and 8-10 broad.
Normally each bears four spores (0.), but basidia with two
spores are frequent (a.),and basidia with three or five spores
HEART-ROT 103
have occasionally been found. Each spore is attached to
the basidium by a very thin extension of the latter called
a sterigma (fig. 41, B, st.), and when ripe the spore which
develops at its extremity is cut off from the sterigma by
a transverse septum. The mature spores are hyaline,
measure 5-5-7 X 4-5-5, and have a slight projection at one
side, where they are attached to the sterigmata.. The
“eee ee = ww ee on
Hy
%
¥
B
Fie. 42.—Diagram showing the necessity for the pores to be vertical.
In A, in which the orientation is correct, all the spores escape. In B,
which. is tilted, only a small proportion of the spores escape from the
pores and the rest stick to the sides. For the sake of clearness the pores
are shown as much wider in proportion to their length than is the case
in Fomus annosus. For correct proportions the pores should be eight
times as long as in the diagram. The loss of spores through tilting would
then be much greater than that shown in the diagram.
paraphyses are similar to the basidia in shape, but remain
sterile, and serve to keep the basidia at appropriate
distances.
The spores, like those of most Hymenomycetes, are
. slightly sticky, so that if they come in contact, during their
fall, with the sides of the pores, they adhere and fail to
escape. To prevent this it is necessary for the pores to be
exactly vertical, and the fungus, by some means not as yet
understood, has the power of making them grow in the
104 HEART-ROT
desired direction. It is expressed botanically by saying —
that the pores are positively geotropic, i.e. they grow
towards the centre of the earth. ‘The necessity for exactitude
in this orientation is demonstrated by the following calcula-
tion. If a tube is 0-25 mm. across and bears spores for
a distance of 2 cm. from the orifice, a displacement from
the vertical of tan *sé3s (= #o), i.e. a displacement of
less than 1°, will prevent any spores growing as‘far as 2 cm.
from the orifice from escaping (fig. 42). Actually the
accuracy has to be far greater than is shown in this calcula-
tion, since the spores do not simply fall from the basidia
but are shot off to a distance of rather less than 0-1 mm.
(vide Buller). The ejection mechanism for the Polyporeae
has been investigated by Buller, and, as is shown in the
accompanying diagram (fig. 41, B), the ejection is accom-
plished by the splitting of the septum between the spore
and the sterigma. The force of propulsion is provided by
the turgidity of the basidium and the spore, each of which
tends to bulge out the septum which separates them. The
fineness of the mechanism is due to the thinness of the
sterigma and the consequent small cross-section of the
septum that crosses it. |
The spores, being very small, present a large surface in
proportion to their weight, so that once they have escaped
from the pores they are easily carried away by the wind.
They germinate at once, either in pure water or damp air,
or in nutrient solution, and under suitable conditions every
spore germinates. The germ tube may emerge from any
part of the wall, even, occasionally, from the small point
where the spore was attached to the sterigma.
2. Conidiophores arise in all cultures. They are borne
on especially broad hyphae, which occur in groups in the
mycelium, and have the form shown in fig. 43. They may
be simple or branched, and have a few cross septa, and the
whole conidiophore is somewhat reminiscent of that of
Aspergillus. 'The conidiophore is rather swollen at its apex,
and the conidia closely resemble the basidiospores. When
the conidia have been ejected the conidiophores are left
|e
HEART-ROT 105
with a number of pointed sterigmata, and even in this state
they are easily recognized.
These conidiophores and conidia were first discovered by
Brefeld, who sought in them support for his theory that
basidia and basidiospores are modified conidiophores and
conidia. He found instances in which the conidiophores
bore three or four large conidia instead of a large number
of smaller ones. But, even so, it is unlikely that they are
homologous with basidia. The
formation of basidiospores is
generally, if not invariably, pre-
ceded by a nuclear fusion ; but if
these conidia are homologous with
the conidia of types whose cytology Oo
has been investigated, the nuclei
which enter them are not derived
at all directly from a fusion nucleus.
This distinction between conidia
and basidiospores has been drawn
subsequently to Brefeld’s work on
Fomes annosus, and it is sufficient
NY Se . : ; Fic. 43.—Fomes annosus :
to justify the disfavour with which 4, conidiophore from which
his synonym ‘ Heterobasidion an- some conidia have fallen,
; , leaving the pointed sterig-
nosum ’ has been received. mata; b, conidia ( x 400).
As stated above, these conidio-
phores occur regularly in cultures, not only on bread and
gelatine, but also on diseased wood and even on fructifi-
cations if kept in a damp chamber. They are usually simple
or slightly branched, but under certain circumstances, as
when a culture is grown on bread soaked in a solution
*of dung and given plenty of room, the conidiophores may
appear in large bundles. A single conidiophore of such
a bundle is exactly like one of the simple conidiophores
and has the same dimensions, but the whole bundle may be
a quarter of an inch high.
As these conidiophores are of such frequent occurrence in
artificial cultures it is extraordinary that they should be so
rarely met with in nature. No doubt the conditions which
106 HEART-ROT
favour their formation are not always present, but probably
the chief reason is that owing to their insignificance they
are generally overlooked. Olsen (Brefeld, 1889, p. 177)
has, however, found layers of the conidiophores on fallen
trees in Norway, growing in such a way as to have a super-
ficial: resemblance to a species of Corticitum. He does not
state the time of the year at which they were found. This
is important, as it is possible that climate plays an important
part in their formation.
Brefeld, who has had unrivalled success in producing the
fructifications of this higher fungi in the laboratory, was
unable to stimulate their growth in the case of Fomes
annosus, and suggested that under pure-culture conditions
a conidial race is produced which becomes incapable of
bearing fructifications. In one of my own cultures, how-
ever, a sterilized larch block, which was infected with
conidia, produced normal, though small, fructifications
(fig. 44), and it must be presumed that Brefeld did not
experiment with the most suitable medium.
The germination of conidia is in every way similar to
that of basidiospores. They germinate at once, and with -
great regularity, both in pure water and in nutrient solu-
tions. The possibility of making inoculations from old,
apparently dried up, cultures of more than a year’s standing
testifies to the longevity of the conidia.
Pure cultures on artificial media. Cultures of the fungus
grow readily in 15 per cent. gelatine or 3 per cent. agar-agar
when appropriate food-stuffs are added. I have found that
meat extract 0-3 per cent., malt extract 3 per cent., give
suitable nutriment. If the medium is at all alkaline it may
be neutralized with citric acid. Cultures were first obtained _
from rotted portions of pumped trees. Small lumps of
wood were sterilized externally by holding for a few seconds
in a weak flame. The central portions were then cut out —
with a sterilized knife and placed on the gelatine in culture
flasks, which had been suitably sterilized prior to inocula-
tion. Cultures grew slowly at first and then fairly rapidly,
and were similar on gelatine and agar. The mycelium is
HEART-ROT _ 107
white, and does not grow out much from the substratum,
and appears finely granular. Conidiophores are produced
in six to ten days, and may constantly be found afterwards.
Cultures obtained from basidiospores or conidia differ in
no way from those grown from diseased wood, but the risk
of including impurities is reduced. Cultures will also grow
on moistened sterilized bread, but they are not very vigorous
on this medium.
Cultures on natural media. Cultures were grown on
. Sterilized larch twigs with bark.
. Sterilized blocks of larch wood, composed of heart-
she or sap-wood or both.
3. Sterilized roots of (a) larch, (b) Wasinoutil pine.
4. Soils from various localities.
Unless otherwise stated, inoculations were made by
placing small pieces of gelatine or agar-agar containing
mycelium on the substratum which was to be infected.
1. Larch twigs 2-3 in. long and 4 in. in diameter were
placed on damp cotton-wool in Erlenmeyer flasks and test-
tubes. On sterilizing in the autoclave the cotton-wool
became slightly stained by liquid running down from the
twigs. Mycelium placed on these twigs grew fairly rapidly,
and at room temperature completely covered them in about
six weeks, and even spread on to the cotton-wool at the
bottom of the flasks. A much more woolly mycelium was
produced on larch twigs than in gelatine or agar-agar
cultures, and conidiophores were everywhere abundant.
Mycelium penetrated the periderm, cortex, phloem, and
wood. |
2. Cultures on larch blocks were very similar. The
mycelium grew much more rapidly on the surface of the
blocks than- inside, especially when the air was thoroughly
damp. When the cotton-wool at the base of the test-tube
was only slightly moistened, the external growth was scarcely
noticeable, though on addition of more water it became
quite normal. It is much greater on blocks of alburnum
than on duramen.
The formation of soluble and insoluble gum and. the
108 HEART-ROT
process of wood destruction in these cultures are described in
the sections dealing with these phenomena. In one of these
cultures two fructifications have now been growing for
more than four years (fig. 44).
3. On small sterilized roots of larch and Weymouth pine
growth was rapid, especially at a temperature of 20°—22° C.
Small bunches of such roots in test-tubes became smothered
in mycelium to a distance of an inch in fifteen days.
4, Altogether over 220 cultures were attempted on soils.
Special care was taken with these because, on discovering
that mycelium would grow readily on sterilized soil, I thought
I might find that it preferred some soil to others, and in
this way immunity and susceptibility to heart-rot might
be in some measure explained. Some results of value have
been obtained from these experiments, but the problem of
immunity and susceptibility has certainly not been solved
by them. No growth was obtained on unsterilized soil,
and growth on autoclaved soil was much more abundant
than on dry sterilized soil, so that the method of treating
soil prior to infection is a matter of great importance.
After some preliminary trials my procedure was as follows :
For each sample of soil twelve or more test-tubes were
prepared. A plug of cotton-wool was placed in the bottom
of each and dry soil poured in to a depth of about 2 in.
The test-tubes were plugged in the usual way. ‘io four of
them sterilized water was added without further treatment
till infection. Four more were moistened with sterilized
water and autoclaved for twenty minutes at 2 atm. pressure.
Yet another four were sterilized in the hot-air sterilizer for
twenty-five minutes at 140° C., and sterilized water was
added when they were cold. Soils were inoculated some-
times from mycelium, but generally by pouring in a drop or
two of water containing conidia. The soils on which such
sets of cultures were made are shown in the following
table.
,
tn. re en ae
ee 2 ee
a attain
— s » aos
RSE RE ROTI We
"VLI9JOVG JO S1O4SNTO 91V (GZ JO [LOS oy UT syoods o41YM OYJ,
‘(LOS pazi[I19jsuN JO Joao dATJIQIyUL yy SMoYs Ydvisojoyd oy], “papp’ SBA [IOs pozt{I10qsun
O[9TT BING ‘AVM SUIS OTT} UL PoJaJUL 9IOM IG-QQz ‘ SNsouUN*T Jo wUNTTeOLUT YALA poyoosUT 910M
F-Z8S_ ‘PEZI[L1948 SVA [IOS VY} AQNj-Js9} YOVE UT [LOS WO serny[NQ—'cF “Yq "SUOTPBOATPONAT
[[VUIs OA} SUTMOYS YOOTG YOAV~ uO oINy[Nd 91INng—'FFH “YLT "SNSOUUD SAWOT JO SaIN4[NO
‘cp OTA ‘PP OLE
(2)
lo
(¢)
(a)
(f)
(9)
(h)
9)
HEART-ROT »
Not
Soil. sterilized.
From pot in which young
larch had been grown:
sandy loam: _ slightly
acid: humus and nume-
rous dead roots of larch
Sand with oak humus:
no lime: markedly acid :
deep black colour
Beech humus : little lime
some roots : brown|
colour
Soil from beneath spruce
containing needles and
roots: much lime: brown
colour
Clay with flints, from
above chalk: — beech
humus
-Well-manured light arable
sand
Moderately manured
heavy arable sand
Sycamore humus
Sandy loam from Douglas
plantation which had
been attacked by Fomes
annosus
Same wood as (k), but
where Douglas was]
healthy
Same wood as (jd),
another part
From larch and_ beech
wood near a fructifica-
tion of Fomes annosus :
loam above chalk
From near spruce at- )
tacked by Ey pore
Schweinitzii : oak above:
loam above chalk |
From near spruce killed)
by Polyporus Schweinit-
zit: very open wood:
loam above chalk
but )
j
The test for the growth of Fomes
was the appearance of conidiophores,
no growth
99
109
Auto- Dry
claved. sterilized.
good growth oo
fair growth —
good growth no growth
fair growth slight
growth
good growth fair
growth
fair growth 2 -
very - poor no growth
growth
fair growth mY a
good growth poor
growth
fair growth fair
growth
good growth no growth
annosus in each case
and if these were not
110 HEART-ROT
present the mycelium was not considered as definitely
belonging to the fungus. This precaution was especially
necessary with cultures on soils which had not been sterilized,
since many saprophytic fungi (especially Penicillium spp.)
as well as bacteria made their appearance. The most
striking feature of this table is the failure of the fungus to
grow on any unsterilized soil and its growth on all soils
when autoclaved.
The changes in the substratum, induced by niitoola vines
which might account for this are :
1. Destruction of rival organisms.
2. Chemical changes induced by boiling.
Both these effects are probably of importance, and they
must be considered separately.
1. Destruction of rival organisms. In cultures on un-
sterilized soil other fungi sometimes appeared, but they
were not constant. A certain species of bacterium, however,
nearly always grew abundantly and formed a white covering
over part of the soil. It seemed possible that this bacterium
was destructive to the mycelium of Fomes annosus, and to
prove this a series of cultures was made in the following
way. Eighteen test-tubes were filled in the usual way with
sand and oak humus (soil 6). Twelve were. autoclaved and
six were not. They were then treated according to the
following table : |
Cultures No. Infected with
276-78. Autoclaved conidia.
279-81. Notsterilized — conidia.
282-84. Autoclaved mycelium.
285-87. Not sterilized mycelium,
288-90. Autoclaved mycelium and a little unsterilized soil.
291-93. Autoclaved conidia and a little unsterilized soil.
The results were :
276-78. Very good growth.
279-81. Bacteria only.
282-84. Very good growth.
285-87. Bacteria only.
288-90, Fair growth at first of mycelium of various fungi, but later
y restricted to mycelium above the soil and bacteria in the
291-93. | soil itself.
HEART-ROT 111
The effect of the addition of bacteria is clearly shown in
fig. 45. It is thus proved that the unsterilized soil contains
some factor which is very strongly inhibitive to the growth
of the mycelium of Fomes annosus, and this factor appears
to be bacterial.
2. It is evident that chemica] changes are induced by
boiling humus soils containing roots and other plant remains
which have not been completely disorganized. .When twigs
and wood blocks are autoclaved in test-tubes, a brown stain
nearly always runs down on to the cotton-wool at the
bottom of the test-tube. When infected with Fomes annosus
the latter grows down on to the cotton-wool and flourishes
on the stained portion. No doubt a similar liquid escapes —
from roots into the soil containing them when the whole is
boiled, and this probably assists the growth of fungus
mycelium. (This, however, cannot apply to such arable
soils as f and g, which contained no organized remains.)
There is also a physical change, for soil which is autoclaved
becomes very evenly moistened, whereas soils which have
been dry sterilized and subsequently wetted are very
difficult to moisten evenly, and this may partially account
for the poor growth of mycelium on dry-sterilized soils.
From the foregoing experiments it is clear that the
mycelium of Fomes annosus will grow on soils under certain
conditions. One condition is the absence of a certain
bacterium which is so frequently present in the soil as to
inhibit the mycelial growth in all my cultures on unsterilized
soils. These experiments were all performed in the summer,
when bacterial activity is at its greatest. It is possible that
in the spring, when bacterial growth is less active,’ more
auspicious conditions for Fomes mycelium may obtain.
_. Infection experiments. A series of infections was made
on two-year-old potted plants of larch and spruce.
These were designed to answer the following questions :
1. Can unwounded roots become infected with the
fungus ?
2. Can infection take place through slight wounds ?
3. Can infection take place through dead roots ?
112 HEART-ROT
4. Can trees be infected through growing on infected
soil ?
In order to infect the trees they were lifted from the
ground and their roots were washed in running water.
Pieces of infected roots, or lumps of infected soil, taken
from pure cultures were tied to roots of the trees by means
of bast. Infection points were tied round with sterilized
moss, and the ‘trees were carefully potted in suitable soil.
On March 30, 1915, twenty-four infections were made with-
out wounding, eighteen with slight. scalpel wounds into the
phloem, eighteen on roots still attached to the trees which
had been placed in nearly boiling water for one minute in
order to kill them, whilst the other roots of the trees remained
alive, and twelve on roots which were both boiled and
wounded. —
None of the infections were effective where there was no
wound, whereas all those where the fungus had touched
a wound were successful. In these successful infections the
mycelium grew into the wood and usually reached the
centre of the root. The hyphae grew both upwards and
downwards in the wood and bark, killing and browning the
tissues on their way. Growth was very slow, but rather
more rapid in the wood than in the phloem, where it was
always hindered by cork layers which were made across the
phloem and cortex as an obstruction. The extreme slowness
of growth is shown by the fact that in most cases the
mycelium had only spread 5-6 mm. upwards and downwards
in the wood and about 1 mm. less in the phloem in six months.
Infections on killed roots were invariably successful. In
five weeks the mycelium had spread to one and a half inches
in the wood, and since infections were mostly made on the
larger roots the hyphae soon spread to the stem and. killed
the tree. With some of the trees, whose experimental roots
had been killed, I was doubtful whether enough healthy
roots had been left to supply water during the summer
months. On this account I cannot be certain that death
was always brought about by the fungus. But some were
examined as soon as the trees began to look unhealthy,
HEART-ROT 113
and in these it was found that the hyphae had penetrated to
‘the stem. _
Thus question 1 can be answered in the negative, ques-
tions 2, 3, and 4in the affirmative. Infection does not take
place through unwounded roots, at any rate under the
circumstances of the experiments (too much confidence
must not be placed in a negative result), but can take place
through a living wounded root or through a dead root.
Mode of attack in nature. The habit of the fungus leaves
no doubt as to the part of the tree which is first attacked.
It is always the roots which first become infected, and in
every case that has been recorded the disease has spread
from the roots to the stem, and not vice versa. Infection
then, must be subterranean, or immediately on the surface
of the soil.
Hartig (1878), who at that time was unaware of the
existence of conidia, recognized two possible means of
infection. One was by the spores of the fungus reaching
the roots of healthy trees, and the other by living roots
coming into contact with roots which contained the fungus.
These two methods will be discussed in turn, and as they
do not seem adequate to account for all the infections which
occur in nature, a third method will be suggested.
(i) Infection by means of spores. The fructifications of
the fungus are usually subterranean, or if they grow above
ground they bear their spores so near to the soil that wind
can play but a small part in the dispersal of the spores.
Often fructifications which begin to develop subaerially
become covered by fallen leaves and shoots of ivy, but this
seems rather to encourage than retard their development.
Thus it is presumed that spores are carried by other means
than air currents, and, as Hartig suggested, rabbits, voles,
and other burrowing animals are probably the chief agency
in dispersal. The fructifications require some open space
for development, and burrows, made by animals under the
trees, afford the kind of station in which they flourish. So
there is reason to think that such animals catch the spores
in their fur or pick them up on their feet, and they may
1888 I
/
114 HEART-ROT
rub them off on to other roots which they pass. As roots
are often broken by rabbits while making their burrows, it’
appears likely that such spores may occasionally come in
contact with wounds by which they can infect the trees.
Hartig attached little importance to this mode of infection,
as he found a very low percentage of germination in the
spores which he examined, but Brefeld found subsequently
that, when ripe, the spores germinate almost invariably.
(ii) Infection by means of healthy xoots coming into contact
with diseased roots. This might happen when a healthy
root grows so as to meet a diseased root, or two roots might
be touching and the disease might spread down one and
cross to the other. In order to discuss this theory it is.
essential to know whether a root can become infected whilst
still undamaged. Unfortunately, Hartig’s experiments leave
this question unsolved. To demonstrate the possibility of
such infection he placed diseased roots in contact with living
roots after removing the outer bark scales from parts of the
latter. It is not clear whether by this treatment the living
tissues of the roots were exposed, i.e. whether the roots
were wounded or not. In my own experiments, infection
did not take place unless the roots were definitely wounded,
and it may be regarded as extremely unlikely that living
roots can be infected except through wounds. If this is
so, this theory of infection demands not only that a diseased
root should be in contact with a living root, but that it
should actually touch a wounded portion of a living root. -
This must greatly reduce the frequency of opportunities
for such infection, and it no longer appears reasonable te
account for the infection of groups of trees, which un-
doubtedly takes place, by this means. The following theory,
though difficult to demonstrate in the woods, seems to have
a wider application. |
(iii) Infection through dead roots. My infection experi-
ments have shown that trees can become infected when
attached dead roots are brought into contact with diseased.
roots or soil infected with the fungus. These experiments
may, however, be criticized on the ground that natural
“HEART-ROT 115
conditions were not accurately reproduced, that by steam-
ing a portion of a root and allowing it to become infected
immediately time was not allowed for the plant to develop
its normal power of resistance against fungal attack. For
when a portion of a root dies, the plant to which it belongs
inserts a screen between the living and the dead parts
which is made as far as possible impermeable to fungal
hyphae. This criticism is valid up to a point. The screen
which the root makes is of two kinds: firstly, a layer of
cork across the phloem and cortex, i.e. all the tissues -
outside the cambium ; and secondly, various devices in the
wood, which, having non-living as well as living elements,
is less able to make a protective layer. In conifers the pro-
tection of this region is chiefly, if not entirely, limited to
plugging the tracheides with resin, and even this amount
-of protection is confined to the sap-wood, which contains
living cells. The heart-wood can adopt no such measure,
and in consequence the heart-wood remains as a free channel
’ for the growth of any fungus which can live in it. It thus
appears that dead roots, if they are small and-contain no
heart-wood, may be safely delimited from the living portions
of the tree; whereas large roots, when dead, present an
open path by which any of the heart-rotting fungi can gain
admission to the trunk.
Dead roots may become infected in a variety of ways—
by spores or conidia, by contact with diseased roots,
_ and, what is probably more important than either of these
methods, by mycelium growing in the soil. Pure cultures
_ of the fungus only proved successful on soils when the
_. latter had been sterilized, and/it was shown that other |
organisms, which were present in unsterilized soils, were
sufficient to prevent the growth of Fomes annosus. But in
spite of being unable to produce a rigid demonstration,
I think it is probable that under suitable conditions the
fungus may grow in forest soil and bear conidia, which,
being washed down by rain, will reach the lower strata,
where, as shown in the next section, dead roots most com-
monly occur.
12
116 HEART-ROT
The frequency of heart-rot in plantations which form the
first rotation on cultivated soil. A large number of observa-
tions haye shown that heart-rot is especially to be appre-
hended in coniferous woods when they are planted as the
first forest rotation on land which has been previously
cultivated. The same applies, though in a lesser degree, to
first plantations on commons and heaths. I have personally
seen plantations of larch damaged in this way on the Tintern
estate, at Terringham Wood near Cinderford, in the Forest
of Dean, on two separate plantations on the Duke of Bed-
ford’s estate at Endsleigh, near Tavistock, and two or three
more near Brentor in Devon, where in every case the larch
was a first forest crop. Other instances have been reported
in Britain, and Sir William Schlich tells me that in Germany
heart-rot commonly affects the spruce, which is planted as
a pioneer rotation, though the second rotation is usually»
free from it. As no adequate explanation has been advanced *
for this phenomenon, I made a special study of a number of
trees at Terringham Wood. Thirteen trees were uprooted
with the object of finding out through what part of the root
system the fungus entered. It was found possible to locate
the path of infection, for, although all the roots of diseased
trees were rotted to a certain extent, the rotted roots could
be divided into two markedly distinct classes. Those which
had what I shall term ‘ primary rot’ were rotted equally
from the centre to the cambium ; the bark was dead and
the wood had reached the pale yellow spongy stage, or had
in some cases been destroyed by worms, the cavity sur-
rounded by the hollow bark being filled with the worm-
casts. Others showed only ‘ secondary rot ’, i. e. they were
rotted only in the centre, like the trunk; the rot was in
an earlier stage, and the rotted wood was surrounded by
a layer of gum. Outside this the young wood and phloem
were living and active. It is reasonable to suppose that
the roots which show the primary rot are those through
* Mathes (1911) correctly ascribed root-rot to the presence of dead
roots and dead roots to poor nutrition and soil aeration, See also Leslie
(1915) and Ribbentrop (1908).
,
a ———
—
Se a
HEART-ROT 117
which the trees have been attacked, whilst those with
secondary rot have been attacked in a reverse direction,
i.e. from the trunk. For the rot on entering the trunk
will spread in all directions in the heart-wood, and will
grow as much down the roots as it does up the stem. The
fact that secondary rot occurs in this way was established
by finding that in such roots the rot at no point reaches
the surface, so that it is clear that the disease cannot have
‘ entered by them (see fig. 37).
It is among the roots with primary rot, then, that we
must look for the source of infection. And these roots
show a very marked localization. In all the roots of the
pumped trees I only found one superficial root which showed
primary rot; all the other roots in this stage had grown
more or less vertically downwards into the subsoil, and
were situated almost directly underneath the trunk itself.
In eight of the trees a tap-root, or other root which had
early taken its place, could be distinguished, and in every.
case but one this root showed primary rot. In two of these
trees no other root was in the same state, in one tree one
’ other root was, in three two other roots, in one four others,
and in one six others. In the tree in which the tap-root
did not show primary rot, two others did, and two trees in
which no tap-root could be found had respectively six and
seven roots showing primary rot. Thus in all thirty-four
roots were found with primary rot, and only one of these
could possibly be called a superficial root. When the trees
were uprooted the surface roots were firm and spread out
from the tree in the usual way, though most of them were
rotted in the core, and the wholly rotted roots had to be
sought on the under-side, where they were usually broken
off short. Their position could generally be located in the
soil from which the trees had been raised, and they could
be seen to continue in their vertical or nearly vertical
course. The conclusion is unavoidable that the trees were
rotted either through their tap-roots or through their deep-
going ‘anchor ’ roots and not through the spreading super-
ficial roots. Similar evidence has been forthcoming in trees
118 HEART-ROT
from other woods.
I picked out were dead. In fact I estimated that in the
hard subsoil 60 per cent. of the larch roots and 25 per cent.
of the oak roots had been killed, though a few of them were
rotted. This wood—a mixture of larch and oak—was
54 years old. For comparison I had a similar trench cut
in a pure oak wood alongside which was 105 years old.
The soil was here composed of a superficial 3 in. of black
leaf-mould and 18 in. of a sandy loam with a few stones
and very many healthy oak roots.. At 21 in. (the same
depth as in the former trench) the sand became much more |
compact, but not nearly so hard as in the younger wood.
There were a few oak roots, all of which were vigorous.
The points that chiefly interest us in comparing the two
trenches are, firstly, that in the older wood the subsoil is
much looser and more broken up than in the younger wood,
and, secondly, that many of the subsoil roots were dead
in the younger wood, whereas all were alive in the older
wood. The cause of death in the former case is almost
certainly lack of air, for, like all other parts of plants, roots
must have access to a supply of free oxygen in order to © |
maintain their vitality.
we n oe = vy
HEART-ROT 119
The discussion must thus be carried back a step farther,
viz. to a consideration of the agencies which facilitate the
aeration of the subsoil in the cases of cultivated land and
: woedland.
These agencies are notably divergent in the two cases.
In arable land the surface soil is very thoroughly aerated.
t6 a depth of 5 to 7 in. by ploughing, and somewhat more
where a steam plough is used, but below this there is very -
little disturbance of the soil particles. Three factors may,
however, assist, though slightly, in bringing fresh supplies
of oxygen to the subsoil. These are—(1) worms, which
burrow deep in cold winters and in dry summers (a depth
of 5 ft. is recorded as not uncommon in Darwin’s Vegetable
Mould and Earthworms, p. 112); (2) rain, which percolates
through to the subsoil carrying with it a small amount of
dissolved oxygen, and allowing room for more air when it
drains away ; and (8) frost, which very rarely affects the
subsoil in our climate. The combined action of these three
factors is very slight in most soils, and only in those with —
very deep drainage is the subsoil at all effectively aerated.
In woé@s, worms are comparatively infrequent, and frost
is entirely negligible. But there is a new factor which is
incomparably more efficient than any in arable land, viz.
roets, which act in a multiplicity of ways. By boring their —
way into the subsoil and by their secondary growth in
thickness they force apart the soil particles, and if they
die and rot the space which the root formerly occupied
must become filled again. Also the organic remains will
improve the soil. Still more important than all these
mechanical disturbances is the absorption of water by the
roots. For when roots draw in water from between the
particles immediately surrounding them the equilibrium
in the soil is disturbed and water is attracted by capillarity
from the more distant, moister soil into that which has
been dried through its propinquity to the roots. This
system by which roots obtain water from a distance is
depicted in the well-known diagram in Sachs’s Physiology
of Plants, which has been copied into most subsequent text-
120 HEART-ROT
books. In this way a large volume of soil is partially dried
by each root, and during a drought water is drawn from all
the subsoil which is penetrated by the root system of the
trees. The rigidity of the roots prevents a wholesale sub-
sidence of the soil, and consequently air must be drawn in
from the outside. When rain comes again, the water slowly
percolates through the interstices of the soil and drives out.
the air, and the same cycle begins anew. This process,
carried on through a number of years, has an important
loosening effect on the soil, and the living conditions of the
roots improve as the forest becomes older. This explains
the fact which is well known to foresters that the subsoil
in woods is more porous than in open ground.
Sufficient evidence has now been advanced on which to
base a theory accounting for the frequency of heart-rot in
the first rotation of larch on land that has previously been
cultivated. The argument for this theory may be given
under three headings :
1. The subsoil in arable or pasture land is very poorly
aerated, and the tree roots which grow more or less vertically —
downwards into this soil not infrequently die from lack of
oxygen.
2. It is shown by experiments described on pp. 111 and
113 that larch trees readily become infected through dead
roots which are still attached to living trees.
3. The pioneer roots which penetrate the subsoil, and
later die from lack of aeration, form suitable infection
points for heart-rotting fungi. Such dead roots are found
especially in the first rotation of trees on new forest land.
This theory may be accepted as a working hypothesis.
Much experience will be needed to prove or disprove it,
and the following points immediately present themselves —
for elucidation. First, it would be interesting to know
whether plantations on arable or meadow land, in which
the subsoil was for some reason well aerated before plant-°
ing, are less liable to heart-rot than new plantations on soil
which is otherwise conditioned. Next, in old woods in
which heart-rot has occurred are there any other causes
HEART-ROT 121
operating which might account for the death of some of the
roots before they became attacked by the fungus? Also
what is the frequency of heart-rot in conifers planted on
heaths ? Where the soil is peaty the aeration of the sub-
soil must be very poor, and heart-rot might be expected.
I-have had no opportunity of investigating these points,
_ and foresters’ notes on the subject would be of great value.
Extreme caution is necessary when attempting to prophesy
the effect of different types of subsoil on the frequency of
heart-rot. In one part of Terringham Wood the subsoil is
a hard strong clay, which must be far inferior in porosity
to that in other parts of the wood. And yet in this portion
several of the larch trees were sound. I thought that
this must be a case in which the theory failed, until I
had one of the trees grubbed up and found that the
roots were all superficial. The subsoil had proved too
hard for them, and consequently the tree had been saved
from infection.
Two points of interest are involved in this theory of the
method of infection. First, the roots which grow vertically
downwards are not those which are most likely to come in
contact with the diseased roots of other trees, so that
Hartig’s contact theory can hardly apply. It seems much
more probable that the roots become infected either directly
by spores and conidia or through the soil being penetrated
by mycelium. The other point is the depth of the soil at
which infection takes place. When a root is killed because
its lower extremities lack sufficient oxygen, it does not
necessarily follow that only those lower extremities die.
Unless the upper part of the root has sufficient vigour to
send out branch roots into the upper layer of soil (and I have
never noticed such branches in roots with primary rot),
the upper part will cease to receive the full supply of water
- and nutriment, and will probably die. In one case I found
a layer of resin across the wood of a vertical root with
primary rot at a point less than a foot below the surface of
the soil, in fact quite near the rootstock, and this probably
represented the upper limit of death before infection.
122 HEART-ROT
I sought for a similar line of resin in other roots, but they
were too far rotted to show any trace of it.. Thus, though
these roots are killed because they grow deeply, it is not
necessary that their point of infection is deep.
The statement that larch is more liable to heart-rot im
pioneer stands than in subsequent rotations is not accepted
by all writers on the subject. In particular, Elwes (Elwes —
and Henry, 1907, vol. ii, p. 363) holds that the rot is more
often found in the second rotation. This runs directly
counter to my experience, and the issue can only be settled
by the counting of woods in which the incidence of heart-
rot has favoured Elwes’s view or mine. In discussing the
question, Elwes does not say which of the heart-rots he
was referring to, whereas my observations are only con-
cerned with Fomes annosus. Ribbentrop (1908) shows that
in Germany the first rotation has given most trouble with
root-rots generally.
‘Methods of prevention. The only preventive measure
which, as far as I know, has been recommended for defence
against Fomes annosus, is digging a trench round all the
infected trees. This method was adopted by Hartig in the
forests near Eberswalde, and the trenches were apparently
dug with the object of cutting off all the roots so that infec-
tion should not pass from root to root from infected trees
to other parts of the forest. Kienitz found that, in one of
the trenches that Hartig had cut, fructifications developed
on the cut and exposed roots, so that in this case the remedial
“measure proved to be a source of danger. He referred to
the treatment as ‘ waldverderblich ’, an expression which
Brefeld quotes with relish. Hartig replied that a trench
could easily be watched for fructifications which could be
removed, and persisted in his recommendation. But the
conidia discovered by Brefeld present a fresh difficulty.
These cannot be removed, for they cannot generally be seen.
And now that we know that the mycelium can grow in the
humus itself, a trench does not seem a likely method of
preventing its further expansion. I have found, however,
that when 10 per cent. of lime is added to humus soil, the
=>
HEART-ROT 123
mycelium is incapable of growing in it. Thus a trench
filled to the surface with lime ‘might prove to be an effective
barrier. But conidia and spore infection through the air
is probably too common to make such a trench worth the
labour of digging. Vigilance and forethought will help us
much more than trenches. Heart-rot is a particularly
insidious disease because its presence is not generally dis-
covered till the trees are cut, so that not only have the
trunks been getting more and more rotted, but they have
been filling for many years a plot that might have been
earning a useful rent. A skilled woodman can generally
distinguish pumped trees by the hollow sound they emit
when struck with the back of an axe or a stick, and by
this time they are often ‘ gouty’ at the base.. But these
means do not aid in the discovery of the disease till much
damage has been done. Testing a tree with an increment
borer, however, will disclose the presence of heart-rot as —
soon as the trunk is affected, and it is recommended that
foresters should test their larch plantations from time to
‘time with this instrument. It should be remembered that
trees usually become pumped in groups, and if every tenth
tree is bored every other year, an epidemic will be dis-
covered before the loss or value becomes serious. Where
one tree is found to be attacked, others in the immediate
vicinity must be bored, and all the trees that are diseased
should be cut out. They will only be wasting themselves
and the land on which they are growing. Sometimes it
may be necessary to clear fell, and in this case it is safer
to grow a rotation of broad-leaved trees before replanting
with larch or other conifer.
Land which is being afforested for the first time presents
a special case. Here pumping must be anticipated, and yet
the exclusion of larch and spruce, and to a less extent
Scots pine, would prove a serious obstacle in afforesting .
new land. KHither the conifers may be grown in a mixture
with hard woods and the former removed in the earlier
thinnings, or, where it is thought advisable to plant pure
coniferous woods, the forester must watch them carefully
124 HEART-ROT
with the increment borer, and be prepared to clear fell as
soon as they reach a remunerative size. The poles can then
be sold for pit props, for which purpose a little heart-rot is
not a serious blemish.
No means of artificially aerating the subsoil is economi-
cally possible. I have seen larch plantations on land that
was previously common, where the ground was said to have
been double-dug before planting. Heart-rot was not very
serious, but it was unmistakably present. Double-digging
will at most affect the top 16 in., and aeration must go down
much deeper than this in order to prevent pumping. Natural
aeration may, however, be encouraged by planting deep-
rooting trees such as oak or false acacia. In Germany,
lupin is also used on heaths for this purpose, and is allowed
to grow for a few years before planting trees.
Destroying all fructifications will no doubt have a palliative
effect, especially in woods where Fomes annosus is not yet
abundant. It removes, at any rate, one of the sources of
infection, and we know that spores are made in great
quantities where fructifications are allowed to appear. In
looking for them it should be remembered that they are
borne particularly on the trunks of dead conifers. They
are always formed near the base, and are often quite hidden
in the débris of leaves, twigs, and weeds that surround the
trunk. By treading lightly all round the tree their presence
can often be discovered.
The remains of rotted trees should be burnt whenever it
is possible, but grubbing up stumps is so expensive that
woods can seldom be effectively cleared in this way. In
some cases, however, such trouble will be well repaid.
There is one wood which I have kept under observation for
many years without ever finding Fomes annosus. The wood
is a fairly large one, and in the last twenty years conifers
have been widely planted in it. Previously it was almost
exclusively composed of broad-leaved trees. This year
(1919) a Corsican pine was found dead and fallen down,
and from its roots I picked the fructification shown in
fig. 40. This pine is almost surrounded by larch trees.
ye lee
a
ar
HEART-ROT 125
In a case like this it is well worth while to dig up all
the roots that can be reached and to cut off as much
of the rootstock as is found to be rotted and to burn the
whole with the utmost care. Such time and trouble spent
now may protect the wood for many years against attack by
the fungus.
CHAPTER VII
HEART-ROT CAUSED BY OTHER FUNGI
Polyporus Schweinitzii, Poria vaporaria, Polyporus sulphureus, Trametes
Pini.
Heart-rot caused by Polyporus Schweinitzii, Fr. Next. to
Fomes annosus this fungus is the most frequent cause of
heart-rot. Though in many respects resembling the two
following diseases, this rot may easily be distinguished from
that caused by the Fomes by the rotted wood béing very
dry, light, and friable, and not spongy or fibrous. It has
marked cleavage surfaces which are at right angles to each
other, so that the wood breaks up easily into more or less
cubical blocks, and, owing to shrinkage, the cracks often
open before the tree is felled and the interstices become
‘filled with white mycelium which frequently becomes
embedded in resin and assumes a chalky consistency. When.
the rot is far advanced the wood is usually light or dark
brown and reeks strongly of turpentine.
The fungus enters by the roots and grows up the stem,
and is in the vast majority of cases confined to the heart-
wood. But dead trees are occasionally found in which all
the wood up to 20 ft. or more from the ground is rotted.
The disease is reported as fairly common on the Con-
tinent, especially on Scots pine, Weymouth pine, and
larch. There are frequent notices of it in the United States
of America on Weymouth pine, balsam fir, white and red
spruce, and arbor vitae, and in New England it is con-
sidered one of the most destructive diseases (Schrenk, 1900).
In Scotland, Murray (1916) reports it as occurring on
Douglas, spruce, and Scots pine. My own observations
have been made in the south of England, where I have
found it on Scots pine, cluster pine, and larch, chiefly in
Sussex, Surrey, Berkshire, and Devonshire, but I have no
a es
——
a SS ey aa a, ee ",
‘
,
. 2 P
x
.
| re
\
a
‘ ;
+
. - 4 : : :
é ‘
= - * ¥ ad —_ 3 Ps 5 ,
:
Fia. 46.—Polyporus Schweinitzii. Stipitate fructification.
Fria. 47.—Polyporus Schweinitzii. Bracket-shaped fructification, showing pore
arrangement on lower surface. The fructification has a distinct stalk.
HEART-ROT CAUSED BY OTHER FUNGI 127
reason to suppose it is confined to those counties. The
parasitology and method of attack of the fungus have not
been worked out. The best papers on the disease are by
Hartig (1878), who described it under the name of Polyporus
mollis, Fr., and gave an account of two hundred-year-old
Scots pines which had been attacked by the fungus, and. by
Schrenk (1900), who described the rot on American trees
and made interesting observations on the fructification.
Fructification. This may arise either from the roots,
often at some distance from the trunk, or on-the trunk
itself up to 10 or 12 ft. When growing on roots that are
buried its connexion with the tree may not be observed until
the roots are bared of earth, but in other cases it may arise
from parts of roots that are exposed to the surface. In
either case it is stipitate, i.e. has a stalk with an expanded
pileus as in fig. 46. Sometimes, as in the figure, two or more
stalks may grow up side by side and their respective pilei
grow into each other, or become congruent. The pores are
borne on the lower side of the pileus, and the upper swollen
part of the stipe also becomes pore-bearing, so that there
is no‘sharp line of demarcation between the pileus and the
stipe. When growing on the trunk the fructification has
a totally different form. It is then bracket-shaped, as in
fig. 47, with a more or less distinct stalk; and often the
brackets are imbricated, borne closely one above another,
in which case there is usually no stalk visible. The fruit-
bodies are generally about 6 in. across, but specimens up to
16 in. have been measured.
Though the two forms of ‘fructification are so different
in their general shape, the details of structure are identical.
The upper surface is dark, reddish brown in colour, and
rough with excrescences. The pore-bearing under-surface
is greenish when young, but changes to red on being touched,
and during the period of active growth it exudes numerous
drops of liquid. . Unlike Fomes annosus, the fructification
is not woody, but soft and fairly light. It hardens some-
what with age and dries up and dies in the autumn. It
generally falls from the tree in late winter unless it has been
128 HEART-ROT CAUSED BY OTHER FUNGI
already eaten and destroyed by insect larvae. It is never
perennial.
The inner part of the fructification is mainly composed
of masses of parallel, very tender, richly septate, red brown
hyphae, filled with protoplasm. On the upper side many
of the masses run up into the scales that project from the
upper surface ; those on the lower side run down into the
trama between the pores, and ac-
cording to Hartig it is the escape
of the contents from these hyphae
that gives the red colour on bruis-
ing. The pores are irregular in
shape (fig. 47), and from 0-5 to
2 mm. in diameter. Their depth
is about 5 mm. Near the stipe
the pores are long and _ labyrin-
thine, but the margin is barren
until marginal growth has ceased,
when pores are formed out to the
edge. Inside, the pores are lined
by the hymenium, which is com-
posed of basidia and various forms
Tid: 48H menial loves of paraphyses. The basidia are
of Polyporus Schweinitzii, colourless or very pale yellow,
na pgp ( b), SS ak 30-40 X 5-Sy. The spores are
as oystidia (c) (300). P), about 4X 6 p» (fig. 48). In addition
to the basidia there are numerous
thin-walled paraphyses and larger specialized thick-walled
paraphyses (cystidia). As shown in this figure the cystidia
are very variable in shape and size. They have deep brown
contents, and the large brown hyphae which carry them can
often be traced for some distance back into the trama. The
cystidia project into the pores beyond the basidia, and
often bear drops of liquid at their extremities to which
spores adhere. Thus it appears that the cystidia obstruct
the normal dispersal of the spores. .
When the fructifications are growing actively the spores
fall from them in clouds, dense enough to be seen with the
fi
Fic. 49.— Section across base of larch-tree showing rot caused by
Polyporus Schweinitzii. (x 4).
Fra. 50.—Section across larch-tree, 15 ft. above the ground,
showing rot caused by Polyporus Schweiniteti ( x ).
HEART-ROT CAUSED BY OTHER FUNGI 129
naked eye. With respect to the secretion of liquid which
occurs at the time of spore dispersal, the following quotation
from Schrenk (1900) is of interest :
‘ At the time of ripening of the spores it was noticed that
hundreds of drops of a yellowish liquid were hanging from
the hymenial surfaces every morning when the fungus in
question was visited. Some of these drops were carefully
collected and were examined. In them floated a number
of spores and flocculent yellowish-brown masses, which
stained yellow with nitric acid. These were present for
several days. Thereafter the liquid was almost clear except
for numberless spores which were in every drop. For three
weeks the drops were collected with a pipette during the
day, and during the night a plate, carefully protected
against dew and rain, was placed under the fungus. In
this way about three-fifths of a pint (300 c.c.) of liquid
were collected. This was poured into an open dish and put
in a cool place, where the water was allowed to evaporate.
A thick brown syrup was left after some weeks, which had
the odour of very impure molasses. The syrup was trans-
ferred to a vial, which was corked and placed in a warm
place. In a few days delicate needle-shaped crystals shot
out, which on examination proved to be melezitose and
mycose, sugars sometimes found in fungi.
‘At the same time that this secretion appeared on the
hymenium, or rather shortly afterwards, a number of small
beetles began to devour the hymenium with great avidity.
So active were they that within three weeks of their appear-
ance the hymenium was entirely destroyed, and of course
with it whatever spores had remained. It is suggested that
the secretion of this sugar and the destruction of the
hymenium by the beetles may have some meaning in con-
nexion with the dispersal of the spores. It is a point worthy
of further observations by local observers in future years.’
Rot in the wood. In all sections of ving trunks which
I have obtained, showing rot with this fungus, the rot is
entirely confined to the heart-wood. Consequently in the
Scots pine with its narrow heart-wood the rot is much
1888
K
130 HEART-ROT CAUSED BY OTHER FUNGI
more confined than in the larch (compare fig. 49 with
fig. 52). This suggests that the fungus is purely sapro-
phytic. At the same time dead trees are not infrequently
found which have become rotted through to the bark, and
it is only then that fructifications are borne on the trunk,
except when large branches have fallen away so as to expose
the heart-wood. For the present it must remain an open
question whether such dead trees are killed by other causes,
or whether P. Schweinitzit can kill the roots, taking advan-
tage of the more moist conditions in the soil, and thus,
secondarily, produce suitable conditions for growth in the
sap-wood of the stem.
In the earliest stages of rot, the heart-wood has a rather
deeper red colour than normally, but it soon loses its reddish
tinge and takes on more the colour of cork (fig. 50). It
also becomes very much lighter in weight. Further decom-
position is generally attended by deepening of the colour
(fig. 49), in which case the wood becomes dark walnut
brown, and has a strong smell of turpentine. It cracks
along transverse, radial, and tangential planes, producing
wedge-shaped or cubical blocks which can be picked out
with the fingers. The cracks are often filled with mycelium
of the fungus, bound together with resin and of cheesy con-
sistency. If a section of a trunk cut in this stage is kept
in a dry laboratory the rotted wood contracts on drying to
such an extent that small pieces fall away by their own
weight, and the section is eventually left with a hollow
centre. The wood is then so far reduced in weight that,
according to Hartig, its specific gravity is only 0-19, as
compared with 0-57 for normal wood.
With the help of a microscope mycelium can be found in
the wood from the earliest stages of rot. But dense agglome-
rations of hyphae, such as occupy the regions of most
intense wood destruction in Fomes annosus and Armillaria
mellea, are nowhere present in wood rotted by this fungus,
except in the cracks as mentioned above. Consequently,
decomposition is less localized and more evenly distributed.
The hyphae present in the wood are mostly colourless and
Fie. 51.—Piece ofslarch wood rotted by Polyporus Schweinitzii, showing crevices
and patches of white mycelium.
Fig. 52.—Section at base of Scots pine stem with rot caused by
Polyporus Schweinitzii.
~
HEART-ROT CAUSED BY OTHER FUNGI 131
of all degrees of thickness from 6» downward, but some of
the largest, in the earlier stages of rot, have brown contents.
These thicker, brown hyphae generally run either vertically
along the tracheides, or horizontally, boring through the
tracheide walls and markedly constricted in the bore-holes.
The finer hyphae, which are much more numerous, branch
frequently and spread in all directions, though the bore-
holes are nearly always transverse to the tracheide walls.
Hyphae may also grow up between
the tracheides. As decomposition
proceeds fewer hyphae are found,
but even in advanced stages of rot
a few colourless hyphae are gener- i WL dowel
ally present. The same variation ( 8 ©) ol
in size and colour of the hyphae ON yay
Ainele
© ©
-©
=
is also a feature of the felted my-
celium which fills up the cracks
in the rotted wood.
Although the mycelium in the
wood lacks those special features
of interest which characterize the
growth of Fomes annosus and yA, 59 ol arcaahigs.” of
Armillaria mellea, the effects pro- Polyporus Schweinitzii in
duced in the wood are peculiar and es eat of ‘the: larch ;0.,
aes : yphal bore-hole; 06.h.,
distinctive. In the first place the brown hypha; 6.p.,_ bor-
wood is never quite delignified, — pit; ¢.h., colourless
eS ypha.
and to the last will give a slight
reaction with phloroglucol and hydrochloric acid. At the
same time the cellulose, which, though present in normal
wood, fails to give normal reactions without special treat-
ment, is so far freed from lignone that rotted wood gives
a blue or purplish colour with chlor-zinc-iodine. Thus
the presence of lignone and the presence of cellulose may
be demonstrated, without special treatment, in the same
cell wall. During the process of decomposition the wood
also undergoes contraction to a very marked extent. In
the final stages this is apparent to the naked eye by the
large cracks and crevices, but before these appear evidences
K 2
132 HEART-ROT CAUSED BY OTHER FUNGI
of a state of tension can be observed in the cell walls them-
selves. The general form of the tracheides is unaffected,
but cracks appear in the walls, always rising from right to
left, as shown in fig. 54, and usually in tiers, one above
another. These cracks appear first in the summer wood,
and in all stages are more conspicuous in the summer wood
than the spring wood. ‘They do not rupture the complete
wall, but the thickening on each
side of the middle lamella cracks
independently, and whereas on
one side the cracks run from right
p. to left, on the other side they
appear (as seen through the wall)
~ to run from left to right, whilst
the middle lamella itself remains
intact. Naturally the tension
breaks away weak places in the
walls, and bordered pits andhyphal
bore-holes are commonly centres
for cracking, and, where both sides
of the wall come within focus, the
cracks appear as a cross as shown
_ in the figure. |
Fia. 54.—Tangential longi-
tudinal section of larch wood Later on, larger cleavages occur
mit aa by Shagcakes in the wood in the transverse,
otha altar iA WyphAl howe: radial, and tangential planes,
hole; b.p., bordered pit;-h.c., and it is these cleavages which
era ae crack; m.r.,medul- Jater become filled with the cheesy
mycelium. Also if a section of
a trunk with a rotten core is allowed to dry, further
irregular cracks occur in all directions in the wood, so that
in such wood the microscopic details are apt to become
obscured.
General remarks on Polyporus Schweinitzii. We know
very little indeed about the mode of infection of this fungus.
In fact the text-books give all the information which is
available on the subject when they say that the fungus
first attacks the roots of a tree and grows up from the roots
hc. 7
bh Tf,
HEART-ROT CAUSED BY OTHER FUNGI = 133
into the stem. This appears to be ipvariably the case, as
no instances have been reported in which the fungus has
attacked the tree through sub-aerial wounds. The mode
- of growth of the fungus in the tree suggests that, if it is
a parasite at all, it is a very feeble one, and it is extremely
‘improbable that the fungus can attack a living root, even
when it is wounded, unless some of the dead wood is exposed.
Very likely its habit is to gain entrance to the tree through
dead roots, but by what means it reaches dead roots is at
present entirely unknown. ae
‘Cure of the disease is, of course, impossible. The forester
must remember that he has a sixth sense in his Pressler
borer, a sense which he should use with enthusiasm and
discernment. If he watches carefully for the incidence of
heart-rot caused by Fomes annosus (see p. 122) he will be
equally guarded against this other heart-rot ; at any rate
he will not receive his first intimation of it when the woods
are cut, but being warned beforehand, he will be able to use
his discretion as to whether the woods should be cut before
normal financial maturity is reached. But apart from the
borer, the presence of P. Schweinitzii can generally be
detected by its fructifications. It fructifies much more
freely than Yomes annosus in the early stage of rot, and as
the fructifications are sub-aerial they can always be seen.
A special look-out should be maintained for the upright
fructifications, like that in fig. 46, which grow from the roots,
and may have no obvious connexion with the tree on which
they are feeding. These may be formed before the trunk is
appreciably affected, whereas fructifications are not borne
on the trunk itself until it is either dead or very severely
damaged. August is the best month in which to look for
these growths, and when they are found they should be
picked and destroyed in order to restrict the further dis-
tribution of the fungus. Of course the removal of the
fructification will not help the tree that is actually attacked,
- and an investigation of the roots of the tree is advisable,
as if the attack is not far advanced it may be possible to
remove all the diseased roots, which are very probably
134 HEART-ROT CAUSED ‘BY OTHER FUNGI
already dead, and thus save the tree, or it may be found
desirable to remove the tree with as many roots as possible,
and thereby save surrounding trees.
These precautions are likely to repay the time spent on
them, as the disease is not yet very common in Britain,
and if taken in time epidemics may be prevented. But it is °
essential that all attacked portions of trees should be burnt,
and not allowed to lie about in the forest, for the fungus
fructifies with great regularity on all exposed surfaces of
rotted wood.
Poria vaporaria, (Pers.) Cooke. This fungus is reported-as
a wound parasite on various conifers both in Germany and
the United States," and it probably occurs on the larch.
But as it is probable that many species of fungi are in-
cluded under this name, and as I have had no opportunity
of studying the fungus on the larch, only a very brief account
of the disease will be given. The information is derived
from Hartig (1878), who found the rot which he attributed
to this fungus several times on the Scots pine and once on
the spruce. The rot so closely resembles that due to Poly-
porus Schweinitzii that the two may frequently have been
confused.
The fungus fructification is reswpinate, i.e. it does not
form a bracket, but is confined to the under-side of the
wood or tree on which it is growing, and does not extend
beyond it. Its upper surface everywhere touches the tree
and is hidden, and its lower surface is covered with small
pores which bear the hymenial surface, as in the genus
Polyporus. The two genera Poria and Polyporus are so
closely allied that they were formerly included in one,
and Hartig describes the fungus under the name of Polyporus
vaporarius. The fructification.is white, thin, and fragile.
The trees which Hartig investigated were mostly 50 to 100
years old. The rotted wood is at first light brown, but
later it becomes dark brown, and at the same time shrinks
so as to cause vertical and horizontal crevices, as in wood
rotted by Polyporus Schweinitzii, Also the crevices become
1 H. von Schrenk (1900).
HEART-ROT CAUSED BY OTHER FUNGI 135
filled up with mycelium as with the latter fungus. But
wood rotted by Poria vaporaria may be distinguished by
the following features :
(i) The rotted wood has the consistency of charcoal,
for which it might be mistaken but for its red-
brown colour.
(ii) It has not the turpentine smell which is so charac-
teristic of wood rotted by Polyporus Schwei-
nitzre.
(iii) The mycelium in the crevices is not chalky, but
woolly, and the mycelium is partly composed
of white mycelial veins, which are made up
of numerous parallel, thick-walled, slightly
branched and sparsely septate hyphae. These
veins grow not only in the crevices, but also
on the outside of blocks of wood, where they
are kept damp, and between the wood and
bark of dead trees.
The microscopic features are also similar to those of
Polyporus Schweimitzi. The hyphae, which are not numerous
in the tracheides, are some thick-walled and some thin-
walled, are poor in branches, and are constricted where
they bore through the tracheide walls. But the bore-holes
are characteristic in that the hyphae digest the middle
lamella across a greater breadth than the rest of the wall,
as shown in fig. 55, B, so that the bore-holes are somewhat
lens-shaped and in surface view appear to be surrounded
by one or more nearly concentric circles. In the summer
wood there are numerous cracks in the tracheide walls
similar to those caused by Polyporus Schweinitzii, but not
- as a rule so long. The hyphae frequently have ‘ buckle
connexions ’ at the points where septa are formed (fig. 55, A).
This rot may either spread up from the roots or may be
initiated above ground where the heart-wood is exposed by
the fall of a branch.
As stated above, there are many varieties of this fungus,
the distinctive features of which are only partially known.
/
136 HEART-ROT CAUSED BY OTHER FUNGI
One form causes a ‘dry rot’ on structural timber not
unlike that produced by the dreaded Merulius lacrymans.
Both occur in houses, especially in cellars and other damp
or ill-ventilated situations, and rot the woodwork to such
an extent that it collapses under any strain that may be
put upon it. Surfaces of the rotting wood are often covered
by a thickish layer of felty mycelium traversed by veins of
denser conducting hyphae. Such layers are made by
Merulius lacrymans as well as Poria vaporaria, but with
increasing age the two fungi may be readily distinguished
by the fact that with Merulius they become grey and silky
s.: y
4 ie
Fie. 55.—Poria vaporaria: A, hypha, showing buckle connexions ;
a, early stage showing how the connexion grows out from one side of the ©
septum ; b, later stage, in which the wall between the connexions and the
part of the hypha on the other side of the septum has been absorbed.
B, hyphal bore-hole ; c, in section; d, in surface view.
on the surface, whereas with Poria they remain white and
felt-like. These mycelial layers are very like those found
in the open on trees attacked by Poria. But the humid
conditions and large area of wood surface afforded by the
under-side of an ill-ventilated floor, or similar situation in
a building, allow of a much more massive mycelium than
is generally found in the forest.
Polyporus sulphureus, Fr. We now come to two fungi
which, though they cause heart-rot in the larch, are definitely
wound parasites, and infect the tree through sub-aerial
wounds or dead branch snags left by the fall of the larger
branches. These two fungi are Polyporus sulphureus and
Trametes Pini. .
Neither of these fungi has yet proved very destructive
to the larch in Britain, and with the former, at any rate,
little fear need be entertained as to its power for evil in
well-regulated woods. The sulphur polypore has been
found in Europe on oak, locust (Robinia pseudacacia),
HEART-ROT CAUSED BY OTHER FUNGI 137
alder, willow, poplar, walnut, pear, and larch (Hartig,
1894) ; in America it is recorded on a similar range of
broad-leaved trees, but appears to be much more general
on conifers, and is reported as a parasite on pine, spruce,
hemlock spruce, &c. (Schrenk, 1900; Atkinson, 1901). It is
not uncommon in England, where it is most frequent on
the oak, but also attacks the larch, yew, and other trees.
Infection generally takes place through wounds left by the
pruning or breaking of large branches, so that single trees
grown in the open are much more liable to attack than
plantations in which side branches are killed before they
attain sufficiently large size. Branches which die naturally
through being shaded by the crown are less likely to intro-
duce the fungus, but Atkinson observed one case in the
American white oak in which the fungus had apparently
entered through a dead leader which had become included
in the gradually thickening trunk. This, however, is excep-
tional, and the occurrence of the disease.in larch woods
may be regarded as evidence of an open canopy which has
allowed undue development of side branches. In parks,
where such development is normal, the snow-break, or
wind-break, of large branches will furnish ‘ infection courts ’
for the fungus, unless the timely treatment of such wounds
is resorted to.
The fungus makes large annual bracket-shaped fructifica-
tions which are usually imbricated, i.e. a number grow
together, one above another, and are generally found on
wound surfaces between May and September. They are
easily recognized from all other polypores by their colour,
being bright orange above and sulphur yellow below. The
soft flesh of young fructifications is full of a clear yellow
fluid, and the upper surface particularly is very moist and
‘turns brown when bruised. This surface is somewhat hairy
and when mature becomes hard and brittle. Drops of water
containing melezitose (Schrenk) collect on the lower side,
which is marked by very fine pores. The whole fructifica-
tion develops very quickly, and soon after maturity is
destroyed by grubs.
138 HEART-ROT CAUSED BY OTHER FUNGI
The fungus produces a red rot in the duramen of the
larch, which spreads indefinitely both upwards and down- |
wards from the point of infection. My own observations of
the rotted wood have been confined to a section from the
base of a large trunk in the School of Forestry Museum at
Oxford. This section (fig. 66, p. 155), which came from
Windsor Park, is hollow, and only small fragments of rotted
wood remain attached to the uninjured wood surrounding it.
These fragments are in many respects similar to wood rotted
by Polyporus Schweinitzii. They differ, however, in show-
ing more regular tangential and transverse cracks, so that
portions broken away are more nearly cubical. These
blocks are heavier and firmer than wood rotted by P.
Schweinitzii, and have a darker and richer chestnut colour. —
The process of decomposition does not appear to have been
described in the larch, though the fungus was carefully —
investigated by Hartig (1878) on the oak. Schrenk’s observa- —
tions on the spruce (1900), although they cannot be appli-
cable to the larch in all details, should be briefly noted in
this connexion.
First the wood turns slightly =e brown, but in longi-
tudinal cuts it is seen that this colour is confined to irregular —
patches. Then small transverse cracks appear which never —
cross from one annual ring to the next, but extend part way
across a ring either from the side of the summer or spring
wood. At this stage microscopic sections show numerous
small breaks and fissures, which are evidence of much
shrinkage having occurred in the wood. Slanting fissures
in the tracheide walls, similar to those shown in fig. 54,
were also observed in the spruce, but they are said to rise
from left to right at an angle of about 45°. The medullary —
rays are often absorbed, so that the tracheides appear dis- —
torted in tangential sections. Later the annual rings separate
from each other, presumably owing to the destruction of the
first-formed spring wood in each ring, and the radial fissures —
become complete, so that the wood becomes divided into
a number of long flat slabs, each the width of an annual —
ring. There is very little mycelial development in the
HEART-ROT CAUSED BY OTHER FUNGI 139
wood at any time. In this respect the spruce apparently
differs from the oak, in which Hartig noted that the mycelium
almost completely fills some of the tracheides and vessels.
I find it very difficult to distinguish microscopically
between old rotted wood of larch decomposed by P. sul-
phureus and P. Schweinitzii. The oblique fissures in the
tracheide walls are very similar in the two cases, though
with each fungus the range of variation in this respect is
considerable. Transverse cracks crossing one or two
tracheides are, however, more frequently associated with
| P. sulphureus than P. Schweinitzii. It should be easier to
control this fungus than any of the root-rotting species.
Infection is sub-aerial, and fructifications when made are
easily seen. Destruction of fructifications as soon as they
are spore-bearing will do much to prevent the spread of
the fungus, but as they can grow again after being removed
early in the season, a constant watch has to be kept, and
it is better to cut down infected trees and, after utilizing
undamaged portions, to burn the remainder. Hartig found
that in the vessels and tracheides of oak wood a form of
conidium was often produced, apparently by the mycelium
of this fungus ; but its connexion with the fungus was not
proved by culture experiments, and the conidia may have
belonged to a saprophytic fungus which gained admission
to the wood subsequently to its destruction by P. sulphureus.
If it is found that these conidia belong to the parasite and
can carry infection, stringent measures would be necessary
to localize the disease in districts where the oak is attacked
by the fungus. For such oaks become hollow and often
open at the side, and the powdered decomposed wood can
be blown about by the wind, and with it the conidia. Con-
sequently all these hollowed old oaks, which are common
‘in parks, would be a source of infection for other trees, quite
apart from the production of visible fructifications. But,
as yet, the relation between these conidia and P. sulphureus
is conjectural. Schrenk found no trace of them in the
wood of the spruce, and Brefeld (1889), who grew pure
cultures of the fungus, states that no conidia were borne
140 HEART-ROT CAUSED BY OTHER, FUNGI
by the mycelium. 1 have found similar conidia in larch
wood, rotted by P. Schweinitzit, which has been allowed to
lie about in the laboratory for some years, but have so far
been unable to induce the conidia to germinate. I hope in the
future to carry out further experiments with them to ascertain
their relationship with the fungus that rotted the wood.
Trametes Pini, (Thore) Fr. This is another wound parasite
of the conifers which rots the heart-wood, and, to a limited —
extent, the sap-wood as well. According.to Hartig it only
gains admission to the tree through wounds caused by the
fall of live branches. Unlike Polyporus sulphureus, it is by
no means confined to trees growing in the open, but has —
proved destructive in plantations of forty years old and
upwards. In woods of this age, owing to thinning, more —
space is given for the development of the crowns of individual
trees, and consequently the branches reach a greater size. —
They are then more liable to be broken by wind or snow, —
especially on the more exposed edges of plantations, and it —
is in such exposed positions that the fungus is most pre-
valent. Hartig (1878) has noted that the frequency of
the disease on its four European hosts, Scots pine, larch, —
spruce, and silver fir, is in the order given, being greatest
in the Scots pine and least on the silver fir, and that the
frequency of broken branches follows the same order.
Wounds left by the breaking off of small branches do not
admit the fungus, as they are almost immediately pro-
tected by a layer of turpentine and resin. But when a large —
branch breaks the broader core of heart-wood, which does —
not secrete these substances, is not so easily protected, and —
it is in the central portion of such a wound that the fungus
first begins to grow. Thus inception of the disease general |
occurs at some height above the ground.
The disease affects the same genera of conifers in America
as in Europe, with the addition of Tsuga, the hemlock —
spruce. It has also been reported as growing on willow
(Stevens, 1913). In Britain, at any rate in the south of
England, it is fortunately uncommon, so that it cannot be
included as one of the more dangerous pests of larch cultiva-
HEART-ROT CAUSED BY OTHER FUNGI 141
tion in this country. Rostrup (1902) does not include it
among Danish fungal diseases. In India it occurs on Pinus
excelsa (Mayes, 1905).
The fructification of the fungus is of Polyporus form, and
may be composed of either a single large bracket (up to
9 in. wide), numerous imbricated small brackets, or ‘ re-
supinate ’ incrustations which may spread for 10 ft. or
more on the under-side of branches or fallen trunks. It is
hard, woody, and perennial, and is most easily distinguished
by the light red-brown colour of the lower surface, which
is pierced by small pores. The pores are round, but appear
elongated where the surface is not horizontal, a feature,
however, which the fungus has in common with many
allied species. Brackets are produced freely on dead trees,
and the spread of the fungus is chiefly secured by these
post-mortem growths. The hymenial surface which lines
the pores bears basidiospores which are 5-6 X 3-4 yp, and
is characterized by elongated, brown, pointed, thick-walled,
spine-like paraphyses or ‘cystidia’ not unlike those figured
in Polyporus Schweinitzii. They persist in the pores for
a long time after the region bearing them has .ceased to
produce spores.
The process of decomposition induced in the wood by
the fungus has been described by Hartig (1878) and Schrenk
(1900). From the infected branch narrow red-brown streaks
spread both upwards and downwards in the trunk. Though
these streaks scarcely spread at all in a radial direction,
they may become tangentially extended in the annual ring
or rings in which they were initiated, and in this way
produce a kind of ring-shake in the tree. Several partial
rings of this nature may arise before the centre of the tree
is appreciably affected. The wood outside the red-brown
area becomes suffused with turpentine and resin, which to
a considerable extent limit the spread of the mycelium.
White flecks then appear in the red rot portions, which are
occasionally, but by no means always, preceded by black
spots similar to those caused by Fomes annosus. These
flecks are much larger than those produced by F. annosus,
142 HEART-ROT CAUSED BY OTHER FUNGI
and are usually the entire width of an annual ring. The
subsequent stages of rot are somewhat variable. The
delignified walls of the white flecks may crumble away,
owing to the digestion of the middle lamellae, leaving holes
arranged in layers associated with definite annual rings,
and if many adjacent annual rings are attacked the wood
becomes honeycombed. This is general in the pine and
spruce and sometimes occurs in the larch. In the Tamarack
(Larix americana) it generally happens that the flecks become —
joined, first longitudinally and later tangentially, so that
entire tangential sheets become rotted and intermediate:
plates can readily be separated out. In the larch and pine
the mycelium is said not to destroy the sap-wood, so that the
water-supply of the upper part of the tree is not materially
reduced ; if a tree is killed by the fungus it is generally
through wind-break at the weakened portion. For the
same reason fructifications are not borne except on or near
the branch stubs. In the spruce and silver fir, which are
poorer in resin, the mycelium can penetrate to the cortex,
and where this occurs brackets may grow from the bark
without any special relation to the branches.
The only remedial measure is to cut down the trees and
destroy infected portions. As the rot is generally confined
to the upper portions of nearly mature trees, the lower part
of the trunk can be utilized, and itis to the forester’s interest
to secure this timber at the earliest possible moment, as the
longer he leaves the tree the greater will be his loss through
the downward spread of the rot. But what is much more
important than his own interest is his clear duty to British
forestry. The fungus is as yet uncommon in Britain, and
we should do all in our power to prevent it from spreading.
Wherever any sign of its presence becomes apparent,
infected trees should be immediately cut down, and every
part of a tree that shows the least trace of rot should be burnt.
To remove the brackets and incrustations is not enough, as
they grow again even from felled logs in the forest, and it
is impossible to maintain a sufficiently careful scrutiny to
prevent the dissemination of the fungus,
i i tal ia — <<
Fie. 56.—Transverse section of larch wood rotted at the centre by
Trametes Pini (x +).
Fie. 57.—Radial longitudinal section of larch wood rotted at the
centre by Trametes Pini (x 4).
HEART-ROT CAUSED BY OTHER FUNGI 148
Note.—Since writing the above I have found a larch near
Oxford in which part of the crown is heart-rotted, apparently
by this fungus. The rot had not advanced very far and no
fructifications were present. The rotted portion shows
incipient ring-shake through the partial decomposition of
the junctions of the annual rings, The holes described at
the junctions occur chiefly in the last-formed elements of
the summer wood, but the first-formed spring wood of the
next year is in places affected. In each annual ring there
is also a number of radially extended delignified patches
and holes. The rotted portion, which occupies the centre
of the stem, is surrounded by a layer of insoluble gum
which resembles that described in connexion with Fomes
annosus. Photographs of the specimen are shown in figs.
56 and 57,
CHAPTER VIII
ARMILLARIA MELLEA, THE HONEY FUNGUS
General. Microscopic details of the fructification. Rhizomorphs.
Effect on the host. The black line and resin flow. The ewes of infection.
Means of prevention.
Armillaria mellea, (Vahl) Sace.," belongs to the large group
of toadstools. There are about a thousand British species
of these toadstools, or Agaricaceae, as the mycologists have
it, a group distinguished from the rest of the higher fungi by
having gills running radially on the under-surface, as in the
mushroom ; and the very large majority of them are purely
saprophytic, living either on the humus in the soil or on
decaying timber or leaves. But a very few can also live
parasitically, deriving their nutriment from the tissues of
the host plants, and thereby causing them damage which
may be more or less fatal. Of these Armillaria mellea is by
far the most destructive. Indeed more trees die, in Europe
at any rate, from attack by this fungus than through any
1 The name Agaricus melleus dates back to 1777, and is apparently due
to Vahl (Florae Danicae, fasc. 12, plate 1013). No doubt the specific
name refers to the honey-coloured pileus. Bulliard (Histoire des cham-
pignons, 1791, plate 377) calls it Ag. annularius, and J. Sowerby (Lnglish
Fungi, vol. i, 1797, plate 101) Ag. stipitis. Fries (Systema Mycologicum,
1821, vol. i, p. 26) fixes Vahl’s name, and places the species in his section
Armillaria (L. armilla, a ring), characterized by its clothed stipe and
partial veil which persists as an annulus, by which points the section is
distinguished from other white-spored agarics. Saccardo(Sylloge Fungorum,
vol. v, p. 80) was apparently the first to raise Armillaria to generic rank
as applied to this species, so that the full title of the fungus is Armillaria
mellea, (Vahl) Sacc. Innumerable figures of the fungus have been published,
of which Vahl’s, though uncoloured, is one of the best. The Flora Batava
contains two figures, viz. vol. x (1849), plate 775, and vol. xi (1853),
plate 815, the latter under the name Ag. mutabilis. Cooke’s coloured
drawing (Illustrations of British Fungi, vol. i, plate 32) is not quite
characteristic,
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THE HONEY FUNGUS . 145
other parasitic agent. It attacks not only the larch but all
other species of conifers, and several species of broad-
leaved trees become subject to it, especially when weakened
by other causes. It is thus of exceptional interest to
foresters, and its life-history has been studied in considerable
detail. This chapter contains most of what is known about
the fungus, and enough has been discovered to give us
a pretty clear idea of the way in which the parasite grows
and spreads. But before going into the details of the disease
I shall give in this first section a summary of the external
features of the fungus and the means by which it may be
recognized.
The toadstool itself is the only part which grows above
ground, and it is thus the most familiar portion of the
fungus. Its sole function is reproduction, and in every
detail it is constructed so as to advance the formation and
dissemination of the spores. At the same time it is an
ephemeral growth, found only in one or two months of the
year, whereas the vegetative part of the fungus grows on
through all the months underground, visible only in its
effects. This toadstool has much the same size and shape
as a mushroom : rather larger, though, at times and always
slimmer in stipe and pileus, and honey yellow in colour.
The stipe or stalk is 3 to 8 in. long, of dull orange or brown,
but varying much in tone. At the base it is usually very
dark brown or even black, and about three-quarters of the
way up is an annulus which is white and rather thick,
though in old specimens it often shrinks to a few whitish
scales. Below the annulus the stipe is roughly. grooved,
but above it is smooth and pale in colour. In the button
state the annulus is continuous from the stipe to the margin
of the pileus, as in the mushroom, but it soon becomes.
ruptured at its circle of attachment to the pileus.
' The upper expanded portion is known as the pileus.
This is honey coloured above, with dark-brown scales
which are clustered near the swollen centre but more
scattered farther out. They are particularly conspicuous
in young fructifications, but with age they become more
1888 ‘
146 ARMILLARIA MELLEA,
obscure. The margin is radially striated. The under-side
of the pileus is beset with whitish gills which run radially
from the stipe to the margin. Those which reach the stipe
are decurrent, i. e. are continued for a short distance, often
a very short distance, down the stipe ; but others start farther
out and fill up the broadening spaces between the rays.
These gills bear on their surface millions of minute white
spores, the formation of which is the sole function of the
toadstool, for each spore is capable under suitable circum-
stances of reproducing the whole fungus. They begin to be
formed while the toadstool is still a button, they ripen as
the pileus expands, and, when mature, fall from between —
the gills and are carried away by the air currents which
pass along the ground surface. It is on account of these —
spores that the toadstool is called a fructification or
sporophore. .
The fungus is very ane, but is not difficult to dis-—
tinguish. The scaly pileus, the decurrent gills, and the
persistent annulus, together with the general colour and
shape, render it one of the easiest of the toadstools to
identify.
It is found from the end of September up to the first
frosts of winter, and generally in woods where the growth
is not too dense. In dark coniferous woods the fungus will
readily grow and spread underground, but it seldom makes
fructifications. On this account it generally remains un-
noticed in such places until its presence is evinced by the
trees it has victimized. :
The details of the toadstools are shown in figs. 58-61.
The fructifications are usually found on stumps or grow-
ing on the ground near by. If a bit of the earth is dug up
beneath a fructification, it will be found to contain one or
more black strands, which resemble leather boot-laces ;
and if the earth is pulled away with care the fructifications
will be seen to be attached to one of the branches of these —
strands, as shown in figs. 60and 61. These strands are called —
rhizomorphs, owing to their superficial resemblance to the
roots of higher plants, and they are composed entirely of
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THE HONEY FUNGUS 147
hyphae woven together into a dense mycelium. But in
function they are entirely different from roots, for they are
incapable of obtaining moisture or nutriment from the soil
in which they are growing. If they are traced back to their
base they will be found to originate in a stump, and though
they may grow to a considerable length, all their food
supplies are drawn from the stump. One of their uses is”
obvious, viz. to bear fructifications at a distance from the
stump; but they have a still more important function, for
when they come into contact with the root of a living tree
under favourable circumstances, they penetrate it and infect
the new host with the disease.
It is these rhizomorphs which make Armillaria so difficult
to eradicate, for they spread far and wide in the soil, and
all the trees in the neighbourhood of an infected stump are
liable to attack.
In addition to the fructifications and the rhizomorphs,
there is the mycelium in the tree itself. In the wood the
mycelium is too fine to be seen by the naked eye, except —
for the black line (fig. 65), which can be found in any
sections across the base of a trunk during the more advanced
stages of attack. In conifers this line is always fine, but in
broad-leafed trees it sometimes becomes fairly broad, and
is very marked in stumps which are thoroughly rotted.
Between the scales of the bark and in the cambium dense
layers of white mycelium are formed -which are the surest
means of diagnosis in the earlier stages of the disease. —
They are much thicker than the layers formed by Lomes
annosus, and are usually veined. This felted mycelium
grows up through the cambium to a considerable height,
but when the tree is dead and the bark has become loosened,
it is replaced by a tangled mass of flattened rhizomorphs
(figs. 63 and 64). These were at one time regarded as a
separate species of fungus under the name of Rhizomorpha
subcorticalis, distinguished from the rhizomorph in the soil,
which was called Rh. subterranea. 7
Though the fungus attacks all kinds of conifers, it favours
some more than others. Scots pine is probably the most
: L 2
1e=™; ARMILLARIA MELLEA,
frequent victim, and after that, according to my experience,
Sitka spruce, Weymouth pine, and Corsican pine. Larch is
not usually attacked till it is more than fifteen years old, —
but is then frequently killed by the honey fungus. I have
also seen trees of deodar, Douglas fir, monkey puzzle, and
common spruce all killed by it, and probably no species of
conifer is immune from attack. Among broad-leafed trees,
oak, beech, chestnut, laburnum, and alder are not infrequently |
destroyed, and I have found a street-planted elm and other
trees killed by it. Wagner (1899) has found the fungus on
twenty-nine species of broad-leaved trees in Germany,
including pear and apple, and in America ! many fruit trees
are fatally attacked. It has generally been held that broad-
leafed trees do not succumb to the fungus unless previously
weakened in some way, but this is always difficult to prove.
Death does not follow on attack so quickly as in the case
of conifers, and the early symptoms of the disease may be
mistaken for inherent weakness in the trees. Certainly oaks
are attacked in Britain as much in the open as in woods,
and there can here be no question of suppression by other
trees. At the same time park trees are especially liable to
all types of root disease, as for instance beech by Fomes
australis and F. resinaceus. This is probably due to grazing
animals damaging the surface roots, and also to the inferior
quality of the subsoil in parks as compared with woods, for
reasons mentioned in the discussion on Fomes annosus,
Larch trees are not usually killed until many years after
infection. The external signs of advanced attack are the
death and fall of the needles and generally a flow of resin
at the base of the trunk. Ifa portion of the bark is removed
from the tree at a point near the roots, thick white layers
of mycelium are disclosed, and on digging the soil away
from the roots rhizomorphs are almost invariably found.
In woods which have an open canopy the toadstools of the
fungus may be found growing from the roots of the trees
for many years in succession before the trees show any
external symptoms of ill health.
1 Pammel (1911), Horne (1912 and 1914), Hey (1914), Long (1914).
THE HONEY FUNGUS 149
Microscopie details of the fructification. The general
appearance of fructifications has been described in the
previous section.
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Pr es A
THE HONEY FUNGUS ESE
They were described by Jos. Schmitz, whose account with
some additions was included by de Bary in the first edition
of his Morphologie der Pilze (1866), and at that time several
different fungi which grew as parasites on the rhizomorphs
were regarded as their fructifications by various authors.
It was not until 1874 that Hartig discovered that both
forms of rhizomorphs are mycelial growths of the fungus
Armillaria mellea, or as it was then called Agaricus melleus.
Brefeld in 1877 obtained rhizomorphs in pure cultures
grown from spores of the fungus, and thereby confirmed
the association. The following account is derived chiefly
from de Bary, Hartig, and Brefeld, who worked out the
development and morphology of the rhizomorphs in great
detail.
It will be best to begin with the germination of the spores.
These germinate in a nutrient solution in a few days, though
germination does not take place in pure water. On a decoc-
tion of plums they produce circular masses of white mycelium
_which grow slowly, reaching a diameter of little more than
5 em. in about eight days, and then stop. Next, closely-
woven masses of hyphae appear near the centre, which are
at first light in colour, but later dark brown. These dense
clumps of mycelium resemble the sclerotia (hard mycelial
growths which function as perennating organs) of many
fungi, and we may follow Brefeld in regarding the rhizo-
morphs as sclerotia which have developed growing points
by which unlimited extension and branching are rendered
possible. |
Of a number of such sclerotium-like bodies only one or
two develop into rhizomorphs ; the rest cease to grow and
become covered with white hyphae. The growing points only
become operative on the lower side of the sclerotia, not on
the upper side where they lie free of the culture substratum.
Once growing points are formed the rhizomorphs quickly
grow parallel to the bottom of the dish, and only cease to
lengthen when the nutrient material is used up. By remov-
ing them to a fresh decoction Brefeld obtained renewed
growth and profuse adventitious branching, and by further
152 ARMILLARIA MELLEA,
removal to very large culture dishes the cultures grew into
more or less compact masses of rhizomorphs. So far, in
development and form, the rhizomorphs agreed with the
subcortical type. They were covered with hyaline hyphae,
which stood out at right angles to the surface. The rind
was white where submerged in the substratum, but brown
or blackish where it rose above it. And each rhizomorph
gave rise to frequent branches. But after a rest of some
months a mass of the Rhizomorpha subcorticalis made
branches which grew out into the damp air and assumed
the form of Rhizomorpha subierranea.
All the authorities cited depict the apical growing points
of the rhizomorphs. Three fairly distinct layers can be
observed in this region. On the outside a fairly loose weft
of thin hyphae which send branches out into the air. Next
is a cortex of longitudinally running hyphae, which are so ~
closely welded together that they have no intercellular
spaces between them, and appear polygonal, not circular,
in section. They have frequent septa, and near the apex
the segments are not much longer than they are broad.
The centre is occupied by the medulla, in which the hyphae
are much broader (up to 20). These hyphae are thin
walled and full of hyaline sap, and are arranged in marked
longitudinal rows with numerous intercellular spaces between
them. Farther from the apex the cortex increases its
periphery through the branching of the hyphae that compose
it, so that more room is left for the medulla. And since the
medullary hyphae do not branch, they are drawn more
apart and large air spaces are left between them. Usually
a large central cavity is formed which becomes the main
aerating system of the inner part of the rhizomorph.
This type of structure only occurs near the growing
point, and in tracing successive stages of maturation from
the apex of the rhizomorph it is found that each of these
layers undergoes modification. The outermost layer, which
is at first slimy, owing to the secretion of gelatinous liquid
from the hyphae, becomes firmer through desiccation. The
outstanding branches disappear, and then the layer becomes
THE HONEY FUNGUS 153
nothing but a firm gelatinous covering to the rhizomorph.
Eventually even this becomes indistinguishable. The most
noticeable change in the cortex, which becomes the rind of
the mature rhizomorph, is the coloration of its outer
layers, which begins a few millimetres from the apex. The
coloration, which is associated with a hardening and
thickening of the outer walls, gradually spreads inwards
until it involves all the cortex and also a part, if not the
whole, of the primary medulla. By the formation of a central
cavity the primary medulla has been restricted to half
a dozen or more layers of hyphae lining the inner side of |
the cortex. But a secondary medulla is formed by branch
hyphae, which grow from the primary medulla or inner
cortex and finally fill up the central cavity.
This description applies to the subterranean form. In
the subcortical rhizomorph the outermost gelatinous layer
does not dry up in the same way, and the branch hyphae,
which arise not only from the superficial hyphae but also
from those more deeply rooted, penetrate the surrounding
tissue of the host, especially through the medullary rays,
and, while destroying the tissues, serve to nourish the
rhizomorphs. Subcortical strands usually remain colour-
less until the severance of the bark from the wood lets in
air, when the cortical walls of the rhizomorphs become
thickened and pigmented, though not so intensely as in the
subterranean form. When bark, already loose, is first
_ pulled off them the rhizomorphs are frequently red, but this
colour gives place to a dull black within a few hours.
Branches arise through a new growing apex being formed
in the inner layer of the cortex, which, in a few days, breaks
through the rind and emerges from the parent strand. The
birthplace of every new branch is indicated, some days
prior to its emergence, by a floccose tuft of hyphae, 3-1 mm.
in diameter, which appears on the surface. The hyphae
forming this tuft arise partly from the surface of the rhizo-
morph and partly from the hyphae more deeply seated.
The effects of the honey fungus on its host. As soon as
a rhizomorph of Armillaria has entered a larch tree, it forms
154 ARMILLARIA MELLEA,
a thick white flaky mass of mycelium which usually spreads
in the region occupied by the cambium. Compared with
the very thin layers of mycelium produced by Fomes annosus
the mycelium of Armillaria is almost leathery in consistency. —
_ Like the rhizomorphs it is composed of a central loose
medulla and a denser cortex. The cortex is composed of —
hyphae running longitudinally and cut up into almost
isodiametric segments by very numerous septa. Branches —
are given off by these hyphae, and these branch hyphae —
grow out into the surrounding tissues, attacking the phloem
. on one side and the wood on the other, chiefly by way of
the medullary rays. It has been stated that the felty —
mycelial layer generally grows in the region of the cambium ; —
sometimes, however, it is outside the phloem in the inner ~
cortex, or among the outer phloem cells. In either case, —
further felty mycelial layers are subsequently made, so that —
in pulling off the cortex and phloem of a root or stem in an
. advanced state of attack, several layers of white mycelium —
are found, one inside another. These layers are veined,and —
at the upper limit present an irregular outline.
The cambium and phloem are usually attacked in the
larch to a height of 3 to 5 ft. In rough-barked pines, such
as Scots and Corsican, mycelium is often found at a higher
level, whilst in Weymouth pine it is usually confined to the
lowermost 2 ft. On the death of the tree the mycelium
continues to grow as long as there is sufficient moisture.
When the bark contracts away from the wood, leaving an
air cavity, the mycelium takes the form of Rhizomorpha —
subcorticalis, the structure of which has already been
described.
In the wood the hyphae spread at first chiefly through
the medullary rays, and they often grow sufficiently fast to
reach to the centre of the tree from one side whilst the tree
is still living on the other. From the medullary rays the —
hyphae grow into the tracheides and spread from tracheide —
to tracheide through fine bore-holes made in the tracheide
walls. The hyphae naturally grow along the tracheides
much more rapidly than across them, so that the rot spreads
Fra. 65.—Section of a larch stem attacked by Armillaria mellea. The black lines ©
can be seen, especially on the right-hand side of the photograph (x }).
Fic. 66.—Hollow larch stem rotted by Polyporus sulphureus ( x 4).
_~ 32
THE HONEY FUNGUS | 155
up a trunk more quickly than transversely. In this first
state of rot the wood is very little affected.
The black line. The second state of rot is distinguished
by black layers in the wood. These layers start from the
cambium and gradually spread inwards, remaining very thin
a
Fie. 67.—Armillaria mellea. Radial longitudinal section of larch wood
_ showing the ‘ black line’, where it crosses a medullary ray (400). For
description, see text.
all the while, so that in a transverse section of the trunk
they appear as lines forming the sides of triangles with the
cambium as base (fig. 65). They also spread upwards
in the wood, forming cone-shaped surfaces, and a section
through a somewhat higher part of the trunk shows them
as irregular circles. °
A black layer,-or black line as it appears in section, is
entirely made up of hyphae. For when the rot has reached
a certain stage in the wood, the hyphae become much more.
frequently septate and bear special short branches. Both
156 ARMILLARIA MELLEA,
the branches and some of the original segments swell up j
into bladder-shaped bodies and their walls become tinted
with a pale brown pigment. This stage is seen in tracheide —
a in fig. 67. The branching continues and more segments
swell up into bladders, so that whole tracheides become
tightly packed with them. At the same time their walls —
become thickened, often to such an extent that the lumen
is nearly obliterated, and the pigmentation becomes much |
more marked.
Some of the swollen cells collapse, and their contents fill —
the interstices between the other bladders and stain the
walls of the tracheides. This stage is seen in tracheide
6 of fig. 67 and also in fig. 68. Next the swollen hyphae
become bleached and empty, their walls again become thin,
and finally they disappear (fig. 67, c). In a radial section
of the larch these three stages can usually be seen in three —
successive tracheides, and the black line itself generally
covers only a single tracheide. It is clear that this series
of changes will secure the forward movement of the line. .
When a section of a trunk containing a black line is looked
at with the naked eye, the wood behind the line is found to
' be different in colour from that in front of it. The purplish —
red colour of the wood in the first stage of rot has given
_ place to a dull yellowish-brown tinge, and the wood takes
a polish with less brilliance. But when looked at with
a microscope it is remarkable how little difference can be
seen in the wood on the two sides of the black line. No
more bore-holes are seen; the tracheide walls stain red
with phloroglucol and hydrochloric acid and do not stain
with the chlor-zinc-iodine reagent; the walls are not ~
appreciably thinner, and even the tori of the bordered pits
remain intact. (Hartig says that after the passage of the
black line the walls stain blue with chlor-zinc-iodine, but
_ in the larch this is certainly not the case.) Nevertheless,
_ at some distance behind the black line marked delignifica-
tion does take place. ‘Comparatively few scattered fila-
mentous hyphae are found in this third state of rot, but in
it the most marked changes occur. Whole tracheide walls
t
THE HONEY FUNGUS 157
are delignified from the outside towards the middle lamella,
leaving a layer of cellulose. Then the cellulose also is
digested so that often whole walls disappear. The walls of
the bordered pits are digested gradually and they often
Fie. 68.—Armillaria mellea. Transverse section of larch wood showing
the ‘ black line’ ( x 450).
crack through shrinkage in the process, and present various
curious figures, some of which Hartig has drawn (1879).
The wood never becomes honeycombed as it does with
Fomes annosus or as crumbly as with Polyporus Schweinitzit,
but it is much too weak to be of any use for structural
so purposes. |
:. The significance of the black line is obscure. It seems
158 ARMILLARIA MELLEA,
to indicate a stage of excessive vigour in the mycelium, and
though it does not directly cause marked changes in the
character of the wood, it appears in some way to transform
the wood into a state in which it is easily acted on by the
hyphae behind.
The dark-brown pigment differs from that. found in the
black specks of wood rotted by Fomes annosus in being
unaffected by hydrochloric acid, though it is bleached by
" concentrated nitric acid. 7
When two black lines approach each other they seldom
unite, but cease to move when about 1 to 5 mm. apart.
This causes the frequent phenomenon of a pair of black
lines running parallel to each other. In some parts a black
line is replaced by a much broader band of colourless,
bladder-like hyphae. These resemble the black-line hyphae ©
in all respects except their colour and the thickness of their
walls, and apparently are much more slowly decomposed.
They may cover a width of as much as 6 to 10 tracheides.
Resin flow. The disease is generally accompanied by an
external flow of resin, and sometimes the quantity of this
substance excreted is so great that the fallen needles, twigs,
and soil round the base of the trunk become compacted
into a hard adherent crust. The resin-arises in two ways.
Firstly, the living cells lining the resin ducts in the cortex
and wood are killed so that the contained resin escapes,
and, taking advantage of the cracks formed by the drying
of the bark, runs down the outside of the trunk. Secondly,
the living tissues in the neighbourhood of the fungus excrete
an abnormal amount of resin, and wood which is formed
after some part of the tree has been attacked is characterized
by containing a large number of irregular resin ducts, so
that in this respect the wood resembles the abnormal wood
made in the region of a canker. On account of this escape
of resin the disease is sometimes known under the name of
‘resin flow’ or ‘ resin glut’, and though the flow of resin
is commonly not so great in the larch as in the Scots or
Austrian pine, it is often the first external symptom of the
disease.
THE HONEY FUNGUS 159
The method of infection. Parasite and host are con-
stantly at war. At one stage in the contest the parasite
has the advantage, at another the host. And it is the
business of the pathologist to find out the line of, defence
where the host has the best chance of success against its
aggressor, as assistance may there be of greatest advantage.
Now, it is generally in the opening stages of the conflict
that the plant is at its strongest. It is the obstacles it
presents to the entrance of fungi, its cuticle and bark,
which gave it from what would otherwise be universal defeat:
and destruction. At the same time, parasites have become
adapted so as to overcome these obstacles in a variety of
ways.
The spores of a few fungi can, on germinating, actually
_ pierce the cuticle of leaves and stems; many germ tubes
have acquired the power of finding stomata and entering
the plant through them, and spores of other fungi germinate
on the stigmas of flowers and grow down into the ovaries
by the same route as the pollen tubes. But those fungi
which attack the trunks of trees have a much more formidable
barrier to pass, since in the older parts of stems the epidermis
is replaced by the bark, which is composed of many layers
of cork cells, which are almost impermeable to fungi.
Though the earlier pathologists seem commonly to have
accepted the theory that fungi pierce the sound bark of
trees, no authenticated instance of this has ever been
recorded, and the trend of recent opinion has been more
and more in the direction of admitting the possibility of
infection only by wounds or by outflanking the bark pro-
tection. Willkomm, for instance, thought that the larch
canker fungus gained admission to the cortex through
undamaged bark, a view that was disproved by Hartig.
But Hartig, in his turn, thought that Fomes annosus could
attack healthy unwounded roots, which now appears ex-
tremely improbable.
The mode of infection employed by Armillaria mellea
has never been very critically examined. The question is
a difficult one, and exceptional care and subtlety will be
160 ARMILLARIA MELLEA,
required in devising experiments to elucidate the problems
connected with it which require solution. But there are
some general considerations which may lead to the adoption —
of a reasonable view, in keeping with the present state of —
our knowledge.
No conidial form of reproduction occurs anywhere in the
life-history of Armillaria, so that the fungus is confined to —
two methods of attack—by spores and by rhizomorphs. In
my opinion the role of these two organs is quite distinct,
as will be shown by the following analysis. In the discussion
the fungus will be regarded as an enemy whose sole object
is to attack living trees. Actually, this is not quite true to
fact, as no doubt the fungus is just as happy living sapro- —
phytically on dead stumps as when sapping the life of
growing trees ; but if it confined its attention to a saprophytic —
existence, we should have no quarrel with it, and it is only —
in its career of depredation that we, as pathologists, are —
interested in it.
Infection by spores has never, to my knowledge, been
experimentally demonstrated, but it is safe to formulate —
the following postulates with respect to spore-infection :
(i) Spores are incapable of infecting a tree through —
healthy bark of root or stem. |
(ii) Infection is not known to take place through stems
or roots which are so young as not to have
developed a bark. |
(iii) Consequently infection can only be effected through ©
wounds or dead roots or dead stems.
Further, dead branches are not a suitable medium for —
infection by a root-attacking fungus like Armillaria, pro-
bably because the surface is not kept sufficiently damp for
the germination of the spores. And dead roots can only
very occasionally become exposed to the air-borne spores.
Thus it is fairly safe to assume that spores can only attack
a living tree through wounds in the rootstock just at the
surface of the soil. Owing to mice and other rodents such
wounds are not uncommon, but, as they soon become
THE HONEY FUNGUS 161
covered by resin, they are only open to infection for a very
short period.
Evidence from observation