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UNIVERSITY OF ILLINOIS
Agricultural Experiment Station.
URBANA, APRIL, 1902.
BULLETIN NO. 70.
CANKER OF APPLE TREES.
BY HEINRICH HASSELBRING, B.S., ASSISTANT IN VEGETABLE PATHOLOGY.
CANKER DISEASES. Throughout the fruit growing- region of
Illinois bark diseases of fruit trees are common. These diseases
are popularly known as "cankers." Their injury consists in de-
stroying more or less extended portions of the bark of living- trees,
thereby causing- serious wounds which interfere with the nutrition
of all parts of the affected limbs above the canker spot, finally re-
sulting- in the death of the limb unless the tree is able to heal
over the wound. These diseases are especially dang-erous, inas-
much as they do not restrict their injury to a single crop or to one
season, but threaten the life of the trees themselves. The
diseases are mostly perennial, and having- once g-ained a foothold
they progress steadily until they destroy all or j a part of the af-
The term "canker" has long- been in use in England to desig-
nate the irregular and knotty excrescences resulting- from the con-
226 BULLETIN NO. 70. \_ApriI,
tinued struggle of an injured part of a tree to heal up wounds
caused by various agencies as sun-scald, frost, or parasites. In
the last case the struggle between the tissues and the parasite
continues for many years and large knotty growths are formed at
the wounded spot. The European canker caused by Nectria di-
tissima is an example of this kind. In America the term ' "canker"
has come to be a general name applied to all diseases involving
more or less extended areas of the bark, although these diseases
differ widely in cause and in the effects produced on the host. It
is, therefore, a generic term covering a wide range of injuries. So
long as this is borne in mind there can be no objection to its use.
Moreover it is difficult to replace popular names by others arbi-
trarily chosen. When, however, it becomes necessary to dis-
tinguish between different cankers, more precise names, as "New
York apple tree canker" may be used.
Some of the common cankers caused by parasites are described
below in order to enable fruit growers to distinguish them and ac-
quire a more accurate knowledge of these diseases. This report is
chiefly concerned, however, with a new canker disease which is
doing serious damage in the apple orchards of Illinois. This
canker is caused by a fungus, Nummularia discreta, Tul., which
has not been reported as a parasite. In order to distinguish this
canker from others it may be designated as the "Illinois apple
The best known of the canker diseases is that caused by
Nectria ditissima. This parasite is common in Europe on beech,
apple and other trees. The mycelium kills a part of the bark
forming cracks which are partially grown over by the neighboring
tissues. The rapid development of the mycelium prevents com-
plete healing and as the process is continued year after year large
knots or cankers are produced. In these the wood is laid bare and
shows the concentric thick ridges caused by the healing tissues.
This canker is not common in the United States.
In New York Paddock investigated a bark disease which he
designated as the New York apple tree canker.* By a long series
of careful investigations and cross inoculations he showed this
disease to be due to the fungus, Sphceropsis Mahritni, Peck,
which causes the Black Rot of the apple. The appearance of the
limbs injured by this fungus is various. Paddock describes the
diseased limbs as having dark enlarged sections with roughened
*Paddock, W. The New York Apple Tree Canker. N. Y. Agr. Exp. Sta.
Bui. 163 (1899) "The New York Apple Tree Canker" (second rep.) do. Bui. 185
IQ02.J CANKER OF APPLE TREES. 227
bark and portions of the wood laid bare. The dead bark clings
tenaciously to the decaying- wood. This canker is very common in
Illinois. The fungus attacks nearly all parts of the tree from the
trunk to the youngest twigs. The bark on the diseased limbs at
first assumes a dingy brown color and is closely appressed to the
wood. It may remain in this condition for a long time, some-
times until the canker spot is nearly a foot in length. Around the
margin of the spot the diseased bark is slightly depressed and is
separated from the sound bark by a distinct line and often by a
narrow crack. Cracks and rifts appear later over the diseased sur-
face and the bark assumes the dark charred appearance character-
istic of the canker.
Stewart, Rolfs, arid Hall* have suggested the possible connec-
tion of Macrophoma Malorum, Berl. & Vogl. and of a species of
Cytospora with canker diseases of apple trees and pear trees.
The Pacific coast canker investigated by Cordley 1 and found
by him to be due to Glceosporium malicorticis, Cordley, does not
occur in this state.
THE ILLINOIS APPLE TREE CANKER,
The most serious canker disease in Illinois at present is that
caused by Nummularia discreta^ Tul. This fungus has been known
for a long time, but it has never been reported as being the cause
of a disease. 2 Its usual habitat, like that of a species closely related
to it, is on dead wood. As a saprophyte the fungus has been fre-
quently reported both from Europe and America on dead parts of
various hosts. It occurs on the wood of apple, sorbus, cercis,
magnolia, and elm. The fungus was first collected in America by
Schweinitz, and described by him as Sphczria discreta. He fre-
quently found it on the large branches of apple trees. This
disease was first observed in this state during the past summer,
but it has evidently existed here for some time. It was found to
be widespread through the southern part of the state and many
individual cankers were found that were at least five or six years
*F. C. Stewart, F. M. Rolfs and F. H. Hall. AFruit Disease Su rvey of
Western New York; N. Y. Agr. Exp. Sta. Bui. 191 (1900).
'A. B. Cordley. Some Observations on Apple-tree Anthracnose; Bot. Gaz.
30:48. Some Preliminary Notes on Apple-tree Anthracnose. Oreg. Exp. Sta.
'-It would seem that this Nummularia must have been observed as a parasite
by Tulasne, who states (Sel. Fung. Carp. II, p. 46). "It grows with us during the
autumn and winter on Sorbus hybrida L. on the thick bark which has recently
228 BULLETIN NO. 70. [April,
The following" report contains the results of some observations
and investigations made on this disease during- the past summer.
These are as yet in no way complete, but some suggestions as to
remedial measures can be given as a result of the work. As the
disease is a more serious trouble than is generally supposed, and
also to furnish an answer to the inquiries of some fruit growers
who have become alarmed about the disease, it was thought best
to publish the results thus far obtained.
Inoculation experiments have been begun but it is yet too
early to form any conclusions in regard to them.
DISTRIBUTION OF THE DISEASE.
The disease under consideration was found during the past sum-
mer in all the apple growing regions of southern Illinois.
It appears to be most severe in the orchard regions near
Neoga, Salem and Centralia. It was, however, observed also in
other parts of the state and it is probable that a closer examina-
tion will reveal it in many localities which it has been impossible
to examine. VVhen fruit growers begin to recognize the canker
better a more accurate knowledge of its distribution will be gained.
Within an infected orchard its distribution is usually scattered,
the infected trees occurring here and there throughout the
orchard. The fact that it does not seem to spread evenly from a
diseased tree to all the neighboring trees would lead to the con-
clusion that its spread depends upon occasional infections under
certain favorable conditions. This will be more fully considered
in the following pages:
The canker-wounds are usually formed on the large limbs
near the trunk of the tree. From here they extend upwards on
the limb and frequently down into the trunk. The larger limbs
may be attacked higher up in the tree, however. The exact position
of the wound depends on the source and manner of infection.
APPEARANCE OF THE DISEASED BRANCHES.
The appearance of the canker varies greatly with age. At first
the canker spots are inconspicuous so that they would easily be
overlooked by the casual observer. In the earliest stages observed
the diseased bark has an unhealthy, dirty brown appearance. It is
usually depressed a little below the living bark. The spots vary
in size, being sometimes six inches in diameter at this stage.
They grow most rapidly in the direction of the long axis of the
limb. If the interior of the bark be examined it will be found to
have a mottled appearance, due to the interspersion of sound areas
CANKER OF APPLE TREES.
BULLETIN NO. JO.
I0g2.j CANKER OF APPLE TREES. 23!
within the dead tissues. The boundary between the dead and sound
bark -is sharply marked. Sometimes cracks appear along- this
boundary. The dead area is usually a little depressed, due par-
tially to drying- and partially to the increase in the thickness of
the sound bark. Often the wound is accompanied by a flow of sap
or by slime flux. This, however, is probably a secondary phe-
nomenon not due directly to the parasite.
In the late summer or autumn the fruiting- stromata, to be
more fully described later, appear near the margin of the diseased
spot. These are produced under the bark, which soon splits,
forming- star-shaped ruptures and exposing- the pale grayish ochre,
spore cushions. These vary from one-eig-hth to one-fourth of an
inch in diameter, rarely exceeding- the latter limit. When the
stromata are mature they have the form of a more or less irregu-
larly circular disc somewhat depressed in the center, or even cup-
shaped. As they are formed in a line near the advancing- margin
of the canker spot those of the different years come to lie in con-
centric rows. This appearance is, however, obliterated in the
older parts of the canker.
As the canker spot increases in size it chang-es its appearance.
The bark of the older parts becomes much roug-hened and blackened
as if it had been charred. Numerous rifts and cracks appear over
the surface of the dead bark, which is very dry and brittle, and
falls off in irreg-ular patches exposing- the dead wood. The circu-
lar stromata are firmly attached to the wood by means of a ring- of
hard fungous tissue, so that they remain seated on the wood
even after the bark has fallen away. The entire blackened
area is dotted over with the circular stromata which form the
most pronounced distinguishing- feature of this canker. The
disease is always easily recog-nized by these stromata, which dis-
tinguish it clearly from the New York apple tree canker. Plate
I shows two large limbs with old canker spots.
GROWTH OP THE MYCELIUM.
The mycelium evidently extends into the wood of the tree
where it grows more rapidly than in the bark. Some limbs were
examined at points several feet away from the canker spot and the
heart-wood was found to be brown and discolored. It is probable
that the fungus can gain an exit where the bark has been broken
and form new canker spots at such points.
The effect of the mycelium on the wood and bark has not yet
been fully worked out, but some facts may be noted here. The
mycelium kills the bark wherever it penetrates, and finally disinte-
232 BULLETIN NO. JO. \_April,
grates the cells. It does not advance evenly through the tissues,
but leaves islands of sound tissue in the midst of the dead bark.
This, of course, is observable only in the young- advancing- part of
the diseased spot, and gives this area the mottled appearance al-
ready mentioned. Finally these spots are also killed. The cells of
the parenchtnya tissue and medulary rays are first attacked. The
hard bast fibers resist the action of the fungus and can be seen as
glistening groups 'of cells arranged in concentric rings in the midst
of the dead tissue. In the recently killed tissue numerous, very
delicate hyaline threads of the fungus can be seen traversing the
cells in all directions and sending numerous branches into the
neighboring cells. The boundary between the sound and the dead
tissues is marked by a very delicate layer of cork cells produced
from the parenchmya cells.
INJURY CAUSED BY THE FUNGUS.
The injury which the fungus does to the tree is at first only local,
being restricted to the area of the canker spot. The death of the
tissues here is due to the direct injurious action of the mycelium.
The rapid advance of the mycelium prevents the formation of any
thick callous or wound-cork on the part of the tree. As a result
the cankered limbs remain comparatively smooth and do not form
the knotty excrescences observed in the case of nectria canker. In
summer a very narrow layer of cork cells, not visible to the naked
eye, is formed on the boundary line between the dead wood and
the sound tissues. This seems to be the only effort on the part of
the tree to stop the advance of the parasite. As the canker spot
increases in size the limb shows the indirect effect of the injury.
The parts above the injury begin to show want of water and nour-
ishment. The leaves assume a sickly appearance and the fruit re-
mains small. The growth of the fungus is most rapid along the
direction of the limb, so that living branches may sometimes be
diseased for a distance of two or three feet before they are finally
and completely girdled. Such limbs show great want of water
and the fruit produced on them is of little value. When a limb is
completely girdled all the parts above the canker-spot die. The
death of limbs usually takes place in late summer and is to a cer-
tain extent dependent upon other conditions. During a dry season
limbs begin to suffer and die even before they have been
completely girdled ; and earlier than during a more favorable
season. The growth of the fungus may continue after the
death of the infected limb, as the lower part is still in un-
interrupted connection with the trunk. Where the canker
1902.] CANKER OF APPLE TREES. 233
spot is near the trunk it rapidly extends down the stem and
threatens the life of the tree itself. Plate II shows a condition
common in Illinois orchards. One of the largest limbs of the tree
has been killed by the canker-fungus.
How THE FUNGUS ENTERS THE TREE.
So far as observations go at present they seem to show that
Nummularia discreta, like Nectria and other fungi of similar
habits, is a wound-parasite. Apparently it is unable to gain an
entrance into the tree except through a previously existing wound
in the bark. The spores which originate in the stromata germ-
inate readily in water, hence when they lodge on the moist sur-
face exposed by a wound their germ-tubes easily effect an entrance
into the tissues of the host. Within the tissues the mycelium ad-
vances more rapidly in the wood than it does in the bark.
A knowledge of the manner in which the parasite enters its
host forms the best foundation for combating diseases of this
nature. Too much attention cannot be given to the various causes
by which wounds are produced in trees. One of the most promi-
nent sources of infection was found to be through wounds neces-
sarily caused by pruning. Too much stress cannot be laid on the
proper pruning of trees; but even if a branch is removed without
leaving a stub, the wound still offers a good point of infection for
the fungus. Yet such wounds heal rapidly and are less dangerous
than the long stubs too frequently observed in the orchards of
In one well cultivated orchard it was a noticeable fact that
most of the canker spots were situated on the under side of the
large limbs near the trunk. In this case it is probable that the in-
juries were caused by some part of the harness in cultivating the
orchard. Often the original wound was entirely obscured by the
subsequent action of the canker. It must, however, be borne in
mind that the first injury which affords an infection-court may be
very slight and easily obscured. Wounds caused by pickers who
injure the bark of the trees by climbing about in them also offer
favorable points for infection. A case of this kind occurred in an
orchard in Neoga, where many of the cankers could be traced
to such injuries.
The foregoing cases represent several classes of injuries which
are likely to occur in orchards. Of course any kind of injury such
as breaking of limbs, twisting and splitting of branches, and per-
haps insect injuries will furnish an infection-court for fungi.
234 BULLETIN NO. jo. [April,
PREVENTION AND TREATMENT.
From the nature of the attacks of Nnmmularia discreta there
seems to be no method of curing- the injury after the parasite has
once gained entrance into the limb. The mycelium extends through
the wood some distance beyond the injured spot. It is thus well
protected within the heart-wood of the tree. If the canker
is found in its first stages, however, it may be useful to cut
away the injured bark and a portion of the wood and keep the
wound covered with Bordeaux mixture or paint. The bark at the
edges of the wound will grow out and heal over the injury.
Limbs which have extended diseased areas should be removed and
burned. When a limb is badly injured it is so weakened that the
fruit borne on it is of little value and the death of the limb is only
a question of time. Meanwhile every diseased spot is a source of
danger to the orchard and cannot be too quickly removed. The
canker spots frequently occur near the trunk so that the life of the
tree is endangered by them. Infections through wounds caused
by pruning can be to a great extent prevented by careful attention
to the details of the operation. The indiscriminate and careless
cutting of branches frequently practiced in this state is detrimental
to the health of the tree aside from the opportunities it offers for
parasites of all kinds to enter. Long stubs should never be left on
the tree. Limbs should be cut close to the parent branch without
making the wound unnecessarily large. Wounds caused by proper
pruning heal without difficulty. They should be kept painted or
covered with Bordeaux mixture. Injuries caused by climbing
about in the trees can be prevented entirely by picking from lad-
ders. There is rarely any necessity for climbing the tree to pick
MINUTE CHARACTERS OF THE FUNGUS.
The young stromata, as has been stated, appear beneath the
surface of the bark. They originate as circular patches of dark
colored tissue formed in the bark from numerous closely inter-
woven fungous threads, Plate III, Fig. A. One of these is called a
stroma. On the upper surface of the stroma are numerous erect
threads which have very small spore-like bodies or conidia. The
conidial layer is at first covered by the epidermis intergrown with
fungous tissue, and is not exposed until this fleshy epidermal
layer ruptures. The conidia are small, one-celled. Attempts to
germinate them have been unsuccessful. Tulasne* figures germi-
*Plate V, 1. c.
CANKER OF APPLE TREES.
2 3 6
BULLETIN NO. JO.
1902.] CANKER OF APPLE TREES. 237
nating conidia of this species, and Bref eld* describes the germina-
tion of the conidia of N. lataniaecola.
As the stroma grows older a ring of black stromatic tissue is
formed beneath the disc. This ring extends through the bark into
the wood of the limb. The stromatic tissue is hard and resistant,
and remains in place after the bark around it has broken loose.
In sectional outline the whole stroma is somewhat cup-shaped.
(Fig. B). The ring of tissue can be seen as a black circular line
in the wood when the stromata are cut off. The upper layer of
the stroma at this time contains numerous flask-shaped cavities
called perithecia, with long necks opening at the surface. (Fig. C).
The spores of the fungus originate within long sacks or asci in
these cavities. (Fig. D).
The spores are nearly round or slightly oblong. They are
rather large and have a thick brown membrane. Along one side
a lighter line can be seen indicating the place where the spore
membrane will rupture at germination. The spores are expelled
from the sacks and come to lie in little black heaps on the surface
of the stromata. It is stated by Tulasne that the spores are ex-
pelled in March or April. They may be found on the discs at
almost any time. It is probable that many cling there for a year
Spores taken from specimens collected in September germ-
inated readily when sown in water or in beet infusion. If these
had been expelled from the perithecia in spring they evidently re-
tained their vitality all summer and for several months more in
the laboratory. In germination the exospore cracks along the
lines previously described. Two germ tubes originate from the
endospore. These turn away from each other and remain at first
closely appressed to the spore. Then they grow out in opposite
directions. Germinating spores are shown on Plate IV.
The germination of the spores is greatly influenced by their
supply of air or oxygen. When many spores were sown in a drop
of liquid those which were near the margin germinated readily in
18 to 20 hours, but those which were submerged grew tardily or
not at all. In the same manner when the drop was spread out over
the slide the spores germinated freely. This may be of significance
in connection with the natural coditions under which the spores
must germinate and grow.
In some instances the young mycelium before it had grown to
any considerable size formed a cluster of branches near the apex,
*Brefeld. Untersuchungen X.-256.
238 BULLETIN NO. 70. [April,
and these produced conidia resembling- those originating- on the
young- stroma. (Fig-. C). More frequently the threads continue
to grow and branch freely, forming- a very compact colony of
mycelium. The growth of the colony is very slow on the media
tried. Often long- threads grow out from the colony and at their
end form a dense brush of branches. None of the colonies
fruited in artificial cultures.
The common term, "canker," includes all diseases involving-
portions of the living bark of trees. These may be due to differ
ent causes. Pp. 225-226
Several canker- diseases caused by funguous parasites are
known in the United States. The most common diseases of this
kind in Illinois are the New York apple tree canker caused by the
Black Rot fungus, and the Illinois apple tree canker caused by
Nummularia discrcta, Tul. Pp.226-227.
The Illinois apple tree canker occurs on the larg-e limbs and
trunks of apple trees. It produces extended blackened areas
within which the bark cracks and finally crumbles away. This
always results in the death of the affected limb. Pp. 227-233.
The fung-us causing 1 the canker is a wound parasite g-aining-
entrance into the tree through wounds caused by pruning or by
accidental injuries. P. 233.
The best means of preventing- the disease is to avoid as much
as possible all injury to the bark, to prune properly, and to paint
wounds with an antisceptic solution. Badly diseased limbs should
be cut and burned. P. 234.
CANKER OF APPLE TREES. 239
EXPLANATION OF PLATES.
Plate I., page 229. (A) An old diseased limb, 4% inches in diameter, showing the
stromata of the canker-fungus. (B) Another somewhat larger limb
showing earlier stages of the disease. The limb was infected through
the wound caused by pruning away a branch. The progress of the
disease is marked by the concentric lines of young stromata. At
the base of the limb the boundary between the dead and the living
bark can be distinguished.
Plate II., page 230. Shows a tree having one of the main branches killed by the
Plate III., page 235. (A) A young stroma of the canker-fungus. (B) A fully
developed stroma, showing the perithecia sunk in the upper layer.
(C) One of the perithecia more enlarged to show the way in which
the spores are borne and expelled. (D) Two of the sacs or asci
containing spores. The spores will escape through the pore at the
upper end of the sac.
Plate IV , page 236. (A) Germinating spores (X 600). (B) Spores more advanced
in germination (X8oo). (C) Condia borne on the young mycelium
(X6oo). (D) Mycelium from a culture 36 hours old, 1X385). These
threads grow through the wood and bark, causing the death of the
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