The blight canker of apple trees

Rew Pork
State College of Agriculture

At Cornell University
Ithaca, N.Y.

—

Library

Blight Canker of Apple Trees

BY
H. H. WHETZEL

Assistant Professor of Botany in New York State College of
Agriculture at Cornell University.

ALBANY
BRANDOW PRINTING COMPANY
State LEGISLATIVE PRINTERS
1906

Cornell University

Library

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

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BLIGHT CANKER OF APPLE TREES.

H. H. Wuerzer, Assisranr Prorgssor or Borany.

Cankers in one form or another on apple trees are familiar to
inust orchardists. As the term is generally understood to-day,
we mean by a canker a dead area in the living bark of the body
or main limbs of trees. This area may be sunken and smooth or
it may be swollen and rough. (Fig. 1 and 2). The term * canker,”
as used to-day, applies usually to diseases of trees that are caused
by parasitic fungi or bacteria. Altho these injuries to the bark
of living trees have been, in the large majority of cases, abso-
lutely proven to be due to the growth of fungi or bacteria, grow-
ers very generally attribute them to “sun scald” or “ winter
injury.” Not only have experiment station men shown that these
injuries are due to the attacks of living organisms rather than
to the results of unfavorable weather conditions, but they have
demonstrated that the different forms of these cankers are due
to distinctly different organisms.

Of the two common cankers in the State the best known is the
New York Apple Tree canker. This is very prevalent in the
orchards of Western New York. It has long been known to
growers but the true cause of it was first worked out by Pad-
dock of the New York State Experiment Station at Geneva in
1899. It is caused by a fungus, Sphaeropsis malorum, and is in
inmost respects very different from the blight canker. In the early
stages of this canker concentric cracks appear in the outer skin
of the diseased area. (Fig. 3.). The cankered surface is always
black and roughened and covered with minute black pimples,
which are the fruit bodies of the fungus, often not so evident in
old cankers. These occur, for the most part, on the main limbs
of trees in old orchards, causing the death of limbs here and
there on the tree (Trig. 4).

There is another canker which ig very severe thruout the State,
especially in certain sections. This we have named the Blight
Canker. The surface of this canker is sunken with a cracked

2

margiv caused by the drawing away of the diseased bark from
the healthy tissue. This differs from the Sphaeropsis canker in
that the surface is smooth, the only cracks being those which
mark the extent to which the canker has spread in a given
period of activity. The Blight Canker is always of a brown
color and never shows any fruit bodies, except when it is old
and the dead tissues have been attacked by rot fungi. Soie-
times these cankers kill a limb, but frequently the disease does
not extend thru the bark into the wood, except in a few places.
It simply kills the outer portion of the bark (Fig. 5).

In the New York Apple Tree canker, the fungus lives from
year to year, gradually spreading sol as to involye a large portion
of the limb, but the Blight cankers are for the most part annual.
living only one year. Only a small percentage of Blight cankers
live thru the winter and serve as infection sources for the spread
of the disease the following vear.

FORMS OF THE CANKER ON DIFFERENT PARTS OF ‘THE TREE.
The chief differences between these two cankers are set forth
in the following parallel columns:

NEW YORK APPDE TREE CANKER.

Caused by a fungus.

Usually found on the
limbs of old trees.

main

Diseased portion more or less
swollen, cracked and rough-
ened. (Fig. 3).

Cankered surface black.

Covered with minute black pim-
ples—the fruit bodies of the
fungus often not so evident
in old cankers. (Fig. 3).

Freshly cankered tissue dry.

Cankers perennial, i. e., living
over and spreading from year
to year. -

BLIGHT CANKER.
Caused ly bacteria.
Occurring most frequently on
the body and limbs of young
trees just coming into bear-

ing.

Diseased area sunken and
smooth not eracked and
checked, (Fig. 7).

Cankered surface brown.

Not showing any pimples or
fungus fruit bodies, except in
old cankers that have been
invaded by — saprophytic
forms.

Freshly cankered tissue watery.

A large per cent. of the eankers
active but one season.

3

DISTINGUISHING CHARACTERS OF THE BLIGHT CANKER.

Blight cankers occur most commonly and are most destructive
on trees 8-14 years old—trees just coming into bearing. Unless
promptly attended to they very frequently result in the death of
the entire tree.

In young trees with smooth bark the cankers are easily de-
tected, even in their first stages. They appear as discolored and
somewhat sunken areas, the margin along the advancing front
being usually slightly raised or blistered. The tissue in actively
spreading cankers is of a darker green than the healthy bark
and is very watery or sappy. On damp cloudy days drops of a
milky, sticky fluid (Fig. 6) exude from the cankered tissues
thru the lenticels or pores in the bark. After a short time the
diseased tissue begins to turn brown and dry out. Unless in a
very active state of progress the margins are very distinct, marked
by a crack where, in drying, the diseased tissue has separated
from the healthy bark (Fig. 1). The older cankers are brown,
somewhat darker than the healthy bark. They are distinctly
sunken. The surface is smooth, never checked or roughened or
beset with pustules or pimples, except in the old cankers, where,
after a time, rot fungi gain entrance and, thriving in the already
dead tissues, produce their fruit bodies on the surface. The
progress of the spreading canker depends largely on the con-
tinuation of favorable weather conditions, which seem to be a
humid atmosphere and cloudy days. With the return of bright
sunny weather, the active spread of the canker is checked ab-
ruptly, often to be resumed again with the return of favorable
conditions. This checking and renewing of activity sometimes
results in large cankers ‘with concentrically arranged cracks
within the cankered area (Fig. 7). This renewal of activity may
take place during the same season or the canker may partially
heal over to spread anew the following year (Fig. 8). A large
percentage of the cankers are active during but one season.
There are always some, however, in which the disease is peren-
nial, living thru the winter-to become active again the following
spring, spreading and enlarging the original limits of the can-
kered area (Fig. 9). The diseased bark is usually killed to the”
wood, to which it clings tenaciously the first season. It grad-
ually decays, however, and falls out, leaving the wood bare and

4

exposed (Fig. 8). In small cankers the cone of diseased bark
may be quickly forced out by the rapidly formed calluses, which
heal and close the canker wound (Fig. 10). In some cases the
canker is superficial, never reaching the cambium, except, per-
haps, in a limited area at the point of infection. Such wounds
heal quickly beneath the dead bark, which clings to the tree as
a sort of scab (Fig. 5).

The canker manifests itself in different forms on different
parts of the tree. Those that grow on limbs are usually small
and circular. If the disease is very severe the canker may be
larger. These we designate as “pit” cankers (Fig. 11). Often
the dead bark remains as a sort of lid to the pit but is easily
removed with the finger or knife blade (Fig. 10). These pit
cankers, in most cases, heal over quickly and by the end of the
second year close the wound entirely. Aside from affording
entrance to rot fungi such cankers unless they enlarge do not
seriously affect the health of the tree.

There is another form of the canker which frequently becomes
serious, called “crotch canker.” It usually appears in the
crotches where the main limbs arise from the body (Fig. 12),
but may also appear in the secondary crotches well up in the
tree. In general characters they are similar to the limb and
body cankers. Owing to their peculiar position, water is re-
tained more readily in the dead bark, thus affording the very
best conditions for the entrance and growth of rot fungi. These
find easier access to the heart wood at the crotch than on the
limbs. It was observed that these crotch cankers heal much less.
readily and successfully than do the limb and body cankers.
Crotch cankers, unless promptly attended to, mean the almost
certain destruction of the trees. In some sections of the country,
especially the Hudson river valley, they have been very serious.

Cankers are frequently formed about pruned stubs (Fig. 18).
These will be discussed further under another heading.

On the bodies of trees are often found large, irregular cankers,.
which originated from the “pit” cankers. They come either
from new infection in the spring or from the organism remaining
in the canker during the winter. Fig 9 shows how such cankers
are formed.

5

The large cankers at the bases of young trees, frequently re-
ferred to by growers as “ collar rot” (Fig. 14) are in many cases
very probably due to the same cause as that of the cankers on
the upper parts of the tree. The well known “collar rot” of
King trees may also be due to the same or a similar organism.

History AND DISTRIBUTION oF THE BLIGHT CANKER.

The blight canker has been known for many years and was
probably first observed by horticultural writers as early as 1780.
It has been repeatedly referred to in horticultural writings
since that time, altho its true nature does not seem to have been
suspected until 1880. In that year Professor T. J. Burrill, of
the Illinois State Experiment Station, while working on the fire
blight of pears and apples, came to the conclusion that the so-
called “sun scald” spots on the bodies and larger limbs of apple
trees are due to the same cause. At a meeting of the Illinois
State Horticultural Society in 1881, in answer to a query regard-
ing the nature of “sun scald,” he said: ‘“ The sun scald on apple
trees is the same as pear blight.” Upon what experimental
evidence, if any, this and other statements were based I have so
far failed to discover. A number of writers since that time have
referred to these cankered patches as “body blight” due to
attacks of Bacillus amylovorus, but none seem to have actually
produced the cankers by the introduction of the bacteria into
the bark of healthy trees. Much of my work has been to try
to prove that these cankers were due to pear blight organ-
isms. In Saratoga county and vicinity, the disease has been very
bad. At least ninety-five per cent. of the young trees which
were just coming into bearing were affected with this canker.
It is a dangerous pest for the very reason that it affects the
trees just at the time that the grower may expect some reward
for his labor. In an orchard of about 400 trees (Fig. 15) there
were not ten trees out of one hundred still alive that were entirely
free from canker and finally the entire orchard was destroyed
altho it was given the best of care. There are also sections in Cen-
tral New York and Jefferson county where orchards are affected
as bady as those in the Hudson River valley (Fig. 16). I believe
the disease is common thruout the State, as well as in numerous

6

states in the United States. Wherever the twig blight of apples
and pears occurs, I believe you will find the cankers on the
bodies of the trees.

CAUSE OF THE DISEASE.

Most orchardists regard the trouble as due to freezing or sun
scald. In seeking to discover the real cause of the trouble I
examined many cankers in different orchards at different times
of the year. One moist cloudy day early in May I discovered a
cankered apple tree from the diseased bark of which were exuding
drops of a sticky milky fluid (Fig. 6). Examined under a micro-
scope this fluid was found to be made up almost entirely of
bacteria—minute rod-shaped plants (Fig. 17). The diseased
tissue within the bark was also found to be alive with these
minute plants. By their rapid growth and multiplication within
the cells of the bark they cause its death. They are not carried
along in the sap, but slowly work their way from cell to cell.
When the canker dries down they die and disappear, so that ex-
amination of the tissues of old ‘cankers does not show them. That
they are the direct cause of the disease was proved in the fol-
lowing way: Bacteria from the cankered tissue were introduced
into the bark on the body of a healthy apple tree and also into
the bark-of a healthy pear tree, with the result that typical
cankers appeared in both cases (Fig. 18). Blossoms and growing
twigs of both pear and apple trees were also inoculated with
bacteria from this same canker. These nearly all developed good
cases of blight in about ten days (Figs. 19 and 20), while twigs
and blossoms punctured with a sterile needle gave no infection.
This last experiment was twice repeated during the summer with
pure cultures of the bacteria from the apple tree canker. The
blight resulted in practically every case. Young fruits of both
the pear and the apple were also inoculated and gave well de-
veloped cases of the disease (Fig. 21). By a comparative study
in various culture media of the bacteria from cankers, twigs and
fruits of both pear and apple secured from different orchards
about Ithaca, the organism of the canker was shown to be iden-
tical with that of the well known “fire blight" of the pear and
“twig blight” of the apple, Bacillus amylovorus.

7

How Trees Become INFECTED.

The next problem was to find out how the trees become in-
fected. Only those ways of infection which have come under my
personal observation will be mentioned, tho there is no doubt
that the bacteria gain entrance to the bark in still other ways.

The bacteria frequently get into the bark of the limbs and
body by way of short spurs and watersprouts (Fig. 22). In
1905 twig blight became very prevalent during July and August,
especially in the region about Ithaca, and it was an easy matter
to find blighted spurs and watersprouts with active cankers about
their bases (Fig. 23). When these watersprouts grow out from
the trunks, as is often the case in young trees, typical body
cankers are formed. The infection of the sprout itself is generally
attributed to the work of insects, which after visiting freshly
cankered sprouts or blighted twigs introduce the bacteria into
the succulent tissues of the rapidly growing healthy shoots. The
blighted watersprout soon dries up and falls away, leaving often
a very indefinite scar in the cankered area so that the following
season it is usually impossible to tell with certainty the manner
of infection. Observation on a large number of trees this past
season, ednvinces me that the blighting of adventitious shoots
on trunk and limb is responsible for most of the cankers in such
locations.

Another source of infection was found to be the pruning knife.
Along one side of an orchard of about 350 trees which was under
observation thruout the season, it was early noticed that the
pruned stubs, of 1904 especially, showed the collars of dead bark
often two or three inches in width (Fig. 18). Instead of forming
a callus and healing over the wound, as would normally occur,
the tissue had died and shriveled up but still clung to the stub.
In most cases the bacteria which had caused the death of the
bark had died out the first season. In a few instances, however,
the canker was observed to be active early in the spring, extend-
ing down the side of the adjoining limb. Two badly diseased
trees on this side of the orchard seem to have been the source of
infection. Owing to their diseased condition, they had been
severely pruned the previous season and very probably the knife
or saw had carried the bacteria to the healthy trees. Flies,

8

which were observed constantly to follow the pruner to suck up
exuding sap, may have been the direct agent in many cases in
transferring the bacteria. The knife itself may convey the dis-
ease, as is shown by the following incident: While making
inoculations into the body of an apple tree on the Station grounds,
I had occasion to remove from near the base a large sprout of
several years’ growth. | This I did with my knife which I had but
shortly before used: to cut bark from a fresh canker. A typical
canker soon developed about this pruned stub (Fig. 24).

Of a similar nature are infections which occur thru wounds or
bruises on the limbs and bodies of trees. These wounds, commonly
results of “barking,” may be made by careless workmen when
plowing or working about the trees or from the gnawing of ani-
mals; one of the worst animals in New York being the woodchuck.
A large percentage of such wounds heal over eventually, but
frequently thru the agency of insects or other means these wounds
serve as infection courts for the canker bacillus. An interesting
case of wound infection came under my observation last season
In cutting a cankered branch, I accidently “barked” a healthy
limb with the cut end of the diseased branch. The tree was not
again visited until some weeks later, when a large and actively
Spreading canker was found to have developed about the abra-
Sion (Fig. 25). The bacteria were found in abundance in the
diseased tissue and pure cultures were secured.

The wounds or punctures of insects seem to be directly re-
sponsible for some of the infections. Sometimes cankers on the
bodies of trees cannot be attributed to infection thru blighted
shoots, and in some cases these cankers have been traced directly
to the wounds made by insects. It is very probable that many of
the cankers at the base of young trees originate in wounds made
by borers. The bacteria are probably carried to these wounds
by flies or other insects which visit these places to feed on the
exuding sap and excrement. The infecting agents in the case
of crotch cankers have not as yet been definitely determined.
It seems likely that insects again are responsible. I have
found them repeatedly hiding in the crevices of the dead bark
that accumulates in the crotches, and one species seems to feeq
to some extent on the living tissue in such. places. I haye also

9

observed this same species feeding on the exuding sap of can-
kered limbs and stubs. That it may carry the bacteria to the
ecrotches seems obvious. Besides this, many of the crotches are
of. such form that they readily retain moisture and thus afford
the best of conditions for bacterial growth.

How tue Disease AFrects THE TREE.

The effect of the canker on the tree is to lower its vitality to
a greater or less degree by cutting off the food supply to the
roots, and thus indirectly reducing the flow of sap to the branches
and leaves. The “collar rot” and “crotch cankers ” seem to be
the most fatal to the tree. The effects of the canker are first
evidenced in the foliage. If there is a large body canker, the en-
tire tree may show the effects of the trouble. More often the first
symptom noted by the grower is the peculiar appearance of the
foliage on one or more of the limbs. Either these branches fail
to leaf out at all in the spring, or if they do, the leaves never fully
expand but remain undersized and curled or inrolled. They never
take on the dark green color of healthy foliage, but remain pale
and gray. Growers often refer to such trees as having “mouse
ear” leaves. As the season advances and the cankers spread,
the leaves often die and dry up on the branches. Sometimes
badly infected trees may live for two or three seasons, but such
trees have scanty foliage, blossom profusely and frequently set
a heavy crop of fruit, which falls prematurely or is small and
inferior in quality.

As I have already pointed out, small cankers may not of them-
selves seriously affect the health of the trees. When the trees
are strong and vigorous they frequently succeed in promptly
healing the wounds. The dead bark of the canker, however,
makes an excellent infection court for the entrance into the tree
of “heart rot” and decay-inducing fungi. Moisture, so necessary
to the germination and growth of the spores of fungi, is retained
for a considerable time in the dead tissue. This is more especially
true of crotch cankers. No doubt these rot fungi are often to
blame for the final death of the tree. The heartwood of badly
affected limbs and trees is commonly found to be soft and rotted,
with only a thin rim of sound sap-wood surrounding it.

10

TREATMENT.

In most cases, it is very difficult to cure a tree after it has
become diseased. However, in the case of young trees that are
just coming into bearing it seems that something should be done
to save them. I believe they can be saved, at least long enough
to bear several good crops.

In the first place, all dead limbs and trees should be promptly
removed from the orchard and burned. Old pear trees in the
neighborhood of young apple orchards are often a constant source
of infection, and, unless kept absolutely free from blight, should
be removed. Also every case of twig blight in both pear and
apple trees should be cut out and burned.

Cut out with a sharp knife all the diseased tissue of the canker
well back into the healthy bark, and treat all the cut surfaces
with corrosive sublimate,—one tablet to one quart of water, or
copper sulfate solution,—one ounce to four quarts of water.
When dry, paint over thoroly with some heavy lead paint. This
should be done early in the season, as soon as the cankers are dis-
covered, for two reasons: Ist, the spreading of the canker and its.
consequent damage to the tree is checked; 2d, the wound is thus
given a long period in which to heal. The painting should be
repeated again toward the close of the season and again the next
year, or until the wound has completely healed. This prevents
a second infection or the entrance of rot fungi. A twice-monthly
inspection of every tree should be made and all cankers carefully
cut out and treated as soon as they appear. Cankered trees so
treated early in the spring of 1905 have formed good calluses
and are fast healing the wounds. Treat all accidental wounds
in the same way.

Keep the body and main limbs of the tree free of watersprouts.
thruout the summer.

In planting, choose trees with open or spreading crotches.

Avoid excessive fertilizing with nitrogenous manures. Apply
some form of phosphoric acid to ripen new growths.

The planting of varieties known to be more or less resistant
to this disease is to be recommended. The Wolf River and Tal-
man Sweet appear to be of this sort, while Baldwin and Ben
Davis suffer most severely. Desirable non-resistant varieties may
be top grafted on resistant stocks.

11

Discussion.

Pror, Baitzy: Are the trees that have been thoroly sprayed
as liable to attqgck as unsprayed trees?

Mr. WuHerzeL: I do not think it makes any difference as the
canker develops where there has been some wound. Unless bor-
deaux was applied to a wound as soon as it was made it would
not prevent infection. We treated a large number of trees with
bordeaux mixture of double strength and I could not see that it
made any difference in the number of infections.

Mr. Sirrine: Are there any ill effects from the use of linseed
oil?

Mr. Wuerzet: The tissue dies back a little but not enough
to cause any trouble.

Mr. Eustace: Have you observed much twig infection?

Mr. Wuerzet: I found practically no active cankers in the
Hudson river valley, where the last two seasons have not been
favorable for the development of this disease. However, around
Ithaca the last two years have been good for the disease and twig
blight and active cankers are easily found. The work of the
disease in the Hudson River valley was chiefly done during the
seasons of 1902 and 1903.

Fie. 1—Showing a characteristic blight canker. This resulted from a wound
infection. Note the crack limiting the diseased area; otherwise the diseased bark
is smooth.

Fig. 2.—Typical New York apple tree canker. Note the cracked and rough-
ened surface as compared with Fig. 1.

Fic. 3.—New York apple tree canker. The first season’s development of the
disease. Note the concentric cracks and cross checks, and also the fruit pus-
tules of the fungus, sphaeropsis malorum.

‘Joyo ver} a[dde YroX Moy; oy Aq por|L] 9013 pfo ue ul sq] peoq—'F ‘DI

Fra. 5.—The outer portion of the bark has been killed by the blight. The
healing tissue beneath is pushing off the dead bark.

Fic. 6.—Active canker showing exuding milky drops. This sticky sap is
made up almost entirely of the blight bacteria.

Fic. 7.—Showing concentric cracks in diseased area due to successive periods
of activity of the canker. (

Fic, 8—Canker in body of tree, resulting from the renewal of activity,"season
after season. 7

I'ic. 9.—Canker spreading the second season, destroying callus of last season
and enlarging area of cankered surface.

Fig. 10.—Pit canker, rapidly forming callus, closing the wound and forcing
out the diseased bark.

Fic. 11.—Typical pit cankers in the body of a tree trom which the diseased.
bark has dropped out.

Fie. 12—Crotch canker. The diseased bark has been removed.

Fic. 13.—Pruned stub canker, infected at the time of pruning. Note the collar
of dead bark.

Fie. 14.—Large body canker near the base of the tree. The tree_has’ made
repeated attempts to heal this wound.

Fic. 15.—Orchard of some 400 trees in the Hudson River Valley. Began to
go out from blight canker in 1903. Less than 50 alive in June, 1905.

.—Map showing the epidemic centers of the blight canker in the State

Fie. 16
in 1905.

Fic. 17.—Bacteria taken directly from active canker on apple tree.

Fic. 18.—Canker on pear tree resulting from inoculation with bacteria from
active’canker on apple tree.

Fie. 19.—Blossom and spur blight of apple resulting from artificial inocula-
tion with bacteria from canker on limb of apple tree.

Fic. 20.—Actively growing pear twigs blighted by artificial inoculation with
bacteria from canker on limb of apple tree.

Fie. 21.—Half grown apple inoculated with pure culture of bacteria ,from can-
ker on limb of apple tree. Note milky drops exuding from diseased tissue. _

Fig. 22.—Recently blighted water sprout on limb of greening. Note the welt
marked canker about its base.

Fic, 23.—Active canker formed about base of blighted spur,

Fie. 24.—Pruned stub canker resulting from cutting away a water sprout with
an infected knife. ,

Fig. 25.—Canker formed about a wound resulting from accidental “barking”
with cut end of diseased limb.

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