440 
er. If a mature canker is examined 
in midsummer, little elevations in the 
bark are easily observed. They are at 
first more or less conical in shape and 
finally burst the outer layer of the bark 
exposing a cream colored mass of fung- 
ous tissue. These are the fruiting struc- 
tures of the fungus and the spores are 
produced in them in great abundance. 
In late summer or early fall the ma- 
ture cankers are found and may be from 
one-half inch in diameter up to eight 
to 10 inches long by three to four broad. 
Frequently large cankered areaS may be 
formed by several smaller ones becoming 
confluent. Mature cankers have a definite 
limiting crack separating them from the 
healthy tissue; there is usually a definite 
ridge surrounding the canker caused by 
the slight formation of callus tissue at 
the edge under the diseased bark. The 
bark is sunken, dry and dead, and darker 
in color than healthy bark. Thickly 
scattered over the surface one finds the 
little cracks described above which are 
formed when the fungus bursts forth. 
These cracks are transverse or triangular, 
seldom if ever, vertical. (Fig. 1-A shows 
a typical canker.) In old cankers the 
fungus tissue exposed by the cracks turns 
black and the bark gradually becomes 
loose at the edges and drops out, leav- 
ing ugly wounds. Bark may cling in the 
cankers, however, for at least’ three 
winters. The wounds, if not too large, 
slowly heal over by the formation of cal- 
lus. 
Injury Caused by the Disease _ 
On account of the nature of the disease 
it is difficult to estimate the amount of 
the injury resulting from its. ravages. 
Under ordinary conditions few to many 
cankers may be found on the branches of 
the trees in infected orchards. Some- 
times twigs are girdled by the formation 
of a small canker, which extends around. 
the stem as is shown in Fig. 2. Not in- 
frequently larger branches are girdled. 
where several cankers grow together. 
Sometimes trunks of young trees are 
girdled in this way. 
It is evident that the disease in any de- 
gree of severity interferes with the nor- 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
Apple Twig Girdled-by Small Canker 
of Anthtacnose. 
Hig. 2. 
mal function of the bark and so, hinders 
the proper distribution of elaborated food 
in the tree. Branches are weakened’ by 
the presence of the cankers and frequént- 
ly break when heavily set with fruit. The 
woolly aphis not uncommonly works un- 
der the bark at the edge of cankers and 
further saps the vitality of the tree and 
interferes with the normal healing over 
of cankers. Large cankers heal slowly 
and the wood may be exposed for con- 
siderable periods, thus affording oppor- 
tunity for the entrance of fungi which 
cause heart rot. 
Distribution 
‘As a serious orchard disease the apple 
tree anthracnose is peculiar to the Pa- 
cific Northwest. It is known to occur in 
British Columbia, Washington, Oregon, 
