APPLE DISEASES 51 
pathogene spreads from year to year until the canker is several 
feet in length. Very early in the formation of the canker a 
crevice is developed at its margin, on the healthy side of which 
corky tissue originates. This plate of cork limits temporarily 
the extent of the lesion. Further spread results in an increase 
of the diseased area with the formation of a second marginal 
crevice. Repetition of this process proceeds from one or more 
points at the edge of the canker until a series of more or less 
concentric crevices is developed (Fig. 13, near top). This 
phenomenon is very similar to that described for frog-eye of the 
leaves. Affected bark remains closely appressed to the wood 
for a year or more, but finally cracks and falls away, exposing 
the wood and a callus about the margin of the wound (Fig. 13). 
Before the bark falls there appears over its surface the same sort 
of black fruiting bodies of the pathogene as described for the 
other affected organs (Fig. 13). Girdling of affected limbs com- 
monly occurs, as a result of which the parts above the lesion 
ultimately die. This is evidenced by a yellowing and browning 
of the leaves and the shriveling of the bark and fruit. Some- 
times there is an hypertrophy of the limb at the upper and lower 
ends of the canker. : 
The above description constitutes external symptoms in 
which the bark appears to be the only part affected. But 
upon removal of the healthy bark immediately above and 
below the canker the sap wood is found to be stained brownish, 
the discoloration appearing in a long slender streak, continuous 
from the canker to a point several inches distant. 
Cause of black-rot canker. 
This disease is of fungous nature, and the causal pathogene 
is Physalospora Cydonia. The fungus passes the winter in 
the old cankers as mycelium and as pycnospores. It is not, 
only found on fruit-trees other than the apple, but also on a 
number of other plants such as alder, ash, basswood, dog- 
wood, elder, hawthorn, hop-hornbeam, lilac, maple, mul- 
