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 Physalospom Cydonice. 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- 



