SOURCES OF APPLE BITTER-ROT INFECTIONS. 9 
writer is unable to find a record of a bitter-rot canker occurring in 
eastern orchards. 
DESCRIPTION OF CANKERS. 
The cankers caused by Glomerella cingulata (Pl. III) are fairly 
characteristic and are not easily confused with any other cankers ex- 
cept those caused by the pear-blight organism (Bacillus amylovorus). 
Especially on varieties which have suffered severely from pear blight, 
bitter-rot cankers and pear-blight cankers are sometimes hard to 
differentiate. The writer does not doubt that in the past bitter-rot 
cankers have many times been mistaken for pear-blight cankers. 
The bitter-rot canker consists of a sunken portion of bark, 
usually somewhat oval in outline, beneath which the wood is dead 
and dry. The dead bark and cambium adhere rather firmly to the 
wood, and in older cankers more or less complete cracks or fissures © 
parallel to the edges of the cankers give a zoned effect to the dead 
bark. The depression (PI. IV), too, in older cankers becomes much 
more pronounced, owing to the fact that because of the death of 
the cambium there has been no increase in the thickness of the branch 
in the infected area. The wood and medullary rays, even in the 
case of young cankers, are colored brown all the way into the pith. 
Often the canker is surrounded by a layer of callus (Pl. IV), which 
successfully prevents further extension of the canker and which 
eventually entirely obliterates the old lesion. 
In many cankers a small dead twig will be found directly in the 
center of the blackened area, doubtless explaining the mode of 
entrance of the fungus. Sometimes this dead twig is the remains 
of a fruit spur, but often it is a water sprout, part of which has 
been broken off. 
Cankers are usually found on branches which are at least 2 years 
of age. The writer has found them on branches which range in 
age from 2 to about 15 years, but has never been able to find them 
on young vigorously growing twigs less than 2 years of age. 
The number of cankers per tree is usually few, even in orchards 
in which the disease has been very serious for a number of years. 
However, in the case of especially susceptible varieties, such as 
Givens, the writer has found trees having more than 30 large bitter- 
rot cankers, besides numerous small ones. 
At first thought it would seem that cankers, being located on 
older branches and hence near the center of the tree, are not advan- 
tageously located for the infection of fruits, but as the apples develop 
and bend the branches downward the position of the cankers is 
often directly above that of large numbers of the apples. Plate II, 
figure 2, shows young fruits badly infected from cankers thus located. 
51135°—18—Bull. 6842 
