SOURCES OP APPLE BITTEK-EOT INFECTIONS. 9 



writer is unable to find a record of a bitter-rot canker occurring in 

 eastern orchards. 



DESCKIPTION 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 diy. 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 (PL 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. 684 2 



