SOURCES OF APPLE BITTER-ROT INFECTIONS. 



17 



Since bitter-rot mummies and bitter-rot cankers are the chief 

 starting points of the disease for the current season, a comparison 

 between them as sources of infection is of interest. (Table III.) 



Table III. — Comparison of cankers and mummies as sources of infection. 



Cankers. 



Mummies. 



Relatively few except on very susceptible varieties. . 



May be few or many. 





poor culture medium. 

 An annual source. 





Not Injurious to the tree directly. 



Fungus comparatively well protected from compe- 

 tition with other organisms. 



Relatively hard to find and remove . . . . . . 



Fungus not well protected. 

 Rather easily found and remov ed. 



Located on older twigs or branches (not so advan- 

 tageous). 



Located among next year's fruits (advantageous). 



CANKERS OTHER THAN THOSE OF BITTER-ROT. 



In 1915 the writer (10) as the result of some work carried on in 

 Arkansas in 1914, showed that the bitter-rot fungus may survive the 

 winter in almost any cankered or dead part of an apple tree, includ- 

 ing Illinois apple-tree cankers due to Nimrnmdaria discreta, dead tips 

 of fruit spurs, dead parts of limbs injured by freezing or death of 

 roots, branches injured by mechanical means, cankers caused by the 

 pear-blight organism {Bacillus amylovorus), and twig cankers 

 caused by the apple-blotch fungus {Phyllosticta solitaria). 



In the case of those Arkansas and southern Missouri orchards in 

 which the disease has been especially destructive over a period of 

 years, it is comparatively easy to isolate the fungus from almost any 

 dead portion of the trees. Such orchards which may have in addi- 

 tion bitter-rot cankers and bitter-rot mummies show how extremely 

 destructive the disease can be and how very important are the agen- 

 cies which carry it over from season to season. The fruit of orchards 

 in which so many sources of infection survive the winter may be 

 utterly ruined at the very first outbreak of the disease. These cases 

 are, of course, exceptional ones; yet the writer has seen orchards 

 the crops of which were totally ruined within a week, each apple 

 averaging approximately 500 infections. In such cases, in which 

 infection from wintering-over sources is so heavy, control by spray- 

 ing alone i*s impossible. 



Cankers due to various causes are important sources of infection 

 in the Middle West, but they appear to play very little part in the 

 infection of fruit in eastern orchards. The writer has found com- 



