72 DISEASES OF ECONOMIC PLANTS 



24) vary in size according to their age, persisting and enlarg- 

 ing year after year. Young cankers are slightly rough, owing 

 to the dead bark, and extend only partly around the twigs. 

 Old cankers are very rough, and may extend several centi- 

 meters longitudinally along the twig, thus giving it a flat- 

 tened appearance. 



The fungus causing the rot upon the fruit was described 

 in 1856 by Berkeley. That the cankers are caused by the 

 same fungus was recognized independently by two research 

 parties, BurriU and Blair, and von Schrenk and Spaulding 

 in 1902. Upon both twigs and fruit the spores are borne 

 in great numbers. The fungus is, moreover, perennial 

 upon the twigs, the cankers serving as initial points of in- 

 fection for each year's epidemic. 



The natural course of the disease is thorough twig infec- 

 tion ; spores from some older lesion are transferred to 

 points of weakness on the twig, such as ruptured bark due 

 to any cause. The fungus here grows rapidly, produces 

 a canker, and forms many spores. These in turn, carried 

 chiefly by rain, partly by insects, wind, or other agencies, 

 reach other susceptible twigs and cause other cankers or 

 fall upon apples, and there initiate spots of rot. Apples 

 thus infected serve as multiplying ground for the further 

 infection of other apples, thus giving rise to the chief part 

 of the fruit rot. Six days after an apple is infected a crop 

 of spores may mature and be ready to further spread the 

 disease. To some extent the causal fungus hibernates in 

 mummified fruits, which thus become sources of initial 

 infection for the succeeding year. 



Bitter rot occurs throughout the United States wherever 

 apples grow, but has been most destructive heretofore in 



