34 Diseases of Economic Plants 



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 through twig infec- 

 tion ; spores from some older lesion are transferred to points 

 of weakness on the twig, such as bark ruptured from any 

 cause. The fungus may also develop and overwinter in old 

 cankers caused by other fungi, on dead twigs or scars on the 

 fruit spurs, or even in the crevices in the bark and at these 

 points produce spores the following year. These in turn, 

 carried chiefly by rain, partly by insects, possibly birds, or 

 other agencies, fall upon apples, and there initiate spots of 

 rot. Apples thus infected serve as multiplying ground for 

 the further infection of other apples. 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 in the succeeding year. 



Bitter-rot occurs throughout the United States wherever 

 apples grow, but has been most destructive heretofore in 

 sections southwest of Virginia between the parallels of 35° 

 and 39° north latitude. It is known also in comparatively 

 harmless form in Europe and Australia. In 1900 it was esti- 

 mated that the loss in four counties of Illinois was $1,500,000. 

 The president of the National Apple Shippers Association 

 estimated the damage to the apple crop of the United States 

 that year as $10,000,000. There is often a loss of from 50 to 

 75 per cent of the crop. 



Two modes of treatment are useful : first, to cut off and 

 burn the cankers in so far as is practicable, and to hand-pick 

 and destroy mummified fruit; second, to spray the growing 

 fruit with a fungicide. The trees should be sprayed fre- 

 quently with Bordeaux mixture, or lime-sulfur (summer 

 strength) from the time the petals fall until the fruit is almost 

 ripe. A non-spotting spray should be used for the last 

 application. Cf. p. 447. 



