PLANT DISEASES 291 



stable manure. Other well-known diseases which, like cotton 

 wilt, may live in the soil are flax wilt, the root rot of tobacco, 

 and the dry rot of the Irish potato. The most practical method 

 of control known for soil-infecting diseases is crop rotation. On 

 badly infected soil, it is sometimes necessary to discontinue a 

 certain crop for a long period, in some cases permanently. 



FIG. 139. Apple blotch 



The leaf has many spore cups showing as reddish spots ; two apples show the char- 

 acteristic blotches ; the twig shows two cankers due to this disease ; the greatly magnified 

 spores at the right show how the spores germinate. (Photographs by D. E. Lewis) 



380. Diseases wholly or in part controlled by spraying. The 



late potato blight is caused by a fungus which attacks the leaves, 

 usually after the blossoming period. This fungus first appears 

 on the foliage and produces an abundance of spores. Some 

 of these spores are scattered by the wind to other plants and 

 some are washed by rain into the soil, where they infect the 

 tubers and produce a dry rot. Some of the diseased tubers when 

 planted the next season may produce diseased plants and an 



