PLANT DISEASES 



277 



years. Losses of ten millions of dollars in one year in 

 the eastern states of the United States have been known. 



Where the conditions are 



>\ 



favorable for the disease, it 

 travels with great rapidity 

 and may destroy a whole 

 field completely in twenty- 

 four to forty-eight hours. 

 This being another type of 

 plant disease, it is com- 

 bated by altogether differ- 

 ent means from those used 

 against smuts of cereals. 

 It can be almost entirely 

 prevented by foresight and 

 the application of methods 

 based on the life story of 

 the fungus. 



The following life history 

 will illustrate : The disease 

 may live through the win- 

 ter in the potato tubers, FIG. 142. Early Blight of Potato. 



DOSSlblv also in the Vines The life story is somewhat different fromkter 

 \ blight, but the treatment is similar. 



Along in July the threads 



work their way into the leaves and here grow very 

 rapidly, killing the tissues of the potato plant. The 

 threads increase in number and soon send branches 

 out through the pores in the surface of the leaf, 

 and these special threads pinch off spores. From 

 this exposed position on the surface of the leaf the 

 spores are blown by the wind and may fall on other 

 leaves. Potato blight can spread rapidly only in damp 

 or wet weather, because these spores falling on the 



