258 DISEASES OF ECONOMIC PLANTS 



it may grow into the seed within the pod. Seeds badly 

 diseased in this manner adhere to the pod; if less badly 

 diseased, they thresh out, are shrunken, and show, accord- 

 ing to the color of the variety, more or less discolored spots. 

 Often seeds are diseased, and yet so slightly disfigured as 

 to escape observation. 



Trial has demonstrated that of badly infested seeds only 

 6 per cent are capable of germination, and that even when 

 they do germinate, the resulting plants seldom reach ma- 

 turity. On young plants the disease often assumes the 

 character of damping off. 



Selby ' makes the following notes as to varietal resist- 

 ance to this disease. 



Very badly affected: French June, Market Garden, 

 Dwarf Telephone, American Wonder. 



Badly affected : Admiral, Telephone, Prosperity, Advancer. 



Slightly affected : Alaska. 



Aside from the pea this disease is said to attack alfalfa, 

 bean, and hairy vetch. The disease was a grave menace 

 in Europe in 1894, in Canada in 1903, in Ohio in 1904. 

 In New York, in 1906, from 50 to 80 per cent of the crop 

 was injured. 



Two years' rotation in nonsusceptible crops lessens the 

 evil very much. Diseased seed should be avoided. 



Powdery mildew (Erysiphe polygoni DC). — This mildew 

 presents the usual character of the powdery mildew as 

 described for grape — a whitish, powdery coating upon the 

 plant surfaces. It develops most conspicuously late in 

 the season, and sometimes becomes so injurious that the 

 plants do not mature seed, though it is not usually very 



1 Selby, A. D., Ohio Agr. Exp. Sta. Bui. 173. 



