MARKET DISEASES OF FRUITS AND VEGETABLES 39 



POWDERY MILDEW 



(Erysiphe polygoni DC.) 



Powdery mildew is often seen on the market in a mild form, but 

 the seriously affected pods of peas and beans generally are discarded 

 by the graders and packers at the shipping point. There is an in- 

 direct effect, however, in that pods not directly infected but from 

 diseased plants may be stunted and of poor quality. Occasionally 

 peas and beans get on the market, even though they are actually 

 severely blemished. Such pods have irregular brown lacelike super- 

 ficial spots and blotches or small areas covered with white powdery 

 spores (pi. 11, D). In the field the disease generally does not 

 become serious, unless an attempt is made to grow peas or beans 

 during late summer. 



The fungus itself, like other powdery mildews, furnishes a positive 

 diagnostic character. It grows on the surface of the affected tissues 

 and forms a dense persistent mycelium, which may cover the leaves, 

 stems, and pods. This mycelium produces in profusion the white 

 spores (conidia) that give the fungus its powdery appearance. 

 Badly affected organs look as though they were dusted with white 

 powder. The plant tissues so affected cannot carry on normal photo- 

 synthesis, and as a result the plants are stunted, develop abnormally, 

 and may even die prematurely. Often in peas and beans no fruit 

 is set. The causal organism is largely superficial in growth, but 

 it sends large feeding organs into the epidermal cells which cause 

 brownish discoloration and death of small areas under the fungus 

 pads. 



Late in the season black spherical fruiting bodies (perithecia) 

 are produced from the mycelium. These bear the spores that serve 

 as primary inoculum in the spring. 



When attacks by powdery mildew are threatened, it is advisable 

 to dust or spray with sulfur. Variations in susceptibility to powdery 

 mildew are shown by commercial varieties of beans and peas. This 

 suggests the possibility of obtaining resistant varieties by selection 

 and breeding. 



(See 80, 89, 101, 129. 148.) 



SCAB 



(Cladosporium pisicolum Snyder) 



Peas from California and Washington sometimes show rather 

 serious blemishes caused by Cladosporium. Dark-brown or black, 

 irregular, raised, scablike spots sometimes occur in great numbers 

 over the surface of the pods (pi. 10, E). Usually the spots do not 

 penetrate the pod wall, but in some instances they may do so, in 

 which case the seeds may be discolored. Seed infections appear as 

 small blisters or brown and black spots. In advanced stages of seed 

 infection the lesions may become dark, sunken, and covered with 

 greenish powdery spores. The inside wall of the pods underneath 

 these scab lesions usually shows white felty or hairlike proliferations. 



In the field irregular discolored spots may be observed on the 

 leaves, and under humid conditions a velvety growth of mold with 

 smoke-colored spores is evident. The spots may eventually turn 



