32 



This disease, like the black-rot and many others, develops most 

 rapidly and does most injury during hot, wet weather. 



Treatment. 



It is desirable to destroy as many as possible of the diseased leaves, 

 shoots, and berries, which may contain the winter spores. Thorough 

 spraying, as recommended for the black-rot, will effectually control 

 this disease. 



POWDERY MILDEW. 



The powdery mildew {Uncinula necator (Schw.) Burr.) rarely 

 causes great loss to American varieties of grapes. It is most severe 

 on the European, or vinifera, grapes. 

 This mildew belongs to a group of fungi 

 quite different from the downy mildew. 



Fig. 13.— The fungus causing downy mildew: a. Fertile 

 filaments of the downy mildew fungus, showing the 

 manner in which the summer spores are borne; b, two 

 summer spores; c, a winter, or resting, spore. (All 

 highly magnified.) 



Fig. 14. — A bunch of young grapes par- 

 tially destroyed by "gray-rot." This 

 is a form of the downy mOdew afl'ecting 

 the very young fruit. 



It differs from all other parasites wliich attack the grape in its 

 superficial habit of growth. The filaments of the fungus do not 

 invade the tissues of the plant to destroy them. The parasite obtains 

 its nutriment by means of sucker-like organs which penetrate the 

 cell walls of the surface layer of tissue only. The fine, white filaments 

 of the fungus, which constitute the vegetative portion of the parasite, 



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