33 



spread over the surface of the leaves, shoots, and fruit, and send up 

 short, hregular branches upon wliich immense numbers of summer 

 spores are produced in short chains (fig. 15, a) . These are most notice- 

 able upon the upper surface of the leaf, giving it a fine gray, powdery, 

 or mealy appearance. Finally the affected part of the leaf becomes 

 light brown, and if the disease be severe the leaves fall. The fimgus 

 produces a similar appearance upon the young shoots. Berries wliich 

 are attacked take on a gray, scurfy appearance, become specked with 

 l)rown, and fail to develop further. Affected grapes when nearly 

 half grown sometimes burst open on one side, exposing the seeds. 

 The fruit does not become softened and shrunken as when attacked 

 by the downy mildew. 



Besides the summer spores, winter, or resting, spores are also pro- 

 duced in the latter part of the season. Tliese are borne in sacs which 



Fig. 15. — The fungus causing powdery mildew {Uncinula necator): a, A fertile filament of the fungus 

 bearing a chain of summer spores; b, a spore case, in which the winter or resting spores are pro- 

 duced; c, a single sac containing winter spores; d, a single winter spore. (All highly magnified.) 



are inclosed in minute black, globose fruiting bodies furnished with 

 slender appendages curled at their tips (fig. 15, h, c, d). These black 

 spore cases are so small that they can scarcely be seen with the naked 

 eye, but by the aid of a hand lens they can be easily observed. The 

 powdery mildew is usually more prevalent during dry, hot seasons 

 than in wet ones. It differs in this respect from most of the other 

 grape diseases. In California tins is the principal fungous disease 

 of the grape. 



Treatment. 



Bordeaux mixture, as recommended for the black-rot, will prevent 

 this disease. Wliere this trouble alone is to be combated it may 

 be successfully done by dusting with flowers of sulphur. East of 

 the Rocky Mountains, however, it should be treated with Bordeaux 

 mixture, as it is rarely likely to occur alone. 



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