222 MANUAL OF AMERICAN GRAPE-GROWING 



does most injury in hot, wet weather. As with practically all 

 diseases of the grape, much can be accomplished in the way of 

 control of the disease by destroying infested leaves, shoots 

 and berries which contain the winter spores, but these sanitary 

 measures are not sufficiently effective and vineyards must be 

 sprayed as recommended for black-rot, except that the first 

 application should be made before the blossom-buds appear. 



Powdery-mildew. 



Less troublesome than downy-mildew in the East, powdery- 

 mildew (Uncinula necator), unless checked, is capable of destroy- 

 ing the entire crop of European grapes on the Pacific slope. 

 In the East it sometimes causes great loss on the several varieties 

 known as "Rogers hybrids" and, curiously enough, is often 

 a rather serious disease of the Concord. The disease is caused 

 by a superficial fungus which passes the winter on fallen leaves 

 and also on the canes. The spores begin to germinate a few 

 weeks after the grape blossoms, but the disease is not often 

 found until the grapes are nearly half grown. The fine white 

 filaments of the fungus, which constitute the vegetative por- 

 tion of the parasite, then attack the leaves, shoots and fruit, 

 sending up short irregular branches on which great numbers 

 of spores are borne. These give the upper surface of the leaf 

 a gray, powdery appearance, hence the name. Eventually 

 the diseased leaves become light brown and if the disease is 

 severe, soon fall. Infected berries take on a gray, scurfy 

 appearance, speckled with brown, are checked in growth and 

 often burst on one side, exposing the seeds. The berries, how- 

 ever, do not become soft and shrunken as when attacked by 

 the downy-mildew. The disease passes the winter in resting- 

 spores produced late in the growing season. Powdery-mildew 

 differs from other fungous diseases of the grape in being more 

 prevalent in hot, dry seasons than in cold, wet ones. 



In eastern America powdery-mildew is controlled by the 



