Note 108 — Chapter III · THE VITICULTURE OF NEW YORK

Powdery mildew (Uncinula necator (Schw.) Burr.) is caused by a fungus which lives on the surface of the leaves. It subsists by means of sucker-like organs which penetrate the walls of the surface layer of cells. The vegetative portion of the parasite consists of fine white filaments which spread over the surface of the leaves, shoots and fruit. In the summer these filaments send up short, irregular stalks upon which large numbers of barrel-shaped spores are produced in chain-like arrangement. These are the summer spores of the fungus. They are borne in greatest quantity on the upper surfaces of the leaves and give the leaf a gray, powdery appearance—hence the name, powdery mildew. Affected leaves finally become light brown and often fall. Diseased fruits are gray in color, scurfy, become specked with brown, fail to develop and often burst on one side thereby showing the seeds. The winter or resting spores are borne in sacs, in the latter part of the season. The spore sacs, in their turn, are borne in small, black, spherical spore cases, each furnished with a number of slender appendages having curled tips. The powdery mildew, unlike most other fungus troubles of the grape, is most prevalent in hot dry weather. The disease is combatted by dusting with flowers of sulphur or by spraying with bordeaux mixture as for black-rot.