54 Farmers’ Bulletin 1220. 
POWDERY MILDEW. 
The powdery mildew *' rarely causes great loss to American varie- 
ties 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. It differs from all other parasites which attack the 
erape in its superficial habit of growth. The parasite obtains its 
nutriment by means of suckerlike 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 para- 
site, spread over the surface of the leaves, shoots, and. fruit, and 
send up short, irregular branches upon which immense numbers of 
summer spores are produced in short chains (fig. 60, a). These are 
3 
i \ 2 = 
i \ — 
; \ \ 
Se : 
i \X @ 
AN 
3 i > 
J tr 9) 
Fic. 60.—The fungus causing powdery mildew: a, A fertile filament of the 
fungus bearing a chain of summer spores; 0, a spore case, in which the =A 
winter or resting spores are produced; c, a single sac containing winter 
spores; d, a single winter spore. (All highly magnified.) 
most noticeable 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 is severe the leaves 
fall. The fungus produces a similar appearance upon the young 
shoots. Berries which are attacked take on a gray, scurfy appear- 
ance, become specked with brown, and fail to mature properly. 
Affected grapes when nearly half grown sometimes burst open on 
one side, exposing the seeds. The fruit does not become softened and 
shrunkenas 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. These are borne in sacs which 
are inclosed in minute, black, globose fruiting bodies furnished with 
slender appendages curled at their tips (fig. 60, 6,¢c,d). These black 
1 Caused by Uncinula necator (Schw.) Burr. 
