246 MANUAL OF FRUIT DISEASES 
seminated. They have been found as late as the month 
of October. 
In the late summer and autumn perithecia begin to appear 
among the hyphe. At first they are yellowish, but they soon 
turn black. They are numerous, more than one hundred thou- 
sarid having been counted on the upper surface of a single leaf. 
Their further history has been discussed. In certain parts of 
California they are found in June. Their formation seems to 
depend upon a peculiar set of weather conditions: if the 
temperature suddenly falls to about 50° Fahr. just after a 
period of warm, moist weather, they are produced in great 
numbers. In other parts of the country where these condi- 
tions do not prevail in early summer they are more scarce. 
The fungus is favored by sheltered, shaded conditions. 
Thus vines in the open are less likely to show powdery-mildew. 
While the fungus requires less moisture than most fungi, it 
will not grow in an exceptionally dry atmosphere. This is 
noticeable in California, mildew being far more abundant along 
the coast than in the drier regions. Vines in low places or along 
streams are often diseased, whereas the remainder of the vine- 
yard is free from powdery-mildew. Rains or fogs, in the spring 
or early summer, accompanied by warm weather are highly 
favorable to the organism. It grows between 50° and 95° Fahr., 
but does so rather slowly except between 75° and 90° Fahr. It 
ceases growth below 50° and above 100° Fahr. These char- 
acteristics of the fungus result in its somewhat erratic appear- 
ance in different seasons and in different localities. 
Control. 
In handling powdery-mildew the following points should be 
taken into account: 
(1) The causal fungus may possibly live for one to one and 
one-half years in the soil. If the perithecia are buried, they are 
only preserved until plowed up again a year later. 
(2) The fungus is favored by moisture in spite of the fact that 
‘ 
