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 hyphse. At first they are yellowish, but they soon 

 turn black. They are numerous, more than one hundred thou- 

 sand 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 



