326 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



For this reason, vines on walls or trellises are more liable to attack, 

 while vines out in the open vineyard, especially where they are sub- 

 jected to the full action of the wind, are less severely affected. 



Though the fungus requires less moisture than most others causing 

 diseases of the vine, it will not grow in an atmosphere that is too dry. 

 In the drier regions of California it is much less frequent than along 

 the coast. Vines in low places or near the banks of rivers or irrigation 

 canals are often attacked when the rest of the vineyard is free. Rains 

 or fogs in the spring or early summer are very favorable to the growth 

 of Oidium, especially if they are accompanied by a warm temperature. 



The fungus will grow at a wide range of temperature. Below 50 

 F., however, its growth is arrested and for 15 or 20 degrees above this 

 its growth is slow. Above 75 F., its growth is rapid and reaches its 

 maximum at about 90 F. or 95 F., providing the air remains suffi- 

 ciently moist. At about 100 F. it ceases to grow and at few degrees 

 above this it is killed. 



These considerations explain the different ways in which the fungus 

 attacks vines in different localities and in different seasons. 



In the coast regions, during the spring, the Oidium attacks most 

 severely the outer leaves and the ends of shoots on the south and east 

 sides of the vine. The parts of the vine which are shaded do not be- 

 come sufficiently warm for the growth of the fungus. As the season 

 progresses and the temperature of the air rises, the west and north 

 sides of the vine become affected and finally, by midsummer, the fun- 

 gus is able to grow on all parts of the vine and the bunches and bases 

 of the canes are attacked. Very near the coast all parts of the vine are 

 liable to attack throughout the summer, but in the main coast valleys, 

 the fungus is usually confined to the interior of the vine after the 

 middle or end of June. 



In the interior valleys, it is very unusual to find the outer leaves of 

 the vines attacked at any season. Vines which are much shaded by 

 neighboring trees or buildings, however, may be attacked in the late 

 spring in a way similar to that which occurs nearer the coast. Usually 

 the fungus is first seen at or just before blossoming time and its attacks 

 are confined to the canes, flower bunches, and interior leaves. This 

 early attack sometimes causes a great loss of crop even when to a cur- 

 sory examination the vine appears perfectly healthy. 



During the summer, the dry air and great heat prevent the devel- 

 opment of the fungus, and in fact destroy the greater part of it. The 

 crop, however, may be seriously injured before sufficiently hot weather 

 arrives. Most varieties of grapes in the hotter parts of the Sacra- 



