328 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



the active substance. Other investigators claim that the active fumes 

 are simply vapors of pure sulfur volatilized by the high temperature. 



The active fumes, whatever their nature, are given off in effective 

 quantities only after the temperature of the air reaches about 75 F. 

 Above this point they are given off the more actively the higher the 

 temperature. Below 75 F. the fungus grows very slowly and the 

 sulfur fumes are little needed, and, as the air warms up to a temper- 

 ature at which the fungus grows rapidly, the fumes are given off in 

 greater abundance. 



The lower the temperature the more slowly the fungus is affected. 

 If the highest shade temperature of the day is between 75 and 80 F., 

 it will require from seven to eight days to destroy all the mycelium. 

 When the temperature rises to 90-95 F., the fungus commences to 

 show signs of injury in 24 hours and in four or five days is destroyed. 

 If the thermometer rises to above 100 F. in the hottest part of the 

 day. one or two days are sufficient to completely rid the vineyard of 

 the disease. 



In order to obtain these perfect results, however, the sulfur must 

 be applied to every part of the vine. The nearer a spore or a piece of 

 mycelium is to a particle of sulfur, the more quickly it is killed. Ex- 

 cept in the hottest weather, the sulfur on one leaf is almost without 

 effect on the fungus on a neighboring leaf. The utility, indeed the 

 necessity, of a perfect distribution of the sulfur, especially in compar- 

 atively cool weather, is apparent. 



The air is always hottest near the soil, and sulfur placed on the soil 

 should give off more abundant fumes. Practice has shown, however, 

 that sulfur placed in this position has almost no effect on the Oidium 

 except in extremely hot weather. In such weather, sulfur is hardly 

 ever needed in California, as it is usually accompanied by such a dry 

 condition of the air that the fungus is killed without the aid of sulfur 

 fumes. 



In exceptional cases, as for instance where very vigorous vines are 

 growing in very moist places in the irrigated regions or when un- 

 usually hot weather occurs in the coast regions, the air may be moist 

 enough for the growth of Oidium even when the temperature rises to 

 considerably above 100 F. In such cases it is sometimes desirable to 

 sprinkle the sulfur on the ground instead of on the vines. The reason 

 of this is that if the air is sufficiently hot the sulfur may injure both 

 the fruit and the leaves of the vines. The fruit and leaves may be 

 spotted and, in severe cases, may fall completely or in part. Such 

 effects are to be feared, however, only when the temperature exceeds 



