OIDIUM OR POWDERY MILDEW OF THE VINE. 317 



immunity to Peronospora, Anthracnose, and Black Rot, and while 

 Oidium requires less moisture than these diseases for its development, 

 it spreads more rapidly and is more difficult to control in a moist at- 

 mosphere than in a dry one. 



At the same time, the Oidium does far more harm, even in Cali- 

 fornia, than is usually suspected, and in certain regions and certain 

 seasons may totally destroy the crop and diminish the crops of follow- 

 ing years by injuring the vine. 



There has been a recrudescence of the intensity of this disease both 

 in Europe and California during the last few years. This is due in all 

 probability not to any change in the fungus or of the seasons, as has 

 been suggested, but to the neglect of regular sulfuring by the grape 

 growers, who have become careless after years of comparative immu- 

 nity due to more or less general treatment for the disease. 



While most grape growers in California sulfur their vines in some 

 way, many of them do not succeed in completely controlling the disease 

 and are often doubtful of the utility of the treatment. Where good 

 sulfur does not give immunity it is because it has not been applied in 

 the right way or at the right time. Many growers who do succeed in 

 keeping their vines free from Oidium do so at the expense of far more 

 sulfur and labor than are necessary. 



The main object of this bulletin, therefore, is to point out the meth- 

 ods of control which have been generally found the best and least 

 costly. 



DESCEIPTION OF THE DISEASE. 



Relationships of tJie Disease. This disease, usually called Mildew 

 in California, is caused by a fungus belonging to the same group as the 

 fungi which cause the mildews of roses, hops, beans, and apples and is 

 generally referred to as Oidium in Europe. The disease known as 

 Mildew or Peronospora in Europe and the East is totally different and 

 caused by a fungus of very different character. In order to distinguish 

 the two, the Oidium is sometimes called Powdery Mildew and the 

 Peronospora, Downy Mildew. As only one exists in California, the 

 use of the word Mildew should lead to no confusion, though perhaps it 

 is best to adopt Oidium as the name of this disease. 



Symptoms and Effect on the Vine. The Oidium attacks all her- 

 baceous parts of the vine, leaves, canes, flowers, and fruit. In the 

 spring, the young leaves which are attacked show at first whitish 

 patches about one-quarter of an inch in diameter on the upper or lower 

 surfaces. (See fig. 2.) 



