OIDIUM OR POWDERY MILDEW OF THE VINE. 325 



single leaf, and as each perithecium contains fifty or more spores, the 

 number produced on a single vine may be hundreds of millions. 



In Europe they are formed in September and October. In Berke- 

 ley they are found in abundance as early as the middle of June. Their 

 formation seems to depend on weather conditions. If after a period 

 of warm, moist weather, which has produced an abundant growth of 

 mycelium, the temperature suddenly falls to near the lowest limit for 

 the growth of the fungus (50 P.), they are produced rapidly and in 

 great numbers. These are weather conditions which often occur at 

 Berkeley and in the valleys nearest the coast. 



In the interior very different conditions exist. The fungus may 

 grow abundantly during the spring and early summer, but there is 

 seldom any fall of temperature at this season sufficient to cause the 

 production of the winter spores. As the summer advances, the air be- 

 comes too hot and dry for the fungus, so that by the time the cooler 

 weather of October arrives, most of the Oidium has disappeared and 

 the abundant growth of strong mycelium .which is necessary does not 

 exist. 



CONDITIONS FAVORING THE DISEASE. 



Variations in Susceptibility of Varieties. There is considerable 

 difference in the susceptibility of various varieties of vines and there- 

 fore in the ease with which they can be kept free from the disease. All 

 species of American vines, Labrusca, Riparia, Rupestris, etc., are 

 much less severely attacked than the European varieties of Vitis 

 vinifera. 



Among the latter there exist, however, various degrees of suscepti- 

 bility. Of the varieties cultivated in California, those most easily and 

 severely attacked are the Carignane, Flame Tokay, Muscat of Alex- 

 andria, Gutedel, Cabernet, Riesling, Clairette, Folle blanche, Ugni- 

 blanc, and Petite Sirah. Tlie more resistant are the Beclan, Duriff, 

 Malbec, Mataro, Grenache, Alicante Bouschet, Petit Bc/tischet, Ara- 

 mon, Mourastel, Vernaccia, and Marsanne. All varieties, however, are 

 capable of being severely injured if the weather conditions favor the 

 growth of the fungus. The Beclan and Duriff are perhaps the most 

 resistant of all mentioned and are for this reason very valuable vari- 

 eties to plant in localities within the influence of the summer ocean 

 fogs. 



Temperature and Moisture Conditions. The fungus requires cer- 

 tain degrees of temperature and moisture for its development. It 

 grows most vigorously and abundantly in sheltered shady positions. 



