GRAPES AND OTHER SMALL FRUITS 13 



masses on the scant mycelium produce the white powdery appear- 

 ance already mentioned, and are the means by which the fungus is 

 spread during the growing season; and (2) winter or resting spores 

 (ascospores) produced in the fall in black bodies (perithecia), that are 

 barely visible to the naked eye. The spores produced in these peri- 

 thecia are the chief means by which the fungus is carried over the 

 winter, although in the hot, interior valleys of California they some- 

 times seem to be replaced in this function by the mycelium. 



The fungus is favored by shade and moisture, but requires less of 

 the latter than do most of the other fungi that cause grape diseases. 

 It does not grow well in a very dry atmosphere, however, hence it is 

 seen less commonly in the interior valleys of California than along the 

 coast. 



CONTROL MEASURES 



On vinifera varieties, the disease can usually be controlled by one 

 to six applications of sulphur dust, the number and time depending on 

 locality, weather, variety, and the exposure of the vine. The larger 

 number will be necessary with susceptible varieties in warm moist 

 seasons or in districts that have a moist climate. In dry seasons or 

 in districts where the climate is regularly dry, a smaller number of 

 sulphurings (sometimes only two) will be sufficient. Under condi- 

 tions most favorable to the growth of mildew, sulphuring should be 

 done (1) in the spring when the vine shoots are 6 to 8 inches long, 

 again when they are 12 to 18 inches long, and a third time when they 

 are 2 to 3 feet long; (2) in early summer; and (3) in late summer, 

 whenever mildew is found on any vine. 



Spraying with bordeaux mixture has been found to give satisfactory 

 control of the disease on American grape varieties in New York. Sul- 

 phur dust causes considerable injury to these varieties and should not 

 be used on them. This is not usually a serious matter, however, since, 

 as stated above, the disease is much less common on them than on 

 European varieties (8, 5, 7, 18, 15, 17, 18, 19). 7 8 



RAIN DAMAGE 



European or vinifera grapes in California differ from American vari- 

 eties grown there or throughout the East in being susceptible to more 

 or less serious damage by rain during the picking season. Practically 

 all varieties are affected, but those that suffer most are Alexandria 

 (Muscat), Sultanina (Thompson Seedless), and Gros Colman. The 

 varieties most commonly subjected to rains are the late ones, such as 

 Emperor and Ohanez, and while they do not show the skin cracking 

 that develops on the Sultanina and Alexandria (Muscat) varieties, 

 fungi gain entrance because of the rain, and secondary damage is done 

 that is sometimes serious. 



The injury consists of (1) loosening of the grape from the cap 

 stem; and (2) cracking on the side or at the stem end, the latter tak- 

 ing the form of partial or complete circles around the cap stem. Both 

 of these make it easier for decay fungi to enter, and in conjunction 

 with the greater abundance of such organisms because of the rain, 

 bring about a third sign of rain damage, namely, mold growth and 

 decay on the fruit even in the vineyard. However, such a condition 



7 Porter, B. A., and Demaree, J. B. See footnote3 . 



8 Duruz, W. P., and Owens, C. E. powdery mildew of grape and its control. Oreg. Agr. Expt. 

 Sta. Cire. Information 123, 4 pp. 1935. (Mimeographed.) 



