12 MISC. PUBLICATION 3 40, U. S. DEPT. OF AGEICULTUEE 



sively on grapes from California. European or vinifera varieties are 

 much more susceptible than those native to the United States. The 

 most susceptible of the commercially important varieties are the Flame 

 Tokay, Carignane, Alexandria (Muscat), Olivette de Vendemain, and 

 Petite Syrah. The more resistant commercial varieties are Alicante 

 Bouschet, Petit Bouschet, and Mataro. However, all vinifera varie- 

 ties may be injured severely if weather conditions are favorable for the 

 growth of the fungus and no effort is made to control the disease (18). 



Powdery mildew is a fungus that attacks all green parts of the vine, 

 appearing as a white powdery growth consisting mainly of spores 

 mixed with or lying upon the delicate threads of the fungus. It may 

 cause any or all of the following: (1) Spotting and discoloration of all 

 parts affected, (2) dwarfing, cracking, and shelling of the fruit, and (3) 

 dwarfing of both canes and leaves. The discoloration on the berries 

 is a light-brown, lacy russeting which is easily distinguishable from 

 the solid brown patches produced by sunburn and from the small 

 black or purplish-brown spots characteristic of black measles (pi. 10, 

 B). On the stems of the bunches, mildew spots are gray at first, 

 then turn brown, and in the latter condition are often called dead 

 mildew by receivers on the market. From the central brown patch 

 on the stems, fine, brown, weblike lines extend in a more or less radi- 

 ating pattern, eventually forming spots an inch long or even longer, 

 since several infections may run together. Stems of normal healthy 

 bunches when picked and held for some time turn brown, but the 

 general opinion among growers and shippers is that mildewed areas 

 under like circumstances turn to a much darker brown. The claim 

 is also made that mildewed stems are more brittle than normal, 

 healthy stems. So far as known, however, there is no scientific 

 evidence to support this contention. 



A distinct growth of white mold in a bunch of grapes in the packing 

 house or on the market is a sign not of mildew but of one or more of 

 the three common rot fungi (blue mold, gray mold, and rhizopus) 

 which probably entered through cracks in the skin or as secondary 

 infection following mildew injury. Diagnosis of mildew without the 

 aid of a microscope must depend on the presence or absence of the 

 brown weblike or lacy russeting and the white mealy growth. 



Claims have been made at various times that mildew leads to decay 

 in transit. It is known to occur on the stems of the bunches, and 

 while sulphuring in the vineyard (in California) and careful culling at 

 the packing house do much to keep mildew out of the packed fruit, 

 there is always a possibility that in a mildew season a small percentage 

 of bunches carrying mildew on the steins may go into the pack. How- 

 ever, even under such conditions there probably is none of the fungus 

 growth on the berries at that time. There is no evidence that the 

 mildew develops further after the fruit leaves the vineyard, or that it 

 predisposes the fruit to decay except possibly by weakening the skin 

 so that cracks are more common and decay fungi find easier entrance. 

 Mildew russeting as seen on the market is usually mild, and such 

 cracks as can be found are very small and have healed over. 



CAUSAL FACTORS 



The fungus Uncinula necator] known as powdery mildew, produces 

 two kinds of spores: (1) Summer spores (conidia) which are borne on 

 numerous short branches of the mycelium and which as they lie in 



