Infection on leaves appears conspicuously on the upper surface as white, powdery patches. Later, the entire leaf surface may be covered with the fungus. Severe leaf infection can result in cupping of leaves. Diseased leaves will scorch or turn brown and fall. Affected berries have a dull, darkened appearance and are usually covered with the light powdery growth of the fungus. In some varieties new fruit infection will cease as the sugar content rises and the fruit matures. When grape cluster stems become covered with the fungus, the stems shrivel and considerable losses from shelling of the fruit may occur. Black specks of the overwintering spore-forming bodies can be seen in the older affected areas. Cluster infections around bloom may lead to poor fruit set, while slightly later infection can cause berry splitting. Fruit infection may also reduce wine quality on varieties intended for that use.
The powdery mildew fungus overwinters inside dormant buds and on the bark of the vine as tiny, black, fruiting bodies (cleistothecia). Spores (ascospores) contained in the cleistothecia are released during rains from bud break until shortly after bloom. They are wind-dispersed to emerging leaves and clusters, and can infect wet or dry tissue at temperatures of 50 degrees F or higher. Infection usually starts soon after the blossom period and will continue on the foliage through the growing season. The fungus develops in dry conditions when the relative humidity is high. Mildew colonies produce masses of white, powdery secondary spores (conidia). Conidia are wind-dispersed throughout the vineyard and do not require rain for release or infection. New colonies that result from these secondary infections produce additional conidia, which can continue to spread the disease. This repeating cycle of infection, spore production, spore dispersal, and reinfection can continue to occur and reoccur in as little as 5 to 7 days. Thus, powdery mildew epidemics can suddenly explode when temperatures are favorable, unless they are managed efficiently.
Recent studies have demonstrated that berries are highly susceptible to infection from the immediate prebloom stage until about 2 weeks after fruit set. Berries of Concord become almost completely resistant to infection after this time. Concord rachises remain susceptible until harvest, but the economic importance of mid- or late-summer rachis infections on processing fruit is questionable. Extensive splitting of berries and severe fruit damage are almost always the result of infections that occurred during the immediate prebloom through fruit set period when berries are most susceptible.
Research has shown that in years when multiple rain events (four or more) have occurred during the period of maximum fruit susceptibility, powdery mildew has been a particularly serious problem regionwide. Thus, for best results, management programs should be at their peak from prebloom through fruit set, especially if weather is wet. Leaf infections that occur beyond the fruit set period are much less serious on Concord and similar cultivars than on V. vinifera and susceptible hybrids. Such infections do not appear to affect Concord crop yield or quality. On V. vinifera and highly susceptible hybrid cultivars, continued suppression of foliar mildew generally is required at least until veraison to avoid early defoliation.
Good management of late-season leaf infection also reduces disease pressure the following year by limiting the number of fungal fruiting structures that overwinter and initiate infection in the spring. For effective management of powdery mildew, sprays may be required as early as 1 to 2 inches of shoot growth on V. vinifera cultivars (depending on rain and temperature) and should not be delayed beyond the immediate prebloom stage on any cultivar. High disease levels the previous year increase early season disease pressure and consequently the importance of early season sprays. Prebloom through fruit set is the most critical period for managing fruit infections on all cultivars, and programs should be especially strenuous if weather is wet during this time. Continued protection through veraison may be beneficial for attaining maximum fruit quality on V. vinifera and susceptible hybrid cultivars and is generally required to prevent premature defoliation.
Cultural practices can reduce disease severity. Plant in sites with good air circulation and sun exposure. Use a training system that allows good air movement through the canopy and prevents excess shading.