Downy Mildew

Downy mildew is a major disease of grapes throughout the eastern United States. The fungus (Plasmopara viticola) causes direct yield losses by rotting inflorescences, clusters (Figure 74) and shoots. Indirect losses can result from premature defoliation. Premature defoliation is a serious problem because it predisposes the vine to winter injury. It may take a vineyard several years to fully recover after severe winter injury. In general, vinifera (Vitis vinifera) varieties are much more susceptible than American types and the French hybrids are somewhat intermediate in susceptibility (Table 6).

Figure 74: Downy mildew fruit infection.

 

Downy Mildew Disease Cycle (Figure 75)

The causal fungus overwinters as tiny oospores in leaf debris on the vineyard floor. In the spring, the oospores serve as primary inoculum and germinate in water to form sporangia. The sporangia liberate small swimming spores, called zoospores, when free water is present. The zoospores are disseminated by rain splash to grape tissues where they swim to the vicinity of stomata and encyst. Stomata are tiny pores through which the plants exchange air and transpiration occurs. Stomata are concentrated on the underside of the leaves. Encysted zoospores infect grape tissues by forming germ tubes that enter stomata and from there invade inner tissues of the plant. The fungus can infect all green, actively growing parts of the vine that have mature, functional stomata.

Figure 75: Downy mildew disease cycle. We wish to thank the New York State Agriculture Experiment Station for use of this figure. Figure taken from Grape IPM Disease Identification Sheet No. 1.

Infected leaves develop yellowish-green lesions on their upper surfaces (Figure 76) 7 to 12 days after infection. As lesions expand, the affected areas turn brown, necrotic or mottled. At night, during periods of high humidity and temperatures above 55°F, the fungus sporulates by forming sporangia on numerous branched structures, called sporangiophores, that protrude out through stomata on the undersides of the leaf. Sporulation only occurs on plant surfaces that contain stomata, such as the underside of leaves, and it gives the surface of the lesion its white, downy appearance, which is characteristic of the disease (Figure 77). Sporangia are disseminated by wind or rain splash. On susceptible tissue they liberate zoospores into water films formed by rain or dew. These zoospores initiate secondary infections which can occur in as little as two hours of wetting at 77°F or up to 9 hours at 43°F. Infections are usually visible as lesions in about 7 to 12 days, depending on temperature and humidity. The number of secondary infection cycles depends on the frequency of suitable wetting periods that occur during the growing season and the presence of susceptible grape tissue. In general, Catawba, Chancellor, Chardonnay, Delaware, Fredonia, Ives, Niagara, White Riesling, and Rougeon are highly susceptible cultivars (Table 6).

Figure 76: Downy mildew on upper leaf surface.

 

Figure 77: Downy mildew on lower leaf surface.

Severely infected leaves may curl and drop from the vine. The disease attacks older leaves in late summer and autumn, producing a mosaic of small, angular, yellow to red-brown spots on the upper leaf surface. Lesions commonly form along leaf veins, and the fungus sporulates in these areas on the lower leaf surface. When young shoots, petioles, tendrils, or cluster stems are infected, they frequently become distorted, thickened, or curled. White, downy sporulation can be abundant on the surface of infected areas. Eventually, severely infected portions of the vine wither and die. Infected green fruit turn light brown to purple, shrivel, and detach easily. White, cottony sporulation is abundant on these berries during humid weather. The fruits remain susceptible as long as stomata on their surfaces are functional. After that, new infections and sporulation do not develop, but the fungus continues to grow into healthy berry tissue from previously infected areas. Later in the season, infected berries turn dull green to reddish purple, remain firm, and are easily distinguished from non-infected ripening berries in a cluster. Infected berries are easily detached from their pedicels leaving a dry stem scar.

Throughout most of the Midwest, downy mildew symptoms often do not appear until after bloom. This is why we often refer to it as a "Late season" disease. The role of oospores in causing early season primary infections is not clearly defined. Although we emphasize the use of fungicides for downy mildew control after bloom, early season fungicide applications can be very important. Especially on highly susceptible cultivars, the early season fungicide program should contain a fungicide that has efficacy against downy mildew. Copper fungicides are highly effective for controlling downy mildew; however, repeated applications of copper can result in some vine damage. Sulfur is not effective. The selection on use of less susceptible varieties is very important in organic production (Table 6).