Downy mildew occurs wherever it is warm and wet during the growing season. There is some cultivar resistance, with V. vinifera cultivars being the most susceptible and V. rotundifolia being the most resistant. This disease is caused by the fungus Plasmopara viticola and infects berries, leaves, and young shoots.

Symptoms

The pathogen attacks all green parts of the vine, especially the leaves. Leaf lesions appear as yellow or reddish-brown areas on the upper leaf surface with corresponding white, downy, or cottony fungal growth on the lower surface. Sometimes lesions are oily, somewhat angular, and are located between the veins. Eventually all lesions become brown and dead with age. Severely infected leaves often fall prematurely. If enough defoliation occurs, the overwintering buds will be more susceptible to winter injury.

Young, infected shoots and cluster stems may curl and are frequently covered with a white, sporulating, fungal mass. Berries on these clusters turn brown and eventually shrivel. Young berries are highly susceptible. Berries that are infected while very young may become entirely covered with a somewhat "fuzzy" white fungal growth that superficially resembles powdery mildew, particularly when weather is wet or in the morning while dew is still present. Cluster infections that occur later in the season cause berries to remain hard with a mottled light-green to red appearance. Berries become less susceptible when mature. Infected berries remain firm compared to healthy berries, which soften as they ripen. Eventually, infected berries will drop.

Downy Mildew Symptoms on Leaves

Disease Cycle

The downy mildew fungus overwinters as dormant spores in fallen leaves on the vineyard floor. The fungus first becomes active in the spring about 2 to 3 weeks before bloom. This fungus has two types of spores, both germinating to give rise to swimming spores. These spores swim to the stomates (breathing pores) of plants and infection takes place. Water is necessary for the spores to swim and to infect, so outbreaks of the disease coincide with periods of wet weather. Spores are then produced during rainy periods if temperatures are above 50 degrees F and are splashed onto susceptible tissues to cause the season's first (primary) infections. Inoculum for such early season infections comes from within the vineyard.

Epidemic disease development can then result from repeated cycles of secondary spread, which is caused by new spores that are produced within the white fungal growth on infected tissues. These spores are produced only at night when the relative humidity is greater than 95 percent. They can be blown relatively long distances and cause infection when they land on susceptible tissues that remain wet.

Disease spread is most severe during periods when humid nights with moderate to warm temperatures are followed the next day by rains or thundershowers, which allow spores to germinate and cause new infections. Frequency of rain and duration of wet periods correlate with the number of additional infections during the growing season. Downy mildew infection can also become a severe problem when a wet winter is followed by a wet spring and a warm summer with a lot of rainfall.

Downy Mildew Symptoms on Fruit

Disease Management

Downy mildew is favored by all factors that increase the moisture content of soil, air, and the plant. Rainfall and high humidity are the most important environmental factors promoting downy mildew epidemics. Some control can be achieved by preventative management practices such as making sure soils are well drained, reducing the sources of overwintering inoculum (fallen leaves), and pruning out the ends of infected shoots. Any practice that improves air circulation and speeds drying within vine canopies will help to control downy mildew. Spring cultivation to bury fallen, infected leaves from the previous year will also help to reduce early season disease pressure.

Properly timed fungicides, however, are still necessary for reliable disease management. Fungicides should be applied just before bloom. Mancozeb, captan, and copper fungicides provide good protection when applied at 10- to 14-day intervals. Ridomil is a systemic fungicide that provides excellent protection against downy mildew when applied at 14-day intervals, but it is not effective against any other diseases of grapes. Ridomil also has limited postinfection activity, but it is prone to resistance development and should not be overused. For cultivars very susceptible to downy mildew or where the disease was severe the previous season, an additional application is suggested about 2 weeks before the first blossoms open.

Downy mildew control programs should focus on (a) controlling primary infections in the prebloom and early postbloom periods and (b) limiting secondary spread during the summer. The timing and intensity of these sprays should be determined by varietal susceptibility, weather conditions, and the availability of fungal inoculum. Because primary infections can first occur 2 to 3 weeks before bloom, protection often needs to start at this time on all vinifera cultivars and on highly susceptible hybrid and native American cultivars (e.g., Chancellor, Niagara). This is particularly true if significant leaf or berry infection occurred the previous year and the weather is wet.

Protection should be provided to all but the most highly resistant cultivars from the immediate prebloom spray through the first or second postbloom spray, depending on the weather. Continued protection against disease spread during the summer should be based on the extent of favorable weather conditions and on the amount of disease already in the vineyard.