Downy mildew is caused by a fungus that can infect berries, leaves and young shoots. It occurs wherever it is wet and warm during the growing season. There is some variety resistance, with V. vinifera varieties being the most susceptible and V. rotundifolia being the most resistant.
The fungus attacks all green parts of the vine, especially the leaves. Lesions on leaves are angular, yellowish, sometimes oily, and are located between the veins. As the disease progresses, a white cottony growth can be observed on the lower leaf surface. Severely infected leaves will drop. If enough defoliation occurs, the overwintering buds will be more susceptible to winter injury. Infected shoot tips become thick, curl, and eventually turn brown and die. Young berries are highly susceptible, appearing grayish when infected. Berries become less susceptible when mature. Infected berries remain firm compared to healthy berries, which soften as they ripen. Infected berries will eventually drop.
The disease is caused by the fungus Plasmopara viticola, which overwinters as dormant spores within infected leaves on the vineyard floor which become active in the spring. 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 cause infection. Water is necessary for the spores to swim and to infect, so outbreaks of the disease coincide with periods of wet weather. Downy mildew is favored by all factors that increase the moisture content of soil, air, and the plant, with rainfall being the principal factor for infection. The frequency of rain and the duration of wet periods correlate with the number of additional infections during the growing season. Downy mildew infection can become a severe problem when a wet winter is followed by a wet spring and a warm summer with a lot of rainfall.
Some control can be achieved by preventative management practices. Spring cultivation to bury fallen, infected leaves from the previous year may help reduce early season disease pressure. Pruning out the ends of infected shoots and practices that improve air circulation and speed drying within the vine canopy will also help to control downy mildew. Fungicides, however, are the most important control measure, especially on susceptible varieties. They should be applied just before bloom, 7 to 10 days later (usually at the end of bloom), 10 to 14 days after that, and, finally, 3 weeks after the third application. For varieties 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.