||Downy mildew on upper (left) and lower (right) surface of grape leaves (Vitis)
Downy mildew is an extremely serious fungal disease of grapes that can result in severe crop loss. It is caused by the fungus Plasmopara viticola.
Symptoms and Diagnosis
The pathogen attacks all green parts of the vine, especially the leaves. Lesions on leaves are angular, yellowish, sometimes oily, and 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. Eventually, infected berries will drop.
The fungus overwinters in diseased leaves on the ground. Spores are released in the spring and spread to the leaves and berries by splashing rain and wind. The fungus has two types of spores, both germinating into swimming spores. These spores swim to the stomates (breathing pores) of plants and initiate infection. Water is necessary for the spores to swim and 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 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 frequent rainfall.
Integrated Pest Management Strategies
1. Maintain plant vigor. Make sure soils are welldrained. Fertilize according to soil test information obtained at least every other year.
2. Sanitation. Remove fallen leaves which are the source of overwintering inoculum.
3. Pruning. Prune out the ends of infected shoots.
4. Fungicides. Fungicides are an important control measure, especially on susceptible cultivars. 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 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 blossom opens. Pesticides registered for use include captan, copper, fosetyl-Al, mancozeb, maneb, and ziram.
5. Cultivars. Select and plant resistant cultivars.
‘Baco #1’, ‘Cascade’, ‘Chelois’, ‘Concord’, ‘Foch (Marechal Foch)’, ‘Himrod’, and ‘Steuben’ are the most resistant varieties.
Strategies 1, 2, 3, and 5 are strictly organic approaches. Of the fungicides listed in Strategy 4, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.