| 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.
Life Cycle
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. |
