How to Manage Pests

Pests in Gardens and Landscapes

Downy mildew—Plasmopara viticola

Identification

Downy mildew causes yellowish and oily or angular, yellow to reddish and brown lesions on leaves. Lesions are limited by the veins. Sporulation of the fungus appears as dense, white, cottony growth in the lesions. Infected shoot tips and petioles thicken and curl and eventually turn brown and die. Young berries appear grayish.

Life cycle

Grape downy mildew occurs mainly in regions where it is warm and wet during the vegetative growth of the vine. Limited rainfall in spring and summer generally reduces the spread of the disease in California. In most regions the fungus survives the winter mainly in fallen leaves. The fungus may also survive in buds and shoot tips. The pathogen is dispersed by splashing rain and wind. The infection process can take less than 90 minutes. Infection generally occurs in the morning and the incubation period is about 4 days. Downy mildew is favored by all factors that increase the moisture content of soil, air, and host plant. Epidemics are promoted principally by rain and irrigation practices.

Solutions

The best way to control downy mildew is by prevention. Preventative management consists of effective soil drainage and reduction of sources of overwintering inoculum, such as pruning out the infected ends of shoots. Extend the interval between sprinkler irrigations as long as possible.

Downy mildew lesions on grape leaves border=
Downy mildew lesions on grape leaves


Statewide IPM Program, Agriculture and Natural Resources, University of California
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