How to Manage Pests
UC Pest Management Guidelines
Pathogen: Plasmopara viticola
(Reviewed 6/06, updated 2/14)
In this Guideline:
The fungus attacks all green parts of the vines, particularly the leaves. Depending on the incubation period and leaf age, lesions are yellowish and oily or angular, yellow to reddish and brown and limited by the veins. Sporulation of the fungus appears as a delicate, dense, white, cottony growth in the lesions. Infected shoot tips thicken, curl ("Shepherd's Crook") and become white with sporulation. They eventually turn brown and die. Similar symptoms are seen on petioles, tendrils and young inflorescences, which, if attacked early enough, ultimately turn brown, dry up and drop. The young berries are highly susceptible. They appear grayish when infected (gray rot) and become covered with a downy felt of fungus sporulation. Berries become less susceptible as they mature, but rachis infections can spread into older berries (brown rot, no sporulation). Infected berries of white cultivars may turn dull gray‑green, while those of black cultivars turn pinkish red. Infected berries remain firm, compared to ripening healthy berries, and drop easily. Portions of the rachis or the entire cluster also may drop.
COMMENTS ON THE DISEASE
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 limits the spread of the disease in California. Surviving inoculum may be present in California at low levels and initially may have been introduced on plant material from outside of California. In most regions the fungus survives the winter mainly as oospores in fallen leaves. However, in California's generally mild winters, survival of the fungus in buds, shoot tips, and persistent leaves may be more important than in other grape-growing regions.
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. Rain and irrigation practices are principal factors in promoting epidemics. The optimum temperature for development of the disease is 68° to 77°F (20° to 25°C) with extremes ranging from 50° to 86°F (10° to 29°C). In California the greatest potential for disease development exists when a wet winter is followed by late spring rains. The potential is high as well in the event of early fall rains.
Preventive management consists of effective soil drainage and reduction of sources of overwintering inoculum. In a vineyard that depends on sprinkler irrigation, extend the interval between irrigations as long as possible.
Fungicides for use against downy mildew can be categorized as either preventive or curative. The preventive fungicides (mancozeb and copper compounds) must be applied before an infection period begins. New growth following application will not be protected. Include a spreader/sticker agent to prevent the material from washing off with rain. In vineyards with a history of downy mildew, apply early season copper sprays as part of a preventive program, especially during wet springs.
UC IPM Pest Management Guidelines: Grape
W. D. Gubler, Plant Pathology, UC Davis