Downy mildew of grape—Plasmopara viticola
Downy mildew is a funguslike oomycete (water mold) that infects succulent green tissues when conditions are warm and wet.
Identification
The pathogen can infect all green parts of the vines, especially the leaves. The lesions it causes on foliage appear oily and are brown, reddish, or yellowish. Lesions are commonly angular because their spread is limited by veins.
Spores of Plasmopara viticola appear as cottony, dense, gray to white growths in the lesions. Infected shoot tips thicken, curl to resemble a shepherd's crook, and become covered with pale spores. Infected parts eventually turn brown and die. Petioles, tendrils and young inflorescences when infected while they are young turn brown, dry up, and drop.
Young berries are highly susceptible to downy mildew. Infected berries can appear grayish and become covered with spores. Berries become less susceptible as they mature, but when the branched stems in a cluster of grape fruit (rachis) become infected, disease can spread into older berries. Infected berries of white grape cultivars may turn dull grayish green, while those of black cultivars turn pinkish red. Infected berries drop easily and are more firm in comparison with ripening, healthy berries. Portions of an infected rachis or the entire cluster may drop.
The gray to white spores of downy mildew can be confused with those of powdery mildew. However downy mildew spores on leaves occur mostly on the under surface whereas powdery mildew spores can be common on both sides of leaves. Examining spores with a hand lens reliably distinguishes the pathogens. Most species of powdery mildew produce spores in chains. Downy mildew spores grow on branched, tree-shaped fruiting structures.
Life cycle
Downy mildew of grapes develops during late winter and spring when conditions are warm and wet during the vegetative growth of the vine. New disease can also develop after rain during early fall.
The funguslike oomycete survives the winter mainly in fallen leaves but also in grape buds and shoot tips. The pathogen's spores are spread by splashing rain and wind.
When conditions are suitable, infection can occur in less than 90 minutes. Infections generally occur in the morning and disease can become apparent in about 4 days. Downy mildew is favored by all factors that increase the moisture content of air, the host plant, and soil. Epidemics are promoted mostly by rain and sprinkler irrigation that wets plants.
Solutions
Prevention is an effective strategy for controlling downy mildew. Provide good air circulation around plants by not planting vines too close together. Prune off leaves around grape bunches soon after bloom. If soil drains poorly (slowly) plant vines on a raised berm or mound of soil. Use drip or furrow irrigation instead of sprinklers. Direct any hand watering to soil around the trunk and avoid wetting foliage. Reduce the frequency of irrigation to the extent that is still compatible with healthy plant growth.
Reduce overwintering inoculum of the pathogen, such as by pruning out the infected ends of shoots. Consider raking up and disposing of fallen grape leaves during the fall. Where downy mildew is a problem, before rains are expected, thoroughly spray green plant parts with Bordeaux mixture or another copper fungicide to reduce the extent of new infections.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources (UC ANR) also available as an eBook, and Pest Management Guidelines: Grape and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
|
Red and yellow lesions on a grape leaf with downy mildew.
Angular, brown lesions and whitish spores of downy mildew.
A shoot tip that's become curled and swollen due to downy mildew and is covered with its pale spores.
Grape berries that are brown and decayed due to downy mildew and are covered with its pale spores.
|
