Downy mildew is an important disease of commercial and backyard grapes in Kentucky. Current warm, wet, humid weather conditions favor infection and disease development. When flowers, clusters, and shoots become infected by downy mildew, yield losses result. This disease may also predispose grapevines to winter injury. Fungicides are available; however, sanitation is a critical step in prevention and management.
Downy Mildew Facts
- Leaf symptoms begin as yellowish-green lesions on upper surfaces. Disease lesions expand, and affected areas turn brown, necrotic, or mottled (Figure 1). On the underside of lesions, white, downy fungal-like growth (sporulation) may be observed (Figure 2). Late season infections result in a mosaic of small, angular, yellow to red-brown spots on upper surfaces and downy growth on the undersides of leaves.
- Infected fruit turn light brown to purple, shrivel, and detach easily. White, cottony growth (sporulation) may be visible during periods of high humidity (Figure 3).
- Severely infected leaves may curl and drop from vines. Infected shoots, petioles, tendrils, or cluster stems may become distorted, thickened, or curled.
- Primary infection occurs in spring during wet periods. Spores are spread by wind or rain-splash.
- Subsequent infections result from additional spore development during periods of high humidity and warm night temperatures.
- Caused by the fungus-like organism Plasmopora viticola.
- The downy mildew pathogen overwinters in fallen leaves.
- Avoid planting highly susceptible grape cultivars such as, Catawba, Chardonnay, Niagara, and White Riesling.
- Select appropriate planting sites with full sun and good air circulation.
- Maintain plant health with proper nutrition and irrigation practices.
- Prune to improve air circulation and reduce canopy humidity.
- Remove and destroy diseased plant material.
- Commercial growers should refer to Midwest Fruit Pest Management Guide (ID-232) for recommended fungicides and application schedule.
- Homeowners may apply fungicides containing copper or mancozeb beginning with new growth and continuing throughout the season. Always follow label directions when utilizing fungicides.
- Backyard Grape Disease Management Using Cultural Practices (with Low Spray, No Spray & Organic Options) (PPFS-FR-S-24)
- Commercial Midwest Fruit Pest Management Guide (ID-232)
- Commercial Grape Fungicide Spray Schedule Worksheet and Sample Spray Guides (PPFS-FR-S-20)
- Downy Mildew of Grape (PPFS-FR-S-13)
- Fruit, Orchard, and Vineyard Sanitation (PPFS-GEN-05)
- Simplified Backyard Grape Spray Guides (PPFS-FR-S-23)
By Kim Leonberger, Extension Associate and Nicole Ward Gauthier, Extension Specialist