It’s Raining A LOT: Stay Ahead of Downy Mildew

By: Melanie L. Lewis Ivey, OSU-Plant Pathology

Once grape clusters reach pea-size, growers may be tempted to relax downy mildew management because the berries themselves are resistant to new infection (referred to as ontogenic or age-related resistance). But when the forecast includes rain, day after day, downy mildew deserves continued attention.

Downy mildew, caused by Plasmopara viticola, thrives under warm, wet conditions. A single rain event can create favorable conditions for infection, but multiple days of rain are especially concerning. Repeated rainfall, high humidity, cloudy conditions, and extended periods of leaf wetness can provide repeated opportunities for the disease to infect new tissue and spread throughout the canopy. When vines have little opportunity to dry between rain events, disease pressure can build rapidly.

Even after berries become resistant to infection, leaves, young shoots, and other susceptible green tissues remain at risk. This is particularly important when repeated rainfall encourages vigorous new shoot and lateral growth. That fresh, succulent tissue can be highly susceptible to downy mildew, providing the disease with new tissue to infect just as conditions become increasingly favorable.

Protecting the canopy as the berries expand is critical because healthy leaves are needed to finish ripening the crop. Severe downy mildew infections can cause premature defoliation, reducing the vine’s photosynthetic capacity at a time when the crop still needs carbohydrates to accumulate sugars and mature properly. Significant loss of foliage can slow ripening, affect fruit quality, weaken vines, and leave them less prepared for winter (i.e., more susceptible to winter injury or early season diseases like Phomopsis cane and leaf spot).

Several rainy days in a row also create challenges for disease management. Frequent rainfall can shorten the effective protection period of some fungicides, make it difficult to get equipment into the vineyard, and create narrow windows for making timely applications. If a spray is delayed while infection periods continue to occur, downy mildew can gain a foothold surprisingly quickly.

For these reasons, the berries reaching pea size should not automatically signal the end of downy mildew management, especially when the rain just keeps coming. During stretches of wet weather, growers should scout susceptible varieties carefully, monitor the leaves for symptoms and signs of disease, watch rainfall and leaf-wetness conditions, and maintain an appropriate disease-management program based on vineyard risk.  During rainy periods try not to extend the spray interval beyond 10 days.

Symptoms of grape downy mildew (Plasmopara viticola) on Catawba grapevine.

Fungicide selection and timing become particularly important under sustained disease pressure. Growers should consider product efficacy, rainfastness, resistance-management guidelines, preharvest intervals, and label restrictions when planning applications. At this time of year (nearing harvest), the pre-harvest interval (PHI) is particularly important to consider.  Rotating among appropriate modes of action and maintaining good spray coverage are also important components of an effective program.  Consult the Ohio Grape Spray Guide or Midwest Fruit Pest Management Guide for guidance on selecting a fungicide.

Symptoms of downy mildew (Plasmopara viticola) on the upper side (yellow spots) of a Catawba grape leaf and signs of the pathogen, seen as whitish-grey areas on the underside of the same leaf.

For more information on grape downy mildew and other grape diseases contact Melanie L. Lewis Ivey, Extension Fruit Patholgist, ivey.14@osu.edu