Weather is ideal for downy mildew on grapes at this time

Frequent rains and high relative humidity are favoring downy mildew in grapes. Choose an effective fungicide to protect clusters and leaves on susceptible cultivars.

Downy mildew, which is caused by the oomycete Plasmopara viticola, has gotten off to a good start this year due to copious rainfall, moderately warm temperatures and high relative humidity over the past weeks. Downy mildew was first seen on ‘Chancellor’ flower clusters of unsprayed sentinel vines in East Lansing, Michigan, several weeks ago and has since increased exponentially at that location, indicating ideal conditions for infection. Infected suckers at the base of the vine were also noticed; these were stunted and covered with a white layer of sporangia. This suggests that disease pressure has been moderate to severe so far this year.

The fungus overwinters as thick-walled spores (oospores) in leaf debris from the previous growing season on the soil surface under vines. Oospores at or near the soil surface will germinate in the spring by producing sporangia that are rain-splashed or wind-blown to susceptible grape tissues. Oospore germination is favored by rainfall of at least 0.4 inches and temperatures over 50 degrees Fahrenheit. Despite the fact that these conditions can occur in April and May, I have never seen downy mildew develop until June in Michigan. At the earliest, I have spotted downy mildew symptoms in early June, and usually these occur on wild grapes that grow close to the ground or on unsprayed Chancellor grapes during bloom as the clusters are particularly susceptible.

Sporangia release swimming zoospores in a film of water – rain or dew – which infect the leaves by penetrating the leaf stomata within several hours. Light-green or yellow lesions, or “oil spots,” on the leaves appear within five to 17 days after infection, depending on the temperature. The incubation period is shortest at temperatures of 20-25 degrees Celsius, or 68-77 F. When the temperatures are lower or higher, the symptoms will take longer to develop. Sporangia and zoospores are actually easily desiccated. They die within two to three hours of exposure to low humidity and sunlight, so most infection occurs soon after their release. However, they may survive on leaf surfaces for more than 24 hours under cool humid conditions. After warm, humid nights, a white downy fungal growth (sporangia) will appear on the underside of the leaves and other infected plant parts.

To look for downy mildew, visually scan leaves and clusters, focusing particularly on leaves and suckers close to the ground, as entire suckers can become infected by contact with oospores in the soil. If you see yellow lesions, turn the leaf over to look for white sporulation on the lower leaf surface. Occasionally, low-level Gramoxone herbicide injury may resemble downy mildew lesions. To confirm that it is downy mildew, simply remove several symptomatic leaves and place them in a plastic bag with a moist paper towel and store in the dark at room temperature overnight. White sporulation should be visible on the underside of the leaf the next day or definitely within two days.

Fungicide sprays for downy mildew at this time are recommended by Michigan State University Extension for susceptible varieties. The following fungicides are excellent options for downy mildew control: Ridomil Gold MZ or Ridomil Gold Cu (consider processor requirements, the pre-harvest interval, and copper sensitivity of vines before spraying) and phosphorous acids like ProPhyt and Phostrol. Both Ridomil and phosphorous acids are systemic and have curative and anti-sporulant activity. Ridomil applications often appear to eradicate downy mildew and can keep it at bay for three to four weeks. Phosphorous acids are not quite as strong as Ridomil, but they are substantially less expensive, and if symptoms and sporulation are already present, you can increase their effectiveness by applying a second “booster” application five days after the first application. Strobilurins like Abound, Pristine and Sovran are also very effective, but don’t have much curative activity and are best used as preventative sprays.

Newer fungicides like Revus, Revus Top, Presidio, Forum, Ranman, Reason and Tanos also generally have good efficacy against downy mildew, but are best used as protectants as they have limited or no curative activity. Some of these, like Forum, are relatively inexpensive and can be used in a preventive tank-mix to provide cost-effective downy mildew protection. Zampro is a new downy mildew fungicide that has shown excellent activity against downy mildew. Be careful with Revus Top as it contains difenoconazole which can be phytotoxic to Concord, Concord Seedless and Thomcord grapes.

For organic growers, Serenade and Sonata are the best options; apply with NuFilm P sticker to extend the longevity of the product.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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