Downy mildew gets an early start in grapes
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Downy mildew, which is caused by the oomycete Plasmopara viticola. has gotten an early start this year due to the high rainfall over the past few months. Beginning oil spots have been sighted on leaves of suckers of unsprayed “Chancellor” grapes, and some growers have mentioned seeing downy mildew-like spots on other varieties. In the “Chancellor,” the leaves showing oil spots were close to the ground, which is where the oospores overwinter. Several systemically infected suckers were also seen: these were partially necrotic with white sporulation on the undersides of down-curled leaves. Suckers can get infected at a very early stage when they grow through or touch the soil and come in contact with germinating oospores. Systemically infected suckers sometimes appear completely white from sporulation and act as a source of disease inoculum if not removed or killed by an herbicide.
The fungus overwinters as thick-walled spores (oospores) in the soil. Only oospores at or near the soil surface will germinate in the spring, which is favored by rainfall (at least 0.4 inch) and temperatures over 50ºF, and typically starts several weeks before bloom in this region. Lesions appear within five to 17 days after infection, depending on the temperature. First symptoms on the leaves may be light green or yellow spots that may have a greasy appearance (oil spots). To look for downy mildew, visually scan leaves and clusters, focusing particularly on leaves and suckers close to the ground. 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.
Fungicide sprays for downy mildew at this time are recommended for susceptible varieties, particularly ‘Chancellor,’ as flower clusters can be infected even before they open. The following fungicides are good options for downy mildew control: Ridomil Gold MZ or Ridomil Gold Cu (consider processor requirements and copper sensitivity of vines before spraying), Abound, Pristine, Sovran, ProPhyt, Phostrol, and Gavel. Newer fungicides like Revus, Presidio, and Tanos are also good options, but most growers are not familiar with these fungicides yet. For organic growers Serenade and Sonata are good options (apply with NuFilm adjuvant).
Dr. Schilder’s work is funded in part by MSU‘s AgBioResearch.