Warm, humid weather is conducive for powdery mildew 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.

Keep scouting vineyards for powdery mildew as warm, humid weather is conducive to disease development. Don’t depend upon a cursory observation of your vines from the seat of your tractor or pick-up truck, but be prepared to “wrestle” with the vine to look at leaves and clusters in dense, shaded portions of the canopy. As a reminder, once powdery mildew gets out of control, it is extremely difficult to arrest the disease. It takes very little diseased fruit (less than five percent) to impart a moldy flavor to the wine. Symptoms of powdery mildew are a white to gray powdery patches on the surface of the leaves and berries.

By now in most areas of the state, the critical period of the highest susceptibility of the fruit clusters to infection should have passed (two to three weeks after bloom). However, there may be late-developing clusters in the canopy that may still be susceptible to infection. There is also a risk of late (diffuse) fruit infections (e.g., between three and five weeks after bloom) that are barely visible but can compromise the integrity of the berry skin by creating small dead spots, which can provide entry points for pathogens that cause Botrytis and sour bunch rots. Soon, though, we will start focusing mainly protection of the foliage as severe powdery mildew infections can reduce the ability of the vine to produce sugars to ripen the fruit. Most grapevines can withstand some foliar disease, especially late in the season, since they are usually not operating at “full capacity”. The susceptibility of the grape cultivar and the crop load are the main factors that determine how much disease the vine can withstand before losses in yield and fruit quality become evident. The weather is also an important factor in this equation.

Unlike other grape pathogens, the fungus does not need free water for infection; moderate to high relative humidity (40-100 percent) is sufficient for germination of conidia. In fact, rainfall is detrimental to survival of conidia as they tend to burst in water. Although infections can occur at temperatures from 59 to 90ºF, temperatures between 68 and 77ºF are optimal for disease development. Temperatures above 95ºF inhibit spore germination, and the fungus may be killed at temperatures above 104ºF.

For infection prevention, good fungicide options include sulfur, sterol inhibitors (Nova, Elite, Procure, Rubigan, Vintage), strobilurins (Pristine, Sovran, Abound, Flint), Endura, and Quintec. Remember that some grape varieties are sensitive to sulfur, Pristine or Flint, and that fungicides differ in their pre-harvest intervals. Also, sulfur applied late in the season may interfere with wine-making so is not advised beyond veraison. Alternating fungicides with different modes of action is important for fungicide resistance development.

If powdery mildew is already present, there are several possible eradicants available: JMS Stylet Oil (paraffinic oil); Armicarb, Kaligreen and MilStop (all potassium bicarbonate salts); and Oxidate (hydrogen peroxide). JMS Stylet Oil (1.5 to 2 percent) appears to offer the best results. Any of the eradicant approaches to powdery mildew require high (at least 100 gallons of water/acre) spray volumes in order to achieve complete tissue coverage as the product needs to come in contact with the powdery mildew hyphae to be effective. One concern with JMS Stylet Oil is that it can delay Brix accumulation, so it is best not to use it after veraison. Also, do not apply oil and sulfur within 14 days of each other. Removing leaves in the fruiting zone for Botrytis bunch rot control can also help reduce powdery mildew severity by increasing airflow, light penetration and fungicide penetration.

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