MSU Extension

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.

Powdery mildew is now showing up on grape leaves and clusters. This disease is caused by the fungus Uncinula necator. The powdery mildew fungus can infect all green tissues, resulting in a whitish gray, dusty or powdery appearance. Powdery mildew colonies are mostly present on the upper leaf surface. Infections of young, expanding leaves can result in distortion or stunting. Early berry infections can result in splitting of berries, secondary rots, and undesirable flavors in wine. Late infections are largely invisible except for a web-like necrotic pattern on the berry surface, which can still predispose the berries to rots. Severe infections reduce vine growth, yield, fruit quality, and winter-hardiness. In late summer, the fungus produces small golden-brown to black fruiting bodies (cleistothecia) on infected plant parts. The cleistothecia overwinter in bark crevices of the vine and release wind-disseminated ascospores in the spring. Leaves in proximity of the bark tend to get infected first. Powdery mildew is favored by high humidity and moderately high temperatures (68°F - 81°F). Temperatures above 95°F inhibit new infections.

Most fungicides used against powdery mildew function to prevent new infections. However, since the body of the fungus is fairly superficial in contrast to most other fungal diseases, it is possible to apply contact fungicides to knock down existing infections. In various trials, JMS Stylet Oil (paraffinic oil) has shown to do a very good job in eradicating existing powdery mildew colonies on leaves and fruit. It apparently works by suffocating the fungal mycelium. This product will also kill spores to prevent new infections and has about a week of residual activity. JMS Stylet Oil is sold as both a conventional and an organic formulation and is labeled for use against powdery mildew, Botrytis, mealybugs, mites, leafhoppers, and whiteflies. The label rate is one to two gallons per 100 gal of water, sprayed with ground equipment every 10 to 14 days. Optimum coverage is very important to get good control since the fungicide has to actually contact the fungus colonies to be effective.

It is important to be aware that JMS Stylet Oil can burn plant tissue if used incorrectly. Be sure to read the label very well before use. Add the oil to tank mix formulations as the last ingredient. Read labels of tank-mix partners to be sure they are compatible with oil. Observe all limitations, precautions, and rate recommendations, in particular with acephate (Orthene), copper ammonium carbonate (CCN), copper hydroxide (Kocide), fenvalerate (Asana XL), Mancozeb, Maneb, methamidophos (Monitor), metalaxyl (Ridomil), and triadimefon (Bayleton). Do not tank-mix JMS Stylet Oil with spreader stickers, Nu-Film, or highly ionized nutrient spray materials (e.g., Nutri-Leaf, Bayfolan). These materials can be sprayed separately from oil. JMS Stylet Oil is compatible with urea (up to five lb per 100 gal) and Epsom salt (three lb per 100 gal). Do not use the following chemicals: captan, Folpet, Morestan, organic tin compounds during, with or following an oil spray. Do not use propargite (Omite) with an oil spray or within 30 days of an oil application. Do not use chlorothalonil (Bravo) or dimethoate (Cygon) in a spray program with JMS Stylet Oil. Do not use the following chemicals with JMS Stylet Oil until at least two weeks have elapsed between use of the oil and the chemicals Dyrene, Bravo, Botran, Kelthane, Dikar, Karathane, Ambush, and Pounce. On grapes, do not use copper and oil together when fruit is present and do not apply sulfur within 10 days of an oil application.

In addition, do not spray at temperatures above 90°F (85°F is a good cut-off). It is okay, however, to spray early in the day when it is cooler, as long as the spray has dried when temperatures rise. Do not spray plants that are under heat or moisture stress. Also, do not spray wet foliage and do not spray when freezing temperatures are anticipated within 48 hours of an oil application. Oil will temporarily remove the bloom on grapes, so it should not be sprayed on table grapes within two weeks of harvest. Since JMS Stylet Oil can delay sugar accumulation in the fruit, do not use more than twice during fruit ripening.

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