Fruiting bodies of fungus
Powdery mildew is a common disease of grapes, caused by the fungus Erysiphe necator. Unlike most fungi, the powdery mildew fungus does not need free water, such as rain or dew, to cause infections. Temperatures for infection are optimal between 68-77⁰F. Susceptibility varies greatly among grape varieties and all green tissue on susceptible varieties can be infected. Most infection occurs during the period from early bloom through 3 to 4 weeks after bloom. Symptoms first appear on leaves and blossom clusters. The fungus overwinters as chasmothecia (tiny fruiting bodies) in bark crevices on the grapevine. Powdery mildew can result in reduced vine growth, yield, fruit quality, and winter hardiness.
Signs and Symptoms
White fungus on leaf
Diseased curled leaves
Symptoms on leaves appear as white or gray patches on the upper (image on the right photo courtesy- tastylandscape.com) and lower leaf surface. Patches on the lower leaf surface can be confused with downy mildew. These patches enlarge until the entire leaf surface is covered with fungal growth. Severely diseased leaves will often curl upwards (image on the far right; photo courtesy-Government of Western Australia, Department of Agriculture and Food) during hot and dry periods. Late in the season, patches may contain tiny yellow or black fruiting structures (chasmothecia) (image on top-right; photo courtesy- cornell.edu).
Symptoms on shoots are not common and appear as dark brown to black patches.
Split diseased berries
Blossom cluster infection can cause the flowers to wither and drop without setting fruit. Diseased berries have patches of white fungal growth that are similar in appearance to leaf symptoms, and resemble a dusting of powdered sugar or flour (image on the far right; photo courtesy- uoguelph.ca). These symptoms are easily confused with downy mildew on grapes. In some cases, the berries will have rust colored spots on the surface. Severely diseased berries may split open (image on the right; photo courtesy- inra.fr) and rot due to infections by secondary pathogens. When purple or red berries are infected they fail to color properly and have a blotchy appearance at harvest.
Powdery Mildew Disease Management
Sanitation practices are very important and can impact the success of fungicide application.
- Pruning and training the shoots provide good air circulation and promote sunlight exposure making conditions less inducive for the pathogen.
- Strategic removal of leaves around clusters also helps in controlling the infection.
Maintain a healthy plant by following good growing practices, more information can be found at Midwest Grape Production Guide.
Cultural practices that improve air circulation through the canopy are important to prevent and reduce infections. Pruning and training shoots to maximize air flow and increase sunlight penetration, as well as the strategic removal of leaves surrounding fruit clusters, will reduce the conditions (humid and warm) that favor disease development. Good pruning and training practices will also improve the effectiveness of a fungicide spray program by increasing coverage of the fungicide within the canopy. The removal of wild grapes surrounding the vineyard is recommended. Powdery mildew spores are dispersed by the wind and wild grapes can be a source of infection.
For highly susceptible cultivars or following a growing season where disease pressure was high, fungicides should be applied early in the season. The most critical time to manage fruit infection with fungicides is immediately before bloom through two to four weeks after bloom.
For commercial wine grape growers implementing low-input spray programs, sulfur is the most commonly used fungicide to manage powdery mildew. Several sulfur-based products are also allowed or allowed with restrictions, for organic grape production. There are also many additional fungicides registered to effectively control powdery mildew. Commercial growers should consult the most current issue of the Midwest Fruit Pest Management Guide for up to date fungicide recommendations and the OMRI Lists for products allowed for organic grape production.
For home grape production, sulfur is very effective at managing powdery mildew, but it can be phytotoxic to the vine and leaves if it is applied when the temperatures are above 86⁰F (30⁰C). Sulfur is inexpensive and can be purchased at a local farm or garden specialty crop stores.
Alternative fungicides such as various oils, potassium salts (monopotassium phosphate, potassium bicarbonate), polyoxin D, and dilute solutions of hydrogen peroxide are effective eradicants and anti-sporulants, but are not effective protectants and have no residual activity. These alternative fungicides should be applied every 5-7 days once powdery mildew symptoms are observed. More frequent applications during rainy periods may be needed. Growers should consult the OMRI Lists for alternative fungicide products that are allowed for organic grape production.
Biocontrol products labeled for powdery mildew control of grapes are available on the market, however, the effectiveness of these products have not been evaluated under Ohio growing conditions. A list of labeled biocontrol products for powdery mildew control is provided below.
- DoubleNickel55 (Certis, Inc.)
- Serenade Max (Bayer CropScience)
- Sonata Max (Bayer CropScience)