Grape Downy Mildew Control – What Causes Downy Mildew On Grapes
By: Mary H. Dyer, Credentialed Garden Writer
Downy mildew on grapes is a serious fungal disease that wreaks havoc in vineyards around the world, especially when the weather is humid, rainy, and mild. The disease affects both wild and cultivated grapes. Grape downy mildew control requires gardening practices that improve growing conditions and minimize water on the leaves. Read on to learn more.
About Grapes with Downy Mildew
Early signs of downy mildew on grapes include small, greenish-yellow spots on the leaves, primarily between the veins. The lesions may be difficult to see, but they will eventually enlarge, and, in severe infections, may turn dark brown and brittle before dropping.
Early symptoms of downy mildew on grapes may also appear on tendrils and stems as shiny, water-soaked depressions with fuzzy fungal growth. Young shoots and tendrils are stunted and distorted. Grapes with downy mildew turn soft and light brown, and may be coated with a dense, gray fungal growth. Infected fruit will never develop normally.
Treating Grape Downy Mildew
Penn State Extension recommends spraying grapevines with a fungicide just before blossoms open, seven to 10 days later, and 10 to 14 days after that, followed by a final application three weeks later. If downy mildew on grapes was severe the previous season, you may want to begin the process a little earlier by spraying the vines about two weeks before the first blooms.
Your county extension office can help you select the best product for treating grape downy mildew.
Additional tips on grape downy mildew control include planting disease-resistant vines, as some varieties are highly susceptible to downy mildew.
Choose a planting site where the grapevines are exposed to sunlight all day. Space vines properly to allow sufficient air circulation.
Be careful not to overwater. If you use overhead sprinklers, extend the time between watering as much as possible. Otherwise, water at the base of the plant.
Ensure the vines are supported so they don’t rest on the soil. Rake the area well at the end of the season to remove diseased plant debris. Cultivate in spring to bury infected leaves and mummified berries that may remain from the previous growing season.
Prune grapevines annually, during the dormant period. Leave only strong, healthy growth from the previous year. Control weeds and tall grass around the plants and in the surrounding area.
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Annemiek Schilder, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - June 19, 2013
When it comes to battling powdery mildew and downy mildew on grapes, the following adage applies: “The early grower catches the fungus!”
Early fruit set heralds a time of extra vigilance when it comes to disease control, as young fruit clusters are highly susceptible to all major diseases, including downy mildew and powdery mildew. Recent rains are likely to favor primary inoculum release for both powdery and downy mildew pathogens and it is possible that we will see substantial powdery mildew pressure on the young fruit this year. As a reminder, it is possible to have powdery mildew fruit infection prior to observing any foliar infections, so protect the fruit of susceptible cultivars even if no powdery mildew has been seen on the leaves. This year, downy mildew appears to have arrived “right on schedule” as the first infected clusters were spotted on unsprayed Chancellor vines in southwest Michigan on June 12, 2013. Therefore, careful scouting is advised on a weekly basis.
Michigan State University Extension strongly advises growers to protect flower and fruit clusters from infection using effective fungicides. The risk of infection is especially high if we experience multiple rain events and moderate to high temperatures (70 to 85 degrees Fahrenheit). As the berries develop, they become naturally resistant to downy mildew and powdery mildew and the need for protection diminishes after the susceptible period ends. This happens quite rapidly for downy mildew – two to three weeks after bloom – whereas for powdery mildew berries build up resistance about four to five weeks after bloom.
Revus Top, Inspire Super and Quadris Top are newer pre-mix fungicides for control of powdery mildew and other diseases in grapes. Difenoconazole, the active ingredient in these products, is one of the more active sterol inhibitor fungicides, but also happens to be phytotoxic on Concord and Noiret grapes under some circumstances. Luna Experience is a new pre-mix fungicide for control of powdery mildew, black rot, Phomopsis and anthracnose.
Serenade and Regalia are options for disease control in organic grapes – their efficacy is based on microbial antagonism and induced resistance, respectively. Add a spreader-sticker (e.g., Nu-Film P) to increase efficacy and longevity of these products. Mineral oils (e.g., JMS Stylet Oil, Purespray Oil) and potassium bicarbonate salts (Kaligreen, Armicarb, MilStop) can be used to eradicate visible powdery mildew colonies. Oxidate (hydrogen peroxide) also has some eradicant activity, but dissipates quickly. If you use eradicants, make sure that spray coverage is thorough (use sufficient spray volume), as only those colonies contacted by the fungicide will be killed.
For most varieties, foliar infections are the main phase to be concerned about. However, cv. Chancellor is like the proverbial “canary in the coal mine” for cluster infections. Both flower and young fruit clusters can be destroyed. As soon as active infections are found, use fungicides with post-infection activity at the highest labeled rate. For downy mildew, Ridomil Gold (MZ or Copper) are the strongest fungicides, followed by phosphorous acid fungicides such as Phostrol and ProPhyt. When using phosphorous acids, applying a “booster spray” five days after the first spray will enhance the curative effect. Strobilurin fungicides have limited post-infection activity and should be used in a preventative mode.
Newer fungicides for downy mildew control are: Zampro, Presidio, Revus/Revus Top (don’t apply Revus Top to Concord or Noiret vines due to risk of phytotoxicity), Gavel (contains mancozeb), Forum, Reason, Ranman and Tanos. While some of these newer fungicides have post-infection (curative) activity, they are best applied on a preventative basis. Forum is one of the least expensive of this group. They are good for integration into a fungicide resistance management program as many of them represent new and different chemistries.
Serenade, Sonata and copper products are organic options for downy mildew control. Among copper fungicides, Cueva and Magna-Bon might be interesting to growers due to their much lower copper content than traditional copper products.
This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).
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It’s possible to use very effective organic downy mildew treatments.
- Bordeaux mixture is the only effective preventive treatment against downy mildew.
- Apply regularly at the beginning of spring and in fall.
- Treat during the entire vegetative phase.
- Treat regularly, on average every two weeks and after every rainfall.
- In the case that downy mildew appears, only a curative treatment will have any effect.
Downy mildew and baking soda
Baking soda is an excellent solution to treat downy mildew.
- Dissolve 1 table spoon baking soda in 1 quart (1 liter) water (ideally rainwater or mineral water, for the water to be softer).
- In dry weather, spray this mix on all the leaves, spraying both on top and on the underside of leaves.
Varieties to Consider
The following are some excellent varieties for homeowners to try:
- Somerset Seedless: Red table grape hardy to -30°F ripens about the same time as Valiant. Strawberry-like flavor. This is the only seedless grape that has performed well in South Dakota.
- Valiant: One of our hardiest grapes, vigorous and very well-adapted to our climate and soils. Has small clusters of dark, blue berries which ripen before most other grapes. Excellent juice, jelly, or table grape may also be used for wine. The 4-cane pruning system works well for this variety. This grape has a tendency to overbear you may need to remove some of the clusters. Bred by Dr. Ron Peterson at South Dakota State University.
- Marquette: An excellent new red wine grape with high sugar and moderate acidity. Open, orderly growth habit. Very good resistance to downy mildew, powdery mildew, and black rot moderate resistance to phylloxera. Tends to break bud early, so avoid sites that warm up early in the spring. Wine is ruby color with pronounced tannins and notes of cherry, berry, black pepper, and spice. Released in 2006 by the University of Minnesota.
- Frontenac: Hardy blue wine grape with cherry flavors. Excellent wine grape for this region, ripens mid- to late September. Tends to have high acidity. Needs good pruning and cluster thinning to keep vine from overproducing. Good disease resistance and some tolerance to 2,4-D herbicide. Minnesota patented cultivar.
- Frontenac Gris: A white sport of Frontenac, with a growth habit similar to Frontenac. Gray/light red fruit. A midseason variety harvested in late September. Minnesota patented cultivar.
- Brianna: White wine grape developed by Elmer Swenson. Berries are greenish gold to gold when fully ripe in early to mid-September. Wine is balanced with pineapple nose and flavor. Also makes a flavorful white juice or table grape. Winter hardy in South Dakota North Dakota.
- LaCrescent: Hardy white wine grape released by the University of Minnesota in 2002. Ripens early (to goldenbrown color), but acidity remains high and may need to be reduced during winemaking. Moderately susceptible to powdery and downy mildew. Loose clusters with occasional fruit set problems. May be susceptible to spring frost in areas that warm up early. Wine has apricot-like flavor and is excellent for blending. Minnesota patented cultivar.
- King of the North: Vigorous, late ripening Concord style grape that originated in Wisconsin. Good for juice, but acidic for fresh eating.
Fungicide Options for Powdery Mildew
You can apply fungicide treatments to infected plants to help manage severe infections. If you are interested in organic fungicides, consider neem oil, sulfur or potassium bicarbonate for powdery mildew. Unlike some chemical options, the fungi don't become resistant to sulfur and potassium bicarbonate. Sulfur should not be used in hot temperatures above 85 degrees Fahrenheit as it may cause scorched leaves. If you use a sulfur spray, wait at least two weeks before applying neem oil.
You can also make a homemade powdery mildew spray with baking soda and water, notes West Virginia University Extension Service. Simply mix 1 teaspoon of baking soda in 1 quart of water and spray it on your plants. This will alter the pH and help to inhibit powdery mildew growth.
Chemical fungicides, such as those containing trifloxystrobin or azoxystrobin, can be used on many plants in your garden to help control powdery mildew. If the fungus is attacking your vegetable plants, make sure any fungicide you use is labeled for use on vegetables. Be sure to follow all warnings and application instructions on the product label.