Protecting young grape clusters from powdery and downy mildew

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.

Powdery mildew

Fungicides such as sterol inhibitors (e.g., Elite, Rally) and strobilurins (e.g., Sovran, Flint, Abound, Pristine) fungicides have the ability to cure early infections, but will not eliminate colonies that are already established. Since strobilurin-resistant powdery mildew isolates have been found in Michigan – mostly in MSU experimental vineyards and wine grape vineyards with a significant history of strobilurin use – and we have circumstantial evidence for sterol inhibitor resistance, we recommend adding a protectant fungicide like Sulfur or Ziram to the tank-mix when using either type of fungicide. Sulfur is the most cost-effective option for non-sulfur-sensitive grape cultivars. Also, alternate fungicides with different modes of action, such as Vivando, Torino, Quintec, Endura, Serenade and Regalia.

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.

Downy mildew

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.

See also

Banning black rot and Phomopsis from young grape clusters

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