Protect grape clusters from powdery mildew and downy mildew

No symptoms have been seen yet, but keep scouting for powdery and downy mildew.

No symptoms of powdery and downy mildew have been seen yet, but may manifest themselves any time from now. It is advised to keep scouting carefully on at least a weekly basis. In 2009, we first observed downy mildew in Chancellor in Fennville during the first week of June and in 2010 during the second week of June. It is safe to assume that these pathogens will be active during the warm, humid weather that is forecast for the coming weeks. Growers are therefore strongly advised to protect flower and fruit clusters from infection using effective fungicides as soon as possible if the vines are not already protected. Also, continue to monitor vineyards for signs of infection.

At this stage, the young clusters are highly susceptible to all major diseases, including downy mildew, powdery mildew, black rot and Phomopsis. Black rot and Phomopsis lesions have been seen for several weeks and indicate that the pathogens are active. Grape anthracnose symptoms are also visible on shoots and leaves and on cluster stems. The risk of infection is especially high if we have multiple or big rain events and moderate to warm temperatures during this time. Prolonged wet conditions during bloom can also allow Botrytis to get a foothold in the clusters of susceptible varieties by promoting growth on senescing flower parts.

If active infections are found, use fungicides with post-infection activity at the highest labeled rate. For downy mildew, Ridomil Gold MZ or Ridomil Gold Copper are the strongest fungicides, followed by phosphorous acid fungicides like Phostrol and ProPhyt. If using phosphorous acids, it is beneficial to apply a “booster spray” five days after the first spray. Strobilurin fungicides have limited post-infection activity and should preferentially be used in a preventive mode. Newer fungicides for downy mildew control are Presidio, Revus and Revus Top (don’t apply Revus Top to Concord or Noiret vines due to phytotoxicity concerns), Gavel (contains mancozeb), Forum and Tanos. While some of these new fungicides have post-infection (curative) activity, they are best applied on a preventative basis. They are excellent for integration into a fungicide resistance management program as they represent new and different chemistries.

It will be especially critical to protect clusters of susceptible varieties from powdery mildew at this time. Sterol inhibitor, like Elite and Rally, and strobilurin, like Sovran and Flint, fungicides have the ability to cure early infections, but will not eliminate already established colonies. JMS Stylet Oil and potassium bicarbonate fungicides (Kaligreen, Armicarb, MilStop) can be used to eradicate visible powdery mildew colonies. Make sure that coverage is thorough (use sufficient spray volume), as only those colonies contacted by the fungicide will be killed. Since strobilurin-resistant powdery mildew isolates have been found in Michigan vineyards – mostly MSU experimental vineyards and wine grape vineyards with a 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. Over the past two years, we have noticed that Ziram as a tank-mix partner did improve control of powdery mildew in a spray program. Also, alternate with fungicides with different modes of action, for example Quintec, Vivando, Endura, Serenade, Sonata and Regalia. Revus Top is a new fungicide for powdery and downy mildew and black rot control in grapes. However, the ingredient that is active against powdery mildew is difenoconazole which belongs to the sterol inhibitor class. This fungicide may be phytotoxic on Concord grapes, so do not use on Concords.

Protect clusters for at least four to five weeks after bloom. As the berries develop, they become naturally resistant to black rot, 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 it is about four weeks after bloom. Concord grapes become resistant to black rot four to five weeks after bloom, but some wine grape varieties may remain susceptible to black rot for up to eight weeks postbloom. However, be aware that the cluster stem (rachis) and berry stems can remain susceptible longer than the berries in most cases. The only disease to which berries remain susceptible throughout their development is Phomopsis, but the risk of infection diminishes after bunch closure because inoculum levels drop off then. Botrytis is just the opposite in that berries actually become more susceptible as they get closer to harvest, especially in tight-clustered varieties. In general, aim to protect the clusters from the major diseases from immediate pre-bloom until four to five weeks after bloom.

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

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