Controlling powdery mildew in tart cherry orchards

Powdery mildew control in tart cherry orchards is best initiated at the first cover timing after shuck split. Here are the most effective fungicides for powdery mildew control in tart cherries.

Powdery mildew can be a problem in tart cherry orchards, particularly in young orchards. This fungal pathogen can also cause defoliation during mechanical harvesting if the infection is severe. Powdery mildew infection tends to occur on the most actively-growing tissue.

The biggest issue for powdery mildew control is the prevention of initial fungal infection. We currently do not have fungicides that will eradicate powdery mildew, so growers need to control this disease before they see it by using protectant products. Thus, by the time you see the white mycelium on the leaves at or before harvest-time, it is too late for control.

The most important spray timing for powdery mildew control is the first cover timing, i.e., the first spray application after shuck split. Prior to shuck split, chlorothalonil (Bravo and other generics) is the fungicide of choice in tart cherry orchards due to its excellent activity in cherry leaf spot control. Chlorothalonil is not effective against powdery mildew. Fortunately, at these earlier timings, the powdery mildew fungus is generally not active. The first cover timing represents the first chance to protect the orchard from the initial powdery mildew infection. This spray is critically important. We’ve shown in our previous research that if this timing is missed, the amount of powdery mildew-infected leaves can increase by at least threefold at harvest. We’ve also found that if fungicides targeting powdery mildew are only applied closer to harvest, powdery mildew infection can get completely out of hand by mid-August (about 70 percent incidence of leaf infection).

The best fungicides currently available for powdery mildew control are the new SDHI premixes Luna Sensation and Merivon and the stobilurin fungicide Gem. Other fungicides including sterol-inhibitors and sulfur provide some powdery mildew control, but are not nearly as effective as Luna Sensation, Merivon or Gem. While copper fungicides or the combination of Syllit + Captan are both excellent for control of cherry leaf spot, they are weak against powdery mildew.

There is a powdery mildew fungicide available called Quintec (quinoxyfen). This fungicide has a mode of action that is different than the SDHIs or strobilurins so it is a good choice for resistance management. Quintec at 7 fl oz per acre has performed very well in powdery mildew trials on cherry in Washington State. However, Quintec has no activity against cherry leaf spot, so that must be remembered if this fungicide is used. The best mixing partner for Quintec for cherry leaf spot control would be Bravo Weather Stik (used under the conditions of the Section 24(c) label). For more information on controlling cherry leaf spot, see the Michigan State University Extension article “Fungicide considerations for cherry leaf spot control at first cover.”

See also

Dr. Sundin’s work is funded in part by MSU‘s AgBioResearch.

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