Controlling powdery mildew in tart cherry orchards with reduced crop

Growers can minimize powdery mildew infections with early, well-timed applications in orchards with light crops.

Powdery mildew can be problematic in tart cherry orchards, and because this disease rarely infects fruit, it still needs to be controlled in years with a light crop. The biggest issue for powdery mildew control is the prevention of initial fungal infection. We currently do not have fungicides that will eradicate a powdery mildew infection, so growers need to control this disease before they see it by using protectant fungicides. If growers observe the white mycelium on the leaves, it is too late to apply a control spray.

The most important spray timing for powdery mildew control is the first cover timing, or 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. At these early timings and in most years, the powdery mildew fungus is generally not active at this time. The first cover timing represents the first and optimal chance to protect the orchard from initial powdery mildew infection. This spray is critically important. We have 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 have also found that if fungicides targeting powdery mildew are only applied later in the season, powdery mildew infection can become seriously problematic by mid-August (about 70 percent incidence of leaf infection). In a year with no crop, this first cover timing is even more important as a cost saving measure – essentially, if a grower applies a fungicide targeting powdery mildew at first cover timing, they will minimize the need to apply fungicides targeting powdery mildew later in the season. This one, well-timed spray will also reduce powdery mildew inoculum for the 2013 season and save money.

The best fungicides currently available for powdery mildew control are Pristine and Gem. Other fungicides including sterol-inhibitors and sulfur provide some powdery mildew control, but are not nearly as effective as Pristine or Gem. While the combination of Syllit + Captan is excellent for control of cherry leaf spot, it is weak against powdery mildew.

The newer fungicide Quintec (quinoxyfen) has a mode of action that is different from the strobilurin in Gem. The strobilurin + boscalid in Pristine works well against powdery mildew, so this fungicide 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. However, Quintec has no activity against cherry leaf spot, and growers must add another product to the tank for leaf spot.

Related MSU Extension News article: Controlling cherry leaf spot in tart cherry ocrhards with reduced crop, George W. Sundin and Nikki Rothwell

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

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