Jun 13, 2011

Powdery mildew control in tart cherry orchards

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. 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 – 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 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 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 plus Captan is excellent for control of cherry leaf spot, it is weak against powdery mildew.

There is a new fungicide available in 2011 called Quintec (quinoxyfen). This fungicide has a new mode of action that is different from the strobilurin in Gem. The strobilurin and boscalid in Pristine works well against powdery mildew, so this new fungicide is a good choice for resistance management. Quintec at 7 fluid ounces per acre has performed very well in powdery mildew trials on cherry in Washington State. However, Quintec has no activity against cherry leaf spot, and growers must add another product to the tank for leaf spot.

By George W. Sundin and Nikki Rothwell, Michigan State University Extension