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Cherry powdery mildew is developing resistance to common fungicides in some Pacific Northwest orchards, according to preliminary data from Oregon State University.

Powdery mildew isolates from ten Washington and Oregon sweet cherry orchards were collected during 2005 and from five orchards in the two states in 2006. The data showed reduced sensitivity to dimethylation-inhibitor fungicides, known as DMIs, said Jill Calabro, OSU graduate student who conducted the studies. Calabro tested resistance to the same five DMI fungicides in both years.

Cross resistance

"Once resistance develops, growers need to be concerned with cross resistance from within the same category," she said during a presentation at the Cherry Institute annual meeting in Yakima, Washington. For example, if the DMI fungicide Rally (myclobutanil) develops resistance, Rubigan (fenarimol) will likely develop resistance also because it is in the same DMI fungicide class and has the same mode of action. Resistance within the same fungicide class can develop even if the fungicides were not used in the orchard.

Resistance to powdery mildew is not new. Other crops, like grapes and cucurbits, have already developed resistance to powdery mildew that is similar genetically to cherry powdery mildew.

In both years, she found no orchard sensitivity to Procure (triflumizole), a newer DMI fungicide. But she did find sensitivity at varying levels to Rally, Rubigan, Elite (tebuconazole), and Orbit (propiconazole). An organic orchard in Mosier, Oregon, showed no resistance sensitivity, however, a conventional orchard in The Dalles, Oregon, exhibited resistance sensitivity to three DMI fungicides.

Management

"We do appear to have some resistance, but there are a lot of things we can do to help manage it," she said.

Growers should practice resistance management by using a diversity of fungicide classes, she suggested. When using fungicides that are at risk for resistance, always use the full label rate and the shortest interval, along with good spray coverage. Alternate between the classes of fungicides, never using the same class for more than two consecutive sprays and never using the at-risk fungicides more than three times in a season.

"Use at-risk fungicides protectively, not as an eradicant," Calabro said, adding that oils and carbonates are eradicants, but not the dimethylation-inhibitor fungicides. Resistance, she explained, can develop quickly. In cucurbits with powdery mildew, resistance to fungicides developed in as short a period as two weeks.