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Fungicide-resistant apple scab arrives in Ont.

Bayer CropScience is warning Ontario apple growers away from one of its products as a treatment for apple scab now that a resistant strain of the disease has arrived in the province.

A recent sampling of scab-infected apple leaves from orchards across the province has revealed the presence of apple scab resistant to quinone outside inhibitors (QoI), including strobilurin fungicides, Bayer said in a release Thursday.

QoI fungicides include Bayer’s trifloxystrobin product Flint and BASF’s kresoxim-methyl product Sovran, both strobilurins, Bayer noted.

Bayer said it took its samples from nine orchards in “almost all major apple growing regions” of Ontario during the 2009 season. Of those, apple scab tissue from seven locations was found to be QoI-resistant in tests at Michigan State University.

“As a result of this study, Bayer CropScience is advising apple growers in Ontario not to use Flint for control of apple scab during the 2010 growing season,” David Kikkert, Bayer’s portfolio manager for horticulture, said in the company’s release.

That warning doesn’t apply to Flint’s other label uses, such as control of powdery mildew, fly speck, sooty blotch and cedar apple rust, Kikkert said.

Flint’s product label already had a warning that resistance to its active ingredient could develop in fungi over time.

“Any fungal population may contain individuals naturally resistant to (Flint) and other Group 11 fungicides,” the company notes on Flint’s label. “A gradual or total loss of pest control may occur over time if these fungicides are used repeatedly in the same locations.”

For apple growers outside Ontario, Bayer now advises tank-mixing Flint with a compatible registered contact fungicide.

Bayer on Thursday described 2009 as the worst scab year on record in Ontario. Cold winters and wet growing conditions favour the disease, which can directly affect yield, fruit quality and storability or indirectly lead to defoliation, which can affect tree vigour and winter survival.

Losses are “typically less serious” in British Columbia’s apple-growing regions but can be “significant” in some fruit-growing areas of B.C.’s interior, the company said.

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