Preliminary results from potato leaf-sampling conducted across Canada shows more than 80 percent of the early blight organisms collected carried a mutated gene for reduced sensitivity to strobilurins.

The reduced sensitivity doesn't mean that strobilurin fungicides don't work at all—it just means they don't provide the control they once did.

Strobilurins, also known as QoIs, are a family of fungicides that have a single mode of action. They include Stratego, Flint, Cabrio EG, Headline, Quadris, Abound, as well as an ingredient of Pristine and Quilt.

Of the 113 isolates of the early blight fungus, Alternaria solani, 80 percent tested positive for the mutation F129L, according to a news release.

The results confirm what many experts expected—the reduced sensitivity of strobilurins to early blight is a widespread issue affecting many potato-growing areas of Canada.

University of Wisconsin plant pathologist also have noted reduced sensitivity to strobilurins, based on laboratory tests of isolates.

Early blight leaf tissue samples were collected during the summer of 2008 in Alberta, Manitoba, Ontario and Prince Edward Island through collaborative efforts of Bayer CropScience sales representatives and provincial potato specialists.

“Initial results confirm that mutant strains are widespread in Alberta, Manitoba and Ontario, which is not surprising given the widespread distribution of mutant strains of the early blight fungus in the United States,” says Rick Peters, research scientist with Agriculture and Agri-Food Canada in Charlottetown, Prince Edward Island. “To date, PEI is the only area that we’ve surveyed where all samples were still sensitive to strobilurins, likely due to the less frequent use of strobilurins in this province.”

The message to potato growers is clear, based on the results of the recent sampling program.

“Develop a sound fungicide management plan to rotate product groups,” says Andrew Dornan, field development representative with Bayer CropScience. “Strobilurins are very susceptible to resistance, and we can’t reverse the effects of the level of reduced sensitivity we are seeing with early blight, and growers need new effective options for early blight control.”