developed in Ontario can be of potential service to other provinces. CAUSES OF RESISTANCE Overuse of fungicide products containing strobilurin has been blamed for resistance development, but in resistance to fungicides like Headline in Tennessee and other U.S. states, the fields in question did not have a history of overuse. "The first thing investigated was management practices and potential overuse," explains Tenuta, "but they started with frogeye leaf spot to isolate the resistant wild type and found that it was quite common in the soybeanproducing areas of the U.S. So, selection for more resistance has been happening in endemic resistant populations — those that were already there." CONTROLLING RESISTANCE As with herbicide resistance, growers can avoid contributing to the development of fungicide resistance by scouting to ensure proper fungicide selection and applying fungicide only when necessary and in response to increased disease risk. "With foliar diseases in soybeans or any crop, we need to understand the disease risk and the pathogen parameters, what it likes and what it doesn't like," says Tenuta. "With frogeye and white mold in soybeans, these pathogens like wetter or cooler or moderate conditions, and the risks are different every year and can be different regionally as well in Ontario." Scouting is important before R1 to full flower to pod set in soybeans and starting before tasselling in corn. Ensure your disease identification is correct so that your product selection is correct. Scout fields again within two weeks after any foliar fungicide application to determine if the fungicide is adequately managing the disease. Mixed modes of action in fungicides are available now, which helps with disease control but also reduces the selection pressure for resistance to develop, says Tenuta. "And don't go below label rates," he says. "The label rates are established to be consistently effective over time and should be closely followed." l ONTARIO GRAIN FARMER 17 SEPTEMBER 2022 Resistance to fungicides is a new issue. The horticulture and greenhouse sector have already had to deal with resistance, but it's new to the field crop sector. AirMix Liquid Fluidizer Flat Floor Stainless Tanks Stainless Hopper Bins Powder Coated Multi-Purpose SmoothWall Bins Meridian is the leader in fertilizer storage systems. Take advantage of low prices, bulk savings and avoid lineups when you’re time matters most by storing fertilizer on your farm. Meridian has a full range of customizable Stainless Tanks and SmoothWall Bins sure to fit your needs. ® © 2021 Meridian Manufacturing Inc. Registered Trademarks Used Under License. (11/2021)
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