A crop consultant working in southeast Missouri recently examined a soybean field that had many plants infected with frogeye leaf spot, and the field had previously been treated with a strobilurin fungicide. The fungicide should have stopped development of frogeye, but had not. The consultant sent frogeye leaf spot-affected soybean leaves from this field to be tested for strobilurin fungicide resistance by Dr. Carl Bradley at the University of Illinois. Dr. Bradley isolated the pathogen from 10 different leaves and tested each for sensitivity to strobilurin fungicide. He learned that the isolates collected from four leaves were sensitive to strobilurin fungicides, but the isolates from six leaves were resistant to strobilurin fungicides. This is the first confirmed finding in Missouri of this pathogen that is strobilurin resistant. Farmers and consultants should observe soybean fields that have been treated with a strobilurin fungicide to determine if frogeye leaf spot is present and still spreading. If so, plants in the field could be treated with a fungicide other than strobilurin to stop the spread of this disease. Current research results show that application of a fungicide to soybean plants at or past the R6 growth stage, when seeds are full sized, will not be as beneficial to yield as an application made at the R3 or R5 growth stages.
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REVISED: September 26, 2011