Isolates of Corynespora cassiicola, Causal Agent of Target Spot of Soybean, in Kentucky Contain the G143A Mutation Conferring Resistance to Quinone Outside Inhibitor Fungicides

Target spot, caused by Corynespora cassiicola, is a soybean disease of increasing importance in the southern United States. Recently, isolates of C. cassiicola with resistance to quinone outside inhibitor (QoI) fungicides have been confirmed with the G143A mutation in multiple southern states, including Alabama, Arkansas, Mississippi, and Tennessee. From 2017 to 2019, a total of 84 isolates of C. cassiicola were recovered from soybean fields in 12 counties in Kentucky. DNA sequencing of the cytochrome b gene of these isolates revealed that 15.5% of the isolates had the G143A mutation that confers resistance to QoI fungicides and were in 50% of the counties in which isolates originated. This represents the first report of QoI fungicide resistance in C. cassiicola isolates from Kentucky soybean fields. Considering these findings, Kentucky soybean growers should adopt target spot management practices that include rotating to nonhost crops, planting resistant soybean cultivars, and applying fungicides from different fungicide classes.