Assessment of Quinone Outside Inhibitor Sensitivity and Frogeye Leaf Spot Race of Cercospora sojina in Georgia Soybean

Y Frogeye leaf spot (FLS), caused by the fungal pathogen Cercospora sojina K. Hara, is a foliar disease of soybean (Glycine max L. [Merr.]) responsible for yield reductions throughout the major soybean-producing regions of the world. In the United States, management of FLS relies heavily on the use of resistant cultivars and in-season fungicide applications, specifically within the class of quinone outside inhibitors (QoIs), which has resulted in the development of fungicide resistance in many states. In 2018 and 2019, 80 isolates of C. sojina were collected from six counties in Georgia and screened for QoI fungicide resistance using molecular and in vitro assays, with resistant isolates being confirmed from three counties. Additionally, 50 isolates, including a baseline isolate with no prior fungicide exposure, were used to determine the percent reduction of mycelial growth to two fungicides, azoxystrobin and pyraclostrobin, at six concentrations: 0.0001, 0.001, 0.01, 0.1, 1, and 10 mu g ml(-1). Mycelial growth observed for resistant isolates varied significantly from both sensitive isolates and baseline isolate for azoxystrobin concentrations of 10, 1, 0.1, and 0.01 mu g ml(-1) and for pyraclostrobin concentrations of 10, 1, 0.1, 0.01, and 0.001 mu g ml(-1). Moreover, 40 isolates were used to evaluate pathogen race on six soybean differential cultivars by assessing susceptible or resistant reactions. Isolate reactions suggested 12 races of C. sojina present in Georgia, 4 of which have not been previously described. Species richness indicators (rarefaction and abundance-based coverage estimators) indicated that withincounty C. sojina race numbers were undersampled in this study, suggesting the potential for the presence of either additional undescribed races or known but unaccounted for races in Georgia. However, no isolates were pathogenic on `Davis', a differential cultivar carrying the Rcs3 resistance allele, suggesting that the gene is still an effective source of resistance in Georgia.

Automated summary

Frogeye leaf spot (FLS), caused by Cercospora sojina, is a major foliar disease affecting soybean production worldwide. In the U.S., management of FLS relies on resistant cultivars and fungicide applications, but fungicide resistance has been observed. A study in Georgia identified C. sojina isolates resistant to QoI fungicides, and evaluated the effectiveness of two fungicides at different concentrations against these isolates.