Assessing Missouri soybean fields for azoxystrobin resistant cercospora sojina

Successful management of Cercospora sojina, the causal agent of frogeye leaf spot, can be achieved through utilizing resistant varieties and fungicide applications. Fungicides in the quinone outside inhibitor (QoI) class are most effective in controlling C. sojina in the field. Unfortunately, azoxystrobin fungicide-resistant isolates of C. sojina have been recovered in many soybean growing areas of the U.S. Fungicide-resistant isolates of C. sojina were first detected in 2011 and 2012 in two counties in southeast Missouri, but no further assessment was conducted. In this study, 121 isolates were collected from 15 surveyed counties between 2019 and 2020 in an effort to understand the geographical distribution of fungicide-resistant C. sojina. Isolates were collected from fields based on the presence of frogeye leaf spot symptoms. Samples were brought to the laboratory and isolates were recovered from individual lesions. A fungicide sensitivity bioassay was conducted to determine which isolates were resistant to the QoI class of fungicides. The fungicide sensitivity bioassay consisted of full-strength PDA amended with technical grade azoxystrobin at five different concentrations and a no fungicide control. Out of 121 isolates, 81 were fungicide-resistant representing 13 of the 15 Missouri counties included in the survey. Notably, the northwest corner of Missouri had the highest concentration of fungicide-resistant isolates, consistent with the recent recovery of fungicide-resistant C. sojina isolates in Iowa and Nebraska. The widespread recovery of fungicide-resistant C. sojina in multiple counties throughout Missouri provides new insight into disease management in the state.