Determining the Distribution of Qol Fungicide-Resistant Cercospora sojina on Soybean from Indiana

摘要

Frogeye leaf spot (FLS) is a foliar disease of soybean (Glycine max) caused by Cercospora sojina. Application of fungicide products that contain quinone outside inhibitor (Qol) active ingredients has been one of the major tools used in the managementof this disease, but, since 2010, Qol-resistant C. sojina isolates have been confirmed in over 20 states in the United States, including Indiana. In summer 2019 and 2020, 406 isolates of C. sojina were collected from 32 counties across Indiana and screened for Qol resistance using a PCR-restriction fragment length polymorphism (RFLP) method. An in vitro fungicide sensitivity test was also performed on a subset of isolates to evaluate their sensitivity to three Qol fungicides: azoxystrobin, pyraclostrobin, and picoxystrobin. A discriminatory dose of picoxystrobin was established as 1 mu,g/ml by testing five concentrations (0.001,0.01,0.1, l,and 10 p,g/ml). Qol-resistant isolates were found in 29 counties, and 251 of the 406 isolates (61.8) were confirmed to be resistant to Qol fungicides based on PCR-RFLP results. Partial nucleotide sequences of the cytochrome b gene from four resistant and four sensitive isolates corroborated the presence and absence, respectively, of the G143A mutation. Results fromthe sensitivity assays with discriminatory doses of azoxystrobin (1 p.g/ml) and pyraclostrobin (0.1 p-g/ml) also supported the findings from the PCR-RFLP assay, because all Qol-resistant isolates were inhibited less than 50 relative to a nofungicide control when exposed to these doses. Resistant isolates harboring the G143A mutation also exhibited resistance to picoxystrobin. The effective concentrations to inhibit mycelial growth by 50 relative to the nonamended control (EC_(50)) in Qol-sensitive isolates ranged from 0.087 to 0.243 mu-g/ml, with an overall mean of 0.152 p-g/ml, while EC_(50) values in Qol-resistant isolates were established as >10 mu-g/ml for picoxystrobin. Results from this study indicated that Qol-resistant C. sojina isolates arespread throughout Indiana and exhibit cross-resistance to Qol fungicides.