Fungicide resistance in Cercospora species causing Cercospora leaf blight and Purple seed stain of soybean in Argentina

SAUTUA, FRANCISCO J; DOYLE, VINSON P; PRICE, PAUL P; PORFIRI, ALEJANDRO; FERNANDEZ, PAULA; SCANDIANI, MARIA M; CARMONA, MARCELO A

PLANT PATHOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2020

0032-0862

Cercospora species cause Cercospora leaf blight (CLB) and purple seed stain (PSS) on soybean. Because there are no resistant soybean varieties available, CLB/PSS management relies heavily upon fungicide applications. Sensitivity of 62 Argentinean Cercospora isolates to demethylation inhibitor (DMI), methyl benzimidazole carbamate (MBC), quinone outside inhibitor (QoI), succinate dehydrogenase inhibitor (SDHI) fungicides and mancozeb was determined in this study. All isolates were sensitive to difenoconazole, epoxiconazole, prothioconazole, tebuconazole and cyproconazole (EC50 values ranged from 0.006 to 2.4 µg mL−1). In contrast, half (51%) of the tested isolates were sensitive (EC50 values ranged from 0.003 to 0.2 µg mL−1), and the other half (49%) were highly resistant (EC50 > 100 µg mL−1) to carbendazim. Interestingly, all isolates were completely resistant to azoxystrobin, trifloxystrobin, pyraclostrobin and insensitive to boscalid, fluxapyroxad and pydiflumetofen (EC50 > 100 µg mL−1). The G143A mutation was detected in 82% (53) of the QoI‐resistant isolates and the E198A mutation in 97% (31) of the carbendazim‐resistant isolates. No apparent resistance mutations were detected in the succinate dehydrogenase genes (subunits sdhB, sdhC and sdhD). Mancozeb completely inhibited mycelial growth of the isolates evaluated at a concentration 100 µg mL−1. All Argentinean Cercospora isolates were sensitive to the DMI fungicides tested, but we report for the first time resistance to QoI and MBC fungicides. Other mechanism(s) than fungicide target‐site modification may be responsible for resistance of Cercospora to QoI and MBC fungicides. Moreover, due to the recent introduction of SDHI fungicides on soybean in Argentina, we speculate Cercospora species causing CLB/PSS are insensitive (naturally resistant) to this group of fungicides.