In vitro sensitivity of Corynespora cassiicola isolated from soybean to different chemical fungicides in northwestern Argentina

REZNIKOV, SEBASTÍAN; BLECKWEDEL, JULIANA; CLAPS, M. PAULA; GONZÁLEZ, VICTORIA; SAUTUA, FRANCISCO J.; CARMONA, MARCELO; PLOPER, L. DANIEL

Congreso; World Soybean Research Conference 11; 2023

Target spot of soybean, caused by the fungus Corynespora cassiicola, has become an important disease in northwestern Argentina (NWA). The Fungicide Resistance Action Committee (FRAC) classifies C. cassiicola as a high-risk pathogen for the development of fungicide resistance. The objective of this study was to determine the fungicide sensitivity of six C. cassiicola isolates to different commercial fungicides. These isolates were collected during the 2014/2015 growing season from soybean fields located in the provinces of Tucumán and Salta, in NWA. The 50% effective concentration (EC50) values were estimated by the relative mycelium growth reduction on PDA medium added with fungicides doses of 0, 0.1, 1, 10, 100 and 1000 ppm of active ingredient. The commercial fungicides used contained: azoxystrobin 25%, trifloxystrobin 50%, pyraclostrobin 25%, difenoconazole 25%, pydiflumetofen 20%, thiophanate-methyl 50%, mixtures containing epoxiconazole 5.0% + pyraclostrobin 13.3%, trifloxystrobin 15.0% + prothioconazole 17.5% + bixafen 12.5%, difenoconazole 12.5% + pydiflumetofen 7.5%, and fluxapyroxad 5.0% + pyraclostrobin 8.1% + epoxiconazole 5.0%. At 7 days, CE50 was estimated for each isolate and chemical product using the methodology by Kataria and Grover (1978). Azoxystrobin and trifloxystrobin were inefficient to control all isolates evaluated (EC50 1,891.3 to 854,673.0 ppm and EC50 3,103.2 to 236,159.7 ppm, respectively). Pyraclostrobin was inefficient for four isolates (EC50 148.8 to 392.1 ppm) and slightly efficient for two C. cassiicola isolates (EC50 19.6 to 49.8 ppm). Difenoconazole was moderately efficient for 5 isolates (EC50 1.6 to 6.2 ppm) and slightly efficient for one isolate (EC50 44.2), pydiflumetofen was moderately efficient (EC50 1.7 to 5.8 ppm), mixtures of epoxiconazole + pyraclostrobin was also moderately efficient (EC50 1.8 to 8.1 ppm) just like trifloxystrobin + prothioconazole + bixafen (EC50 1.1 to 4.1 ppm). Thiophanate-methyl was inefficient for one isolate (EC50 10,471.3 ppm), moderately efficient for one isolate (EC50 3.6 ppm), and highly efficient for four isolates (EC50 9.7x10-4 to 0.3 ppm). The mixtures difenoconazole + pydiflumetofen and fluxapyroxad + pyraclostrobin + epoxiconazole were highly efficient for all isolates (EC50 7.7x10-4 to 4.9x10-1 ppm and EC50 2.9 x10-6 to 5.7 x10-2 ppm, respectively). Since C. cassiicola showed variability in sensitivity to the fungicides evaluated, there is a high risk of selection for resistance. Further work needs to be carried out to detect possible mutations, increasing the number of isolates tested.