Fitness Penalty of Qoi-Resistant Isolates of C. cassiicola and Sensitivity Profile to Five Fungicides

The fungal pathogen known as Corynespora cassiicola causes a known disease on cotton and soybean plants, target spot. Fungicides are a crucial tool in disease management but reported cases of C. cassiicola resistance to different fungicide groups have been reported. The objectives of this study were (i) to determine if there is a fitness loss on C. cassiicola isolates with the G143A mutation in the cytochrome b gene;  (ii) to determine the sensitivity profiles for C. cassiicola from cotton and soybean to thiophanate-methyl, prothioconazole, pyraclostrobin, mancozeb, and the combination fluxapyroxad + pyraclostrobin. Mycelial growth of twelve C. cassiicola isolates were compared, four isolates QoI-resistant and eight QoI-sensitive. No fitness penalty was found, whether QoI-resistant or QoI-sensitive. EC₅₀ values of QoI-resistant isolates were statistically higher than QoI-sensitive isolates for all fungicides, except for thiophanate-methyl. EC₅₀ values for cotton and soybean isolates were statistically different for pyraclostrobin, mancozeb, and the mixture fluxapyroxad + pyraclostrobin but not for thiophanate-methyl and prothioconazole. Our study characterized EC₅₀ values for five fungicides of C. cassiicola isolates from cotton and soybean in the United States, and these values can be used as a reference for further studies. These results will be useful to monitor sensitivity of U.S. populations of C. cassiicola from cotton and soybean, and to facilitate fungicide resistance management through detection of shifts in fungicide sensitivity.