In vitro Effect of Fungicides on Corynespora cassiicola Isolates from Cotton and Soybean in Alabama

Corynespora cassiicola is a pathogen with increasing importance in cotton and soybean producing countries. Severe disease symptoms and significant yield losses can occur when the pathogen is not properly controlled. Fungicides have been a crucial tool in disease management; however, reported cases of C. cassiicola resistance to MBC, SDHI, and QoI have been published from tomato, cucumber and soybean isolates. Specific genes known as cassiicolin-encoding genes (Cas2, Cas6, Cas2+6) were found in C. cassiicola isolates in Alabama, reflecting an overall genetic differentiation. For this reason, we explored the effectiveness and determined the sensitivity (EC₅₀) of fungicides over different isolates of cotton and soybean from Alabama based on the cassiicolin-encoding genes. Cotton and soybean leaf samples were collected from different locations of Alabama and 12 isolates were obtained. All isolates were submitted to DNA extraction and PCR with specific primers for gene detection. Sensitivity (EC₅₀) of C. cassiicola to Headline (pyraclostrobin) and Priaxor (pyraclostrobin+fluxapyroxad) was determined using a mycelial growth and EC₅₀ values for each isolate were estimated by logarithmic regression analysis (P ≤ 0.05). EC₅₀ for the fungicide Headline ranged between 17.71–66.01 mg/L for C. cassiicola isolates from cotton and ranged between 50.03–94.50 mg/L for C. cassiicola isolates from soybean. For the fungicide Priaxor, EC₅₀ ranged between 0.57–1.03 mg/L for C. cassiicola isolates from cotton and ranged between 1.45–11.80 mg/L for C. cassiicola isolates from soybean. The results suggested that soybean isolates have been exposed to these fungicides more often than cotton isolates. High EC₅₀ values found for the fungicide Headline indicate a loss of sensitivity of C. cassiicola isolates to their active ingredient. A potential loss of sensitivity of C. cassiicola isolates to fluxapyroxad was observed when Priaxor was the fungicide tested. Our results will be useful to monitor resistance to fungicides of C. cassiicola and help with fungicide resistance management.