Fungicide sensitivity screening of Corynespora cassiicola in US soybean and cotton

Target spot, caused by the fungus Corynespora cassiicola, is a foliar disease of cotton and soybean. Target spot has become a disease of concern in soybean and cotton production systems. Data for fungicide sensitivity and understanding potential impact on yield is lacking for C. cassiicola. Fungicide resistance is also a concern in C. cassiicola with the increase in resistant pathogens in soybean. With the lack of knowledge about baseline fungicide sensitivity and the concern of fungicide resistance the objectives of this study were: (i) to evaluate fungicide products and mixes in small plot soybean and cotton trials, (ii) develop baseline fungicide sensitivity data on C. cassiicola isolates from Tennessee soybean and cotton, (iii) investigate the interaction of the insecticide malathion with demethylation inhibitor fungicides on target spot management, and (iv) screen C. cassiicola isolates for possible fungicide resistance mutations that are non-sensitive to quinone outside inhibitor (QoI) fungicides.

Small plot field trials were conducted in soybean and cotton in 2018-2020 to evaluate different fungicide products on management of target spot. A wide variety of fungicides were used in soybean and cotton. Miravis Top was the only product to protect yield and decrease target spot severity in the soybean trials. In cotton, fungicides provided no impact to disease severity and yield in 2018 and 2020, whereas Miravis Top protected yield and decreased disease severity in 2019. Pyraclostrobin, thiophanate methyl, and azoxystrobin had the highest EC50 values when evaluating baseline sensitivity. In vitro and field evaluations suggest that products that contain FRAC groups 7 and 3 have the potential to better protect yield from target spot compared to products containing groups 11 or 1 fungicides. Due to the presence of the G143A mutation in two C. cassiicola isolates from soybean, fungicide resistance to the QoI fungicides has been confirmed in Tennessee. The interaction between malathion and two DMI fungicides was evaluated over three separate trials: in small plots, in a detached leaf assay, and in vitro. Malathion had little to no efficacy on management of C. cassiicola and also did not impact the efficacy of the two DMI fungicides.