G143A Mutation in the Cytochrome B Gene Detected from Corynespora cassiicola Isolates in Alabama

Corynespora cassiicola, the causal agent of target spot disease, is a fungal pathogen with increasing importance in cotton and soybean producing countries. Quinone outside inhibitors (QoI) is one of the most important classes of fungicides widely used for disease management of foliar diseases in field crops; however, there are several reported cases of C. cassiicola isolates from tomato, cucumber, and soybean resistant to these fungicides. QoI-resistance has been detected in the cytochrome b (cytb) gene which a nucleotide substitution in a specific codon, such as G143A, F129L, and G137R, confer resistance to QoI fungicides. Therefore, we investigated the occurrence of a point mutation in the cytb gene associated with QoI-resistance of 12 isolates of C. cassiicola from cotton and soybean collected in Alabama. Multiple alignment analysis revealed a mutation at position 143 (G143A) in four isolates from soybean plants. The mutation changed the codon 143 (GGT to GCT), resulting in an amino acid change from glycine to alanine (G143A), which is known to be associated with QoI-resistance. The G143A mutation was not found on isolates from cotton plants. Other mutations, F129L and G137R, were not found in our isolates. The present study is the first report of the occurrence of the G143A mutation associated with C. cassiicola in the U.S. The dynamic of mutations associated with fungicide resistance in C. cassiicola should be monitored for management of resistance to C. cassiicola in the U.S.