Defining fungicide resistance mechanisms in the Corynespora cassiicola population from Mississippi soybean

Target spot, caused by Corynespora cassiicola, is a common lower canopy soybean disease in the southern United States. Recently, target spot has resurged in importance especially following the identification of resistance to the quinone outside inhibitor (QoI) fungicides. As a result, a survey of C. cassiicola from soybean throughout Mississippi began in 2018. A total of 819 C. cassiicola monoconidial isolates were obtained from 228 fields in 75 counties. The molecular mechanism of QoI resistance was determined, resulting from an amino acid substitution from glycine (G) to alanine (A) at position 143, using a PCR-RFLP method and comparing nucleotide sequences of the cytochrome b gene. Five previously defined geographic regions were used to present the distribution of the G143A substitution and included the Capital, Coast, Delta, Hills, and Pines. The Capital had the greatest proportion of G143A-containing isolates (95.0%) followed by the Coast (92.9%), Delta (89.8%), Pines (78.8%), and Hills (69.4%). In all, 85.8% of the C. cassiicola isolates carried the G143A substitution. In addition, the effective fungicide concentration (EC50) of randomly selected C. cassiicola isolates to azoxystrobin was used to characterize isolates as resistant (n=14), based on presence of the G143A susbstitution and EC50 values > 52 μg/ml, or sensitive (n=11), based on absence of the G143A susbstitution and EC50 values < 46 μg/ml. The EC50 values varied among isolates (p < 0.0001) with QoI-sensitive isolates exhibiting lower EC50 values than QoI-resistant. The current study revealed that a reduction in sensitivity to QoI fungicides has likely resulted based on the percentage of C. cassiicola isolates containg the G143A substitution identified in Mississippi.