Defining Fungicide Resistance Mechanisms in the Population from Mississippi Soybean.

Target spot, caused by , 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 from soybean throughout Mississippi began in 2018. A total of 819 monoconidial isolates were obtained from 228 fields in 75 counties. The molecular mechanism of QoI resistance was determined, which resulted 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 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 isolates carried the G143A substitution. In addition, the effective fungicide concentration (EC) of randomly selected isolates to azoxystrobin was used to characterize isolates as resistant ( = 14) (based on the presence of the G143A substitution and EC values >52 μg/ml) or sensitive ( = 11) (based on the absence of the G143A substitution and EC values <46 μg/ml). The EC values varied among isolates ( < 0.0001), with QoI-sensitive isolates exhibiting lower EC values than QoI-resistant isolates. The current study revealed that a reduction in sensitivity to QoI fungicides has likely resulted based on the percentage of isolates containing the G143A substitution identified in Mississippi.