Comparison of quinone outside inhibitor fungicide-resistant and -sensitive isolates of Cercospora sojina

Isolates of Cercospora sojina, causal agent of frogeye leaf spot of soybean (Glycine max), that are resistant to quinone outside inhibitor (QoI) fungicides have been reported in the United States. Radial mycelial growth and sporulation of QoI-resistant and -sensitive C. sojina isolates were compared in the laboratory, and virulence of QoI-resistant and -sensitive isolates on a susceptible soybean cultivar (Blackhawk) and on a cultivar (Davis) with the Rcs3 resistance gene that confers resistance to frogeye leaf spot were compared. No differences in amount of sporulation between QoI-resistant and -sensitive isolates were observed. On cultivar (cv.) Blackhawk, frogeye leaf spot caused by QoI-resistant isolates was more severe than disease caused by QoI-sensitive isolates 7-8 days after inoculation, but no differences in disease between QoI-resistant and -sensitive isolates were observed 9 days after inoculation. On cv. Davis, QoI-resistant isolates caused significantly greater disease severity than sensitive isolates 8-14 days after inoculation. Although cv. Davis was affected more severely by QoI-resistant isolates, disease severity was less on cv. Davis than cv. Blackhawk, indicating that the Rcs3 gene was still effective against the C. sojina isolates tested. Our findings indicate that no fitness costs associated with QoI resistance in C. sojina were observed in the characteristics measured in our research.