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Oral: Fungicide Resistance


Assessing potential virulence differences between QoI-sensitive and -resistant Cercospora sojina isolates from Mississippi soybean
N. BROCHARD (1), M. Tomaso-Peterson (2), T. Allen (3), R. Melanson (3) (1) Mississippi State University, U.S.A.; (2) Mississippi State Unversity, U.S.A.; (3) Mississippi State Unversity, U.S.A.

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Frogeye leaf spot (FLS) of soybean is caused by the fungus Cercospora sojina Hara. Quinone outside inhibitor (QoI) fungicides have historically been the standard for FLS management in MS. However, widespread QoI resistance has been reported in the MS soybean production system. QoI resistance may be associated with a fitness cost that includes a loss of virulence. A comparison of virulence was assessed between two C. sojina sensitivity groups based on the presence or absence of the G143A substitution and termed QoI-sensitive or -resistant. Twenty four C. sojina isolates, 11 QoI-sensitive and 13 QoI-resistant, were assayed in an in planta virulence assessment. Soybean seedlings were inoculated with conidial suspensions of each C. sojina isolate. Percent FLS severity assessments were made 21 days post inoculation. QoI-sensitive and -resistant C. sojina isolates were pathogenic to soybean seedlings based on the production of typical FLS lesions. Disease severity significantly differed among isolates within sensitivity groups. No differences in disease severity were noted between sensitivity groups; however, a 4.5% increase in severity was noted for the QoI-resistant group. This suggests QoI-resistant isolates are equally virulent as QoI-sensitive isolates, indicating no fitness cost associated with QoI-resistance. Failure to control C. sojina with QoIs dictates the need for alternative management practices when high-yielding FLS-susceptible cultivars are planted in MS.