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Oral: Fungicide Resistance


Fitness and competition studies of QoI resistant and sensitive Cercospora sojina isolates, the causal agent of frogeye leaf spot
B. LIN (1), H. Kelly (1), H. Yu (2), A. Mengistu (3) (1) University of Tennessee-Entomology & Plant Pathology, U.S.A.; (2) University of Tennessee-Entomology & Plant Pathology, U.S.A.; (3) United States Department of Agriculture, Agricultural Research Ser

Frogeye leaf spot (FLS), caused by Cercospora sojina, is a foliar disease affecting soybean, often managed by applications of Quinone outside inhibitor (QoI) fungicides. Because C. sojina isolates have developed resistance to QoIs, a better understanding of the biology and aggressiveness of isolates is important. Spore suspension inoculum prepared from QoI-resistant and -sensitive isolates were mixed in six different ratios. Greenhouse plants (cultivar Blackhawk) were treated: water (control), Quadris, or Quadris Top. Results were:1) Disease severity increased as the proportion of QoI resistance increased, indicating that QoI-resistant isolates are more aggressive; 2) Quadris inhibited FLS caused by QoI-sensitive isolates only, and when the proportion of QoI-resistance was ≤ 10%; 3) Quadris Top provided more effective control of FLS by reducing symptoms and delaying disease development. Fitness parameters measured were (i) mycelial growth and sporulation capacity, (ii) conidial germination and growth inhibition on fungicide amended media. No significant differences of the fitness parameters were observed between resistant and sensitive isolates, except for sporulation capacity and growth inhibition. Moreover, qPCR results showed resistant isolates dominated the population in all mixed inoculation tests. These results indicate there are differences in QoI resistant and sensitive C. sojina isolates in-planta that will influence effective disease management of FLS.