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TECHNICAL SESSION: Chemical Control of Fungal Diseases

Fungicide sensitivity screening for Corynespora cassiicola and field evaluations
Ty Smith - University of Tennessee. Heather Kelly- University of Tennessee

Target spot (TS), caused by the fungus Corynespora cassiicola, is a foliar disease of cotton and soybean. Over recent years, TS has become a disease of concern. in both production systems. Data for fungicide sensitivity and understanding potential impact on yield is lacking for C. cassiicola. The objective of this study is to conduct fungicide screening to monitor sensitivity in C. cassiicola in Tennessee soybean and cotton production. The sensitivity of 18 C. cassiicola isolates to 8 technical grade fungicides across multiple fungicide groups (FRAC Groups 1, 3, 7, and 11) was evaluated based on mycelial growth inhibition assays. The EC50 of each fungicide was calculated. All active ingredients had EC50 > 100 µg/mL with the exception of difenoconazole which EC50 varied among isolates from 3 to 40 µg/mL. Field trials were also conducted at 3 locations for soybean and 1 location for cotton. Five fungicide tank mixes were evaluated for control of TS among 3 soybean varieties of differing susceptibility and 6 fungicide products were evaluated among 1 cotton cultivar. While Miravis Top and Quadris Top SBX reduced TS compared to the non-treated check, only Miravis Top significant increased soybean yield. No treatments were significantly different in the cotton trial. Based on the EC50 values and lack of control with products not containing difenoconazole one can conclude there is lack of efficacy and/or resistance already in the isolates tested.