Multi-Year Regional Evaluation of One and Two Applications of Fungicides for the Management of Target Spot on Two Cotton Varieties

Corynespora target spot (Corynespora cassiicola) is a re-emerging disease of cotton in the Southeastern and Mid-Southern U.S. Variability in incidence and severity of target spot among years and geographic location suggests disease development is driven by extended periods of  moisture and warm temperatures. Following disease onset, premature defoliation occurs as lesions develop and spread throughout the canopy. Our objective was to evaluate the effectiveness of various fungicide application programs for managing target spot as well as yield response and lint quality. This study was conducted over 3 years with trials established in 4 locations across 4 states (FL, GA, LA, and VA) in 2014, 8 locations across 7 states (AL, FL, GA, LA, MS, TN, and VA) in 2015, and 10 locations across 6 states (AL, FL, GA, LA, MS, and TN) in 2016. Two varieties hypothesized to vary in susceptibility to target spot (Phytogen 499 and DeltaPine 1137) and four fungicides (Headline SC, Priaxor, Quadris, and Topguard) were evaluated. Fungicide treatments consisted of a single application at disease onset or an application at disease onset followed by a second spray 14 days later. Treatments were applied to plots in a full factorial, randomized complete block design with 2 varieties, 2 application timings, and 5 fungicide treatments (4 fungicides plus a non-treated control). Disease onset, incidence, and defoliation were evaluated bi-weekly following the first observation of lesions. Lint yield and quality were assessed following harvest. Date of disease onset and severity varied among years and locations. Disease incidence and defoliation were greater on PHY499 than on DP1137. Fungicides delayed disease progress and reduced overall defoliation, but yield responses to fungicide applications were infrequent. However, fungicides did reduce defoliation, which was correlated with yield. Implications of this study for the epidemiology and management of target spot in cotton will be discussed.