Target Spot-Incited Defoliation and Yields of Selected Cotton Varieties over a Three Year Period in Southwest Alabama

A study was conducted at the Brewton Agriculture Research Unit in 2014, 2015, and 2016 to assess the reaction of selected mid- and full-season commercial cotton varieties to target spot, incited by Corynespora cassiicola, and disease impact on yield.  The experimental design was a factorial arranged in a split plot with the cotton varieties Phytogen 499WRF, Phytogen 575WRF, Deltapine 1137B2RF, Deltapine 1252B2RF, Fibermax 1944GLB2, and Stoneville 6448GLB2 as whole plots and a fungicide program consisting of four or five applications of Headline SC at 9 fl oz/A + Bravo Ultrex at 1.5 pt/A as the split plot treatment designed to minimize target spot incited defoliation and subsequent yield loss or a non-fungicide treated control.  The site was irrigated as needed in all study years and fertilized for maximum yield.  Final defoliation levels and rAUC values differed by cotton variety and fungicide program.  While final defoliation and rAUC for all the above varieties were lower for the fungicide than non-fungicide treated controls, lower values for each of these variable on the fungicide treated cotton were obtained for Phytogen 575WRF, Deltapine 1252B2RF, Stoneville 6448GLB2, and Fibermax 1944GLB2 compared with Phytogen 499WRF.  For the non-fungicide treated controls, higher defoliation and rAUC values were obtained for the latter variety than for the other five remaining varieties, which had similar lower values for both disease variables.  Over the study period, equally higher yields were noted in 2014 and 2015 than 2016, when elevated hard lock counts were recorded.  In addition, Fibermax 1944GLB2 had higher seed yields than Phytogen 499WRF, Phytogen 575WRF, Deltapine 1137B2RF, Deltapine 1252B2RF, and Stoneville 6448GLB2 with all of the latter varieties having similar yields.  Finally, seed cotton yields were higher for the fungicide- than non-fungicide treated control.  Data for total boll, rotted, and hard locked bolls will be presented.