An Estimate of Yield Loss to Corynespora Leaf Spot On Cotton

Potential impact of Corynespora leaf spot on cotton yield was evaluated in parallel studies conducted at the Gulf Coast Research and Extension Center (GCREC) in Fairhope, AL and Plant Breeding Unit (PBU), in Tallassee, AL on sites cropped the previous year to cotton.  The experimental design was a split plot design with cotton variety as the main plot and Headline 2.09SC application number as the split plot treatments.  Phytogen 499 and DPL 1050 cotton varieties were planted at both study sites. Headline 2.09SC at 6 fl oz/A treatments started at first bloom and were repeated at 14-day intervals as specified for a total of one, two, three, four, and five applications at the former site and from one to six applications at the latter site.  A non-fungicide treated control was included on both varieties at each study site.  Corynespora leaf spot intensity was rated starting 2 to 3 weeks after first bloom at one to two week intervals through mid-September using the Florida 1 to 10 peanut leaf spot rating scale. Cotton was irrigated at PBU but not at GCREC.  Corynespora leaf spot intensity was higher on Phytogen 499 than DPL 1050.  At both sites, Corynespora leaf spot intensity on  Phytogen 499 and DPL 1050 gradually declined, particularly on the former variety, as Headline application number increased.  At GCREC, no further decline in defoliation occurred with more than three and four Headline applications on DPL 1050 and Phytogen 499, respectively.  Least defoliation was seen on the above varieties at PBU with six Headline 2.09SC applications. For seed yield at GCREC, application number and the variety x application number interaction were significant.  Regression analysis showed a significant yield response due to Headline application number on Phytogen 499 but not DPL 1050.  The relationship between Corynespora leaf spot incited defoliation and yield at the PBU study will also be discussed.