Efficacy of Registered and Candidate Fungicides Against Corynespora Leaf Spot Compared

Studies were conducted at the Gulf Coast Research and Extension Center in Fairhope, AL on a site previously cropped to cotton to assess the efficacy of registered and candidate fungicides for the control of Corynespora leaf spot caused by Corynespora crassiicola on cotton.  In study 1, registered fungicides Twinline at 7 and 8.5 fl oz/A, Headline 2.09SC at 6 and 9 fl oz/A, and Quadris 2.08E at 6 and 9 fl oz/A were applied at first bloom and 14-days later to Phytogen 499 and DPL 1050.  For study 2, registered fungicides Twinline at 7 and 8.5 fl oz/A, Headline 2.09SC at 6 fl oz/A, and Quadris 2.08E at 6 fl oz/A along with Headline AMP at 9 fl oz/A, Quilt EXCEL at 14 fl oz/A, Muscle 3.6E at 7.2 fl oz/A, Bravo Weather Stik at 1.5 pt/A, and Stratego YLD at 5 fl oz/A were applied to Phytogen 499 at first sign of disease on July 26 and again 14-days later.  Corynespora leaf spot intensity was rated on September 13 using the Florida 1 to 10 peanut leaf spot rating scale.  In study 1, Phytogen 499 suffered heavier leaf spotting and defoliation than DPL 1050.  While defoliation was reduced with Twinline, Headline 2.09SC and Quadris 2.08E, none provided a high level of protection from Corynespora leaf spot on either variety.  Also, disease control did not improve at the higher rates of the above fungicides.  For study 2, sizable reductions in defoliation were obtained with the recommended fungicides Twinline, Headline 2.09SC, and Headline AMP when compared with the non-treated control which suffered 80% defoliation.  Least efficacious fungicides were Bravo Weather Stik, Muscle 3.6E, and Stratego YLD.  When compared with the control in study 1, only Headline 2.09SC at 9 fl oz/A significantly increased seed yield.  For study 2, seed yield for the control and fungicide treatments were similar.