Influence of Weed Control Programs in Glyphosate -Resistant Cotton on Insects and Seedling Disease.

Research conducted from 1997 to 1999 in cotton (Gossypium hirsutum L.) and in non-crop areas evaluated early season insect control and weed control with labeled rates of several insecticides and the herbicide glyphosate applied alone and in combinations. Dicrotophos plus glyphosate improved aphid (Aphis gossypii Glover) control, and thrips (Frankliniella spp.) control was improved with the addition of glyphosate to acephate, dicrotophos, or dimethoate. Redweed (Melochia corchorifolia L.) control was reduced with glyphosate plus endosulfan, and addition of oxamyl, endosulfan, dicrotophos, and imidacloprid reduced prickly sida (Sida spinosa L.) control compared with glyphosate alone. Only the glyphosate plus lambda-cyhalothrin combination controlled less hemp sesbania [Sesbani exaltata (Raf.) Rybd. ex A. W. Hill] compared with glyphosate alone. For both the insect and weed control studies, responses associated with the combinations were not consistent over experiments and differences observed were not of practical significance. Field studies were conducted to evaluate the interaction between simulated thrips injury (manual removal of cotton terminals) and glyphosate postemergence weed control programs. Removal of cotton terminals delayed appearance of first square and first flower and decreased seedcotton yield in two of four experiments whether or not glyphosate was applied. Results suggested that stress imposed on cotton plants from thrips damage should not result in greater injury from glyphosate when applied according to label to glyphosate-resistant cotton. Field experiments evaluated the influence of preemergence herbicides and postemergence glyphosate application on seedling diseases in cotton. In one year, use of fluometuron with pendimethalin, pyrithiobac, or metolachlor preemergence followed by glyphosate (0.84 kg ai/ha) to cotyledon cotton increased hypocotyl disease severity 12% compared with the same program without fluometuron. Disease severity on hypocotyls was 7% greater when glyphosate was applied at 4-leaf than at cotyledon. In contrast, disease severity on roots was 46% greater when glyphosate was applied at cotyledon. Results of greenhouse experiments using soil inoculated with Rhizoctonia solani supported those of the field study showing that certain preemergence herbicides can predispose cotton to greater seedling disease injury and that glyphosate applied to cotyledon glyphosate-resistant cotton can reduce hypocotyl disease severity