Epidemiology of Verticillium Wilt of Cotton: Effects of Disease Development on Plant Phenology and Lint Yield

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Abstract
The major effect of Verticillium wilt on cotton plants was the reduction of growth and development, which resulted in reduced plant height, lateral branching, and dry natter accumulation in leaves, stems, roots, squares (flower buds), and bolls. The number of nodes was not greatly reduced in diseased plants and plant stunting was mainly due to decreased internode elongation. One of the first visible disease symptoms was a reduction in growth rate that could be detected approximately 2 wk prior to the appearance of foliar symptoms. Squares were shed before or after foliar symptoms appeared and rarely developed to flowering size on diseased plants unless high air temperatures occurred. Set bolls, however, generally remained attached to the plant and eventually opened, even when defoliation occurred. At the time foliar symptoms appeared, the dry weight of taproots and main lateral roots was greater in plants with foliar symptoms than in those that were apparently healthy. Cotton lint yields and the number of open bolls per plant were directly related to the growth period before foliar symptoms appeared. During periods of high air temperatures, the effects of Verticillium wilt were reduced while plant growth and dry matter accumulation were resumed. Isolates of Verticillium dahliae from diseased cotton leaves in the field under study ranged from nondefoliating to defoliating pathotypes . Approxinately 19% of the isolates tested were classified as nondefoliating, 72% as intermediate types, and 9% as defoliating.