Epidemiology of Verticillium Wilt of Cotton: A Relationship Between Inoculum Density and Disease Progression

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Abstract
Inoculum density of Verticillium dahliae in field soil in May was related to mid-Sept. incidence of foliar symptoms of Verticillium wilt in cotton over a period of 7 yr within 2 adjacent fields in California [USA]. Disease progress curves for each growing season showed a highly significant straight line relationship with time when high air temperatures did not inhibit disease development. Slope values of the disease progress curves were related to inoculum density of V. dahliae at population densities below 40 propagules/g (p/g) of soil. In weekly samplings throughout the season, percentage of plants with foliar symptoms was always less than the percentage of plants with vascular discoloration. The differences between these 2 symptoms increased during periods of high air temperatures when further development of foliar symptoms was arrested. Inoculum density of soil sampled in May increased on the average of 13-15 p/g per year in soil continuously cropped to cotton. The naturally occurring pathotypes present in the experimental field sites ranged from nondefoliating to partial defoliating. Approximately 19% of the leaf isolates tested were classified as nondefoliating, 72% as intermediate types, and 9% as defoliating.