INCIDENCE OF VERTICILLIUM WILT AND YIELD LOSSES OF COTTON CULTIVARS (GOSSYPIUM-HIRSUTUM) BASED ON SOIL INOCULUM DENSITY OF VERTICILLIUM-DAHLIAE

Inoculum density (propagules per gram of soil) of Verticillium dahliae at planting time, when plotted against the disease incidence (percent foliar symptoms) for two cotton cultivars for three successive years, closely followed the negative exponential curve Y = 57.09*(1 - e-0.351*X), Correlation coefficients for each of the 3 yr separately and all 3 yr cumulatively were 0.71, 0.96, 0.84, and 0.85, respectively. The highest correlation between soilborne populations of the pathogen and ratios of lint yields of a Verticillium wilt-tolerant cultivar (Acala GC-510) over a susceptible cultivar (Acala SJ-2) was expressed by the power equation Y = 97.08*X0.038. Correlation coefficients for the 3 yr separately and for all 3 yr together were 0.88, 0.95, 0.86, and 0.87, respectively. The fitted line predicts that with about three propagules per gram of soil both cultivars should produce equal yields; at higher inoculum density, tolerant cultivars, such as Acala GC-510, should have higher lint yields than susceptible cultivars, such as Acala SJ-2.