Inoculum Densities of Fusarium oxysporum f. sp. vasinfectum and Meloidogyne incognita in Relation to the Development of Fusarium Wilt and the Phenology of Cotton Plants (Gossypium hirsutum)

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Abstract
Development of Fusarium wilt in upland cotton (Gossypium hirsutum) usually requires infections of plants by both Meloidogyne incognita and Fusarium oxysporum f. sp. vasinfectum. In this study, the soil densities of M. incognita and F. oxysporum f. sp. vasinfectum and the incidence of Fusarium wilt in three field sites were determined in 1982-1984. Multiple regression analysis of percent incidence of Fusarium wilt symptoms on population densities of M. incognita and F. oxysporum f. sp. vasinfectum yielded a significant fit (R 2 = 0.64) only on F. oxysporum f. sp. vasinfectum. Significant t-values for slope were also obtained for the interaction of M. incognita and F. oxysporum f. sp. vasinfectum, but densities of M. incognita and F. oxysporum f. sp. vasinfectum were also related on a log10 scale. The physiological time of appearance of first foliar symptoms of Fusarium wilt, based on a degree-days threshold of 11.9°C (53.5°F), was used as a basis for determining disease progress curves and the phenology of cotton plant growth and development. Effects of Fusarium wilt on plant height and boll set were determined in three successive years. Increases in both of these plant characteristics decreased or stopped before foliar symptoms were apparent. Seed cotton yields of plant cohorts that developed foliar wilt symptoms early in the season (before 2,000 F degree-days) were variable but not much different in these years. This contrasted with cohorts of plants that first showed foliar symptoms late in the season (after 2,400 F degree-days) and cohorts of plants that showed no foliar symptoms of wilt. Regression analyses for 1982-1984 indicated moderate to weak correlations (r = 0.16–0.74) of the time of appearance of the first foliar symptoms and seed cotton yields.