Abstract
ABSTRACT Races 0 (Foc-0) and 5 (Foc-5) of Fusarium oxysporum f. sp. ciceris differ in virulence and induce yellowing or wilting syndrome, respectively, in chickpea. We modeled the combined effects of soil temperature and inoculum density of Foc-0 and Foc-5 on disease developed in chickpea cvs. P-2245 and PV-61 differing in susceptibility to those races, using quantitative nonlinear models. Disease development over time in the temperature range of 10 to 30 degrees C and inoculum densities between 6 and 8,000 chlamydospores g(1) of soil was described by the Weibull function. Four response variables (the reciprocal incubation period, the final disease intensity, the standardized area under the disease progress curve, and the intrinsic rate of disease development) characterized the disease development. Response surface models that expressed the combined effect of inoculum density and temperature were developed by substituting the intrinsic rate of disease development in the Weibull or exponential functions with a beta function describing the relationship of response variables to temperature. The models estimated 22 to 26 degrees C as the most favorable soil temperature for infection of cvs. P-2245 and PV-61 by Foc-5, and 24 to 28 degrees C for infection of cv. P-2245 by Foc-0. At 10 degrees C, no disease developed except in cv. P-2245 inoculated with Foc-5. At optimum soil temperature, maximum disease intensity developed with Foc-5 and Foc-0 at 6 and 50 chlamydospores g(1) of soil respectively, in cv. P-2245, and with Foc-5 at 1,000 chlamydospores g(1) of soil in cv. PV-61. The models were used to construct risk threshold charts that can be used to estimate the potential risk of Fusarium wilt epidemics in a geographical area based on soil temperature, the race and inoculum density in soil, and the level of susceptibility of the chickpea cultivar.
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