Analysis of stem canker epidemics in irrigated and nonirrigated conditions on differentially susceptible soybean cultivars

Incidence and severity of stem canker caused by Diaporthe phaseolorum var. caulivora were quantified from an inoculum point source in irrigated and nonirrigated plots of resistant, moderately resistant, and susceptible soybean cultivars (Bay, Wilstar 550, and Bedford, respectively) during the 1988-90 seasons. Stem canker severity at each disease assessment was higher on all cultivars in irrigated plots. In general, disease progress was described better by the logistic model than by the monomolecular model for all cultivars in both irrigated and nonirrigated plots. The apparent infection rates of the logistic model were higher on all cultivars in irrigated plots compared with those in nonirrigated plots. Similarly, the gradient and velocity of spread were greater in the irrigated plots than in the nonirrigated plots. Greater stem canker severity and higher apparent infection rates occurred on the susceptible cultivar Bedford than in the moderately resistant and resistant cultivars. Pycnidia and perithecia occurred on the infected plants during each growing season, indicating that the fungus produces more than one cycle of inoculum within a season. This secondary inoculum was capable of infecting soybean plants in the greenhouse at reproductive growth stages. Better fit of the disease progress data to the logistic model also indicates that secondary infections do occur in the field. However, in the field symptoms from infections by the secondary inoculum ate irregular during the crop season because of the prolonged incubation period. Therefore, stem canker should be classified as a both primary inoculum-dependent and infection rate-dependent disease for apparent lack of regular symptom expression from secondary infections and the confounding effects of symptomless, secondary infections