Spatial and Temporal Development of Common Rust in Susceptible and Partially Resistant Sweet Corn Hybrids

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Abstract
The spatial and temporal development of common maize rust (Puccinia sorghi) was studied in isolated field plots of sweet corn hybrids Florida Staysweet (susceptible) and Sugar Loaf (partially resistant) at Urbana, IL [Illinois, USA] in 1984 and 1985. A single plant at the eight-leaf stage was inoculated in each plot to serve as an infection focus. Selected plants were evaluated for incidence and severity two times per week from the first observation of uredinia at the foci until fresh market harvest. Incidence was measured as the proportion of diseased leaves per plant. Severity was measured as the percentage of the total plant leaf area infected. Three-dimensional response surfaces of rust incidence or severity in time and by distance were plotted. Average epidemic rates and gradients also were calculated. The Gompertz model (gompit transformation) gave the best statistical fit of the regression of incidence and severity on time and log10 (distance), respectively. Average epidemic rates (k) obtained from the regression of gompit(incidence) and gompit(severity) on time provided a good method of comparing the partial resistance of the hybrids and was consistent over years. The regression coefficient of gompit(incidence) and gompit(severity) on log10 (distance) (i.e., the gradient, b) was not a consistent method of comparing partial resistance. The use of infection rates in conjunction with response surfaces appeared to offer the best method of comparing partial resistance of cultivars based on disease progress and spread. In both years, some leaves of Sugar Loaf were rust free at all distances from the focus, and rust severity was below levels that would be expected to result in substantial yield loss. All leaves of Florida Staysweet were infected by 33 and 36 days after inoculation in 1984 and 1985, respectively.