Epidemiology and Yield Losses Associated with Alternaria Blight of Sunflower

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Abstract
Epidemics of Alternaria blight, caused by Alternaria helianthi, were initiated at different plant growth stages on two sunflower genotypes (cms HA89 and hybrid 894) during the 1981 and 1982 growing seasons at Brookings, SD [USA]. Yield losses as great as 51 and 60% were observed on hybrid 894 amd cms HA89, respectively, when seed yields were compared to those of check plots sprayed with fungicide. The 100-seed weights and seed oil contents were also significantly reduced in some inoculated treatments. Yield losses, losses in 100-seed weight, and oil content were consistently greater on the inbred line cms HA89 than hybrid 894, indicating that Alternaria could be a greater problem in seed production fields than in commercial hybrid fields. The logistic growth model consistently fit Alternaria blight progress curves better than the Gompertz model in 1981, but in 1982 no consistent difference between the models was detected. Selected critical-point, and multiple-point regression yield loss models gave coefficients of determination of 0.65 and 0.70, respectively, when disease severity was used to predict percent yield reduction due to A. helianthi.

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