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Progress and Spread of Dark Leaf Spot in Cabbage. D. A. Fontem, Former Graduate Student, Plant Pathology Department, University of Florida Agricultural Research and Education Center, Hastings 32405. R. D. Berger, D. P. Weingartner, and J. A. Bartz. Professor, Plant Pathology Department, Associate Professor, University of Florida Agricultural Research and Education Center, Hastings 32405; and Associate Professor, Plant Pathology Department, University of Florida, Gainesville 32611. Plant Dis. 75:269-274. Accepted for publication 15 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0269.

The progress and spread of dark leaf spot (Alternaria brassicicola) in three cultivars of cabbage (Brassica oleracea var. capitata) were characterized in two seasons. The intensification of disease in time at each of six points along a gradient from a line source of diseased plants resulted in disease progress curves that were almost symmetrically sigmoidal. Three growth models (logistic, Gompertz, and Weibull) were fitted to the severity values by nonlinear regression. The shape parameter (c) for the Weibull function averaged 3.61 (ranged from 2.40 to 6.36). The average daily epidemic rates with the logistic model were k1 = 0.06 in the winter and k1 = 0.11 in the spring. Final disease severity (yf) at the source averaged 0.52 in the winter and 0.97 in the spring. In both seasons, gradients of disease were very steep from the source to 1 m. The nearly flat gradients from 1 to 6.7 m were fit satisfactorily by each of eight gradient models, but the total gradient was described adequately only by the modified model of Gregory and the Hoerl function. Initial disease (time of appearance and amount), epidemic rate, and yf were influenced by distance from the source of inoculum. The three cultivars of cabbage differed in apparent susceptibility to dark leaf spot as determined by average epidemic rates, yf, isopathic rates, areas under disease progress curves (AUDPC) (temporal), and volumes under disease progress surfaces (VUDPS) (temporal and spatial); however, initial disease severities and slopes of disease gradients were similar for the three cultivars. In the spring, yield was correlated negatively with yf and AUDPC.