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Ecology and Epidemiology

Structural Characterization of Bean Root Rot Epidemics. C. Lee Campbell, Graduate fellow, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; L. V. Madden(2), and S. P. Pennypacker(3). (2)(3)Graduate assistant, and associate professor, respectively, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 70:152-155. Accepted for publication 22 August 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-152.

A 0.4-ha field of snapbeans (Phaseolus vulgaris ‘Tendercrop’) was divided into 100 contiguous quadrats. Twelve plants were removed from each quadrat at 10, 20, 30, 40, and 50 days after planting to assess disease severity and incidence of bean root rot induced primarily by Rhizoctonia solani, but also in part by Fusarium solani f. sp. phaseoli. A disease progress curve was developed to depict the epidemic occurring in each quadrat. The curve elements used to characterize these disease progress curves were: Weibull distribution function scale parameter; Weibull distribution function shape parameter; area under the disease progress curve; final disease severity; first-difference regression linear coefficient; first-difference regression quadratic coefficient; percent plants infected; and estimated time of disease onset. The structure of the 100 disease progress curves was examined by conducting a multivariate Principal Axis Factor Analysis followed by a Varimax rotation. Four factors, accounting for 90% of the total variance, were selected to express the disease progress curve elements more simply and without confounding interactions among elements; ie, to obtain greater parsimony. Factor 1 was an overall description of disease progression including shape, level, and rate parameters. Factor 2 showed a relationship between area under the disease progress curve and level of the epidemic. Factor 3, an indication of epidemic temporal location, identified the uniqueness of the estimated time of disease onset. Factor 4 identified a relationship between disease severity and disease incidence.