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Ecology and Epidemiology

Progression Dynamics of Hypocotyl Rot of Snapbean. C. Lee Campbell, Former graduate fellow, Department of Plant Pathology, The Pennsylvania State University, University Park, 16802, Present address of senior author: Department of Plant Pathology, North Carolina State University, Raleigh, 27650; S. P. Pennypacker(2), and L. V. Madden(3). (2)(3)Associate professor, and graduate assistant, respectively, Department of Plant Pathology, The Pennsylvania State University, University Park, 16802. Phytopathology 70:487-494. Accepted for publication 29 October 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-487.

Severity of snapbean hypocotyl rot, induced primarily by Rhizoctonia solani and in part by Fusarium solani f. sp. phaseoli was evaluated in a total of 22 fields in Clinton County, Pennsylvania, in which snapbeans were grown for commercial use in 1977 and 1978. The 11 fields selected each year were planted to six cultivars in 1977 and five cultivars in 1978. Disease severity was estimated at weekly intervals for each field as the proportion of hypocotyl surface covered by lesions; the assessments were made on 50 arbitrarily selected plants from each field. The “simple interest” disease model was not appropriate for describing the epidemics as indicated by the shape parameter of the Weibull distribution function. Disease progress was, however, adequately described in all cases with a quadratic first-difference regression model and in some cases with only a linear term in the regression model. A hierarchical cluster analysis performed by using six disease progress curve elements identified the presence of at least two epidemic types for each year. The two types could be differentiated largely on the basis of rate of disease progression.