Progression Dynamics of Hypocotyl Rot of Snapbean

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Abstract
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, [USA] in which snapbeans were grown for commericial use in 1977 and 1978. The 11 fields selected each year were planted to 6 cultivars in 1977 and 5 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 adequately decribed in all cases with a quadratic first-difference regression model and in some cases with only a linear term in the regression model. A hierarchial cluster analysis performed by using 6 disease progress curve elements identified the presence of at least 2 epidemic types for each year. The 2 types could be differentiated largely on the basis of rate of disease progression.

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