Twenty fungal isolates of Rhizoctonia solani AG-3 sclerotia were isolated from diseased potato tuber samples that were obtained from potato-producing areas in the Lower Egyptian governorates. The pathogenicity of the 20 isolates of R. solani AG-3 was tested on nine potato cultivars under greenhouse conditions. The black scurf disease incidence (BSDI) and stem canker index (SCI) were used as the criteria to evaluate the pathogenicity. An analysis of variance (ANOVA) showed that the isolate, cultivar and interaction between isolate and cultivar were very highly significant sources of variation for all the tested parameters. The statistical significance between the isolate and cultivar suggests that a physiological specialization exists among the R. solani AG-3 isolates that cause pathogenicity in potato. Additionally, the pathogenicity of the tested isolates was found to result from a combination of virulence and aggressiveness, and the isolates significantly differ for both types of pathogenicity. Similarly, the resistance of the tested cultivars was a mixture of both vertical and horizontal resistance, and there were significant differences among the cultivars for each type of resistance. A cluster analysis (isolates) of all of the tested parameters differentiated the isolates into four groups based on their virulence patterns in the nine potato cultivars, but there was no relationship among these groups and their geographical origin. The cluster analysis for the cultivars using all of the tested parameters differentiated the three groups based on the cultivar’s reaction pattern to the twenty R. solani AG-3 isolates.
Key words: Potato, cultivars, Rhizoctonia solani, pathogenicity, cluster analysis
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