Abstract
The pathogenicity of eight isolates of Rhizoctonia solani from different anastomosis groups was evaluated on five cotton (Gossypium barbadense L.) cultivars under greenhouse conditions. Damping-off, plant height and fresh weight were used as criteria to evaluate pathogenicity. Analysis of variance (ANOVA) showed that the cultivar and cultivar x isolate interaction were always non-significant sources of variation in all the tested parameters. The isolate was always a highly significant (P=0.000) source of variation in all the tested parameters. The results of the ANOVA in the present study suggest that physiologic specialization did not occur within R. solani isolates pathogenic on cotton. They also imply that resistance of the tested cultivars was only horizontal, and there were no significant differences among cultivars in this type of resistance. Similarly, pathogenicity of the tested isolates was only aggressiveness, and the isolates significantly differed in this type of pathogenicity. A hierarchical cluster analysis was conducted in order to group the isolates according to disease variables measured on the tested cultivars. Cluster analysis divided the isolates into groups; however, grouping the isolates was not related to their AGs.
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