INFLUENCE OF GROWTH MEDIUM ON THE PATHOGENICITY OF SOME ISOLATES OF RHIZOCTONIA SPP. ON COTTON SEEDLINGS

The nutritional status of seven multinucleate isolates of Rhizoctonia solanl (AG4) and one binucleate isolate of Rhizoctonia was varied by growing the isolates on four growth media with var­iable nutritional values (water agar, potato dextrose agar, Cza­peck's-dox agar, and sorghum extract agar). Effect of growth me­dium on pathogenicity of the isolates, on cotton cultivars Gin 80 and Giza 89, was evaluated under laboratory and greenhouse condi­tions. Percentage of seed germination and radicle length were used as criteria for judging pathogenicity of the isolates under la­boratory conditions. Pre-emergemce damping-off, post-emergence damping-oll, survival, and dry weight were used for judging pa­thogenicity under greenhouse conditions. Growth medium was a highly significant (p50.01) or a significant (p50.05) source of vari­ation in all the laboratory or greenhouse variables used for evalu­ating pathogenicity of the isolates. This result confirms the im­portance of the mycelium nutritional status in determining its pathogenicity. Pathogenicity showed differential responses to the growth media that is, a single isolate can be highly pathogenic when it is grown on a particular medium, but may show only low pathogenicity on another medium. Significant positive and negative correlations were observed between laboratory and greenhouse var­iables. Two regression models, derived from stepwise multiple re­gression analysis, were constructed for each cultivar to predict pa­thogenicity of the isolates under greenhouse conditions. The results indicate that laboratory tests by using sorghum extract agar as growth medium can be used as a rapid method for evaluating differ­ences among Rhizoctonia isolates; however, this should not replace pathogenicity tests with seedlings under greenhouse conditions.