Effect of Rhizoctonia solani AG group,previous crop, flax cultivar,and their interactions on the incidence of flax seedling blight were studied under greenhouse conditions in autoclaved soil.All the main effects of AG group,previous crop,and cultivar were highly significant sources of variation in survival. Of the first order interactions, AG group x previous crop was the only significant source of variation. The second order interaction of AG group x previous crop x cultivar was a nonsignificant source of variation. All the previous crops significantly reduced survival within each AG; however,the magnitude of reduction varied from one AG to another. Thus, within AG-2,corn,rice, and cotton reduced survival by 32.42,16.69, and 6.82% respectively, while withinAG-4, con, rice, and cotton reduced survival by 4. 68,11.55, and 11.55%, respectively.

Forty eight isolates of Rhizoctonia solani were obtained from flax seedlings showed seedling blight. Anastmosis tests revealed that 12 isolates (25%) belonged to AG-2, while 36 isolates (75%) belonged to AG-4. Pathogenicity test on flax cultivar Shkha 1, under greenhouse conditions, showed that the pathogenic isolates in the pre-emergence stage represented 91.67 and 77.72% within AG-2 and AG-4, respectively. However, the pathogenic isolates of AG-4 representing 58.23% of the total isolates as well as the highest percentage of the pathogenic isolates (71.79%). This observations held true in the case of plant height and dry weight. These results indicate that both AGs 2 and 4 are important in the etiology of flax seedling blight. The importance of AG-2 is due to its high virulence, while the importance of AG-4 is due to its high prevalence .Cluster analysis suggested that, within each AG, isolates could be separated into subgroups with specific virulence patterns.

Rhizoctonia solani is an plant pathogenic fungus with a wide host range and worldwide distribution. Double diffusion technique was used to differentiate among Rhizoctonia solani isolates from flax. In this technique, antisera of isolate 23 (AG-2) and isolate 24 (AG-4) reacted against antigens of 24 isolates belonging to AG-2 and Ag-4. The antiserum of isolate 23 (AG-2) was insensitive to differentiate among the isolates of AG-2, while it was highly sensitive to differentiate among some isolates of AG-4. The antiserum of isolate 24 (AG-4) was highly sensitive as a serotaxonomic tool to differentiate among AG-2 and AG-4, while it was of limited value in differentiating among isolates of within each AG.