The fungus Rhizoctonia solani is causing diseases on a wide range of host plants. Biotypes of R. solani are divided into anastomosis groups (AG), of which AG 2-2 and AG 4 are patho¬genic for sugar beet. AG 2-2IIIB, very common in the main sugar beet growing regions of Germany, is causing root and crown rot on sugar beet, AG 4 is causing damping-off of seedlings. We tested the pathogenicity of anasto¬mosis groups AG 2-2IIIB, AG 2-2IV and AG 4 on sugar beet cultivars of different degree of resistance and age (seedlings and young plants 5 and 8 weeks of age) at different temperature regimes (25°/18°C and 15°/8°C day/night). Our aim was to discrimi¬nate characteristics of the AGs and to assess tempera¬ture and age conditions favourable to an induction of disease. Warm temperatures increased the degree of infestation, which was most pronounced in sugar beet plants eight weeks of age inoculated with AG 2-2IIIB. Oldest plants also expressed the strongest differentiation between suscep¬tible and resistant genotypes, indicating a general suscep¬tibility of young plants and a develop¬ment of resistance with age in resistant genotypes. In a bioassay with sugar beet and linseed (Linum usitatissimum L.) we compared the selectivity and symptoms of an infection with isolates of AG 2-2IIIB, AG 2-2IV and AG 4 and additionally observed disease symp¬toms after inoculation with field soil where R. solani infection had previously been found. The aim of this test was to develop a bioassay which allows a rapid R. solani screening of field soil. A detection of an infection with pure cultures was achieved both with sugar beet and linseed, but sugar beet was more sensitive and better suited to estimate the occurrence of inoculum in the soil.