Characteristics and pathogenicity of isolates of Rhizoctonia spp. associated with damping‐off of sugar beet

Rhizoctonia solani and R. cerealis were isolated from diseased sugar‐beet seedlings in Ireland. Isolates of R. solani were assigned to anastomosis groups AG‐2, AG‐4, AG‐5 and an unidentified group that did not anastomose with recognized tester isolates. Cultures of AG‐2 were similar to those of AG‐5 on oatmeal agar (OA) and potato‐dextrose‐marmite agar (PDMA). Cultures of AG‐4, the unidentified group and R. cerealis were morphologically distinct from one another, AG‐2 and AG‐5. The optimum temperature for growth of AG‐2 was 225 C, with optimum growth of AG‐4, AG‐5 and the unidentified group at 275‐C. R. cerealis grew slower than all groups of R. solani, with optimum growth at 225°C. Hyphae of R. cerealis were significantly narrower than those of the groups of R. solani studied. In glasshouse pathogenicity tests, some AG‐2 and all AG‐4, AG‐5 and isolates from the unidentified group caused damping‐off of beet seedlings. In controlled environments of 10‐25°C, an AG‐2 isolate was the most aggressive at 10 C whilst AG‐4, AG‐5 and the unidentified group caused most disease at or above 15°C. R. cerealis was also pathogenic to beet seedlings, causing damping‐off at 10 and 15 C.