Characterization and Pathogenicity of Thanatephorus cucumeris from Sugar Beet in Minnesota.

In 1993, hymenia of Thanatephorus cucumeris occurred on petioles of sugar beet leaves, but disease was not observed on leaves, crowns, or roots. Of 33 cultures isolated from sugar beet, 28 were identified as Rhizoctonia solani AG-3 (from four fields planted to potatoes in 1992) and five isolates were AG-5 (from one field planted to wheat in 1992). These isolates of R. solani AG-3 and AG-5 were nonpathogenic to moderately pathogenic on sugar beet seedlings (stands ranged from 49 to 95%). The same isolates were nonpathogenic when inoculated on 8-week-old sugar beet roots (root rot indices were ≤1 [0 to 7 scale]). All isolates of R. solani AG-3 (but none of AG-5) formed sclerotia on roots. Disease indices (0 to 4 scale) on potato sprouts at 10°C were low, did not differ significantly (P = 0.05) among isolates and the control in either of two experiments, and averaged 0.9 for 14 isolates of AG-3, 0.5 for three isolates of AG-5, and 0.5 in the control. All isolates of AG-3 (but none of AG-5) formed sclerotia on potato seed pieces. When potato sprouts were grown at 25°C, disease indices were low (averaged 0.4 in each of two experiments), but four isolates of AG-3 and three of AG-5 had disease indices significantly (P = 0.05) higher than those of the uninoculated control. Sclerotia were not observed. The presence of hymenia of T. cucumeris is significant in that sexual reproduction and inoculum production occurred on a nonhost crop and were related to AGs of R. solani associated with the previous crop (AG-3 for potato and AG-5 for wheat).