CHARACTERIZATION AND PATHOGENICITY OF THANATEPHORUS-CUCUMERIS FROM SUGAR-BEET IN MINNESOTA

In 1993, hymenia of Thanatephorus cucumeris occurred on petioles of su gar beet leaves, but disease was not observed on leaves, crowns, or ro ots. Of 33 cultures isolated from sugar beet, 28 were identified as Rh izoctonia solani AG-3 (from four fields planted to potatoes in 1992) a nd 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 modera tely pathogenic on sugar beet seedlings (stands ranged from 49 to 95%) . The same isolates were nonpathogenic when inoculated on 8-week-old s ugar beet roots (root rot indices were less than or equal to 1 [0 to 7 scale]). All isolates of R. solani AG-3 (but none of AG-5) formed scl erotia on roots. Disease indices (0 to 4 scale) on potato sprouts at 1 0 degrees C were low, did not differ significantly (P = 0.05) among is olates 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 th e control. All isolates of AG-3 (but none of AG-5) formed sclerotia on potato seed pieces. When potato sprouts were grown at 25 degrees C, d isease 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 significa ntly (P = 0.05) higher than those of the uninoculated control. Sclerot ia were not observed. The presence of hymenia of T. cucumeris is signi ficant in that sexual reproduction and inoculum production occurred on a nonhost crop and were related to AGs of R. solani associated with t he previous crop (AG-3 for potato and AG-5 for wheat).