RELATIONSHIP OF PLANT-AGE, CULTIVAR, AND ISOLATE OF RHIZOCTONIA-SOLANI AG-2-2 TO SUGAR-BEET ROOT AND CROWN ROT

Roots of cultivars Maribo Ultramono (MU, susceptible) and ACH 184 (mod erately resistant) of sugar beet (Beta vulgaris) were inoculated with two AG-2-2 isolates of Rhizoctonia solani (from sugar beet and pinto b ean [Phaseolus vulgaris]) at 6, 8, 10, and 12 wk after planting. Two p rocedures were followed to inoculate roots at four ages. Seeds were pl anted on the same day and roots were inoculated at 2-wk intervals (con secutive inoculations); and seeds were planted at 2-wk intervals and r oots were inoculated on the same day (simultaneous inoculations) in 19 90-1991 field trials. From 2 to 8 wk after inoculation, roots were rat ed for disease (0-7 scale) at 2-wk intervals. Root rot severity was ab out two disease ratings higher at each evaluation in 1991 than in 1990 . In three of four trials, disease decreased as root age at time of in oculation increased. In four trials, MU had a higher root rot rating ( 4.5) than ACH 184 (3.6); and in three trials, the R. solani isolate fr om pinto bean gave a higher root rot rating (4.9) than the isolate fro m sugar beet (3.8). In culture, the former isolate grew faster by 0.01 -8.0 mm/24 hr at 25-35 C than the latter isolate. Average weekly air t emperatures were between 25 and 35 C for at least 4 of 8 wk following inoculations in the field. Overall, the least amount of Rhizoctonia ro ot and crown rot occurred on the moderately resistant sugar beet culti var as plant age increased.