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Relationship of Plant Age, Cultivar, and Isolate of Rhizoctonia solani AG-2-2 to Sugar Beet Root and Crown Rot. CHERYL ANN ENGELKES, Former Research Assistant, Department of Plant Pathology, University of Minnesota, St. Paul 55108. CAROL E. WINDELS, Associate Professor of Plant Pathology, Northwest Experiment Station, University of Minnesota, Crookston 56716. Plant Dis. 78:685-689. Accepted for publication I I April 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0685.

Roots of cultivars Maribo Ultramono (MU, susceptible) and ACH 184 (moderately resistant) of sugar beet (Beta vulgaris) were inoculated with two AG-2-2 isolates of Rhizoctonia solani (from sugar beet and pinto bean [Phaseolus vulgaris]) at 6, 8, 10, and 12 wk after planting. Two procedures were followed to inoculate roots at four ages. Seeds were planted on the same day and roots were inoculated at 2-wk intervals (consecutive inoculations); and seeds were planted at 2-wk intervals and roots were inoculated on the same day (simultaneous inoculations) in 1990-1991 field trials. From 2 to 8 wk after inoculation, roots were rated for disease (0-7 scale) at 2-wk intervals. Root rot severity was about 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 inoculation 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 from pinto bean gave a higher root rot rating (4.9) than the isolate from sugar beet (3.8). In culture, the former isolate grew faster by 0.01-8.0 mm/24 hr at 25-35C than the latter isolate. Average weekly air temperatures were between 25 and 35C for at least 4 of 8 wk following inoculations in the field. Overall, the least amount of Rhizoctonia root and crown rot occurred on the moderately resistant sugar beet cultivar as plant age increased