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Histopathology of Resistant and Susceptible Sugar Beet Roots Inoculated with Rhizoctonia solani. E. G. Ruppel, Research Plant Pathologist, ARS, USDA, Crops Research Laboratory, Colorado State University, Fort Collins 80521; Phytopathology 63:123-126. Accepted for publication 28 July 1972. DOI: 10.1094/Phyto-63-123.

Resistant and susceptible sugar beet cultivars were penetrated by Rhizoctonia solani directly by individual hyphae or by infection cushions. Lesion diameter and depth were greater in susceptible roots than in resistant roots. Necrosis and some tissue degeneration preceded hyphal advance in all roots after penetration. Hyphae in resistant roots usually were observed only in the periderm or outer secondary cortex, whereas in susceptible roots the hyphae often transected several vascular rings. Hyperplasia of cortical cells occurred at the margin of necrotic and healthy tissue, but no wound periderm or cicatricelike cell layers were evident in resistant roots. Resistance in sugar beet to R. solani was not found to be due to mechanical barriers to the pathogen.