Comparison of Techniques and Inoculum Sources in Evaluating Peas (Pisum sativum) for Resistance to Stem Rot Caused by Rhizoctonia solani. Randy J. McCoy, Research Assistant, Vegetable Crops Production, Agricultural Research Service, U.S. Department of Agriculture, Prosser, WA 99350. John M. Kraft, Research Plant Pathologist, Vegetable Crops Production, Agricultural Research Service, U.S. Department of Agriculture, Prosser, WA 99350. Plant Dis. 68:53-55. Accepted for publication 26 July 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1984. DOI: 10.1094/PD-68-53.

Differences in disease severity were compared in peas inoculated by planting seeds in soil infested with 20 sclerotia of Rhizoctonia solani per gram of soil or cornmeal-sand (cm-s) inoculum and by direct inoculation of seedling epicotyls. Similar disease severity ratings occurred in peas planted in soil infested with 20 sclerotia per gram or 3% cm-s inoculum. Increasing the cm-s concentration to 6% resulted in higher disease ratings. Direct inoculation of seedling epicotyls with mycelial disks of R. solani from dextrose-asparagine broth (DAT) caused more severe epicotyl rot than sclerotial inoculum did. Disks of R. solani grown on PDA or V-8 juice agar resulted in low disease ratings even on susceptible lines (plant introduction [PI] numbers 140165, 166159, and 223285). Although stem rot severity ratings for 20 pea lines differed among inoculation treatments, the overall relationship of these lines within an inoculum treatment (high to low) remained about the same. Incubation of R. solani on peptone-sucrose-yeast extract broth (PSY) containing 1 or 2% peptone and 2% sucrose resulted in abundant uniform sclerotial (250–425 μm) production within 9 days. Incubation of R. solani in potato-dextrose broth, V-8 juice broth, and DAT broth resulted in fewer sclerotia. Higher disease severity ratings occurred when pea seedlings were exposed to sclerotia produced in PSY medium containing 2% peptone and sucrose than in PSY medium containing 1% peptone or sucrose. One breeding line (B77-634-4), two PI accessions (189171, 197990), and Dark Skin Perfection were considered resistant.