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Induced Resistance in Alfalfa to Corynebacterium insidiosum by Prior Treatment With Avirulent Cells. R. B. Carroll, Graduate Assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802, Present address of senior author: Plant Science Department, University of Delaware, Newark, Delaware 19711; F. L. Lukezic, Associate Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 62:555-564. Accepted for publication 20 December 1971. DOI: 10.1094/Phyto-62-555.

Resistance to virulent isolates of Corynebacterium insidiosum was induced in four susceptible alfalfa cultivars (DuPuits, Cardinal, Sonora, Moapa) by leaflet infiltration with suspensions of avirulent mutants from the same original single-celled culture. Similar tests on small root sections of greenhouse and gnotobiotically grown alfalfa, maintained in a moist chamber, gave the same results as those obtained with infiltrated leaves. Challenge tests showed that partial protection occurred as early as 6 hr, and protection was maximum or near-maximum by 12 hr. Challenge tests on companion leaflets indicated that the protection factor(s) is not translocated or light-dependent. Protection in leaflets and root sections was also induced by suspensions of avirulent cells that had been Formalin-treated, sonicated, or sonicated and then filtered through a .45-µ Millipore membrane. Protection was not elicited by cell-free filtrates of nonsonicated avirulent cells or by suspensions of virulent cells treated in the same manner as the avirulent cells. Protection was not induced in root sections by effective or noneffective isolates of Rhizobium meliloti, or by four isolates of unidentified bacteria obtained from alfalfa roots. Protection was obtained when roots of seedlings and mature plants grown under gnotobiotic conditions were preinoculated with avirulent C. insidiosum cells.