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Expression of Genetic Susceptibility, Host Resistance, and Nonhost Resistance in Alfalfa Callus Tissue Inoculated with Phytophthora megasperma. Sally A. Miller, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address: DNA Plant Technology Corporation, 2611 Branch Pike, Cinnaminson, NJ 08077; Leen C. Davidse(2), and Douglas P. Maxwell(3). (2)(3)Department of Plant Pathology, University of Wisconsin, Madison 53706, (2)Present address: Department of Phytopathology, Agricultural University, Wageningen, The Netherlands. Phytopathology 74:345-348. Accepted for publication 12 October 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-345.

An alfalfa (Medicago sativa) callus tissue culture system was developed in which host resistance and susceptibility to the alfalfa pathogen, Phytophthora megasperma f. sp. medicaginis (Pmm), and nonhost resistance to the soybean pathogen, P. megasperma f. sp. glycinea (Pmg), were expressed. Hyphae of Pmm completely covered 4- to 6-wk-old calli derived from a plant susceptible to Pmm (M269) within 5 days after inoculation, but growth was limited on calli derived from a plant resistant to Pmm (M194). Colonization of both callus types by Pmg was more restricted than colonization of M194 by Pmm. Calli in all interactions became discolored within 3 days after inoculation, but resistant and nonhost calli were more discolored and darker brown than susceptible ones. Both callus morphology and expression of disease resistance were influenced by the concentration of kinetin in the growth medium: as the kinetin concentration decreased, the consistency of the calli appeared more fluid and they were more susceptible to both Pmm and Pmg. A kinetin concentration of at least 2.0 mg/L of growth medium was necessary for a clear differentiation of resistant and susceptible genotypes.