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Isoflavonoid Phytoalexins and Nonhypersensitive Resistance of Beans to Xanthomonas campestris pv. phaseoli. J. G. Wyman, Department of Plant Pathology, Cornell University, Ithaca, NY 14853; H. D. VanEtten, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 72:1419-1424. Accepted for publication 25 March 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1419.

Bean phytoalexins, phaseollinisoflavan and kievitone, strongly inhibited Xanthomonas campestris pv. phaseoli in liquid culture while phaseollin and coumestrol were not inhibitory. When phytoalexin accumulation and xanthomonad multiplication in unifoliate leaves of the susceptible bean cultivar Dark Red Kidney and the resistant bean cultivar Tara were compared, none of the known isoflavonoid phytoalexins were found in Dark Red Kidney tissue exhibiting typical common blight symptoms. Hypersensitively reacting tissue of Tara contained significant amounts of phaseollin and phaseollinisoflavan, lower amounts of coumestrol, and an unidentified, moderately inhibitory coumestan. However, Tara tissue which did not react hypersensitively did not contain detectable levels of any of the isoflavonoids 5 days after inoculation with 106 colony-forming units of X. campestris pv. phaseoli per milliliter. At this time populations of X. campestris pv. phaseoli had attained stable stationary levels, indicating that resistance had been induced and was well established. The limited antibacterial activity of phaseollin and coumestrol and the absence of all four characterized isoflavonoids in nonhypersensitive resistant Tara tissue suggest that the four phytoalexins studied do not contribute significantly to nonhypersensitive resistance of beans to X. campestris pv. phaseoli.