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Correlation of Medicarpin Production With Resistance to Phytophthora megasperma f. sp.medicaginis in Alfalfa Seedlings. A. Vaziri, Department of Plant Pathology, University of California, Riverside 92521; N. T. Keen(2), and D. C. Erwin(3). (2)(3)Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 71:1235-1238. Accepted for publication 16 March 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-1235.

The resistance of 18 alfalfa cultivars to Phytophthora megasperma f. sp. medicaginis (Pmm), as determined in greenhouse tests, was correlated with the accumulation of medicarpin, one of three antifungal compounds produced in seedling assays. Two of the induced compounds were isolated and identified as 3-hydroxy, 9-methoxypterocarpan (medicarpin) and 4,7-dihydroxyflavanon (liquiritigenin), on the basis of ultraviolet and mass spectral data. The third compound was not identified. Medicarpin was the major antifungal compound formed. Inoculated seedlings initially contained traces of medicarpin, which increased rapidly by 12 hr, and reached maximum concentrations at about 72 hr. Medicarpin was produced following inoculation of both resistant and susceptible cultivars, but the concentration was always less in susceptible cultivars (correlation coefficient = 0.95 for the degree of susceptibility determined in the greenhouse with medicarpin concentrations produced in the seedling assay). Bioassays showed that Pmm was sensitive to medicarpin (ED50 for inhibition of mycelial growth = 97 μg ml1).