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Independent Inheritance of Genes Conditioning Resistance to Phytophthora Root Rot from Diploid and Tetraploid Alfalfa¹

Phytophthora root rot, caused by Phytophthora megasperma Drechs. f. sp. medicaginis Kuan and Erwin (Pmm), is a major cause of decline of established alfalfa (Medicago sativa L.). Three different inheritance modes for resistance to Pmm have been proposed and independently inherited resistances could be combined into a single alfalfa population. The objective of this study was to determine if genetic factors conditioning resistance to Pmm from diploid (2n 2x = 16) and tetraploid (2n = 4x = 32) alfalfa are independently inherited. Resistance to Pmm was transferred from the 4x alfalfa plant M193 to the 2x level by use of triploid-bridge crosses, and the inheritance of resistance in the derived diploids DDR-850, DDR- 1049, DDR- 1051, and DDR-1070 was studied. Each derived diploid was crossed as the female and male parent with susceptible 2x plants. Progeny were grown in the greenhouse in pots containing a peat:sand mix and inoculated at $ weeks with a chopped mycelial suspension of Pmm. Resistance in DDR-850 and DDR-1049 was conditioned by a dominant allele at both of two independently segregating complementary loci, Pm₁ and Pm₂. Resistance in DDR-1051 and DDR-1070 was conditioned by a dominant allele at either of two independently segregating loci, Pm₅ and Pm₆. Plants DDR-850 and DDR-1051 were each crossed as the female parent with each of five 2>x Medicago falcata L. plants resistant to Pmm. Testcross segregations revealed that the Pm₃ and Pm₄ loci from M. falcata are inherited independently of at least one and maybe both of the Pm_l and Pm₂ loci from DDR-850 and independently of at least one and maybe both of the Pm₅ and Pm₆ loci from DDR-1051. This paper is the first known example of successful transfer of disease resistance from 4x to 2x alfalfa, combination of genes conditioning disease resistance from diverse sources, and completion of inheritance studies at the 2x level.

Key Words: Interploid crosses • Analytic breeding • Phytophthora megasperma f. sp. medicaginis • Medicago sativa L. • Medicago falcata L.

² Former research assistant (Present address: USDA-ARS, Washington State Univ., Pullman, WA 99164-6421), professor, and former Wisconsin-Madison, Madison, WI 53706.

Received for publication October 8, 1986.