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

Abstract

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 8 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 2x 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 Pml 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.