Inheritance of Resistance to Phytophthora megasperma in Tetraploid Alfalfa1

Abstract

Phytophthora root rot (PRR) of alfalfa (Medicago sativa L.) is one of the most important diseases of that crop. Previous studies on the inheritance of resistance to PRR in ‘Lahontan’ and ‘Vernal’ clones indicated susceptibility was conditioned by a single incompletely dominant tetrasomic gene, designated Pm. This paper reports studies on the inheritance of resistance to PRR in two different tetraploid breeding lines: North American Plant Breeders (NAPB) No. 0310 and Hunter River P3 (Australian breeding line) which had both shown different responses to recurrent selection than would have been expected if susceptibility was conditioned by a single incompletely dominant gene.The observed segregations for disease reaction in S1 populations and in F1 populations from crosses of two resistant NAPB 0310 plants and two susceptible plants from ‘Saranac’ and ‘Vernal’ suggested that resistance was conditioned by two incompletely dominant complementary genes, Pm1 and Pm2. An arrangement o[ at least a duplex genotype at one locus (Pm1 or Pm2) and a simplex genotype at the other locus was required before resistance was expressed. S2 and F2 family segregations for disease reaction provided further evidence to support the two locus model. An intercross between a resistant plant from NAPB 0310 and a resistant plant from Hunter River P3 indicated that different genes conditioned disease reaction in these two sources. Susceptibility was incompletely dominant in Hunter River P3.These studies have identified a new genetic system conditioning resistance to PRR. Due to the high degree of dominance expressed for the resistant reaction in plants from NAPB 0310, this type of resistance could be readily utilized in breeding alfalfa with multiple pest resistance.