An Association between Resistance to Bacterial Wilt and Nitrogen Fixation in Alfalfa1

Abstract

Bacterial wilt (BW), a major disease of alfalfa (Medicago saliva L.), is caused by the bacterium Corynebacterium insidiosum (McCull.) H. L. Jens. Symbiotic nitrogen (N) fixation of alfalfa involves the bacterium Rhizobium meliloti Dang. The objective of this research was to determine if factors conditioning resistance to BW were associated with plant characteristics related to N2‐fixation in alfalfa. In one study, alfalfa gene pools segregating for different types of resistances to BW were grown in nil‐nitrate greenhouse sandbenches. Each plant was evaluated for acetylene reduction rate, top dry weight, nodule mass score, nodule number score, fibrous root score, and secondary root score. The same plants were then inoculated with C. insidiosum and later evaluated for resistance to BW. In two other studies, breeding lines that had been selected in the greenhouse for acetylene reduction rate, top dry weight, nodule mass, and fibrous root mass were evaluated in the field for resistance to BW.Correlation coefficients between resistance to BW and acetylene reduction rate and between BW and nodule mass were low but significant in one gene pool where resistance to BW was conditioned by an additive gene system. In the second gene pool where a dominant gene conditioned resistance to BW, there was no apparent association between BW resistance and N2‐fixation. In the field studies, lines selected for high levels of N2‐fixation‐associated characteristics, especially nodule mass and top weight, were generally more susceptible to BW than lines selected for low levels of N2‐fixation. Selection for high levels of N2‐fixation did not affect either fall dormancy response, resistance to Phytophthora root rot caused by Phytophthora megasperma Drechs., or resistance to Fusarium wilt caused Fusarium oxysporum Schlecht. f. sp. medicaginis (Weimer) Synd. & Hans. Breeding for high levels of N2‐fixation could increase susceptibility to BW unless plant breeders monitor resistance or utilize the dominant gene (BW1) source of resistance.