Characteristics and Frequency of Aphanomyces euteiches Races 1 and 2 Associated with Alfalfa in the Midwestern United States

Aphanomyces euteiches Drechs. causes economically important root rot diseases of several legume species when wet soil conditions prevail. This soilborne oomycete pathogen causes severe root rot damage to pea (Pisum sativum L.), alfalfa (Medicago sativa L.), snap bean (Phaseolus vulgaris L.), and red clover (Trifolium pratense L.) (12,21,22,24,25,29). A. euteiches was first described as an important pathogen of pea in 1925, and was first associated with root rot of alfalfa in 1927 (12,13). Isolates of A. euteiches that expressed host preference to alfalfa were first described in 1964 (24), but it was not until the 1980s that Aphanomyces root rot was first reported to be an important and widespread disease of alfalfa (3,8). Aphanomyces root rot of alfalfa has been reported in North America from Quebec, Canada and several U.S. states, including Iowa, Kentucky, New York, Virginia, and Wisconsin (1,6,19, 24,30). Disease control remains challenging in spite of the protracted awareness of this damaging disease. Chemical controls for Aphanomyces root rot are not available for legumes, and this disease is best managed by avoiding highly infested fields (7). Aphanomyces root rot is managed more successfully in alfalfa than pea because breeding for disease resistance has been much more successful for alfalfa than pea (7,9,26,31,32). Variation in host preference and virulence in populations of A. euteiches has likely contributed to the challenges of controlling Aphanomyces root rot. A. euteiches is composed of subspecific groups based on host of origin and host preference for pea, alfalfa, bean, pea and alfalfa, alfalfa and red clover, or none of these hosts (6,10,15,22,29). The bean, alfalfa, and alfalfa and red clover groups also are genotypically distinct based on random amplified polymorphic DNA (RAPD) analysis (15). Variation in virulence among isolates of A. euteiches to different genotypes within a host species, also termed physiologic specialization or race, has been reported for isolates from pea and alfalfa. Most studies have focused on pea. Six pea lines were used to describe two races among 15 Michigan isolates, four races among 14 Norwegian isolates, and two races among 17 New Zealand isolates (2,17,27). In another study, five isolates of A. euteiches were used to detect specific pea lines that expressed different levels of resistance within a group of pea germ plasms (16). Variation in virulence among isolates from alfalfa has been reported for isolates from several states, including Maryland, Minnesota, North Carolina, Virginia, and Wisconsin (6). Most of the isolates reported, which are now designated race 1 (R1), were highly virulent on the susceptible alfalfa cv. Saranac and expressed low virulence on WAPH-1, an alfalfa population with R1 resistance similar to resistant commercial cultivars (6). Isolates from a number of different states, however, were virulent on Saranac and WAPH-1 and are now designated race 2 (R2). It was unknown whether these R2 isolates represent a widespread risk to resistant commercial cultivars, which were developed using R1 isolates for screening (6). The purpose of this study was to characterize the distribution and traits of R1 and R2 isolates of A. euteiches. The study was focused on Wisconsin, but limited sampling was also conducted in Minnesota and Kentucky. R2 isolates are significant because they overcome most commercially available sources of resistance to Aphanomyces root rot in alfalfa; however, only very limited information has been reported for R2 isolates (6,20). Objectives were to determine the frequency and distribution of R2 isolates, to determine if the frequency of R2 isolates is influenced by cropping history or resistance of the baiting host, and to compare genotypic and phenotypic characteristics of R1 and R2 isolates.