Department of Plant Pathology, Iowa State University, Ames 50011
Iowa State University Cooperative Extension, 107 E. Benton, Albia 52531
Department of Agronomy, Iowa State University, Ames 50011
701 Ridge St., Madison, WI 53705
Department of Agronomy, University of Wisconsin
Department of Plant Pathology, University of Wisconsin, Madison 53706
Aphanomyces euteiches has become recognized as an important root rot pathogen of alfalfa in the north-central United States, and resistant cultivars are now commonly planted. Recent evidence indicates the existence of A. euteiches strains, designated as race 2, that are virulent on resistant cultivars, but there is little information on the prevalence of such strains or their impact on the performance of A. euteiches-resistant cultivars. The purpose of this study was to assess the virulence of A. euteiches isolates obtained from Iowa and Wisconsin soils and to determine the frequency of isolates virulent on race 1-resistant alfalfa populations. In addition, the yield performance of susceptible and resistant alfalfa populations was compared in four Iowa locations and one Wisconsin location. Fourteen isolates of A. euteiches from different Iowa locations were used to challenge two race 1-resistant cultivars (Paramount and Quantum), a susceptible cultivar (Agate or Vernal), and two resistant breeding populations (WAPH-1 and WAPH-2). Fifty-nine isolates of A. euteiches from one location in Wisconsin were used to challenge one susceptible cultivar (Saranac) and WAPH-1 and WAPH-2. Every isolate was virulent to one or more alfalfa cultivars or populations. Emergence of seedlings in growth chamber experiments did not differ significantly among isolates or alfalfa populations. Alfalfa population and A. euteiches isolate had significant effects on disease severity index (DSI, 1-5 scale), but there were significant interactions (P < 0.05) between these two effects. All 14 Iowa isolates of A. euteiches were virulent (DSI ≥ 3.0) on Agate (mean DSI = 4.4, range 3.8 to 4.9), WAPH-1 (mean DSI = 3.9, range 3.0 to 4.4), and the two commercial resistant cultivars (mean DSI = 3.9 and 4.1, range 3.2 to 4.4). On WAPH-2, only three isolates were virulent (mean DSI = 2.5, range 1.8 to 3.2). Of 59 Wisconsin isolates, all were virulent on Saranac (mean DSI = 4.6, range 3.9 to 5.0), 21 were virulent on WAPH-1 (mean DSI = 2.9, range 1.8 to 4.8), and only four were virulent on WAPH-2 (mean DSI = 2.3, range 1.8 to 3.4). In field studies, we compared yield performance of alfalfa cultivars that were resistant or susceptible to A. euteiches or Phytophthora medicaginis at four Iowa locations for one to three harvest years, and one Wisconsin location for two harvest years. Mean yields of cultivars with resistance to one or both pathogens were significantly higher than those of susceptible cultivars in only one of the four Iowa locations. In Wisconsin, WAPH-4, a Race 2-resistant alfalfa population, expressed a significant yield advantage when compared with both WAPH-1, a Race 1-resistant alfalfa population, and Columbia 2000, a cultivar susceptible to both race 1 and 2 of A. euteiches. These results indicate that race 2 of A. euteiches is prevalent in Iowa and Wisconsin soils and may be limiting the yield benefits of currently available race 1-resistant alfalfa cultivars. Incorporation of race 2 resistance is likely to improve the performance of alfalfa cultivars in A. euteiches-infested soils.