Virulence of Aphanomyces euteiches isolates from Iowa and Wisconsin and benefits of resistance to A-euteiches in alfalfa cultivars

Aphanomyces euteiches has become recognized as an important root rot pathog en of alfalfa in the north-central United States, and resistant cultivars a re now commonly planted. Recent evidence indicates the existence of A. eute iches strains, designated as race 2, that are virulent on resistant cultiva rs, but there is little information on the prevalence of such strains or th eir impact on the performance of A. euteiches-resistant cultivars. The purp ose of this study was to assess the virulence of A. euteiches isolates obta ined from Iowa and Wisconsin soils and to determine the frequency of isolat es virulent on race I-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-r esistant cultivars (Paramount and Quantum), a susceptible cultivar (Agate o r Vernal), and two resistant breeding populations (WAPH-1 and WAPH-2). Fift y-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. Emerg ence of seedlings in growth chamber experiments did not differ significantl y among isolates or alfalfa populations. Alfalfa population and A. euteiche s isolate had significant effects on disease severity index (DSI, 1-5 scale ), but there were significant interactions (P < 0.05) between these two eff ects. All 14 Iowa isolates of A. euteiches were virulent (DSI <greater than or equal to> 3.0) on Agate (mean DSI = 4.4, range 3.8 to 4.9), WAPH-1 (mea n 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 v irulent on WAPH-1 (mean DSI = 2.9, range 1.8 to 4.8), and only four were vi rulent on WAPH-2 (mean DSI = 2.3, range 1.8 to 3.4). In field studies, we c ompared yield performance of alfalfa cultivars that were resistant or susce ptible to A. euteiches or Phytophthora medicaginis at four Iowa locations f or one to three harvest years, and one Wisconsin location for two harvest y ears. Mean yields of cultivars with resistance to one or both pathogens wer e significantly higher than those of susceptible cultivars in only one of t he four Iowa locations. In Wisconsin, WAPH-4, a Race 2-resistant alfalfa po pulation, expressed a significant yield advantage when compared with both W APH-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 t hat 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 alf alfa cultivars. Incorporation of race 2 resistance is likely to improve the performance of alfalfa cultivars in A. euteiches-infested soils.