Improved seedling health, yield, and stand persistence with alfalfa resistant to Aphanomyces root rot

Alfalfa (Medicago sativa L.) breeders have made substantial progress in recent years to develop cultivars with resistance to Aphanomyces root rot (ARR, caused by Aphanomyces euteiches), yet data showing the agronomic benefits of this resistance under natural field conditions are limited. Two trials were seeded into naturally infested soils with alfalfa cultivars ranging from susceptible to highly resistant to ARR. The trials provided a test of the hypothesis that ARR-resistant cultivars would provide improved performance. In one test, the combination of high rainfall during the 4 wk following seeding on a soil with a slow percolation rate led to a severe outbreak of ARR, with symptoms typical of a syndrome commonly observed in commercial alfalfa fields in Kentucky. Under these conditions, the ARR-resistant cultivars provided dramatically improved seedling health, yield, and persistence. Cultivars having a resistance (R) or high resistance (HR) rating provided the most consistent performance. In the other test, a near-normal rainfall amount on a deep soil with good internal drainage led to moderate disease pressure. In that case, the ARR-resistant cultivars exhibited a slight improvement in seedling health, but yield trends were not as clear as in the former trial. Based on these findings and previous research, we conclude that the use of cultivars with R or HR ratings to ARR may solve a common stand-establishment problem in spring-seeded alfalfa in Kentucky.