Alfalfa Cultivar Performance with Differing Levels of Resistance to Phytophthora and Aphanomyces Root Rots

Abstract

Although Phytophthora root rot (Phytophthora medicaginis [Hansen and Maxwell]; PRR) and Aphanomyces root rot (Aphanomyces euteiches [Drechs]; ARR) have been identified as serious diseases of alfalfa (Medicago sativa L.), research has not been conducted to determine what level of resistance is required to adequately protect the crop. Alfalfa cultivars with various levels of PRR and ARR resistance were evaluated for plant stand and vigor during the seeding year in a naturally infested field at Marshfield, WI, over 3 yr (1990-1992). Plant stands of the susceptible cultivar Saranac averaged 50 % less than the dual resistant germplasm WAPH-1. In 1990, alfalfa stands were reduced from 8 plants/ft of row for the dual resistant germplasm to less than 2 plants/ft of row for Saranac. Average vigor score (1 to 5 = best) for resistant lines was 3.6 compared with 1.8 for susceptible cultivars. In a 1993 study at Marshfield, severe root rot stress occurred, resulting in yield increases of 325 % for cultivars having high resistance (HR) ratings for PRR and ARR compared with the dual susceptible cultivars. Regression equations relating level of PRR or ARR resistance and forage yield were developed from summary data of alfalfa cultivar evaluations conducted at 26 Wisconsin locations. Forage yield increased as the level of PRR resistance increased from 0 to 40 % (susceptible [S] to resistant [R]). Yields of cultivars increased linearly with increasing level of ARR resistance. These data verify that PRR and ARR are serious diseases, especially in wet soils. We recommend that farmers select cultivars classified as R or HR (> 30 % resistance) for both PRR and ARR for adequate crop protection and maximum forage productivity in Wisconsin