ALFALFA CULTIVAR PERFORMANCE WITH DIFFERING LEVELS OF RESISTANCE TO PHYTOPHTHORA AND APHANOMYCES ROOT ROTS

Although Phytophthora root rot (Phytophthora medicaginis [Hansen and M axwell]; PRR) and Aphanomyces root rot (Aphanomyces euteiches [Drechs] ; ARR) have been identified as serious diseases of alfalfa (Medicago s ativa L.), research has not been conducted to determine what level of resistance is required to adequately protect the crop. Alfalfa cultiva rs with various levels of PRR and ARR resistance were evaluated for pl ant stand and vigor during the seeding year in a naturally infested fi eld at Marshfield, WI, over 3 yr (1990-1992). Plant stands of the susc eptible cultivar Saranac averaged 50% less than the dual resistant ger mplasm WAPH-1. In 1990, alfalfa stands were reduced from 8 plants/ft o f 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 a t Marshfield, severe root rot stress occurred, resulting in yield incr eases of 325% for cultivars having high resistance (HR) ratings for PR R and ARR compared with the dual susceptible cultivars. Regression equ ations relating level of PRR or ARR resistance and forage yield were d eveloped from summary data of alfalfa cultivar evaluations conducted a t 26 Wisconsin locations. Forage yield increased as the level of PRR r esistance increased from 0 to 40% (susceptible [S] to resistant [R]). Yields of cultivars increased linearly with increasing level of ARR re sistance. These data verify that PRR and ARR are serious diseases, esp ecially in wet soils. We recommend that farmers select cultivars class ified as R or HR (> 30% resistance) for both PRR and ARR for adequate crop protection and maximum forage productivity in Wisconsin.