ROOT HEAVE OF ALFALFA CULTIVARS WITH DIFFERING LEVELS OF RESISTANCE TO APHANOMYCES ROOT-ROT

Aphanomyces root rot (ARR) (caused by Aphanomyces euteiches Drechs.) a nd phytophthora root rot (PRR) (caused by Phytophthora sojae M.J. Kauf mann & J.W. Gerdemann) are important diseases of alfalfa (Medicago sat iva L.) grown in wet soil, hut few studies have quantified the effects of ARR on alfalfa performance. An alfalfa cultivar trial of 82 cultiv ars with differing levels of ARR resistance was seeded in July 1993 in a naturally infested soil at Marshfield, WI. Excessive precipitation in 1993 and 1994 led to widespread expression of ARR and PRR, and larg e cultivar yield differences were measured. There was differential roo t heaving among cultivars in the 1993-1994 winter. Fall 1993 forage yi eld was low, but cultivars with high ARR resistance yielded > 400% mor e than did susceptible cultivars. Root heaving in 1994 was inversely c orrelated with level of resistance to ARR (r = - 0.68), 1993 forage yi eld (r = - 0.72), and 1994 forage yield (r = - 0.81). Resistant and hi ghly resistant cultivars produced 2 Mg ha(-1) more forage than suscept ible or low resistance cultivars. Forage yields in 1995 (a relatively dry year) did not differ with ARR resistance level. Under wet soil con ditions, ARR-resistant alfalfa cultivars can produce higher forage yie lds than susceptible cultivars. In addition, resistance to ARR reduces damage due to root heaving.