Relationships of Increasing Levels of Resistance of Phytophthora Root Rot in Alfalfa to Stand Longevity and Yield. F. A. Gray, Associate Professor, Department of Plant, Soil, and Insect Sciences, University of Wyoming, Laramie 82071. W. H. Bohl, and D. S. Wofford. Assistant Professors, Department of Plant, Soil, and Insect Sciences, University of Wyoming, Laramie 82071. Plant Dis. 72:1064-1067. Accepted for publication 30 June 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-1064.

The relationships of increasing levels of resistance in alfalfa of Phytophthora root rot (PRR), incited by Phytophthora megasperma f. sp. medicaginis, to stand longevity and forage yield was studied over 4 yr (1981–1984) in a field naturally infested with P. m. f. sp. medicaginis. Cultivars Baker and Vernal, Futura and WL 220, Vancor and WL 312, and Peak and Agate were used to represent the PRR susceptible (S), low resistance (LR), moderate resistance (MR), and resistant (R) categories, respectively. Data from the two cultivars in each category were combined before analysis. Phytophthora root rot ratings (1–5; 1 = none, 5 = very severe), made 2 mo after seeding, for the S, LR, MR, and R cultivars were 3.6, 3.0, 2.7, and 2.9, respectively. Resistant cultivars produced significantly (P = 0.05) more forage than either the MR, LR, or S cultivars by the fourth year. Total forage yields of cultivars over 4 yr were: 30.2, 41.1, 43.6, and 46.9 t/ha for the S, LR, MR, and R cultivars, respectively. When the study was terminated after 4 yr, yields of the resistant cultivars were still increasing while the yields of the susceptible cultivars were declining. Stands remaining in the fall of 1984 for the S, LR, MR, and R cultivars were 21, 31, 46, and 54 plants/m², respectively. Disease and stand in 1981 were significantly related to total yield with correlation coefficients of r = –0.51 and r = 0.53. One hundred 2-yr-old transplants (50 healthy and 50 with severe root rot) of the PRR-susceptible cultivar Skyline 200, were studied for 2 yr to determine the effect of disease on yield and winter survival. A significant stand reduction occurred in diseased plants in the spring of the second year, following a severe winter. No loss occurred in the healthy plants. Yield of plants with PRR was reduced by 68% when compared with healthy plants over the 2 yr.