Phytophthora Root Rot of Alfalfa in Wyoming. F. A. Gray, Associate Professor, Plant Science Division, University of Wyoming, Laramie 82071. W. H. Bohl and R. H. Abernethy, Assistant Professors, Plant Science Division, University of Wyoming, Laramie 82071. Plant Dis. 67:291-294. Accepted for publication 2 August 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-291.

Phytophthora root rot was found for the first time in all major alfalfa-producing areas in Wyoming. Phytophthora megasperma f. sp. medicaginis was isolated from 3-mo- to 3-yr-old alfalfa plants that exhibited typical symptoms of Phytophthora root rot (PRR). An isolate of P. megasperma obtained from alfalfa caused damping-off of alfalfa and sainfoin seedlings and root rot of mature alfalfa plants in greenhouse tests. Alfalfa cultivars with resistance to PRR had better stands, less disease, and higher forage yields than susceptible cultivars when grown in a field naturally infested with P. megasperma. A stand reduction of 75% in the PRR-susceptible cultivar Skyline 400 was attributed to the disease during the first year of production.