Phytophthora root rot (PRR) is an important disease of alfalfa (Medicago sativaL.) in wet soils. Excess soil water (ESW) stress is required for the successful infection and development of PRR. The effects of P at 0, 40, and 80 μg.g‐1soil and K at 0, 120, and 240 μg.g‐1soil (in factorial combinations) on the severity of PRR or ESW stress was investigated on a PRR‐susceptible (Iroquois) and a resistant (Oneida) alfalfa cultivar in a greenhouse trial. P and K applications significantly increased the dry weight of tops prior to imposition of PRR or ESW stress. However, plants subjected to PRR or ESW stress did not respond significantly to P or K and there was no significant effect of P or K on the severity of PRR or ESW injury. The plants subjected to PRR or ESW stress had lower tissue P concentration than those under stress‐free conditions. Increasing P additions resulted in a significant increase in P concentration of Iroquois plants, while in Oneida the response was non‐significant. The response of K concentration in the plant to increasing K additions was dependant on P levels.