Excess Soil Water and Phytophthora Root Rot Stresses of Phytophthora Root Rot Sensitive and Resistant Alfalfa Cultivars1

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Abstract
The adaptation of alfalfa (Medicago sativa L.) to well‐drained soil has limited its range of culture. The poor performance of this crop in wet soils has often been attributed to the effects of Phytophthora root rot (PRR). However, excess soil water itself may reduce alfalfa growth. The impact of continuous excess soil water stress (ESWS) and PRR stress (PRRS) and the growth potential after elimination of the excess soil water was studied on PRR‐resistant (Oneida) and PRR‐susceptible (Iroquois) alfalfa cultivars. The impact of ESWS on the two cultivars was similar. No visible root injury symptoms were apparent despite significantly reduced root growth when compared to stress‐free (SF) plants. Shoot growth was less consistently affected than root growth. For the plants subjected to continuous PRRS the severity of PRR was 4.7 for Iroquois and 3.5 for Oneida on a scale of 1 to 5 representing increasing damage. The PRR symptoms were 42 and 15% less for the Oneida and Iroquois cultivars, respectively, after a recovery period following the elimination of excess soil water. Shoot and root growth after the recovery period were significantly greater than that of continuously stressed plants of Oneida, but the growth of Iroquois was not significantly different. Manganeses concentration in the plants under continuous stress was twice that of SF plants or of stressed plants after the recovery period. The concentrations did not reach toxic levels for the plants. Alfalfa response to wet soil is not a simple phenomenon that may be attributed to the same single factor in each situation. It may best be described as a “wet‐soil syndrome” the cause and intensity of which will depend on the magnitude and balance of all the individual stresses that are involved.