Response of Susceptible and Resistant Alfalfa Cultivars to Phytophthora Root Rot in the Absence of Measurable Flooding Damage¹

Abstract

Alfalfa (Medicago sativa L.) growth and development is known to be adversely affected by Phytophthora megasperma Drechs. f. sp. medicaginis (Pmm), a root rotting fungus most active under waterlogged soil conditions. The response of the plant to the pathogen under low levels of flooding stress is not well-established, but that relationship is important during alfalfa establishment on soils flooded for short periods by rainfall. Our main objective was to determine if yield reductions caused by the disease occurred in the absence of yield reductions caused by flooding on seedling alfalfa plants. Controlled environment studies were conducted to quantify the effects of flooding or inoculation with Pmm, or both, on alfalfa yield and disease severity, and to examine the differential response of a susceptible ('Iroquois') and a resistant ('Oneida') cultivar. At 26°C, one or two 3-day cycles of flooding with subsequent 4-day drainage periods did not reduce dry matter yields of alfalfa shoots grown in either soil (mixed, mesic Typic Udipsamment) or vermiculite. Inoculation with Pmm significantly reduced the shoot yield of Iroquois, but not that of Oneida. The presence of an interaction between Pmm inoculation and cultivar in the absence of a flooding X cultivar interaction indicates true disease resistance and not simply increased flooding tolerance in Oneida. Oneida also showed fewer and less severe root lesions than Iroquois. Flooding increased both the incidence and severity of root lesions. The pathogen, Pmm, could be reisolated from root tissue in which disease symptoms were observed. Redox potentials indicated that anaerobic conditions developed quickly in both soil and vermiculite following flooding and that aerobic conditions were reestablished within one day in both media upon draining. Re~ sistance to Phytophthora root rot is ofvalue on new stands subjected to marginal flooding stress.·----------------