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Disease Note

Phytophthora Root Rot of Alfalfa in Québec. C. Richard, Agriculture Canada Research Station, Sainte-Foy, Québec G1V 2J3. Plant Dis. 69:905. Accepted for publication 29 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-905d.

Symptoms typical of Phytophthora root rot (yellow-brown to black patches of rot, rot of taproots) have been observed in alfalfa (Medicago sativa L.) in Québec for many years. Moreover, Phytophthora megasperma Drechs, was isolated from soil in alfalfa fields in five Québec counties (Bagot, Rouville, Saint-Jean, Napierville, Châteauguay) in 1978. In 1984, the pathogen was isolated from severely diseased alfalfa roots for the first time in Québec, in Papineau County. The pathogen has been identified as the small-spored type of P. megasperma, namely, P. megasperma f. sp. medicaginis, on the basis of oogonium size and pathogenicity tests on cvs. Saranac (susceptible) and Agate and Dekalb 120 (resistant). Among the cultivars commonly grown in Québec. Apollo was resistant and Iroquois, Apica, and Vernal were susceptible, after inoculation with the pathogen under waterlogging conditions.