Strains of Rhizobium leguminosarum biovar viceae have antifungal activity against the pathogen Pythium ultimum. Compositions and methods for treating or protecting plants susceptible to Pythium ultimum damage, and Pythium sp. "group G" damage in particular, are provided. Such strains include, for example, the strains deposited in the International Depository Authority of Canada under accession numbers IDAC 200704-01, IDAC 200704-02, IDAC 200704-03, and IDAC 200704-04.