Biological contaminants in North American legume inoculants

A survey of 40 commercial North American rhizobial inoculants was made to determine the number of contaminant cells relative to rhizobial cells. All of the inoculants were manufactured for sale during the 1993 season and were made using non-sterile peat as carrier. Products from three major North American manufacturing companies were examined. All of the inoculant samples were collected as part of the Canadian Legume Inoculant and Pre-Inoculated Seed Product Testing Program. Only one of the inoculants contained more rhizobia (Rhizobium or Bradyrhizobium spp) than contaminants. Three others contained 1–2 times more contaminants than rhizobia, 21 of the samples contained 2–10 times more contaminants than rhizobia, 8 contained 10–100 times more contaminants than rhizobia, and 3 contained more than 1000 times more contaminants than rhizobia. Four of the samples contained too few rhizobia to be detected in the standard most-probable-number (MPN) test used by the Canadian regulatory inoculant testing program. The most common contaminants were bacteria (109–1010 g−1), actinomycetes (108–109 g−1), and fungi (105–107 g−1). Contaminant bacteria capable of inhibiting growth of R. leguminosarum bv. trifolii or R. meliloti were isolated from inoculant products of all three manufacturers.