Persistence of a streptomycin-resistant variant of Corynebacterium insidiosum in soil and alfalfa roots in soil

Summary A pathogenic, streptomycin-resistant variant of Corynebacterium insidiosum, the alfalfa wilt bacterium, was used to determine the persistence of the bacterium in soil and in infected alfalfa roots buried in soil subjected to fluctuating temperatures and different moisture regimes. Use of streptomycin in the agar medium reduced the numbers of soil micro-organisms significantly and enabled colonies of C. insidiosum to be detected at low soil dilutions. In soils incubated at 20°C and moistened to field capacity, the pathogen was not recovered after 7 days from a Malmo silt loam but remained viable for 20 days in the A and C horizons and for 31 days in the B horizon of a Lethbridge silt loam. In excised alfalfa roots held in a Lethbridge silt loam, the pathogen persisted for 50 weeks in soil at the wilting point, regardless of temperature, and in soil subjected to temperatures fluctuating between -5 and +5°C, regardless of moisture. However, in excised alfalfa roots held at a mean temperature of 20°C and under saturated moisture conditions, C. insidiosum did not persist beyond 16 weeks. re]19730925