Experiments with a non‐pathogenic strain of Xanthomonas campestris pv. graminis¹

Abstract

Isolates of Xanthomonas campestris pv. graminis are known to vary in pathogenicity, which may be lost more or less rapidly in culture on artificial media. Inoculation of Lolium multiflorum with mixtures of a highly pathogenic strain and increasing proportions of a nonpathogenic strain of X. c. graminis resulted in a decreasing disease expression and mortality. Re‐isolation from plants inoculated with 1% pathogenic and 99% non‐pathogenic bacteria resulted mostly in highly pathogenic monobacterial colonies. Of about 200 colonies, 3% were non‐pathogenic. A few isolates had an intermediate pathogenicity. It has not been established whether they were pure strains or mixtures of pathogenic and non‐pathogenic bacteria. All isolates from plants inoculated with the non‐pathogenic strain were non‐pathogenic. The nonpathogenic bacteria multiplied inside the plants, but they were mostly found in the inoculated leaves. Thirty pots containing each about 20 plants of Italian ryegrass were pre‐inoculated with non‐pathogenic strains and, 4 weeks later, they were cut with scissors contaminated by pathogenic X. c. graminis. The disease was highly reduced in about half of the pots. Further experiments are needed to judge the expediency of such a ‘premunition’ of grasses.