Abstract
A mixture of 6 phage-type strains of streptomycin resistant Erwinia carotovora was inoculated into several cruciferous plants and isolation frequency of each strain from diseased tissues was examined to test possible competition among strains in the host tissues. The strains I (phage-type: A), II (B), III (C), IV (D), V (E), and VI (F) were used in this experiment. After the symptom appearance, the pathogenic bacteria were periodically isolated by the dilution plating with modified Drigalski's medium which contains 100ppm streptomycin sulfate. All isolates were then phage-typed by spot test to estimate the isolation frequency of each phage-type strain. Strain III (C), IV (D), and VI (F) were isolated in early stage of infection. In every plants, strain VI (F) became predominant with passing of time after inoculation. The other strains such as I (A), II (B), and V (E) were not detected in later stage. To determine the predominance order among strains, the midribs of chinese cabbage were inoculated with different combination of strains. By testing the isolation frequency of each strain at 3 days after inoculation, it was made clear that predominance order among strains was VI (F), III (C), IV (D), V (E), and I (A), or II (B). When each strain was inoculated alone, only the original strain was isolated without any change in phage-type even in the later stage of disease development. These results suggest that when several strains of E. carotovora were inoculated simultaneously, some competition could occur among strains during disease development.
