Biological characterization of various strains of Pseudomonas tolaasii that causes brown blotch disease

Brown blotch disease of cultivated mushrooms is caused by Pseudomonas tolaasii , which secretes the bacterial toxin, tolaasin. Tolaasin is a peptide toxin that causes pore formation in the plasma membrane of mushroom cells. Forty-two strains of pathogenic bacteria causing brown blotch or similar diseases were isolated from mushrooms showing disease symptoms. To characterize these bacteria, the genes of 16S rRNA were sequenced and analyzed. Thirty-three strains were identified as five different species of Pseudomonas . Of these, 23 were identified as P. tolaasii and named as P1-type pathogens. Because the strains identified as P. tolaasii were major pathogens that cause the brown blotch disease, phylogenetic analyses of these pathogens were conducted by the neighbor-joining method. These strains comprised three phylogenetic subtypes, P1α (6 strains), P1β (16 strains), and P1γ (1 strain). Biological characterizations of the isolated bacteria were performed and confirmed that all three subtypes were able to cause the disease by forming blotches on the surface of the mushroom tissue. However, hemolytic activities were observed in the P1α and P1γ strains, but not in the P1β strains. These results imply that remarkable diversity exists among the various strains of P. tolaasii , each strain showing distinct biological characters.