Isolation of a Gram-positive bacterium effective in suppression of brown blotch disease of cultivated mushrooms, Pleurotus ostreatusand Agaricus bisporus, caused by Pseudomonas tolaasii

A Gram-positive bacterium was isolated from a rotting Pleurotus ostreatusfruiting body that markedly reduces the level of extracellular toxins (i.e., tolaasins) produced by Pseudomonas tolaasii, the most destructive pathogen of cultivated mushrooms, The isolated bacterium is saprophytic but not parasitic nor pathogenic to P. ostreatus.A low ratio, ca. 10-3cells of the isolated bacterium for one P. tolaasiicell, was sufficient for detoxification in vitro. Inoculation of the isolated bacterium prevents the development of bacterial disease in P. ostreatusand Agaricus bisporus.The suppression of the disease development, however, requires the initial cell density equivalent to ca. 10-1cells of the isolated bacterium for one cell of the pathogen. The effect is ascribed to the inactivation of tolaasin by the live suppressive bacterial cells, and not to metabolites secreted from the organism into culture media. Examination by conventional bacteriological tests and with testing kits, i.e., MicroStationTMSystem Release 3.5 (Biolog Inc., Hayward, CA), ATB Expression (bioMerieux Inc. Japan) and VITEK (bioMerieux inc. Japan), failed to assign the organism to any defined bacterial genus. The suppressive bacterium may be useful in future for the development of biocontrol system and/or the construction of genetically modified edible fungi resistant to the disease caused by P. tolaasii.