Various Pathogenic Pseudomonas Strains that Cause Brown Blotch Disease in Cultivated Mushrooms

Brown blotch disease in cultivated mushrooms is caused by Pseudomonas tolaasii, which secretes a lipodepsipeptide, tolaasin. Tolaasin is a pore-forming toxin in the cell membranes, thus destroying the fruiting body structure of mushroom. In this study, we isolated pathogenic bacteria from mushrooms that had symptoms of brown blotch disease. In order to identify these bacteria, their 16S rRNA genes were sequenced and analyzed. Pathogenic bacteria identified as Pseudomonas species were thirty five and classified into five subgroups: P1 to P5. Each subgroup showed different metabolic profile measured by API 20NE kit. Fifty percent of the bacteria were identified as P. tolaasii (P1 subgroup). All five subgroups caused the formation of brown blotches on mushroom tissues and the optimum temperature was 25oC, indicating that they may be able to secrete causal factors, such as tolaasin and similar peptide toxins. These results show that there are at least five different pathogenic Pseudomonas species as blotch-causing bacteria and, therefore, strains from the P2 to P5 subgroups should be also considered and studied as pathogens in order to improve the quality and yield of mushroom production.