Taxonomically diverse Pseudomonas species induce bacterial blotch of edible mushrooms around the world. Pseudomonas tolaasii, [Pseudomonas gingeri], and Pseudomonas agarici are dominant mycopathogenic pseudomonads in mushroom production farms. In this study, among 216 mycopathogenic bacterial strains isolated from edible mushrooms in Iran, 96 strains were identified as Pseudomonas spp., while only three strains were preliminarily identified as P. agarici. Multilocus sequence analysis showed that only one strain (FH2) authentically belonged to P. agarici, while the other two strains either belonged to [P. gingeri] or represented a unique phylogenetic clade. The three strains also differed from each other in phenotypic characteristics e.g., production of fluorescent pigment and the reaction to tolaasin produced by P. tolaasii. Pathogenicity assays under controlled environment showed that the symptoms induced by authentic P. agarici were far less severe than those caused by the predominant species P. tolaasii. Furthermore, co-inoculation of P. agarici with three bacterial pathogens that are prevalent in Iran on mushroom caps i.e., P. tolaasii, Ewingella americana and Mycetocola sp. resulted in the development of combined symptoms representing characteristics of both pathogens. Antibiosis assay showed that tolaasin-producing strains of P. tolaasii could inhibit the growth of P. agarici, while tolaasin-negative strains of the same species were unable to do so. This led us to the hypothesis that the inhibitory effect of P. tolaasii on P. agarici is driven by tolaasin production in the former species. This inhibitory effect also associated with the rarity of P. agarici in natural conditions.
Keywords: Agaricus bisporus; Brown blotch; Button mushroom; Drippy gill; Pseudomonas tolaasii; Yellow blotch.