Ewingella americana: An Emerging Multifaceted Pathogen of Edible Mushrooms

Abstract

Mycopathogenic bacteria play a pivotal role in the productivity of edible mushrooms grown under controlled conditions. In this study, we carried out a comprehensive farm survey and sampling (2018-2021) on button mushroom (Agaricus bisporus) farms in 15 provinces in Iran to monitor the status of bacterial pathogens infecting the crop. Mycopathogenic bacterial strains were isolated from pins, stems, and caps, as well as the casing layer in 38 mushroom farms. The bacterial strains incited symptoms on mushroom caps ranging from faint discoloration to dark brown and blotch of the inoculated surfaces. Among the bacterial strains inciting disease symptoms on bottom mushroom, 40 strains were identified as Ewingella americana based on biochemical assays and phylogeny of 16S rRNA and gyrB gene. Ewingella americana strains differed in their aggressiveness on mushroom caps and stipes where the corresponding symptoms ranged from deep yellow to dark brown. In the phylogenetic analyses, all E. americana strains isolated in this study were clustered in a monophyletic clade closely related to the non-pathogenic and environmental strains of the species. BOX-PCR-based fingerprinting revealed intraspecific diversity. Using the cutoff level of 73-76% similarity, the strains formed six clusters. A chronological pattern was observed where the strains isolated in 2018 were differentiated from those isolated in 2020 and 2021. Taking together, due to the multifaceted nature of the pathogen, such a widespread occurrence of E. americana in mushroom farms in Iran could be an emerging threat for mushroom industry in the country.