Microbial succession in casing layer shapes bacterial blotch disease associated communities in cultivated white button mushroom: From casing layer to disease

Background: Bacterial blotch is a major disease of cultivated white button mushroom (Agaricus bisporus) traditionally attributed to individual Pseudomonas pathogens. However, the recurrent detection of diverse bacterial taxa in blotch-affected mushrooms suggests that disease may involve broader changes in microbial community organization. We characterized bacterial communities associated with symptomatic and asymptomatic mushrooms and examined bacterial succession in the casing layer across early, pinning, and harvest stages at two commercial mushroom farms in the United States using complementary 16S rRNA gene amplicon sequencing and shotgun metagenomics. Results: Mushroom-associated communities were dominated by Pseudomonas regardless of disease status, indicating that bacterial blotch was not simply associated with increased abundance of the dominant genus. Instead, symptomatic mushrooms exhibited significant community restructuring, enrichment of specific taxa, and markedly reduced microbial network complexity. Species-level metagenomics revealed extensive reorganization within Pseudomonas, with contrasting shifts among multiple blotch-associated lineages, accompanied by changes in non-Pseudomonas taxa, including Mycetocola and Ewingella. Despite these taxonomic shifts, dominant Pseudomonas populations retained broadly conserved functional profiles, with disease-associated enrichment of pathways related to central metabolism, O-antigen biosynthesis, and peptidoglycan maturation. Casing communities underwent pronounced directional succession, shifting from early dominance by Exiguobacterium and Leuconostoc toward enrichment of Flavobacterium, Pedobacter, and Pseudomonas at later stages. Multiple blotch-associated Pseudomonas lineages were detected throughout casing development, while Pseudomonas increased from approximately 2% in early casing to 51% at harvest at the farm with higher disease incidence. Succession occurred without significant changes in alpha diversity, indicating that community development primarily reflected taxon replacement and redistribution. Conclusions: Our findings support a microbiome-centered framework for bacterial blotch in which disease is associated with host and stage dependent microbial succession, species-level community restructuring, and altered microbial connectivity. This framework extends beyond the single-pathogen paradigm and highlights bacterial blotch as a community-level disease process shaped by dynamic interactions between the mushroom host and its surrounding microbiome.