Postharvest Discoloration of the Cultivated MushroomAgaricus bisporusCaused byPseudomonas tolaasii, P.reactans, andP.gingeri

A postharvest discoloration of the cultivated mushroom Agaricus bisporus in Pennsylvania was associated with three pathotypes of fluorescent pseudomonads. Pathotype A strains caused pitted, dark-brown blotches on mushroom caps, formed precipitates in agar ('white line' reactions) with Pseudomonas tolaasii strain ATCC 14340 (reclassified as P. 'reactans'), were phenotypically like P. fluorescens biovar V, and fit descriptions of P. tolaasii. Pathotype B strains caused pitted, yellow- brown, sometimes slimy lesions on mushrooms, formed no 'white line' reactions, were phenotypically like P. fluorescens biovars III and V, and fit descriptions of P. 'gingeri'. Pathotype C strains caused mild, lightbrown discoloration on mushrooms with little tissue collapse, formed 'white line' reactions with P. tolaasii ATTC 33618 (type strain), had P. fluorescens biovar III and V phenotypes, and fit descriptions of P. 'reactans'. Isolations from mushroom casing material yielded all pathotypes including both pathogenic and nonpathogenic strains of P. 'reactans' with distinguishing phenotypic or chemical characteristics. Cellular fatty acid analysis suggested pathogenic strains of P. 'reactans' were more similar to P. tolaasii and P. 'gingeri' than to saprophytic strains of P. fluorescens, and nonpathogenic strains were more similar to saprophytic P. fluorescens than to P. tolaasii or P. 'gingeri'.