The infecting bacteria, Pseudomonas tolaasii, produces the toxin tolaasin that causes brown spots to cover the surface of the mushroom.
Brown blotch disease is especially problematic on common mushroom farms, where it can spread quickly and cause huge economic losses.
[1] The most common symptom of brown blotch disease in A. bisporus is the formation of round lesions on the cap and stalk of the mushroom.
[2] As the disease progresses, these lesions go from a pale yellow to a dark brown and cause the quick deterioration of mushrooms after harvest.
[1][3] Any external growth such as the fruit bodies (spore-producing structures) formed by infected mushrooms are completely brown.
[3] However, P. tolaasii is able to survive in a variety of environmental conditions and successfully compete with other bacterium populations due to several biological mechanisms, such as its ability to switch between a smooth and rough phenotypic strain.
The infection can also spread by airborne dust, splashing during watering, the tools of farmworkers, nematodes and mushroom flies.
[1][2] To prevent the spread of brown blotch disease on mushroom farms, various disinfectants and antibiotics have been used, but none are fully effective and nontoxic to humans.
[2] Introducing an antagonistic bacterium such as P. reactans, which forms the white line-inducing principle that neutralizes tolaasin, was also seen as a possible solution.
[2] Research indicates that the incubation of harvested mushrooms at low temperatures for four days inhibited the browning symptom, but this method has not yet been tested at a large scale.