Ginger blotches on Agaricus bisporus due to monoacetylphloroglucinol production by the pathogen Pseudomonas 'gingeri'.

Background: Agaricus bisporus is the most widely cultivated and consumed mushroom worldwide. Pseudomonas 'gingeri' is the only pathogenic causative agent of ginger blotch in A. bisporus. Current research on mushroom pathogenic biotoxins is limited to P. tolaasii, which causes brown blotch, while understanding of P. 'gingeri' is lacking, therefore identifying the toxins produced by P. 'gingeri' and evaluating their toxicity on A. bisporus is essential for understanding its pathogenic mechanisms.

Results: A pathogenic bacterium isolated from fruiting bodies of A. bisporus with ginger blotch was identified as P. 'gingeri', and its main toxin identified as 2', 4', 6'-trihydroxyacetophenone monohydrate, also known as monoacetylphloroglucinol (MAPG). Its first known extraction from a mushroom pathogen is reported here. MAPG at 250 μg/mL significantly inhibited the host's mycelial growth, increased branching, caused the structure to become dense and resulted in folds appearing on the surface. An MAPG concentration of 750 μg/mL MAPG led to mycelial death. P. 'gingeri' had high MAPG production in medium containing 0.1 mol/L of either glucose or mannitol (4.30 and 1.85 μg/mL, respectively), and mycelia were inhibited by 69.6% and 41.1%, respectively. The MAPG content was significantly lower in other carbon source media.

Conclusion: This work provides a detailed description of the structure and virulence of the P. 'gingeri' biotoxin, which has implications for understanding its pathogenic mechanism and for exploring precise control strategies for A. bisporus ginger blotch disease, such as the development of MAPG inhibitory factors. © 2023 Society of Chemical Industry.