Characterization of Pseudomonas species causing brown blotch of Agaricus bisporis.

Bacterial blotch is occasionally causing damage in the production of common mushroom (Agaricus bisporus). The disease is found worldwide and can be caused by different fluorescent Pseudomonas species present in casing material. For identification of the causative agents of blotch in the Netherlands and Belgium, fluorescent Pseudomonas species were isolated from mushrooms with mild or severe brown blotch symptoms collected at six growers. Strains were tested for pathogenicity by streaking the mushroom head with bacterial slime using a cotton swab and incubation of mushrooms for eight days at 15°C. In total 16 isolates were collected varying in virulence. These, together with 14 other Pseudomonas strains present in different culture collections (LMG and at WUR), including the type strains of P. gingeri, P. tolaasii, P. agaricus and P. reactans were characterized and tentatively identified by multilocus sequence analysis based on four genes (rpoD, rpoB, gyrB, glyA) and by average nucleotide identity using whole genome sequences . We found three species, P. tolaasii, P. gingeri and an unnamed species able to cause severe symptoms whereas other unnamed species were able to cause mild symptoms. The information can be used for the development of diagnostic tools and in studies on epidemiology and disease management.