Abstract:
An unusual postharvest spotting disease of the commercial mushroom, Agaricus
bisporus, which was observed on a commercial mushroom farm in Ontario, was found to
be caused by a novel pathovar of Pseudomonas tolaasii.
Isolations from the discoloured lesions, on the mushroom pilei, revealed the presence
of several different bacterial and fungal genera. The most frequently isolated genus being
Pseudomonas bacteria. The most frequently isolated fungal genus was Penicillium. Of
the bacteria and fungi assayed for pathogenicity to mushrooms, only Pseudomonas
tolaasii was able to reproduce the postharvest spotting symptom. This symptom was
typically reproduced 1 to 7 days postharvest, when mushroom pilei were inoculated with
101 to 105 cfu. Of the fungi tested for pathogenicity only a Penicillium sp. and
Verticillium fungicola were shown to be pathogenic, however, neither produced the
postharvest spotting symptom.
The Pseudomonas tolaasii strain isolated from the postharvest lesions differed from a
type culture (Pseudomonas tolaasii ATCC 33618) in the symptoms it produced on
Agaricus bisporus pilei under the same conditions and at the same inoculum
concentration. It was therefore designated a pathovar. This strain also differed from the
type culture in its cellular protein profile. Neither the type culture, nor the mushroom
pathogen was found to contain plasmid DNA. The presence of plasmid DNA is therefore
not responsible for the difference in pathogenicity between the two strains.
