Phenotypic variation and survival of genetically marked Pseudomonas tolaasii in mushroom compost

The role of Pseudomonas tolaasii as an important pathogen of the common mushroom Agaricus bisporus is difficult to study in the microbially complex growth medium used for mushroom production. Two strains of P. tolaasii that had been marked with kanamycin resistance and xylE genes were introduced individually into casing soil over mushroom compost. Survival studies revealed that P. tolaasii numbers in casing soil over mushroom compost decreased a 1000-fold in the first 9 days and then remained relatively stable over the rest of the monitoring period. The presence of the pathogenic colony form and the nonpathogenic colony variant was monitored on mushroom caps and in mushroom compost to detect any phenotypic variation while incubated in these environments. Reversion from the nonpathogenic to pathogenic form was detected following isolation and culture from diseased mushroom caps. Inoculation of the marked strains directly onto the cap or into compost beds seeded with A. bisporus resulted in the appearance of brown blotch symptoms.