Characterization of fluorescent pseudomonads responsible for the yellowing of oyster mushroom (Pleurotus ostreatus)

Fluorescent pseudomonads isolated from different lesions on caps and/or stipes of cultivated Pleurotus ostreatus were identified as strains of Pseudomonas tolaasii or showed the White Line Assay (WLA) feature of P. ‘reactans’ or were WLA-negative fluorescent Pseudomonas spp. Pseudomonas tolaasii was consistently associated with brown-reddish blotches on P. ostreatus pseudo-tissues, and in the pathogenicity assays caused depressed dark brown lesions with deliquescence on Agaricus bisporus pseudo-tissues blocks and brown-reddish blotches and yellow discoloration on P. ostreatus sporocarps. Pseudomonas ‘reactans’ and the WLA-negative fluorescent Pseudomonas spp. were mostly associated with superficial yellow lesions on P. ostreatus sporocarps, and in pathogenicity assays caused light or dark brown discoloration, depending on the isolates, on A. bisporus pseudo-tissues blocks and the yellow discoloration of P. ostreatus sporocarps. The results of this study indicate that the aetiology of lesions on cultivated P. ostreatus involves a complex composed of interactions between P. tolaasii, P. ‘reactans’ and Pseudomonas spp., but that individually these bacteria cause different symptoms. This is the first report where the pathogenicity features of these pathogens has been clearly ascertained, and that has fully satisfied Koch’s postulates for the bacteria on the host mushroom. On the basis of virulence, biochemical and physiological characters, the isolates of P. ‘reactans’ and Pseudomonas spp. responsible for yellowing of oyster mushroom belong to several species of Pseudomonas.