Abstract
Bacterial diseases of the edible white button mushroom Agaricus bisporus caused by Pseudomonas species cause a reduction in crop yield, resulting in considerable economic loss. We examined bacterial pathogens of mushrooms and bacteriophages that target them to understand the disease and opportunities for control. The Pseudomonas tolaasii genome encoded a single type III protein secretion system (T3SS), but contained the largest number of non-ribosomal peptide synthase (NRPS) genes, multimodular enzymes that can play a role in pathogenicity, including a putative tolaasin-producing gene cluster, a toxin causing blotch disease symptom. However, Pseudomonas agarici encoded the lowest number of NRPS and three putative T3SS while non-pathogenic Pseudomonas sp. NS1 had intermediate numbers. Potential bacteriophage resistance mechanisms were identified in all three strains, but only P. agarici NCPPB 2472 was observed to have a single Type I-F CRISPR/Cas system predicted to be involved in phage resistance. Three novel bacteriophages, NV1, ϕNV3, and NV6, were isolated from environmental samples. Bacteriophage NV1 and ϕNV3 had a narrow host range for specific mushroom pathogens, whereas phage NV6 was able to infect both mushroom pathogens. ϕNV3 and NV6 genomes were almost identical and differentiated within their T7-like tail fiber protein, indicating this is likely the major host specificity determinant. Our findings provide the foundations for future comparative analyses to study mushroom disease and phage resistance.
Highlights
Agaricus bisporus is the most commonly cultivated mushroom in Europe and North America, most frequently known as the “button mushroom” or “Portobello mushroom”
72 h, the mushrooms infected with P. tolaasii NCPPB 2192T had begun to display darkening of the tissue and pitting of the surface, confirming the ability of the strain to cause brown blotch disease
During an experiment to enumerate P. tolaasii from the surface wash of A. bisporus mushrooms showing no symptoms of disease, a different colony morphology was identified on culture plates in addition to that expected for P. tolaasii
Summary
Agaricus bisporus is the most commonly cultivated mushroom in Europe and North America, most frequently known as the “button mushroom” or “Portobello mushroom”. Several pathogenic pseudomonads are known to cause discolouration of A. bisporus, termed “blotch disease”, including Pseudomonas tolaasii, that causes brown blotch [2] and P. agarici, that causes both yellow blotch and a disease of the mushroom gill structures called drippy gill [3]. A variety of different Pseudomonas bacteria can cause disease symptoms in mushrooms [8], including Pseudomonas gingeri [9], Pseudomonas fluorescens and Pseudomonas putida [10], Pseudomonas protegens [11], Pseudomonas NZI7 [12] and Pseudomonas constantinii [13]. The best characterised strains are P. tolaasii and P. agarici, two species known to cause discolouration and blotch disease and drippy gill disease, respectively, in the edible white mushroom, A. bisporus. Most disease studies have worked on the small peptide toxin, tolaasin [14,15], while there have been various management strategies and control regimes [8] including some studies on biocontrol using bacteriophages [16,17]
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