Some effects of cultural treatments on virus diseases of cultivated mushroom, Agaricus bisporus

SUMMARY Inoculating crops of a white strain of mushroom with virus‐infected cultures delayed cropping and decreased yields; the effects were progressively less the later that infection occurred, and/or the larger the amount of healthy spawn used. Crops in trays inoculated at a single site usually developed three zones: (1) a barren zone about the site of inoculation, progressively enlarging as the crops aged; and surrounded by (2) a band of stunted mushrooms bordering an outer area (3) of apparently healthy crop, in which a few sporophores might show hard‐gill or watery‐stipe abnormalities. Mycelial isolates taken at different distances from the sites of inoculation grew at different rates, growth being inversely proportional to the virus content of the fruit‐bodies as estimated by electron microscopy and serology. Isolates from virus‐free mushrooms grew rapidly on agar media, producing white fluffy colonies with many coarse strands, whereas those obtained from the few fruit‐bodies near sites of inoculation were brown, adpressed to the medium, and grew very slowly. Between these extremes a continuous range of intermediates occurred, most of which remained constant when subcultured. Isolates taken at successive intervals from the same site in a tray had progressively smaller growth rates. Unsterilized mushroom‐growing equipment (e.g. trays) is thought to carry viruses in infected spores and mycelial fragments which infect later crops without inducing characteristic zones. Instead, cropping of the whole tray declines with each successive flush. Such infection was prevented by heat‐sterilizing the trays between crops.