The present study mainly concerns Verticillium malthousei, a parasite of the cultivated mushroom Agaricus bisporus. To a lesser extent, Mycogone perniciosa, another pathogen is also examined. After describing the pathogens and disease symptoms, a study was made of the histopathology of diseased mushroom sporophores, resulting in the conclusion that while producing intercellular mycelium, M. perniciosa causes host cell breakdown, a condition not observed for V. malthousei which prevents normal differentiation in sporophore tissue. A limited host range of both pathogens was indicated by failure to find naturally occurring wild hosts. The significance of spore dispersal of both parasites, from infected mushroom sporophores, on wind-borne dust and debris, by watersplash, flies and contact are discussed in terms of commercial growing practice, the mechanism of mist dispersal being shown to be inoperative for V. malthousei. Soil fungistasis, causing the failure of most conidia of V. malthousei to germinate in soil, results in the long-term survival of spores of the organism, which is atypical of the genus. The nature of soil fungistasis for V. malthousei is shown to be dependant on a requirement of exogenous nutrients for spore germination and the unavailability of such materials in field soil. The low competitive saprophytic ability of V. malthousei and susceptibility to soil fungistasis is overcome in soil in the presence of mushroom mycelium, but infection of sporophore tissue only was observed. The ineffectiveness of dithiocarbamate fungicide treatment in controlling disease caused by V. malthousei is examined in terms of fungal resistance, fungicide stability, toxicity and selectivity. The apparent greater efficiency of benomyl is discussed with respect to possible hazards in its use.
|Date of Award||1971|