Investigation of the epidemiology of green mould of mushroom (Agaricus bisporus) compost caused by Trichoderma harzianum

In 9 years of advisory experience in the British Isles and in virulence tests in the laboratory and field, only one of three distinct taxa of T. harzianum, namely Th2, consistently colonized mushroom compost either in vitro or in mushroom houses. A single particle of contaminated spawn initiated colonies up to 30cm in diameter. Four other Trichoderma species common in the mushroom environment rarely caused problems. Circumstantial evidence suggested airborne dust to be a main source of contamination of compost or its packing machinery, along with transmission on workers' clothing, pallets, load covers, trailers and by vectors such as mites, mushroom flies and mice. The Th2 isolate of T. harzianum was recovered from all of these sources but, in two tests, it did not survive peak heating. The control measures described here, based on strict hygiene, proved effective. Timing of colonization was important, also temperature of spawn run. Type of compost, as judged by analysis, appeared not to be important. In the field, spawning with pear based 'spawn' instead of grain spawn failed to prevent colonization, whereas in vitro, it reduced it. In the absence of any spawn, Th2 only grew strongly in microwaved or autoclaved compost. Reasons for this are discussed.