Development of a fungal bioassay to investigate detoxification and treatment of conifer barks for UK horticulture: a retrospective report

In 1972 the commercial mushroom Agaricus bisporus was chosen as the main target for assaying the potential toxicity of different conifer barks for UK horticultural use. A non-contact assay was developed that involved exposing A. bisporus colonies to volatiles of fresh or treated pulverised barks and measuring colony growth. Bark treatments tested were: ageing at 2°C for six weeks, steam sterilising for 20 min and heat treatment at 50°C for 14 days to mimic thermophilic composting. Fresh barks of Norway Spruce, Sitka Spruce. European larch and Douglas Fir significantly reduced colony growth and could even be fungistatic to the point of lethality. Fresh Japanese Larch, Scots Pine and Corsican Pine barks were moderately inhibitory to innocuous. Aged barks were consistently more inhibitory than their fresh counterparts. All steam sterilised barks were either innocuous or (Scots Pine and Corsican Pine) mildly stimulatory to growth. All the 50°C-treated barks were either marginally or strongly stimulatory. In a basic gas chromatography analysis volatile oil yields by dry weight of fresh barks ranged from 0.11% (Corsican Pine) to 0.4% (Norway Spruce) and 0.62% (Sitka Spruce). The three most inhibitory fresh barks, Norway Spruce, Sitka Spruce and European larch, also produced the largest quantities of monoterpenes (0.52%, 0.3% and 0.12% dry weight respectively) and, together with European larch, probably the highest levels of the specific monoterpene α-pinene. The monoterpene content of a mixed conifer commercial bark stack declined from an initial 0.01% to only 0.0025% after composting. Collectively, the results provided circumstantial evidence for a link between bark toxicity and monoterpene content and supporting evidence for the practice of commercial thermophilic composting as a means of monoterpene removal. Subsequently a new and expanding industry suppling conifer bark for horticulture emerged in the UK.