The role of micro-arthropods in the defacement of sawn lumber by sap-stain and mould fungi

Fungal sap stain, mould, and basidiomycete defacement of boards sawn from the sapwood of freshly felled logs of Corsican pine (Pinus nigra var. maritima (Aiton) Melville) was assessed during a 14-week field trial at a sawmill in southern England. The boards were placed in nonstickered stacks to reduce moisture loss, and stacks were constructed in a nest arrangement to allow experimental untreated board samples to be positioned centrally in each stack. The experimental samples were placed in plastic tanks, and the open tank faces were left either uncovered or were covered with a fine-mesh nylon screen to prevent entry of microarthropods. All the experimental samples were sterilized by gamma-radiation prior to the tanks being installed into the board stacks. The outer casing boards, which surrounded the experimental tanks in each stack, were dip treated with a fungicide, a broad-spectrum insecticide-acaricide, a combination of the fungicide and the insecticide-acaricide, or water, which served as a control. After 8 and 14 weeks of exposure, casing boards were inspected for fungal infestation. The experimental sample boards removed from the centre of the stacks at the 8-week inspection were also assessed for fungal infestation. Boards that were accessible to arthropods (i.e., in open tanks surrounded by non-insecticide-treated boards) suffered severe sap stain. Experimental boards protected from arthropods by a mesh screen and (or) casing timbers treated with the insecticide chemical had minimal sap stain. In contrast, colonization by moulds was not reduced when either physical or chemical barriers were in place, suggesting that mould spores were mainly transmitted by air currents. Micro-arthropods were clearly implicated as primary agents of sap-stain transmission on green sawn lumber. They may also graze the moulds and therefore limit the intensity of defacement by these fungi.