Effect of inoculum density and borate concentration in a stump treatment trial against Heterobasidion annosum

The effectiveness of disodium octaborate tetrahydrate (DOT) as a stump trea tment chemical for Sitka spruce in Britain was tested on sis occasions by i noculating treated and untreated stumps with basidiospores of Heterobasidio n annosum at three concentrations ranging from an average 49 viable spores/ ml to 4.9 x 10(5)/ml of water. The extent of colonization of heartwood by H . annosum was measured and, along with the incidence of infected stumps, pr ovided a measure of the combined effects of spore concentration and of the two DOT treatments (15 and 30 g/m(2)) on the trial results. On untreated st umps, both the incidence of infection and the cross-sectional area of stump heartwood colonized by the fungus increased with inoculum density. The sam e effect was evident in treated stumps, but it was reduced by increasing DO T application. Infection was at its lowest in stumps treated with DOT at 30 g/m(2), being entirely absent from those 60 stumps that were inoculated wi th the fewest spores. The implications of these findings for the design of trials of control agents that rely on artificial inoculation with H. annosu m and for the selection of dose rates to use in harvesting operations are d iscussed.