The efficacy of chemical thinning treatments on Pinus sylvestris and Larix kaempferi and subsequent incidence and potential impact of Heterobasidion annosum infection in standing trees

Chemical thinning can be used where conventional felling and extraction of trees is impractical or uneconomic. An experiment was established to examine the efficacy of a range of thinning treatments and to investigate subsequent H. annosum infection on Scots pine and Japanese larch. Ring barking, traditional applications of liquid glyphosate and glyphosate encapsulated as Ecoplug Max (R) were compared. Ecoplugs were more effective and faster acting on larch compared with conventional glyphosate treatments, killing 100 per cent of trees within 17 months, whilst both glyphosate treatments killed 88 per cent of pines. Most treatments allowed ingress of Heterobasidion annosum, although this did not directly relate to treatment wound size. Persistence of the pathogen within the trees, and the possibility of using prophylactic fungicidal products alongside thinning treatments requires further study. Ultimately, the speed of kill achieved is probably too slow for defoliating trees infected with Phytophthora ramorum in Britain, where sanitation felling remains the recommended option. Where thinning treatments are used, the impacts of H. annosum infection are unlikely to be significantly greater than those represented by conventional harvesting operations but this possibility requires further study.