Infection of Sitka spruce stumps by spores of Heterobasidion annosum: control by means of urea

The ability of a 17 per cent solution of urea to protect Sitka spruce stump s against infection by spores of H. annosum was tested on 24 sites througho ut Scotland and north England. After treatment, stumps were either inoculat ed with basidiospores or exposed to natural infection. Urea failed to reduc e the incidence of infection arising from inoculation but it reduced the le vel of natural infection by two-thirds from a mean of 8.8 per cent to 3.5 p er cent. In inoculated stumps that became infected, urea significantly incr eased the extent of colonization, particularly on peat soils, but this effe ct was not detected in naturally infected stumps. The fungus was largely co nfined to the heartwood and it is suggested that colonization may have been stimulated by the addition of nitrogen. It is concluded that urea provides an effective means of control against present ambient spore loads.