Circumstantial and experimental evidence suggests that the bark beetle Ips cembrae acts as a vector for Ceratoeystis laricieola sp. nov. which it introduces into larches during breeding attacks. The fungus kills bark and cambium, and invades the sapwood causing the fotmation of areas of'blue stain' surrounded by dry wood. Multiple inoculations resulting from numerous beetle attacks may totally disrupt conduction causing dieback and death of whole trees. Attacks seem frequently to be associated with drought and other forms of stress. Trees damaged by /. cembrae and C. laricicola may be attacked subsequently by the woodwasp Urocerus gigas which introduces a sapwood decay fungus, Amylostereum chailletii. This is the first record of an association between /. cembrae and a species of Ceratocystis and the first record of dieback in larch caused by /. cembrae and C. laricicola. I. cembrae was introduced into Britain from mainland Europe and a similar association may occur there.

In an inoculation experiment to test the effect of wood moisture content on infection of Sitka spruce {Picea sitchensis) stumps by basidiospores of Heterobasidion annosum, exposure to high rainfall increased infection in the heartwood and reduced sapwood infection compared to covered stumps. This was associated with a greater moisture content in both wood types. Within-treatment variation in the amount of infection was high and it is suggested that endogenous factors in stumps may have a greater influence on infection than the environment or the availability of spores.

No abstract

Infection of Pinus contorta and Picea sitcbensis stumps by basidiospores of Heterobasidion annosum is extremely variable, both within and between sites, but in general P. sitchensis stumps are less susceptible than those of P. contorta. Measurement of the cross-sectional area occupied by H. annosum on each stump provides a more sensitive test of species susceptibility than assessment of the proportion of stumps infected. P. sitchensis stumps become infected on a variety of soils but there is evidence to suggest that infection may be reduced by high rainfall.In some infected stumps, H. annosum is confined to the lower stump tissues. Its absence from the upper portion of the stump may be due to replacement by other micro-organisms or, alternatively, physical conditions in the upper stump tissues may prevent its continued survival after infection has taken place. In both species, but more commonly in P. sitchensis, some stumps remain alive for at least two years after felling, particularly on peat soils, due to the presence of root grafts with neighbouring trees. Results for P. sitcbensis suggest that infection occurs more readily in living stumps than in those which die rapidly after felling.The viability of H . annosum basidiospore suspensions can be determined more accurately and more rapidly on a selective agar medium than on conifer stem sections.