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Summary In the Western Italian Alps (WIA), the three European species of the forest pathogen Heterobasidion spp. can coexist in the same area. Heterobasidion parviporum Niemela ¨ & Korhonen and Heterobasidion abietinum Niemela ¨ & Korhonen are normally found in areas with a significant presence of their respective primary hosts, spruce (Picea spp.) and fir (Abies spp.). The host/niche occupied by Heterobasidion annosum (Fr.) Bref. in the region still remains unclear. Although Scots pine (Pinus sylvestris), a major host for this fungal species in other parts of Europe, is abundant in the region, little or no evidence of disease caused by H. annosum is visible in this tree species. Two different, but not mutually exclusive, hypotheses can explain the presence of H. annosum: (1) Scots pines are infected but largely asymptomatic and (2) H. annosum has adapted to different hosts. An analysis of Heterobasidion species was performed in two natural, mixed-conifer forests using traditional isolation techniques and novel direct molecular diagnosis from wood. In a subalpine stand of mixed spruce (Picea abies), larch (Larix spp.), and Swiss stone pine (Pinus cembra), 18 naturally infected spruces and larches only yielded H. parviporum. A Swiss stone pine in the same stand was extensively colonized by both H. parviporum and H. annosum. In a second subalpine stand, an analysis of 18 spruce stumps and nine Swiss stone pine stumps yielded both H. parviporum and H. annosum isolates. Pine stumps had been mostly colonized by H. parviporum prior to tree felling, suggesting that this species may be secondarily infected by the locally predominant Heterobasidion species (i.e. H. parviporum). Results of our analysis also indicated that primary colonization of spruce stumps (e.g. through basidiospores) was caused by both H. parviporum and H. annosum, while secondary infection of such stumps was mostly because of H. parviporum.

Summary Tree survival and causes of mortality were studied in an experimental afforestation in the upper subalpine forest zone in the Swiss Alps. A total of 59.8% of Pinus cembra and 45.6% of Pinus mugo were killed by Gremmeniella abietina during the first 20 years after planting, compared to 1.5% of Larix deddua trees. The mortality rates caused by G. abietina were highly correlated with the duration of snow cover in spring. Tree losses were lowest at sites where the snow melted early and highest at sites where the snow ablation was delayed in spring. Tree mortality varied greatly between years. In the year after the coldest summer of the observation period mortality due to G. abietina infections was highest, suggesting high susceptibility of trees in poor condition. Phaddium infestans, which was the second most important factor for mortality in P. cembra, killed trees irrespective of their condition. Other biotic and abiotic causes of tree mortality had negligible influence compared with the impact of G abietina and P. infestans. Excluding anthropogenic impacts, the diverse spatial pattern of forested and treeless sites close to the subalpine timberline may predominantly result from the action of parasitic fungi, depending on the ablation pattern of the snow cover in spring.

Ectendomycorrhizae of Pinus mugo + Boletus edulis ssp. edulis and Pinus cembra + Suillus variegatus formed in pure culture. These associations have not been suspected hitherto in nature because sporocarps do not occur.

Pure culture synthesis of Pinus cembra mycorrhizae + Suillus placidus. The fungus forms an extremely thick mantle and a strongly developed Hartig net.

Gomphidius glutinosus a powerful antagonist of Fomes annosus in pure culture. F. annosus isolated from spruce was strongly inhibited by Gomphidius glutinosus found under Picea abies. In presence of the pathogen, Suillus placidus, a very probable mycorrhizal fungus of Pinus cembra and a moderate antagonist of F. annosus, produces a yellow reacting antibiotic substance.

Studies on the physiology and parasitism of Lachnellula flavovirens (Bres.) Dennis. From laboratory and field studies L. flavovirens was identified as a facultative parasite on several alpine conifer species (Pinus cembra, P. montana, Larix decidua). In the course of the work, factors affecting its development were investigated, especially with regard to temperature and nutrients.

Resistant progenies of Pinus monticola, improved in the northwestern USA, and Asian and European white pines were inoculated with the blister rust fungus, Cronartium ribicola, of Hokkaido. No resistance was observed in P. monticola but the other pines remained healthy. This shows that the resistance of white pines to the blister rust fungus is limited to a particular pathogenic race and a different race of the rust fungus exists in Hokkaido.

Epicuticular wax composition and its variation with needle age was analysed chemically. Needles were taken from conifers at the highest altitude of their local distribution. Quantitative and qualitative differences in the composition of the three conifer waxes were found. Increasing needle age influenced the wax composition in Pinus cembra and Picea abies.

Infection experiments with Ascocalyx abietina and A. laricina, on cembra pine, mountain pine, and larch between 1979 and 1985 gave very varying results from one year to another. Moderate to very severe disease only occurred after incubation on favourable sites in the subalpine zone. The intensity of disease may depend on local site and microclimatic conditions for infection. Successful infections occurred especially in autumn. An annual infection rate of 10 to 15 percent can endanger an afforestation: in an afforestation in Avers, 50 percent of the trees were killed in some 5 years.