Pine trees are an integral part of the forest landscape across Switzerland (see Fig. 4). In recent years, they have come under increasing pressure, not least due to new fungal diseases that have been introduced from abroad. Needle and shoot diseases play a major role for pine health. The Swiss Forest Protection Group has been documenting diseases affecting Swiss tree species since 1984.
When the appearance (habitus) of an infested pine changes, the tree attracts attention. Needle and shoot diseases are clearly visible and thus easily change the appearance of the tree. Pines in particular are susceptible to many such diseases, also in comparison with other conifers such as Norway spruce (Picea abies) and silver fir (Abies alba). This is also reflected by the comparatively high level of attention devoted to the pine tree both in research and in the advisory activities of the Swiss Forest Protection Group.
Needle diseases not only change the appearance of the tree, however. They can also weaken the tree, and thus increase its susceptibility to other biotic and abiotic influences. Conversely, biotic and abiotic stress factors make a tree susceptible to infection with needle disease: the weaker the tree, the more easily additional infections occur. Examples of abiotic stress factors include drought, hail or sudden waterlogging. An example of a biotic damaging influence is the infection with a root pathogen such as the honey fungi Armillaria, to which Scots pine (Pinus sylvestris) is particularly susceptible (Nierhaus-Wunderwald et al. 2012).