Rust fungi belong to the class of the Basidiomycota (Basidionmycetes) and to the order of the rust fungi (Uredinales). Worldwide, they are represented by 5000 species. All rust fungi are specialized for particular host plant species. Their development rarely takes place on a single plant species. Mostly, the cycle is combined with a change between two systematically unrelated host plants.
Spruce rust fungi (Chrysomyxa) live on spruce species and often on flowering plants from the family of the Ericaceae. In Europe, there are six spruce rust fungus species of which half occur in Switzerland:
The most common rust fungus in Switzerland is the spruce Chrysomyxa rhododendri. This species changes between the host species of rhododendron and spruce (Fig. 3). The fungus is able to endure and to reproduce on both of the native rhododendron species. The rhododendron species are the hibernation sites of the fungus. The second host is the spruce where the young trees are preferably infected. The spruce Chrysomyxa does not necessarily depend on the spruce for its cycle. This is why it also occurs on spruce outside of the area of distribution of spruce. The spruce Chrysomyxa is common in the Alps as well as in other European mountain regions. In Switzerland, the fungus can be found between 1’000 and 2’000 m. In particular, it can often be found in the alpine valleys of Grisons and Ticino.
The spores of the fungus are formed during spring at the lower side of the leaf of rhododendron. The wind disperses them over long distances. Chrysomyxa rhododendri only infests the new and recently emerged needles. After the rust fungus has spread over the needles, typical symptoms develop: a striking yellowish shine and discolored needles. The spores which develop on spruce infest the winter host, the rhododendron, during autumn and thus contribute to the dispersal of the spruce Chrysomyxa.
Spruce needle rust fungus (Chrysomyxa abietis) lives without a change between hosts and only on the needles of spruce. It prefers to infest dense stands of spruce. Unlike the spruce needle rust Chrysomyxa rhodendri, it occurs at lower elevations between 400 m and 1’100 m. The first visible symptoms for an infestation with the Chrysomyxa abietis are yellow-green and, later, orange-yellow horizontal stripes on the needles. The fungus is often found in Christmas tree plantations.
Spruce cone rust fungus (Chrysomyxa pyrolata) occurs considerably less frequently than the other two native Chrysomyxa representatives. It changes between the spruce and evergreen (Pyrolaceae) species and populates only the cone scales. The Chrysomyxa pyrolata overwinters on Pyrolaceae species and seems to propagates below ground with these evergreen species.
The spruce needle rust Chrysomyxa rohododendri has been known to the alpine region over more than 100 years. Still, an outbreak which has led to dying trees has never been documented. The fungus hardly affects rhododendron either. Chrysomyxa abietis on the other hand, may cause significant damage within Christmas tree plantations. Chrysomyxa pyrolata may reduce the seed production by 20-60% but still does not pose a threat to forestry.
Translation: Marcus Schaub (WSL)