Microstrobilinia castrans, a new genus and species of the Sclerotiniaceae parasitizing pollen cones of Picea spp.

Abstract

Abstract The fungal pathogens of spruce are well known not only in Europe but worldwide. Therefore, it was all the more surprising to discover a new fungal species and genus in Central Europe that attacks the pollen cones of three spruce species. The new ascomycete forms apothecia on stromatized pollen cones of Norway spruce (Picea abies) and Serbian spruce (P. omorika) in mountain areas as well as on West Himalayan spruce (P. smithiana) planted in urban lowland regions of Switzerland and Germany. It was also detected in France, based on sequences found in the GlobalFungi database of fungal occurrences from high-throughput-sequencing metabarcoding studies. The different origins of the host trees in Europe and Asia leave the origin of the fungus unclear. The new fugus might be a neomycet for Europe. A phylogenetic analysis using SSU, LSU, ITS, RPB2 and TEF1 sequences classified the fungus to the family Sclerotiniaceae (Helotiales, Leotiomycetes). However, it differs morphologically to the other genera in this family by an ascus without apical apparatus and four citriform spores. Furthermore, it is the only known cup fungus that parasitizes pollen cones of conifers by stromatizing their tissue and infecting pollen grains. The fungus does not seem to cause major damage to the spruce populations, as only a few pollen cones per tree are affected. All this leads us to describe the newly discovered fungus as the new species and new genus Microstrobilinia castrans, the fungus that castrate pollen cones of spruce.