Ash dieback : the pathogen Hymenoscyphus fraxineus and other fungi associated with stem collar necroses of common ash

The ascomycete Hymenoscyphus fraxineus has caused devastating damage to the common ash (Fraxinus excelsior) in Europe in recent decades, resulting in considerable economic and ecological losses. The ash dieback pathogen is native to Asia, where it lives mainly as a saprobiont in peaceful coexistence with the Asian ash species Fraxinus mandshurica. Symptoms of the fungus in common ash include wilting of the leaves with subsequent dieback of shoots, twigs and branches, as well as damage to the stem base. These stem collar necroses contribute significantly to the mortality of ash trees. After the mostly primary infection by H. fraxineus, secondary wood-destroying fungi can colonise the stem base and the roots. Reduced stability leads to hazards for forest workers and forest visitors, as well as massive dieback and fallen ash trees. Stem collar necrosis is therefore an important aspect of ash dieback and requires comprehensive research. In the studies of this work stem collar necroses of ash trees were examined in the mycological laboratory. A culture-based method was used to determine and identify the isolated fungi morphologically and by DNA. The results showed, that the composition of the fungal community was different in each stem collar necrosis and at each sampling site due to the large number of species by single isolates. However, fungi of the Ascomycete division made up the largest proportion. The fungi associated with the examined stem collar necroses also included fungal species not previously described on common ash, such as Cryptostroma corticale, the causal agent of maple sooty bark disease, or the recently described species Vexillomyces fraxinicola, which was found for the first time. Nevertheless, the most common isolated species were also frequently recognised in other studies and were present at most of the sampling sites investigated in Germany. In addition to the ash dieback pathogen H. fraxineus, these include especially Armillaria species and Diplodia fraxini. As weak parasites or pathogens, they can lead to an extension of the damage – Armillaria spp. in particular due to its ability to cause intense white rot and D. fraxini because of its nature as a very aggressive pathogen. Multilocus genotype analyses showed, that multiple infections of the stem collar by different strains of H. fraxineus are not necessarily related to the size of the stem collar necrosis or the progression of damage. However, they are an important indication for the infection pattern of the pathogenic fungus. The multiple dominance of one genotype per stem collar necrosis suggests that a single strain of H. fraxineus initiates the stem collar necrosis and that other strains become established over time through successful infections. The role of the number of genotypes, the pathogenic aggressiveness of the individual strains or even intraspecific competition should be investigated in further studies.