In recent decades the vitality and physical stability of European ash trees in Germany have been reduced by European ash dieback, especially when associated with stem collar necroses and rots. This study was carried out to investigate the composition of the fungal communities associated with stem collar necroses. Filamentous fungi were isolated from 58 ash trees out of nine forest stands in northern, eastern, and central Germany. Obtained isolates were identified to a genus or species level by means of morphological and molecular analyses. In total 162 morphotypes including endophytic, saprotrophic, and pathogenic fungi were isolated. For 33 species found no prior reports from Fraxinus excelsior were recognised, including Cryptostroma corticale and Diplodia sapinea. None of the identified species were found at all studied sites, though Diplodia fraxini was the most common fungus with regard to frequency within all isolates, occurring at seven sample sites. This species is followed by Hymenoscyphus fraxineus, Armillaria spp., Neonectria punicea, Diaporthe cf. eres, Fusarium cf. lateritium, and Paracucurbitaria sp. in order of frequency within all isolates. The aforementioned species are characterised and analysed in respect to their occurrence in stem collar necroses and at sample sites. The influence of site conditions on the fungal composition was described for five intensively sampled sites with a minimum of five studied trees (Schwansee, Rhüden, Berggießhübel, Satrup, and Schlangen). The sampling site of Schlangen was further subdivided into four subplots with different positions in the terrain. In the remaining four extensive sample sites, either one or two trees, respectively, were sampled and analysed (Oranienbaumer Heide, Woltershausen, Wolfenbüttel, and Neuhege). Over all sample sites, fungal communities of symptomatic stem tissue are similar concerning the most frequent fungi, but vary greatly according to singularly isolated fungi.

During a survey on fungi associated with wood necroses of Prunus trees in Germany, strains belonging to the Leotiomycetes and Eurotiomycetes were detected by preliminary analyses of ITS sequences. Multi-locus phylogenetic analyses (LSU, ITS, TUB, EF-1α, depending on genus) of 31 of the 45 strains from Prunus and reference strains revealed several new taxa, including Arboricolonusgen. nov., a new genus in the Helotiales (Leotiomycetes) with a collophorina-like asexual morph. Seven Cadophora species (Helotiales, Leotiomycetes) were treated. The 29 strains from Prunus belonged to five species, of which C. luteo-olivacea and C. novi-eboraci were dominating; C. africanasp. nov., C. prunicolasp. nov. and C. ramosasp. nov. were revealed as new species. The genus Cadophora was reported from Prunus for the first time. Phialophora bubakii was combined in Cadophora and differentiated from C. obscura, which was resurrected. Asexual morphs of two Proliferodiscus species (Helotiales, Leotiomycetes) were described, including one new species, Pr. ingenssp. nov. Two Minutiella species (Phaeomoniellales, Eurotiomycetes) were detected, including the new species M. pruni-aviumsp. nov. Prunus avium and P. domestica are reported as host plants of Minutiella.

Prunus belongs to the economically most important genera of fruit crops in Germany. Although wood pathogens possess the capability to damage the host substantially, the knowledge of the fungal pathogenic community and the mycobiome of Prunus wood in general is low. During a survey in important fruit production areas in Germany, branches with symptoms of fungal infection were sampled in Prunus avium, P. cerasus and P. domestica orchards, and 1018 fungal isolates were obtained primarily from the transition zone of symptomatic to non-symptomatic wood. By a combination of blastn searches and phylogenetic analyses based on ITS and LSU sequences with a strong focus on reliable reference data, a diversity of 172 fungal taxa belonging to Ascomycota, Basidiomycota and Mucoromycota were differentiated. The majority of the strains belonged to three classes of Ascomycota, namely Sordariomycetes, Leotiomycetes and Dothideomycetes. The dominant species were Aposphaeria corallinolutea (Dothideomycetes) and Pallidophorina paarla (Leotiomycetes) that were isolated more than a hundred times each, while all other taxa were isolated ≤ 30 times. Only part of them could be identified to species level. Because of the high plasticity of species boundaries, the identification certainty was divided into categories based on nucleotide differences to reference sequences. In total, 82 species were identified with high and 20 species with low (cf.) certainty. Moreover, about 70 species could not be assigned to a known species, which reveals Prunus wood to represent a habitat harbouring high numbers of potentially new species, even in a well-explored region like Germany.

From 2018 to 2020, Germany experienced periods of exceptional weather conditions. Extremely high summer temperatures and precipitation deficits induced stress and mortality in forest trees. Acer pseudoplatanus (sycamore) was one of the affected tree species. Symptoms of sooty bark disease (SBD) and severe damage of entire stands, both caused by the fungal species Cryptostroma corticale, were reported more frequently. To explore the non-symptomatic distribution of C. corticale, wood cores from visibly healthy sycamore stems were sampled and all outgrowing fungi were identified and recorded. In total, 50 trees, aged 30–65 years, were sampled at five different forest stands, from which 91 endophytic filamentous morphotypes could be isolated. The fungal endophytic community in the woody tissue of the sycamore trees varied greatly at the different sites and between the trees. The number of isolated morphotypes at the different sites ranged from 13 to 44 and no morphotype was found at all sites. At 1.20-m stem height, 3.3 fungi could be isolated from woody tissue per tree on average. The most abundant species isolated from visibly healthy sycamore in regard to both occurrence at the studied sites and continuity was C. corticale. It was recorded at four of the studied forest stands, from 26% of all studied sycamore trees, and had a frequency of 7.85% relative to the 293 isolated filamentous strains that were isolated. The second most abundant species was Xylaria longipes followed by Lopadostoma turgidum. In this study clear evidence for the endophytic lifestyle of C. corticale is presented which thus appears to be spread further than expected based on visible SBD symptoms.

Vitality loss of beech is a complex disease of European beech that has been occurring all across central Germany since the summer of 2018, leading to growth loss and causing damage to affected trees. Using a culture-based method, 13 matured European beech trees affected to varying degrees by Vitality loss of beech in central Germany were sampled. The isolation of fungi from each test tree was conducted in a standardised manner, with subsequent culturing, morphotyping and identification of filamentous species. All in all, 181 morphotypes were isolated from twigs, branches and trunks of symptomatic and asymptomatic tissue and identified. Fifteen different orders of Ascomycota and four orders of Basidiomycota were detected. Isolated species and corresponding orders differed greatly depending on the test tree, the sampled tree compartment and the tissue type. However, it could not be shown that the vitality status of the host tree had an effect on the fungal community in asymptomatic tissue, possibly because the effect was superimposed by the site or individual tree characteristics. While, depending on the individual tree, a large number of different fungal species probably contributed to the damage caused by Vitality loss of beech, Biscogniauxia nummularia and Neonectria coccinea were present throughout the whole study area, confirming their high relevance in Vitality loss of beech. Biscogniauxia nummularia was isolated more frequently from the asymptomatic tissue of damaged trees than from the asymptomatic tissue of undamaged trees and is therefore possibly suitable as a bioindicator for the beech vitality.

Leaf fungal pathogens alter their host species’ performance and, thus, changes in fungal species composition can translate into effects at the tree community scale. Conversely, the functional diversity of tree species in a host tree’s local neighbourhood can affect the host’s foliar fungal infestation. Therefore, understanding the factors that affect fungal infestations is important to advance our understanding of biodiversity-ecosystem functioning (BEF) relationships. Here we make use of the largest BEF tree experiment worldwide, the BEF-China experiment, where we selected tree host species with different neighbour species. Identifying fungal taxa by microscopy and by high-throughput DNA sequencing techniques based on the internal transcribed spacer (ITS) rDNA region, we analysed the fungal richness and infestation rates of our target trees as a function of local species richness. Based on the visual microscopic assessment, we found that a higher tree diversity reduced fungal richness and host-specific fungal infestation in the host’s local neighbourhood, while molecular fungal richness was unaffected. This diversity effect was mainly explained by the decrease in host proportion. Thus, the dilution of host species in the local neighbourhood was the primary mechanism in reducing the fungal disease severity. Overall, our study suggests that diverse forests will suffer less from foliar fungal diseases compared to those with lower diversity.