The Endophytic Mycobiome of European Ash and Sycamore Maple Leaves – Geographic Patterns, Host Specificity and Influence of Ash Dieback

Abstract

The European ash (Fraxinus excelsior) is threatened by the introduced ascomycete Hymenoscyphus fraxineus, the causal agent of ash dieback. Endophytic fungi are known to modulate their host’s resistance against pathogens. To understand possible consequences of ash dieback on the endophytic mycobiome, F. excelsior leaves were collected in naturally regenerated forests and the fungal communities analyzed by classic culture and Illumina amplicon sequencing using a newly developed and validated fungal-specific primer. Collections were done in the area infested by ash dieback north of the Alps, and in the disease free area on the south side. Sycamore maple (Acer pseudoplatanus) was additionally collected, as well as the flowering ash (F. ornus), which occurs naturally in the south and shows tolerance to ash dieback. Both cultivation and amplicon sequencing revealed characteristic endophytic fungal communities dominated by several strictly host specific Venturia species. On A. pseudoplatanus, a hitherto undescribed Venturia species was identified. Due to its dominance on F. excelsior, V. fraxini is unlikely to go extinct in case of reduced host densities. A majority of species was not strictly host specific and is therefore likely less affected by ash dieback in the future. Still, shifts in community structure and loss of genetic diversity cannot be excluded. The potentially endangered endophyte Hymenoscyphus albidus was rarely found. In addition to host specificity, species with preferences for leaf laminae or petioles were found. We also detected considerable geographical variation between sampling sites and clear differences between the two sides of the Alps for endophytes of F. excelsior, but not A. pseudoplatanus. Since sycamore maple is not affected by an epidemic, this could point toward an influence of ash dieback on ash communities, although firm conclusions are not possible because of host preferences and climatic differences. Furthermore, the mycobiota of F. excelsior trees with or without dieback symptoms were compared, but no clear differences were detected. Besides methodical refinement, our study provides comprehensive data on the ash mycobiome that we expect to be subject to changes caused by an emerging disease of the host tree.

Forest Tree Microbiomes and Associated Fungal Endophytes: Functional Roles and Impact on Forest Health

“…Outside the native range of trees (Fraxinus ornus L., Fagus sylvatica L.), they harbor distinct species diversities compared to native range where more host-specific fungi can be observed [139,140]. Some dominant fungal endophytes show a strong host specificity (e.g., Venturia fraxini Aderh., [136,141,142]. These results suggest that diverse forest ecosystems, due to site-specific factors (climate, host species and latitude), have an effect on the species distribution and frequency of fungal endophytes [86,135].…”

“…The abundance and diversity of tree-associated fungi have further been shown to be affected by increases in latitude [122,123]. Differences were documented in the endophyte community of Fraxinus excelsior L collected from the north and south side of the Alps [87,136]. The presence of diverse tree species in a mixed forest that consisted of Alnus glutinosa (L.) Gaertn, Larix sibirica Ledeb., P. abies, Pinus sylvestris L.) was shown to have an effect on a foliar fungal community of Betula pendula Roth.…”

An overview of ash ( Fraxinus spp.) and the ash dieback disease in Europe.

“…Although some fungal ash endophytes with high in vitro antagonistic activity against H. fraxineus have been identified [93][94][95][96], an influence of endophytic mycobiomes on the health status of ash trees (i.e. the susceptibility to ash dieback) or on the establishment of the pathogen has not yet been demonstrated [77,94,96,97].…”

Analyzing Ash Leaf-Colonizing Fungal Communities for Their Biological Control of Hymenoscyphus fraxineus

“…To assess the role of fungal endophytes in ash dieback tolerance, the structure of leaf-and twig-associated fungal communities of tolerant and diseased ashes was investigated over a series of studies (Cross et al, 2017;Hanackova et al, 2017a;Schlegel et al, 2018;Kosawang et al, 2019). Different approaches showed significant variations in the community structure depending on the plant location, season and plant genotype.…”