Mycobiota Associated with Symptomatic and Asymptomatic Fraxinus excelsior in Post-Dieback Forest Stands

Abstract

In Lithuania, the dieback of European ash (Fraxinus excelsior L.), caused by alien ascomycete Hymenoscyphus fraxineus, started in the mid-1990s, resulting in a large-scale decline of F. excelsior and its dominated forest habitats. Nevertheless, the recent inventories show the presence of several hundred hectares of naturally regenerated F. excelsior stands. We used seven naturally regenerated sites and three planted progeny trials of F. excelsior to collect leaves, shoots, roots, and the surrounding soil to study ash-associated fungal communities based on high-throughput sequencing. Results showed that fungal communities associated with F. excelsior in re-emerging stands in post-dieback areas were composed of 1487 fungal taxa. Among these, 60.5% were Ascomycota, 37.5%—Basidiomycota, 1.7%—Zygomycota, and 0.2% were Chytridiomycota. Revealed mycobiota was largely composed of endophytic fungal communities as these were dominated by Cladosporium sp., Fraxinicola fraxini (syn. Venturia fraxini) and Vishniacozyma foliicola. Identified mycobiota also included a range of ash-specific fungal taxa. Hymenoscyphus fraxineus occurred in all stands but was not frequent. Cladosporium sp. showed strongest negative correlation with the presence of H. fraxineus. This ascomycete, given its dominance in leaves, shoots and in the organic soil layer, might be the limiting factor for the infection rate or spread of H. fraxineus. Although fungal communities in asymptomatic and symptomatic samples of F. excelsior differed significantly from each other, the majority of the most frequently found fungal taxa were not host-specific, suggesting that these were negligibly affected by ash dieback. Investigated stands in natural F. excelsior habitats exhibited larger diversity of fungal taxa (especially ash-specific), than progeny trials planted on former grasslands, indicating the importance of natural habitats in F. excelsior restoration programs.

Fungal Communities in Re-Emerging Fraxinus excelsior Sites in Lithuania and Their Antagonistic Potential against Hymenoscyphus fraxineus

“…Cladosporium sp. (inhibition factor 76%) was relatively infrequently isolated in the present study ( Table 2 ), but it was among the dominating fungi in a high-throughput sequencing study [ 15 ], therefore this particular fungal taxon can be regarded as frequent in nature.…”

“…The fungal taxa detected in shoots and leaves of F. excelsior were similar to those reported in numerous other culture-based studies [ 11 , 12 , 13 , 14 ]. However, high-throughput sequencing of the shoot and leave samples collected in the present study revealed a very contrasting fungal community structure [ 15 ]. Evidently, each of the aforementioned fungal detection methods has its advantages and shortcomings, suggesting that the use of both methods can provide valuable complementary information.…”

“…In support, a negative correlation between H. fraxineus and Cladosporium sp. was also found in shoots of F. excelsior [ 15 ]. Cladosporium sp.…”