Mycobiome of Fraxinus excelsior With Different Phenotypic Susceptibility to Ash Dieback

Abstract

For the last two decades, large-scale population decline of European ash (Fraxinus excelsior) has occurred in Europe because of the introduction of the alien fungal pathogen, Hymenoscyphus fraxineus, from East Asia. Since European ash is a keystone species having critical importance for biodiversity, and only a small percentage of the ash population appears to show some tolerance against the pathogen, the loss of ash trees means that other associated organisms, especially those with high or obligate associations to ash, are at risk of further species declines. In this study, we used high throughput DNA sequencing and multivariate analysis to characterize: (i) the mycobiome in aerial tissues (i.e., leaf, bark, and xylem) of ash trees showing different phenotypic response to ash dieback, (ii) the temporal variation in fungal communities across the growing season, and (iii) the similarity in fungal community structure between ash and other common trees species that may serve as an ecological niche substitute for ash microfungi. Results showed that fungal communities differed among the three tissue types, susceptibility classes, in time and between sites. Trophic analysis of functional groups using the FUNGuild tool indicated a higher presence of pathotrophic fungi in leaves than in bark and xylem. The share of pathotrophic fungi increased along a gradient of low to high disease susceptibility in both bark and xylem tissue, while the proportion of symbiotrophic fungi correspondingly decreased in both tissue types. Neighboring, alternative host trees did not share all the fungal species found in ash, however, most microfungi uniquely associated to ash in this study are generalists and not strictly host specific. The progressive disappearance of ash trees on the landscape imposes a high risk for extinction of Red-listed macrofungal species, and breeding for resistance against ash dieback should help sustain important biodiversity associated to ash. Microfungal diversity though may be less prone to such demise since most ash-associated endophytes appear to occur on a broad range of host species.

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

“…In general, our study revealed a high diversity of fungal taxa in both leaf and shoot samples, which was in agreement with earlier findings of culture-independent studies [34,39,59]. Species composition was clearly dominated by Ascomycota, which included representatives of endophytic and other plant colonizing fungi [42].…”

“…Mycological studies associated with ash dieback have changed over time, i.e., from identification of causal agent [3,31,32], to the assessment of seasonal patterns of the disease [11,33], fungal diversity, and its seasonal and spatial dynamics in planta [34]. Numerous studies on fungal diversity in and near ash trees covered myciobiota diversity in healthy [35][36][37], as well as diseased [32,34,[37][38][39] stands. The number of studies on ash-associated mycobiome (especially endophytes) boosted during several past years.…”

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

“…On the other hand, A. pullulans and Cladosporium sp. were frequently reported as fungi associated with F. excelsior [ 13 , 22 , 23 ], indicating that H. fraxineus can possibly co-exist with these ascomycetes in vivo or it is able to avoid the direct confrontation due to seasonal succession of these fungi on tissues of F. excelsior [ 13 ]. It should also be noted that fungal taxa, which showed the strongest reaction of growth inhibition of H. fraxineus in dual-culture assays, were fast-growing fungi.…”

Living With The Impact Of Ash Dieback – Local Mitigation Practices Against<em> Hymenoscyphus fraxineus</em> on the Island of Ireland

“…(3) alter the environmental conditions that promote a desirable microbiome, (4) genetically modify ash trees to impact signalling or selection traits that determine microbial community and composition (Griffiths et al, 2020). Recent studies into the ash microbiome include comparisons between communities present on healthy and infected ash trees, comparing different ash tree tissues and analysing their temporal variation (Kowalski et al, 2016;Cross et al, 2017;Agostinelli et al, 2021;Lahiri et al, 2021). For example, the study by Griffiths et al (2020) identified a significant association between severity of infection by H. fraxineus and the composition of the fungal and bacterial communities associated with ash leaves.…”