Ash Dieback Research Articles

Lessons to learn for better safeguarding of genetic resources during tree pandemics: The case of ash dieback in Europe

Ash dieback (ADB) has been threatening populations of European ash (Fraxinus excelsior & F. angustifolia) for more than three decades. Although much knowledge has been gathered in the recent past, practical conservation measures have been mostly implemented at local scale. Since range contraction in both ash species is likely to be exacerbated already in the near future by westward expansion of the emerald ash borer and climate change, systematic conservation frameworks need to be developed to avoid long-term population-genetic consequences and depletion of genomic diversity. In this article, we address the advantages and obstacles of conservation approaches aiming to conserve genetic diversity in situ or ex situ during tree pandemics. We are reviewing 47 studies which were published on ash dieback to unravel three important dimensions of ongoing conservation approaches or perceived conservation problems: i) conservation philosophy (i.e. natural selection, resistance breeding or genetic conservation), ii) the spatial scale (ecosystem, country, continent), and iii) the integration of genetic safety margins in conservation planning. Although nearly equal proportions of the reviewed studies mention breeding or active conservation as possible long-term solutions, only 17 % consider that additional threats exist which may further reduce genetic diversity in both ash species. We also identify and discuss several knowledge gaps and limitations which may have limited the initiation of conservation projects at national and international level so far. Finally, we demonstrate that there is not much time left for filling these gaps, because European-wide forest health monitoring data indicates a significant decline of ash populations in the last 5 years.

Ash dieback: A single-species catastrophe or a cascade of ecological effects in the ground flora?

Climate change and human activities have accelerated the spread of non-native species, including forest pests and pathogens, significantly contributing to global biodiversity loss. Pathogens pose a significant threat to forest ecosystems due to a lack of coevolution with native hosts, resulting in ineffective defence mechanisms and severe consequences for the affected tree species. Ash dieback, caused by the fungus Hymenoscyphus fraxineus, is a relatively new invasive forest pathogen threatening ash (Fraxinus excelsior) with mortality rates in northern Europe reaching up to 80 %. The loss of ash due to dieback has severe ecological implications, potentially leading to an extinction cascade as ash provides crucial habitats and resources for many organisms. Despite this, the consequences of ash dieback on associated communities are largely unknown. To address this, we analysed changes in species richness, vegetation structure, and composition in 82 permanent vegetation plots across 23 Norwegian woodlands. We compared data collected before and 10–14 years after the emergence of ash dieback. In these woodlands, ash significantly declined in cover, leading to changes in tree species composition and facilitating the establishment of other woody tree species like hazel (Corylus avellana) and the invasive species sycamore (Acer pseudoplatanus). Despite these changes in the tree species composition, no significant alterations were observed in the understory plant community, indicating a degree of ecosystem resilience or a lagging community response. At this point, and with our focus on the vascular plants, we do not find support for cascading effects due to ash dieback. However, our findings demonstrate that one invasive species is facilitating the expansion of another, raising concerns about potential ecological imbalance and cascading effects in the future.

Applying palaeoecological analogues to contemporary challenges: community-level effects of canopy gaps caused by systematic decline of a prevalent tree species

Temperate woodland vegetation is initially determined by spatiotemporal and historical factors, mediated by complex biotic interactions. However, catastrophic events such as disease outbreaks (e.g., sweet chestnut blight, ash dieback), infestations of insect pests, and human-accelerated climate change can create canopy gaps due to systematic decline in, or loss of, tree species that was once an important part of the canopy. Resultant cascade effects have the potential to alter the composition of woodland ecosystems quickly and radically, but inherent lag times make primary research into these effects challenging. Here, we explore change in woodland vegetation at 10 sites in response to canopy opening using the Elm Decline, a rapid loss of Ulmus in woodlands across northwestern Europe ~ 5800 years ago that coexisted alongside other stressors such as increasing human activity, as a palaeoecological analogue. For arboreal taxa, community evenness significantly decreased, within-site turnover significantly increased, and richness remained unchanged. Changes in arboreal taxa were highly site-specific but there was a substantial decline in woody climbing taxa, especially Hedera (ivy), across the majority of sites. For shrub taxa, richness significantly increased but evenness and turnover remained consistent. Interestingly, however, there was a significant increase in abundance of shrubs at 70% of sites, including Calluna (heather), Ilex (holly) and Corylus (hazel), suggesting structural change. Surprisingly, there was no change in richness, evenness or turnover for herb taxa, possibly because change was highly variable spatially. However, there was a marked uptick in the disturbance indicator Plantago (plantain). Overall, these findings suggest that woodlands with sustained reduction in, or loss of, a tree species that once formed an important part of the canopy has profound, but often spatially idiosyncratic, impacts on vegetation diversity (richness), composition (evenness), stability (turnover), and on abundance of specific taxa, especially within the shrub layer. Use of this palaeoecological analogue, which was itself complicated by cooccurring changes in human activity, provides a valuable empirical insight into possible cascade effects of similar change in canopy opening in contemporary settings, including Ash Dieback.

Unravelling a hidden synergy: How pathogen-climate interactions transform habitat hydrology and affect tree growth

Interactions between multiple global change stressors are a defining characteristic of the Anthropocene. Tree-associated pathogens are affecting forested ecosystems worldwide and occur in the context of increased frequency and intensity of extreme climate events such as heat waves, droughts, and floods. The effects of these events, along with subsequent changes in environmental conditions, on remaining and regenerating trees, are not well understood but crucial for the restoration and conservation of forested habitats.In this study, we investigate ash (Fraxinus excelsior) dieback in a temperate broadleaf woodland as a case study to explore the processes influencing non-infected trees during pathogen-induced mortality events. Utilising an experimental setup, we examine tree growth rates at different chronological stages of the disease, including naturally progressing ash dieback (4–5 years since disease outbreak), accelerated ash dieback where ash trees have been girdled (10–15 years), and negligible ash dieback (<20 % ash trees).During a year with typical climatic conditions (2021), soils in accelerated ash dieback plots remained saturated throughout the summer due to insufficient transpiration (57 % higher in the accelerated dieback plots), suggesting a significantly increased risk of summer run-off and floods. However, tree growth rates in these plots were not affected (t-test, t = −0.3 to 1.2, p > 0.05). Conversely, anomalously dry years, such as the 2022 summer drought, saw higher soil moisture in the accelerated ash dieback plots (t-test, t = 4.8, p < 0.01) acting as a buffer, resulting in normal tree growth during drought compared to greatly reduced growth in plots with weaker dieback.These findings emphasise the complex interactions between extreme climate events and pathogen outbreaks. Better understanding of the relationships between pathogens and hydrology on tree growth is imperative and detailed long-term studies on tree growth and hydrology will facilitate and improve mitigation strategies.

Exploring the interaction between aminobutyric acid and epigenetics in modulating ash dieback response in european ash (Fraxinus excelsior)

AbstractEuropean ash populations face a significant threat from the invasive fungus Hymenoscyphus fraxineus, leading to ash dieback disease. The absence of resistant individuals and restrictions on biotechnological solutions hinder the ability to combat this widespread illness in the near term. Priming strategies offer an alternative approach to boost stress tolerance not only in this scenario but also in other endangered tree species by triggering plant defense mechanisms. The non-proteinogenic amino acid β-aminobutyric acid (BABA) has demonstrated potential in enhancing resistance to diverse stressors in plants. Despite limited research on forest tree species, the current study evaluated BABA’s effectiveness in mitigating ash dieback disease severity and explored potential correlations between priming treatments and epigenetic modifications. The results indicated that BABA enhanced stress tolerance in ash seedlings following inoculation with Hymenoscyphus fraxineus. BABA effectively decreased the development of necrosis associated with ash dieback disease in seedlings five months post-inoculation. Additionally, treatments involving BABA were linked to observed epigenetic alterations. Elevated levels of the non-canonical deoxynucleosides 5-(hydroxymethyl)-2'-deoxycytidine (5-hmdC) and 5-(hydroxymethyl)-2'-deoxyuridine (5-hmdU) were confirmed subsequent to the treatments. This study highlights the potential of BABA and other priming strategies in enhancing disease tolerance in forest tree species like European ash. Short-term improved stress tolerance and epigenetic changes were confirmed. Yet, the exact priming conditions for inducing long-term effects in plants, including long-living forest trees, remain unknown, posing a challenge for applying priming strategies to manage ash dieback and protect many other endangered tree species.

Detection of Viruses in Special Stands of Common Ash Reveals Insights into the Virome of Fraxinus excelsior

Plant diseases are mostly multicausal with several factors influencing the health status of affected hosts. Common ash (Fraxinus excelsior), a significant tree species of European forests, is currently mostly endangered by ash dieback, caused by the invasive fungus Hymenoscyphus fraxineus. However, contributing factors, including pathogenic viruses, are poorly understood. Here, we report the results of a virus screening conducted on selected special stands of F. excelsior. Over three consecutive years, ash trees from different origins were tested, including leaf material from mature seed trees, young trees and ash seedlings from the natural regeneration. Using RT-PCR, we screened for five viruses, including the generalist species ArMV (Nepovirus arabis) and CLRV (Nepovirus avii), as well as newly discovered viruses in ash, including the emaravirus ASaV (Emaravirus fraxini), the idaeovirus PrLBaV (Idaeovirus ligustri), and cytorhabdoviruses. The results revealed a high virus diversity in common ash. An association of ASaV detection with specific leaf symptoms, including shoestring, chlorotic ringspots, and vein yellowing, was documented. An analyses of relevant gene products of cytorhabdoviruses obtained from ashes of different sites revealed sequence diversities and two distinct phylogenetic groups present in ash populations. Signatures of novel viruses from different families have been identified by high-throughput sequencing. Together, our results provide insights into the virus diversity and distribution of viruses in ash and expand our knowledge about the virome of this endangered tree species.

Current understanding and future prospects for ash dieback disease with a focus on Britain

Abstract Hymenoscyphus fraxineus is an introduced ascomycete fungus which causes ash dieback and has resulted in widespread mortality of ash throughout Europe. Although H. fraxineus has been present on the continent for at least four decades, it was not identified until 2006. The first record of the pathogen in Britain came in 2012 although it was probably present a decade earlier. The most common host European ash (Fraxinus excelsior L.) is economically and ecologically important to Britain where the cost of ash dieback is estimated at billions of pounds. The impact of ash dieback has stimulated a major research response which we review with the aim of providing up-to-date information relevant to Britain and identifying knowledge gaps where research would contribute to improved disease mitigation. Hymenoscyphus fraxineus is an outcrossing fungus with high genotypic diversity; ascospores produced via sexual reproduction are critical to aerial dispersal and infection. Temperature, moisture, and ground cover influence pathogen fruit body development, the timing of ascospore release, and extent of ascospore germination; they also interact together to affect the likelihood of infection. In addition, stand characteristics, including tree density, tree height, and landscape fragmentation, affect disease dynamics with increased disease severity on moist sites with high ash density. Efforts at finding natural resistance in ash have identified genetic markers associated with disease tolerance, and gene expression analysis is providing insights into the basis of that resistance. Mainland European findings indicate that ash dieback makes trees more vulnerable to other pathogens, whilst endophytes in the ash phyllosphere can suppress infection by H. fraxineus. Possible tools for long-term control of ash dieback include (1) deployment of resistant ash, (2) quantitatively informed management practices based on microclimate models and better understanding of the pathogen life cycle, and (3) manipulation of biocontrol agents from the ash microbiome or pathogen mycoviruses.

Possibilities for reproduction and archiving resulting in a clone collection of a unique grey poplar (Populus × canescens Aiton Sm.) population in the Czech Republic

Abstract Floodplain forests as one of the most endangered ecosystems in Europe have recently been impacted by fungal pathogens Phytophtora sp. and Hymenoscyphus fraxineus (chalara). Grey poplar (Populus × canescens Aiton Sm.) can be used as one of well adapted tree species for the reforestation of withering stands of ash (Fraxinus sp.). A unique population of grey poplar (Populus × canescens Aiton Sm.) characterized by desirable phenotypic traits was used for this purpose. The gender distribution was asymmetric; out of 155 individuals, 113 were female. Out of 33 different genotypes determined in the population, 15 were used as a source of approved forest reproductive material. Vegetative reproduction methods (ex situ clonal reproduction by micropropagation and cuttings) were developed and used, to rapidly initiate the recovery of forest stands of grey poplar. In total, 940 explants were successfully micropropagated and adapted to natural conditions, to ensure a genetically diverse source of viable plants used for reforestation. Moreover, we used methodological procedures of micropropagation for setting up cryopreservation technique. With respect to long-term storage of valuable grey poplar genotypes, modulation of gene expression by cold hardening during cryopreservation revealed significant changes in a few candidate genes involved in plant cellular processes.

Load of the ash dieback pathogen hymenoscyphus fraxineus differs in soil

AbstractThe ascomycete Hymenoscyphus fraxineus causes the devastating ash dieback disease of European ash (Fraxinus excelsior L.). Spore traps are often used to measure the amount of ascospores in the environment, but the pathogen-load of the soil in ash stands has not been recorded so far. This is of particular interest with regard to the occurrence of ash stem necrosis, a decisive factor for the severe course of the disease. In order to gain a more differentiated insight into the pathogen-load in ash stands, we analysed soil samples from four ash tree sites in southern Germany, covering a clone plantation, two seed orchards and a forest. The pathogen-load was determined using a quantitative TaqMan real-time PCR assay for ten to twenty plots per stand. Results obtained by the species-specific assay highlighted that the pathogen-load is heterogeneously distributed in the ash stands. H. fraxineus DNA targets were detected in 17% of the soil samples. The pathogen-load differed according to soil depth, with the highest pathogen abundance in the top 5 cm, followed by 5–10 cm and finally 10–15 cm. Pathogen-load and thereby infection pressure were found to be highly variable for the individual trees in one stand. Overall, the study discovered detectable levels of H. fraxineus in the soil of all four study sites, which supports the hypothesis that H. fraxineus can be found in the soil of ash stands. The qPCR approach was found to be an effective method for monitoring the load of H. fraxineus in soil and for demonstrating the successful application of the method on the sample type of custom-made spore traps. Results suggest the implication of site-specific pathogen-load determination in future H. fraxineus-monitoring and selection of less susceptible ash trees for breeding and seed production.

Mother trees of common ash (Fraxinus excelsior) disperse different sets of mycobiome through their seed wings

ObjectiveThe endophytic mycobiome is present in all studied plant compartments, including fruits and seeds. Here, we studied the mycobiome of seed wings as they are transferred with seeds in common ash and tested whether the mycobiome differs among trees. To achieve this, we used ITS1-based amplicon sequencing and two genotypes of F. excelsior as a model to compare the mycobiome of mother trees and their wings.ResultsWe compared the mycobiome of 57 seed wings to the seed stalks (57) collected from two genotypes of F. excelsior using three ramets of each genotype. Alpha diversity indices (ACE, Fisher and Observed OTUs) suggested a higher richness of the mycobiome associated with the seed wing than the seed stalk within each genotype. However, there were neither significant differences in Shannon diversity between the mycobiomes from the two tissue types nor the two genotypes. PERMANOVA revealed significant differences in the mycobiome composition between tissue types (P < 0.001). It also showed a significant difference between seed wings (P = 0.04), but not between seed stalks of the two genotypes. Our results suggest that Fraxinus excelsior mother trees disperse different sets of mycobiomes with their seed wings, which may be important for germination and seedling establishment—especially in the light of ash dieback.

Effects of ash dieback on leaf physiology and leaf morphology of Fraxinus excelsior L.

Key messageAsh dieback causes alterations in leaf physiology and morphology, particularly affecting the specific leaf area, which can be used to discriminate between different degrees of damage.Since the introduction of the invasive fungal pathogen Hymenoscyphus fraxineus in Europe, the European common ash (Fraxinus excelsior L.) has been threatened by ash dieback. An infection leads, for example, to typical symptoms of dying shoots, but changes of leaf physiology and morphology are still largely unexplored. Therefore, five physiological and morphological traits, chlorophyll content, chlorophyll fluorescence, specific leaf area, leaf thickness, and fluctuating asymmetry, were investigated in four different study sites in southern Germany regarding possible changes due to ash dieback and their relationship to different degrees of damage. Both higher and lower levels of chlorophyll with increasing damage due to ash dieback were observed. Chlorophyll fluorescence and fluctuating asymmetry proved to be less suitable indicators of damage. Leaf thickness showed the tendency (however not significant) of an increase in more severely damaged trees. The specific leaf area was identified as a suitable indicator of the damage severity, with significant smaller values in less healthy trees. Therefore, ash dieback can also result in notable alterations in leaf physiology and morphology.

Stakeholder exposure to and knowledge of tree pests and diseases and their management in urban areas

Urban trees and forests provide many benefits to the urban environment and are important for climate change adaptation. Yet, they are increasingly threatened by insect pests and diseases, hereafter tree pests/diseases. There is little evidence of the risk awareness and knowledge of different urban stakeholders of this growing threat, how they are affected by tree pests/diseases, and how they might respond to it. To fill this gap, we undertook an online survey of different key stakeholder groups associated with urban trees and forests in Germany. A majority of 75.8 % of the 186 respondents consider urban tree pests/diseases a severe problem and 51.1 % reported high knowledge of tree pests/diseases. There was a lack of knowledge of certain reportable quarantine pests/diseases (e.g., canker stain of plane, emerald ash borer, Xylella) and pest/disease management options (e.g., manual treatment methods and tree diversification). Respondents were most affected by the horse chestnut leafminer (61.3 %), ash dieback (58.1 %) and oak processionary moth (50.0 %). The most widely used pest remedial measures were improvements of tree living conditions (60.8 %) and purchases of plants from certified or trusted local sources (59.7 %). Multiple correspondence analysis showed a significant association between levels of knowledge of tree pests/diseases and pest management responses (11.7 %). Our results suggest that future efforts to improve urban tree health should be enhanced and tailored to the different requirements of various stakeholder groups. The findings of this first comprehensive study with a purely urban focus will inform the development of future activities that prevent or reduce the spread of tree pests/diseases in urban areas. Data AvailabilityThe survey respondents did not give permission for the full data to be accessed or used by third parties.

Changes in plant diversity of European lowland forests: Increased homogenization and expansion of shade-tolerant trees

European lowland forests have undergone significant transformations over the past century due to increased canopy closure, eutrophication and plant invasions. The new millennium has brought plant invasions, alien fungal pathogens, and extreme weather conditions, all contributing to forest diebacks. However, the response of plant diversity remains poorly understood. We examined changes in understorey vegetation following a decade (2012–2021) of severe drought in four temperate lowland forest types (dominated by oaks, hornbeams, ashes, and willows) to understand how water availability, canopy openness, and alien fungal pathogens altered diversity in interaction with climate change. Following an exceptionally warm decade, the vegetation exhibited increased homogeneity, with denser shrub and herb layers, likely resulting from diminished tree canopy, particularly in hornbeam and ash forests. Homogenization was manifested by the colonization of plants with conservative life-history strategies (higher leaf dry matter content) to cope with increased drought. Consequently, communities with fewer species increased their species richness, while species-rich communities experienced impoverishment. Conversely, herbaceous species with higher specific leaf area were declining. Compositional changes were particularly prominent in hornbeam and oakwoods at the drier end of moisture gradient where the understorey became dominated by woody plants. Ashwoods were opening up due to ash dieback by fungal pathogens, causing replacement of wetland forbs by nitrophilous forbs and woody saplings. The overall number of species decreased in oakwoods but increased in hornbeam and ashwoods due to increased cover of grasses and shade-tolerant woody species, mostly Acer campestre. Conversely, willow understorey vegetation at the wetter end of the moisture gradient remained relatively stable. Our findings suggest shifts towards more homogeneous vegetation. Conservation plans for managing the remnants of species-rich lowland forests should consider the accelerated environmental changes linked to canopy alterations, biological invasions, and extreme climate events.

Evaluating the impact of an invasive pathogen on tree population decline: An evidence based modelling approach

An upsurge of invasive forest pathogens (IFPs) has been causing widespread damage to forest ecosystems worldwide. Modelling future forest loss caused by IFPs is challenging, as it requires a sophisticated understanding of the pathogen-hosts-surrounding interactions. We developed a complexity-appropriate model using an evidence-based approach to predict the decline of the European ash (Fraxinus excelsior L.) population caused by ash dieback (Hymenoscyphus fraxineus) in a British deciduous woodland. Our model predicts that (1) the ash population will decline by ∼26 % in the next 10 year; (2) an ±10 % relative error in mortality survey would cause a ∼8 % bias in 10-year population decline; and (3) a 5 % increase in resistant trees would save ∼3 % population over 10 years. Our research demonstrates the merit of systematic reviews in balancing model complexity against generalisation. By scaling up the methodology to other IFPs, it is possible to forecast forest health with various management scenarios.

Strong antagonism of an endophyte of Fraxinus excelsior towards the ash dieback pathogen, Hymenoscyphus fraxineus, is mediated by the antifungal secondary metabolite PF1140.

Ash dieback, caused by the fungal pathogen Hymenoscyphus fraxineus (Helotiales, Ascomycota), is threatening the existence of the European ash, Fraxineus excelsior. During our search for biological control agents for this devastating disease, endophytic fungi were isolated from healthy plant tissues and co-cultivated with H. fraxineus to assess their antagonistic potential. Among the strains screened, Penicillium cf. manginii DSM 104493 most strongly inhibited the pathogen. Initially, DSM 104493 showed promise in planta as a biocontrol agent. Inoculation of DSM 104493 into axenically cultured ash seedlings greatly decreased the development of disease symptoms in seedlings infected with H. fraxineus. The fungus was thus cultivated on a larger scale in order to obtain sufficient material to identify active metabolites that accounted for the antibiosis observed in dual culture. We isolated PF1140 (1) and identified it as the main active compound in the course of a bioassay-guided isolation strategy. Furthermore, its derivative 2, the mycotoxin citreoviridin (3), three tetramic acids of the vancouverone type (4-6), and penidiamide (7) were isolated by preparative chromatography. The structures were elucidated mainly by NMR spectroscopy and high-resolution mass spectrometry (HRMS), of which compounds 2 and 6 represent novel natural products. Of the compounds tested, not only PF1140 (1) strongly inhibited H. fraxineus in an agar diffusion assay but also showed phytotoxic effects in a leaf puncture assay. Unfortunately, both the latent virulent attributes of DSM 104493 observed subsequent to these experiments in planta and the production of mycotoxins exclude strain Penicillium cf. manginii DSM 104493 from further development as a safe biocontrol agent.IMPORTANCEEnvironmentally friendly measures are urgently needed to control the causative agent of ash dieback, Hymenoscyphus fraxineus. Herein, we show that the endophyte DSM 104493 exhibits protective effects in vitro and in planta. We traced the activity of DSM 104493 to the antifungal natural product PF1140, which unfortunately also showed phytotoxic effects. Our results have important implications for understanding plant-fungal interactions mediated by secondary metabolites, not only in the context of ash dieback but also generally in plant-microbial interactions.

Bibliometric Analysis of Trends and Future Directions of Research and Development of Seed Orchards

Seed orchards are important seed resources for producing improved tree crops for future plantations, forest restoration, and forestry practices (i.e., gene conservation) and for transmitting current gene diversity to future generations. Seed orchards are a major sub-division in forest science. The establishment and management of a seed orchard involves many steps, from the selection of superior trees to the harvesting of a seed crop. Studying the trends and future directions of seed orchards using different analysis methods is critically important, especially to establish resistant forests via the production of climate-smart, biotic/abiotic-stress-resistant seedling materials. Published papers related to seed orchards should be analyzed to determine the current trends in this field and to contribute to its future directions. Bibliometric analysis has been used for different purposes in various scientific fields. However, it has not been performed for publications in seed orchards. This study was carried out to analyze the current trends of research on seed orchards and to determine the future directions of these orchards based on published papers. For these purposes, 1018 published papers were obtained from the Science Citation Index, Science Citation Index Expanded, and citation index databases of “Web of Science” using the keyword “seed orchard”. The papers were published between 1980 and 2022 and were subjected to bibliometric analysis based on the most prolific contributors, references, countries, and keywords. CiteSpace software 6.1 R6 was applied to visualize information about seed orchard research. The average number of citations per publication was 13.05, and the 4 H-Index of the publication set was 48. The most prolific contributors with the strongest citation bursts, the highest centrality, and the greatest numbers of published papers were from Canada, Sweden, South Korea, Finland, and Czech Republic, while Canada (186 published articles), the USA (140), and Sweden (115), together with China, Brazil, and Germany, were active countries, especially based on citations from recent years. The “keywords” of the papers were the core of the research. “Mating pattern”, “Swedish forestry”, “fertility variation”, “Hymenoscyphus fraxineus”, “threatened Pacific sandalwood”, “outbreeding depression”, “climate change”, “management”, and “growth”, together with others such as “genetic improvement” and “effective size”, were active study areas and keywords, based on results of the analysis. They also guided the literature search and inventory and classification of early studies and served as predictors for future studies. The results of this study are discussed based on the trends and future directions of the research and development of seed orchards.

Hymenoscyphus fraxineus persistence in the ash litter

AbstractHymenoscyphus fraxineus causes ash dieback in Europe. It overwinters on ash leaf residue (rachis) within the forest litter. In late spring, the fungus produces apothecia on the rachises and releases ascospores to infect leaves. Previous studies reported that H. fraxineus was able to produce apothecia on the rachis for 5 years after the leaf infection under artificial conditions. However, ash litter is known to decompose rapidly in situ. We therefore monitored the decomposition kinetics of ash leaf debris and the persistence of the pathogen, as well as its ability to produce apothecia in the forest litter. For this, leaves shed in autumn in stands affected by ash dieback were placed in mesh bags and left in the forest litter for 6, 18 and 30 months. At each sampling period, litter mass loss and level of colonization of the rachises by H. fraxineus were measured, as was the pathogen's ability to produce apothecia on them. Despite high fragmentation, about 14% of the rachis dry weight remained in the mesh bags after 30 months, and the pathogen retained the ability to produce apothecia on these rachises. A simulation estimating the age composition of the colonized rachises present in the litter during the fruiting period was developed from these results. It shows that the persistence of H. fraxineus in old rachises of the litter represents a reservoir of inoculum that could compensate for poor colonization of autumn leaves and revive local outbreaks after years of unfavourable weather for the development of the pathogen.

Tools for pathogen genetic surveillance: Lessons from the ash dieback invasion of Europe.

Tools for pathogen genetic surveillance: Lessons from the ash dieback invasion of Europe.

Differential gene expression of Hymenoscyphus fraxineus grown on Fraxinus excelsior and F. mandshurica supplemented media

AbstractAsh dieback is caused by the invasive fungal species Hymenoscyphus fraxineus and leads to the loss of the common ash (Fraxineus excelsior) in many areas. In contrast, the pathogen seems to colonize the local ash species Fraxinus mandshurica (Manchurian ash) symptom-free in its native environment East Asia. In order to gain insight into the differences in the substrate dependence of the pathogen, two H. fraxineus strains (1511 and 1431) were grown in media supplemented with leave material of F. excelsior or F. mandshurica and without supplement. A total of 95 and 916 genes were upregulated in media containing F. excelsior leaf material for strain 1431, compared to F. mandshurica and media without plant supplementation. The differential expression analysis of strain 1511 revealed 483 and 875 upregulated genes. Oxidative phosphorylation is highly upregulated in strain 1431 cultivated in F. excelsior-supplemented media indicating an active metabolism but probably also linked to the release of reactive oxygen species. In both strains during growth on F. excelsior, transcripts with similarity to genes of toxin biosynthetic pathways of other Ascomycota are enriched. In strain 1511, a gene cluster with similarity to depudecin cluster is upregulated in addition. For both strains, the upregulated genes, if grown in media supplemented with leaf material from F. excelsior, include a gene of the biosynthetic cluster of phytotoxin viridiol. The results suggest increased metabolic activity and toxin production in the presence of F. excelsior tissue compared to F. mandshurica.

Alien invasive pathogens and pests in the changing environment: Focus on North Europe

On 12–13 September 2023, a panel of forest pathologists and entomologists gathered in Visby, Gotland Island, Baltic Sea, Sweden. The participants, a total of 20, represented all five Nordic Countries, three Baltic States, and Ukraine, and also joined by the local stakeholders representing the Swedish Forestry Agency and Gotland Administration. The purpose was to conduct a scientific workshop focusing on invasive tree pests and diseases emerging in each respective country, as well as to participate in an excursion to damaged woodland sites of Gotland. The Nordic Forest Research (SNS) financially supported the event (https://nordicforestresearch.org/n2023-01/). Keywords: Corinectria / Neonectria cankers; Hymenoscyphus fraxineus; Emerald Ash Borer; Diplodia sapinea; Acute Oak Decline; Polygraphus proximus; emerging birch pests; Bronze Birch Borer; Thekopsora areolata; seedborn conifer fungi