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ContextIt might be possible to establish a new generation of Fraxinus excelsior which is insusceptible towards ash dieback (agent: Hymenoscyphus pseudoalbidus) by efficient breeding. However, a considerable number of highly tolerant individuals which have the ability to pass on this trait to their progeny are needed.AimsThe aim of this study was to...
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... the coefficients of genetic correlation were smaller than the coefficients of the correlation of clonal means. Moreover, there were high correlations between the years 2012 and 2013 for the two traits in the two orchards with repeated assessments (Table 4). The coefficients of genetic correlations were smaller than the coefficients obtained from correlation of clonal means and were approximately the same for the two traits. Genetic correlation was higher in the orchard of Landstuhl compared to the orchard of Kusel. The correlation of clonal means was slightly higher for the portion of epicormic shoots than for crown defoliation. Prevalence of collar rots in 2012 was 3.6 % in Kusel and 10.9 % in Landstuhl. In Emmendingen and Schorndorf, no collar rots could be detected. In 2013, In the present study, the potential of seed orchards in collar rot prevalence increased to 8.2 % in Kusel and southwestern Germany as a source of breeding for re- 16.9 % in Landstuhl. Differences in mean crown defo- sistance against H. pseudoalbidus was analysed by sur- liation and mean portion of epicormic shoots between veying the two traits crown defoliation and portion of trees with and without collar rots are presented in epicormic shoots. The study reports the current fraction Table 5. The difference in crown damage in 2013 was of potentially resistant trees in four seed orchards, pro- clearer when the trees which were already affected by vides H 2 and CV G values of susceptibility and investi- collar rots in 2012 were compared to the unaffected gates the level of susceptibility of clones from different individuals. nearby populations. Further, it reveals novel insights in In the present study, the potential of seed orchards in southwestern Germany as a source of breeding for resistance against H. pseudoalbidus was analysed by sur- veying the two traits crown defoliation and portion of epicormic shoots. The study reports the current fraction of potentially resistant trees in four seed orchards, pro- vides H 2 and CV G values of susceptibility and investigates the level of susceptibility of clones from different nearby populations. Further, it reveals novel insights in the interdependency between crown defoliation and the formation of epicormic shoots. The H 2 and CV G values (Table 2) indicate a relative strong genetic determinism of susceptibility to ash dieback in southwestern German populations. Possibly, the H 2 values of the present study are even to a small part underestimated, as it cannot be guaranteed that the upper part of all trees consisted of the scion due to grafting mistakes or resprouting of rootstocks. However, we believe that wrong genotype affiliation, if at all, had a marginal influence on the genetic estimates in this study since the grafting success was thoroughly examined several times during the first growing seasons of the orchards, and all trees which exhibited indications of unsuccessful grafting during the survey, i.e. when the graft union was not clearly visible, were excluded from the analyses. Moreover, the results of the present study are quite in agreement to former studies from northern Europe. McKinney et al. (2011) scored damage due to ash dieback by assessing the percentage of loss of crown foliage, which is comparable to crown defoliation, over a time of 3 years in two stands. They reported H 2 values ranging from 0.25 to 0.54 and CV G values ranging from 38 to 87 %. From progeny tests in Denmark and Lithuania, quite similar results were reported for narrow-sense heritability ( h 2 ), ranging from 0.37 to 0.52 and from 0.40 to 0.49, and for additive coefficients of variation (CV A ), ranging from 37 to 61 % and from 30 to 39 % (Pli ū ra et al. 2011; Kjær et al. 2012). Results of a Swedish study on two sites revealed H 2 values ranging from 0.07 to 0.57 and CV G values ranging from 17 to 55 %, but these values are not directly comparable since they were calculated from ordinal data of ash dieback damage in different parts of the crowns (Stener 2013). As in the former studies (McKinney et al. 2011; Pli ū ra et al. 2011; Kirisits and Freinschlag 2012; Kjær et al. 2012; Stener 2013), no clone was completely unaffected. This confirms the suggestion that there is no genetically determined total resistance, although some genotypes exhibit a very low degree of susceptibility against ash dieback (Stener 2013). The observed decrease in the CV values from 2012 to 2013 can be explained by the increase of the mean of the traits. Slight annual fluctuations of heritability could also be detected in other studies (McKinney et al. 2011; Kjær et al. 2012; Stener 2013). Previous studies revealed high correlations between crown defoliation and disease intensity (Metzler et al. 2012; Enderle et al. 2013). However, other reasons for defoliation, such as abiotic factors, can also be important. Moreover, vigorous trees can countervail the loss of twigs as a result of ash dieback by forming epicormic shoots (Fig. 3). Thus, crown defoliation de- pends to a considerable degree on stand properties and micro-location and is rather an indicator of overall tree vigour than of susceptibility to ash dieback. This is why we believe that the portion of epicormic shoots in the crown, as defined in Section 2.2, is a better indicator of disease infection or susceptibility. Assessing the portion of epicormic shoots was not more time-consuming (after some time of practice) than assessing the crown defoliation. Although epicormic shoots are positively correlated with crown defoliation (Table 3), there are considerable differences in total values as well as in the estimates of H 2 and variance components (Table 2). The H 2 value for crown defoliation was highest in the orchard with the most homogenous stand conditions and considerably smaller in the orchards of relatively low stand homogeneity (Tables 1 and 2). This reflects the influence of the environment on the phenotype. The H 2 values for portion of epicormic shoots differ less between the orchards, indicating that the environment has less influence on this trait. Assuming that the portion of epicormic shoots is an indicator of susceptibility and crown defoliation is an indicator of overall tree vigour, the positive correlations between the two traits imply that more susceptible trees are less vigorous and/or that trees of lesser vigour are more susceptible. The coefficients of genetic correlations and correlations from clonal means between the traits were higher on the orchards of relative low disease severity (Table 2 and 3 and Fig. 4), indicating that the interdependency between the two traits becomes lesser with increasing disease severity. The coefficients of genetic correlations were, in all orchards but the orchard of Schorndorf, smaller than the coefficients of correlations from clonal means. Apparently, the interdependency between the traits is only partly shoots than trees without collar rot, although the differ- due to genetic causes. ence in crown defoliation in 2013 was not significant The highly significant correlations of clonal means be- (Table 5). This might be due to reduced tree vigour in tween the years for crown defoliation and portion of epicormic collar rot-affected trees. The difference in crown damage shoots (Table 4) show that the relative rank in the level of in 2013 was clearer when the trees which were already susceptibility of the clones was more or less stable over the affected by collar rots in 2012 were compared to the years, although the total values of these traits increased. This is unaffected individuals, indicating that the influence of in agreement with a Danish study, where crown damage was collar rots increases with time. In another study, no surveyed over a period of 3 years (McKinney et al. 2011). The influence of collar rots on radial increment could be genetic correlations between the years confirm this result, detected, possibly because the phenomenon of collar although they were smaller than the correlations of clonal rots was too recent to already have a measurable impact means (Table 4). (Enderle et al. 2013). The following aspects complicate The subprovenances in the orchards of the present the interpretation of the present results: (i) the age of study are spatially not far apart from each other the necroses is unknown (although it appears likely that (Fig. 1). In the model, the influence of the necroses formation took place within the last 4 or subprovenances was significant, but very small. Thus, 5 years), (ii) the total lesion sizes and their extent in susceptibility to ash dieback seems to not be determined relationship to the diameter of the stems are unknown by subprovenances of closely located origins. However, and (iii) the tissues of the crowns and of the trunk differences in susceptibility were detected in a prove- collars do not belong to the same genotype. Thus, we nance trial between provenances from southern Germany would like to refrain from drawing further conclusions which differ more in their geographic origin (Enderle from these results. et al. 2013). Pli ū ra et al. (2011) reported differences in The present study shows that there is high genetic variation susceptibility between Lithuanian and western European in susceptibility to ash dieback in the assessed clones. It provenances, which they traced back to processes of highlights that there is a considerable genetic potential for adaption which had already commenced in the future breeding for resistance in provenances of southwestern Lithuanian populations. Germany. However, it is necessary to monitor the develop- As expected, our data indicates a rapid increase of ment of ash dieback in the next years to figure out if clones disease severity over time (Fig. 4). The number of trees that appear resistant to H. pseudoalbidus at the moment will without visible symptoms declined drastically, which prove to stay unaffected over a longer period of ...
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... collection in the seed orchards took place in July and early August 2012. During this time of year, there is a maximum of foliage, as the formation of Lammas shoots is completed and premature leaf fall has not started. In the orchards of Kusel and Landstuhl, the surveys were repeated in July 2013. Because of the construction of a gas pipe in the orchard of Kusel, 42 ash trees had to be removed during the winter 2012/2013, and these trees were not included in the analyses of the data of 2013. The removals of trees led to a minimal reduction of the status number from 73.4 in 2012 to 73.2 in 2013. Trees were divided into the following classes of crown defoliation: class 1=1 – 10 %, class 2=11 – 25 %, class 3=26 – 60 % and class 4=61 – 99 % crown defoliation. Trees without any visible symptoms of ash dieback were assigned to class 0. Crown defoliation has often been used in recent studies to quantify the degree to which a tree is affected by the ash dieback disease (e.g. McKinney et al. 2011; Husson et al. 2012; Kirisits and Freinschlag 2012; Lenz et al. 2012). However, some trees were highly affected by ash dieback but showed a relatively low degree of crown defoliation due to extensive formation of epicormic shoots (Fig. 3). On the other hand, some trees were highly defoliated but showed only moderate symptoms of ash dieback. Thus, in order to record the degree of disease affection, trees were additionally divided into classes of the estimated portion of epicormic shoots in the crowns, using the same division of classes as for crown defoliation. We defined epicormic shoots in a broad sense, also including small side twigs that substitute for affected terminal shoots of branches. This could be accomplished easily by observing the crowns from opposite sides. The assessment was carried out by the same person for the orchards in Emmendingen and Schorndorf, whereas three teams consisting of two people each assessed the trees in Kusel and Landstuhl. Prior to the Carlo techniques using the MCMCglmm package for R (Hadfield ...
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The UK needs to develop effective policy responses to the spread of tree pathogens and pests. This has been given the political urgency following the media and other commentary associated with the arrival of a disease that causes ‘dieback’ of European Ash (Fraxinus excelsior) - a tree species with deep cultural associations. In 2014 the UK governme...
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... This study was conducted at two study sites ( Fig. 1), located in the south of Germany in the federal state of Baden-Wuerttemberg. Both study sites have previously been included in research on ash dieback (Enderle et al., 2015, Buchner et al., 2022, thus detailed information on the health status of the planted individuals already exists. ...
The extended spread of ash dieback in Europe has far-reaching consequences for Fraxinus excelsior L. populations. The progression of the disease leads to characteristic symptoms, particularly within the tree crowns. To date, assessing the damage severity of each individual tree typically requires in-field inspections. However, UAVs equipped with RGB, thermal, and multispectral sensors offer cost-effective and objective possibilities. This study relied on such analyses and focused on two ash seed orchards in Baden-Wuerttemberg, Germany, where visual inspections were compared with multisensorial data obtained in spring, summer and autumn of 2022 and 2023. The calculated RGB and multispectral vegetation indices were able to significantly discriminate between different degrees of damage due to ash dieback; in contrast, thermal data were less reliable and linked to different dynamics. Novel thresholds applied to the vegetation indices enabled a classification of mild and severe damage with an overall accuracy of 74.9 % for the multispectral index DVI (Difference Vegetation Index) and 73.0 % for the RGB index GRVI (Green-Red Vegetation Index). Combining RGB and multispectral indices further improved the overall accuracy to 77.2 %. The presented workflow offers forest practitioners an accessible toolset for evaluating the health status of ash populations affected by ash dieback.
... Furthermore, differences in the resistance of families in progeny trials pointed toward its heritability (McKinney et al. 2011;Pliūra et al. 2011;Kjaer et al. 2012;Muñoz et al. 2016). In progeny and clonal trials, the narrowsense and broad-sense heritability of ash dieback resistance was estimated in the ranges of 0.37 -0.53 and 0.1 -0.57, respectively (McKinney et al. 2011;Pliūra et al. 2011;Kjaer et al. 2012;Enderle et al. 2015;Muñoz et al. 2016;Stener 2018). Specific single nucleotide polymorphism (SNP) and gene expression markers were found to be associated with the resistance to ash dieback in a Danish clonal trial (Harper et al. 2016;Sollars et al. 2017). ...
... In planted trials, as summarized by Plumb et al. (2020), mortality rates can be 85 percent, susceptibility is highly variable, ash dieback symptoms can be found on almost all trees, and there are additive genetic effects on ash dieback resistance. So far, the evaluation of ash dieback resistance and its heritability is based on annual assessments and analyses of crown damage (McKinney et al. 2011;Pliūra et al. 2011;Kjaer et al. 2012;Enderle et al. 2015;Muñoz et al. 2016;Stener 2018). These evaluations were made in one to five separate years, thus evaluating the actual susceptibility and not the individual long-term resistance. ...
... These evaluations were made in one to five separate years, thus evaluating the actual susceptibility and not the individual long-term resistance. Depending on the disease pressure or the formation of epicormic shoots that mask actual crown damage, the levels of crown damage might fluctuate across years Enderle et al. 2015). Consequently, authors indeed stress the need for long-term observations of the stability of ash dieback resistance (McKinney et al. 2011;Kjaer et al. 2012). ...
Common ash (Fraxinus excelsior L.) has been considered an important candidate species for climate-resilient forest management in Germany. The occurrence of ash dieback, caused by the invasive fungus Hymenoscyphus fraxineus, severely limits its use. However, less damaged ash trees in heavily infested stands are observed. This suggests a tree-specific and genetically determined susceptibility to ash dieback. The potential of individual trees for selection and conservation programs has been investigated on 35 clones in a clonal trial and 30 families in a progeny trial since 2014. Ash dieback-related mortality, crown damage, and height growth were examined. Resistance to ash dieback was evaluated based on multi-year observations. Mortality of ramets occurred rapidly and crown damage of clones stabilized, whereas steadily increasing in progeny during the study period. About 34 percent of all offspring (0-56 percent per family) showed high resistance to ash dieback and 10 percent showed also good height growth. Broad-sense heritability and narrow-sense heritability were moderate and comparable to previous studies. We assume that at least some individuals from specific progenies will resist the disease in forest stands. In situ and ex situ conservation strategies should be promoted. Seed orchards with more resistant genotypes could provide less susceptible reproductive material, and breeding programs could help to accelerate the fixation of favorable alleles.
... The seeds were collected in 2011 from a seed orchard with tree provenance classified as "81,108" in Schorndorf (Baden-Württemberg, Germany). The seeds were known to be susceptible to ash dieback as the seed plantation was already heavily affected by 2013 (Enderle et al. 2015). Seeds were collected and germinated in vitro to produce multiple genotypes. ...
Ash dieback is a tree disease caused by the fungal pathogen Hymenoscyphus fraxineus. Since its introduction into Europe, it has caused widespread and significant losses of the European ash, Fraxinus excelsior . Inoculations of F. excelsior with a low virulent H. fraxineus isolate were assessed as a promising method for reducing symptoms associated with ash dieback, presumably by triggering systemic induced resistance. Two strains of H. fraxineus were chosen based on observations of high and low in planta virulence. Crude extracts obtained from cultures of the highly virulent strain were more phytotoxic in a leaf puncture assay than ones obtained from the low virulent strain. UHPLC-DAD-MS/MS data identified the phytotoxin viridiol and the potential phytotoxin hyfraxin A in both cultures. However, the production of these compounds in vitro did not correspond with virulence in planta . To test the effects of priming, saplings of F. excelsior were first inoculated with the low virulent strain and subsequently with the highly virulent strain. On average, necrosis expansion on the stems was reduced by 53% in primed saplings at the end of the 14-week monitoring period, thus providing proof of the priming concept. These results contribute to our understanding of a possible integrated biological disease control approach for increasing resistance in saplings and reducing potential damages associated with pathogens, particularly during nursery propagation, out-planting and through the establishment phase. We discuss results in the context of relevant literature and summarise the limited availability of literature on priming and underlying principles in trees.
... Quantitative genetic studies revealed narrow sense heritability for ADB tolerance of 0.37-0.72 (Pliura et al. 2011;Kjaer et al. 2012;Lobo et al. 2014;Muñoz et al. 2016;Seidel et al. 2024) and broad sense heritability of 0.1-0.57 (McKinney et al. 2011;Stener 2013;Enderle et al. 2015;Seidel et al. 2024) among different ash trials in Europe. Furthermore, the level of crown damage due to ADB is genetically weakly correlated to autumn leaf colouring (McKinney et al. 2011;Stener 2013) and significantly correlated to spring bud burst (Stener 2013). ...
... (Pliura et al. 2011;Kjaer et al. 2012;Lobo et al. 2014;Muñoz et al. 2016;Seidel et al. 2024) and broad sense heritability (H bs 2 = 0.10-0.57) (McKinney et al. 2011;Stener 2013;Enderle et al. 2015;Seidel et al. 2024). The set of SNPs can only capture the genomic component of variation, which given this level of heritability will account for approximately 50% of the observed variation among trees, even in managed homogenous field trials. ...
Common ash (Fraxinus excelsior) is under intensive attack from the invasive alien pathogenic fungus Hymenoscyphus fraxineus, causing ash dieback at epidemic levels throughout Europe. Previous studies have found significant genetic variation among genotypes in ash dieback susceptibility and that host phenology, such as autumn yellowing, is correlated with susceptibility of ash trees to H. fraxineus; however, the genomic basis of ash dieback tolerance in F. excelsior requires further investigation. Here, we integrate quantitative genetics based on multiple replicates and genome‐wide association analyses with machine learning to reveal the genetic architecture of ash dieback tolerance and of phenological traits in F. excelsior populations in six European countries (Austria, Denmark, Germany, Ireland, Lithuania, Sweden). Based on phenotypic data of 486 F. excelsior replicated genotypes we observed negative genotypic correlations between crown damage caused by ash dieback and intensity of autumn leaf yellowing within multiple sampling sites. Our results suggest that the examined traits are polygenic and using genomic prediction models, with ranked single nucleotide polymorphisms (SNPs) based on GWAS associations as input, a large proportion of the variation was predicted by unlinked SNPs. Based on 100 unlinked SNPs, we can predict 55% of the variation in disease tolerance among genotypes (as phenotyped in genetic trials), increasing to a maximum of 63% when predicted from 9155 SNPs. In autumn leaf yellowing, 52% of variation is predicted by 100 unlinked SNPs, reaching a peak of 72% using 3740 SNPs. Based on feature permutations within genomic prediction models, a total of eight nonsynonymous SNPs linked to ash dieback crown damage and autumn leaf yellowing (three and five SNPs, respectively) were identified, these were located within genes related to plant defence (pattern triggered immunity, pathogen detection) and phenology (regulation of flowering and seed maturation, auxin transport). We did not find an overlap between genes associated with crown damage level and autumn leaf yellowing. Hence, our results shed light on the difference in the genomic basis of ADB tolerance and autumn leaf yellowing despite these two traits being correlated in quantitative genetic analysis. Overall, our methods show the applicability of genomic prediction models when combined with GWAS to reveal the genomic architecture of polygenic disease tolerance enabling the identification of ash dieback tolerant trees for breeding or conservation purposes.
... Since the ash dieback disease has been documented to be widespread in Germany [43], the pathogen was sampled, molecularly detected, and characterized at a few different locations ( Figure 1). Isolated not from necrotic ash tissue but from ash leaf rachises, the obtained fungal isolates in this study are supposed to represent a broad range of the natural population from the sampling location Waldsieversdorf [44]. ...
The ash dieback disease caused by Hymenoscyphus fraxineus is widespread in Germany and is the subject of intensive research efforts. The fungus identification is based on the genomic internal transcribed spacer (ITS) region, which can also be the site of intragenomic variability. In Germany, despite intense research efforts, only a few numbers of H. fraxineus sequence data are recorded. Therefore, this study aims to characterize H. fraxineus isolates obtained from diseased ash leaves in Brandenburg (Germany). Fungal isolates from infected ash leaf tissue were analyzed molecularly using species-specific primers and based on sequencing the ITS region of rDNA. The analysis of the two identified sequences revealed two base substitutionscompared to the reference sequences. Thus, they show an identity of 98.8%–100% to the reference sequences and support the assumption that H. fraxineus has multiple copies of the ITS region. The phylogenetic grouping with reference sequences did not show a distinct cluster for the European, particularly the German, sequences. This indicates that the evolvement of the genetic variability of the ITS region is still an ongoing process.
... Silvicultural measures can be used to incentivise DBH growth and tree vigour, especially in resistant trees [40], and alter the air conditions close to the ground, making them less favourable for the fungi [24,41]. Another option is resistance breeding if there is a suitable genetic potential [42]. Other ecologically acceptable options, such as the use of biocontrol agents, are still in perspective [41]. ...
Ash dieback is a major issue affecting European ash populations, including narrow-leaved ash (Fraxinus angustifolia Vahl). An important factor contributing to the decline of narrow-leaved ash is the fungal disease caused by Hymenoscyphus fraxineus. However, the mortality of trees also depends on stand structure that may influence the disease dynamics. In 2020, we analysed the stand structure of middle-aged, extensively managed, narrow-leaved ash stands growing on Humogley soil (Cariceto remotae-Fraxinetum angustifoliae, Jov., et Tom. 1979). This permanent sample plot is located in Posavina (Serbia), where we observed reduced tree vitality and mortality. The stand originates from natural regeneration after a succession of marsh habitats. At ages 20–25 years (1996) and 30–35 years (2006), selective thinning was carried out. Until the age of 45–50 years (2020), the stand was left unthinned, and the presence of ash dieback fungus was recorded in Serbia. In 2020, we measured the diameter at breast height (DBH) of each tree on the plot and assessed their crown class and degree of isolation. These parameters were evaluated in relation to the crown defoliation of the trees. The results indicate that reduced vitality and mortality of trees manifest in conditions of strongly expressed intraspecific competition in the stand, particularly during the stem exclusion stage. Healthy trees were observed primarily within the predominant/dominant crown class and exhibited the highest mean DBH. Trees classified as dead or dying (81%–100% defoliation) had a lower mean DBH compared to both healthy trees (<25% defoliation) and significantly defoliated trees (26%–80%). This was observed at both the stand level and in predominant/dominant trees, suggesting that tree mortality is primarily linked to their poor growth.
... Although approximately half of these early selections have died, they will be replaced and further supplemented by newer selections that have been recently grafted. Tolerance to ash dieback is widely reported to be moderately heritable (McKinney et al., 2014;Enderle et al., 2015;Muñoz et al., 2016;Semizer-Cuming et al., 2019) and durable (Stener, 2018), and therefore the possibility for a breeding programme for tolerance to ash dieback is feasible. However, such programmes need to be based on hundreds of individuals in order to retain genetic diversity (Kjear et al., 2017). ...
... Notably, Fraxinus excelsior exhibits natural variation in ADB susceptibility and several studies have shown that part of this variation is heritable with estimated heritabilities of 0.25-0.57 (Pliura and Baliuckas 2007;McKinney et al. 2011McKinney et al. , 2012McKinney et al. , 2014Pliura et al. 2011;Lobo et al. 2014Lobo et al. , 2015Enderle et al. 2015;Harper et al. 2016;Muñoz et al. 2016;Plumb et al. 2020). Several dierent phenotypes are presumed to be connected to ADB susceptibility. ...
Common ash, Fraxinus excelsior , is threatened by the invasive pathogen Hymenoscyphus fraxineus , which causes ash dieback. The pathogen is rapidly spreading throughout Europe with severe ecological and economic consequences. Multiple studies have presented evidence for the existence of a small fraction of genotypes with low susceptibility. Such genotypes can be targets for natural and artificial selection to conserve F. excelsior and associated ecosystems. To resolve the genetic architecture of variation in susceptibility it is necessary to analyze segregating populations. Here we employed about 1000 individuals of each of four single-tree progenies from potentially tolerant mother trees to identify full-sibling (full-sib) families. To this end, we first genotyped all 4000 individuals and the four mothers with eight SSR markers. We then used the program COLONY to predict full-sibs without knowledge of the paternal genotypes. For each single-tree progeny, COLONY predicted dozens of full-sib families, ranging from 3–166 individuals. In the next step, 910 individuals assigned to full-sib families with more than 28 individuals were subjected to high-resolution genotyping using over one million genome-wide SNPs which were identified with Illumina low-coverage resequencing. Using these SNP genotyping data in principal component analyses we were able to assign individuals to full-sib families with high confidence. Together the analyses revealed five large families with 73–212 individuals. These can be used to generate genetic linkage maps and to perform quantitative trait locus analyses for ash dieback susceptibility. The elucidation of the genetic basis of natural variation in ash may support breeding and conservation efforts and may contribute to more robust forest ecosystems.
... Numerous studies have revealed that a small proportion of European ash throughout Europe (including Britain) possess genetically based, heritable resistance to H. fraxineus, also referred to as disease tolerance (McKinney et al. 2011;Stener 2013;Enderle et al. 2014;Lobo et al. 2015;Havrdová et al. 2016;Sollars et al. 2017;Stocks et al. 2017). The existence of heritable resistance under strong genetic control therefore provides opportunities to design breeding strategies and produce resistant ash trees, although the genetic basis of resistance to H. fraxineus is not yet fully understood. ...
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.
... In 2023, from initially 416 individuals, only 120 living ash trees remained on the plantation. Both ash seed plantations have already been the subject of ash dieback research in other studies (Enderle et al. 2015;Buchner et al. 2022;Eisen et al. 2022Eisen et al. , 2023Eisen et al. , 2024. ...
Key message
Ash 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.
Abstract
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.
