Search

Ash dieback caused by the invasive fungus Hymenoscyphus fraxineus leads to massive mortality among common ash (Fraxinus excelsior) in Europe. To select tolerant genotypes, inoculation experiments are frequently conducted with isolates of the fungus. The aim of this study was to improve the inoculation methodology for evaluating susceptibility of ash genotypes to ash dieback through (i) testing the virulence of different isolates of Hymenoscyphus fraxineus and (ii) investigating temporal lesion development using three widely applied cultivars: Altena, Atlas and Westhof's Glorie. First, an experiment was conducted with a repeated measure design in which virulence of five different isolates was tested. Second, a subset of two isolates that induced the longest downward lesion length in experiment one was used in combination with a slightly adjusted inoculum protocol. Significant differences were found between isolates for downward lesion length, but a significant interaction effect of isolates and cultivars was absent. Also, the inoculation position within the stem affected lesion length; the largest lesions were found on the highest inoculation position within the stem. Furthermore, we found that cryopreserved isolates can remain virulent over years. The timing of inoculation at the end of the growing season was effective as large lesions already occurred during winter dormancy. For future inoculation studies, we propose to use: (i) isolates that induce large lesions, as less virulent isolates induced not only shorter but also fewer lesions; (ii) a similar inoculation position to better compare inoculations within and between experiments; (iii) cryopreserved isolates for testing over years; (iv) mycelial suspensions for inoculum preparation to cover wood chips more evenly; (v) reference clone—like the studied cultivars—to standardize research outcomes between years and research groups. [ABSTRACT FROM AUTHOR]

Background: The dieback epidemic decimating common ash (Fraxinus excelsior) in Europe is caused by the invasive fungus Hymenoscyphus fraxineus. In this study we analyzed the genomes of H. fraxineus and H. albidus, its native but, now essentially displaced, non-pathogenic sister species, and compared them with several other members of Helotiales. The focus of the analyses was to identify signals in the genome that may explain the rapid establishment of H. fraxineus and displacement of H. albidus. Results: The genomes of H. fraxineus and H. albidus showed a high level of synteny and identity. The assembly of H. fraxineus is 13 Mb longer than that of H. albidus', most of this difference can be attributed to higher dispersed repeat content (i.e. transposable elements [TEs]) in H. fraxineus. In general, TE families in H. fraxineus showed more signals of repeat-induced point mutations (RIP) than in H. albidus, especially in Long-terminal repeat (LTR)/Copia and LTR/Gypsy elements. Comparing gene family expansions and 1:1 orthologs, relatively few genes show signs of positive selection between species. However, several of those did appeared to be associated with secondary metabolite genes families, including gene families containing two of the genes in the H. fraxineus-specific, hymenosetin biosynthetic gene cluster (BGC). Conclusion: The genomes of H. fraxineus and H. albidus show a high degree of synteny, and are rich in both TEs and BGCs, but the genomic signatures also indicated that H. albidus may be less well equipped to adapt and maintain its ecological niche in a rapidly changing environment. [ABSTRACT FROM AUTHOR]

The invasive ascomycete Hymenoscyphus fraxineus has been threatening Fraxinus excelsior populations throughout Europe for over two decades. Since the infection and first colonization by the pathogen occurs in leaves, leaf-colonizing microorganisms have been discussed as a barrier and as possible biocontrol agents against the disease. To identify fungal groups with health-supporting potential, we compared the fungal microbiota of compound leaves from susceptible and tolerant ash trees in four ash stands with high H. fraxineus exposure. The fungal communities were analyzed both culture-independently by ITS2 amplicon sequencing and by the taxonomic classification of 1,704 isolates using matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) or sequencing of the entire ITS region. The fungal community structure did not show significant differences depending on the health status. However, for several OTUs and a MALDI group, a significantly higher abundance was found in tolerant ash trees. Thus, the yeast Papiliotrema flavescens was significantly increased and accounted for 12.3% of the mycobiome of tolerant ashes (OTU0003), and it had also a distinctly higher abundance among the isolates. The filamentous ascomycete Sarocladium strictum was increased 24-fold among the isolates of tolerant trees, but its abundance was comparably low. An in vitro screening for the growth inhibition of the pathogen via cocultivation resulted in 28 yeast-like isolates and 79 filamentous fungi with antagonistic activity. A statistical cocultivation test on two H. fraxineus strains confirmed six of the yeast-like isolates that suppressed H. fraxineus significantly, from 39–50%, two of them through a fungicidal effect. The highest inhibition rates among the yeasts were found for three isolates belonging to Aureobasidium pullulans and P. flavescens. The cocultivation test of the filamentous isolates revealed higher effects compared to the yeasts. Four isolates showed significant inhibition of both H. fraxineus strains with a rate of 72–100%, and five further isolates inhibited only one H. fraxineus strain significantly. The most effective isolates were members of the genus Cladosporium. During the next step, in planta tests will be necessary to verify the efficacy of the antagonistic isolates and to assess their suitability as biocontrol agents. [ABSTRACT FROM AUTHOR]

With ongoing climate change, episodes of severe flooding are predicted to become more frequent despite a general trend towards increasing summer drought. We investigated how wood growth of adult trees of two species characteristic of floodplain forests in Central Germany (Fraxinus excelsior L., Quercus robur L.) and two less-typical species (Acer pseudoplatanus L., Carpinus betulus L.) responded to both maximum stream water level and climate, with a special focus on the effects of the extraordinary flood of the Saale River in June 2013 and the extreme drought in summer 1976. Tree-ring widths were measured on wood cores, and standardized ARSTAN tree-ring chronologies were produced. Using variance partitioning as well as linear mixed-effects models, we compared the effects of monthly values for maximum water level, temperature and precipitation sum on tree-ring width. Further, we calculated resistance, resilience and recovery of the tree species to the extreme events of flooding in 2013 and drought in 1976. Wood growth of all species studied, and particularly of F. excelsior, responded positively to the extraordinary flooding in June 2013. However, in the best models for the characteristic floodplain forest species (F. excelsior and Q. robur), mainly precipitation (F. excelsior) or a combination of precipitation and wood growth of the previous year (Q. robur) acted as drivers of wood growth of the current year. In contrast, growth of the less habitat-specific species (A. pseudoplatanus) mainly showed a significant response to the combination of temperature and wood growth of the previous year. C. betulus was the only species studied that benefited from the extreme drought in 1976. However, two years afterwards, only the wood growth of A. pseudoplatanus was still reduced, while F. excelsior and Q. robur fully recovered. In comparison to other regions in Central Europe, the moderate flood regime of the Saale River seems to have the potential to mitigate effects of summer drought in this region, which is one of the driest in Germany. Thus, increased flooding frequency might, to some degree, reduce drought effects brought about by climate change as well. [ABSTRACT FROM AUTHOR]

Ash dieback caused by an alien, invasive fungus Hymenoscyphus fraxineus is a serious disease of European ash species in many parts in Europe. In Poland, the disease was recorded in the beginning of the 1990s. This study was performed in 2016–2017 with the aim to identify fungi isolated from ash shoots showing dieback symptoms in the Wolica Nature Reserve in Poland, as well as from shoots of two years-old ash seedlings inoculated with H. fraxineus in the greenhouse. The most frequently isolated fungi from shoots of common ash (associated with the pathogenic fungus H. fraxineus) were identified on the basis of sequencing of the internal transcribed spacer region (ITS1) of fungal rDNA. In total, 19 fungal taxa were identified for ash shoots as follows: H. fraxineus, Fusarium avenaceum, Alternaria spp., Phomopsis oblonga, Diplodia mutila and other Phomopsis spp. The pathogen H. fraxineus was not found for all the shoots samples; one year after inoculation the aforementioned fungi and other species as: Alternaria alternata, Bionectria ochroleuca, Epicoccum nigrum, F. acuminatum, F. avenaceum, and Paraphaesphaeria neglecta were identified in inoculation point, as well as H. fraxineus. The same quantitative and qualitative changes of organisms were observed in the case of artificially colonised ash seedlings in the greenhouse, as well as in the shoots of adult ash trees in the forest. [ABSTRACT FROM AUTHOR]

Societal Impact Statement: Damage to ash trees by ash dieback caused by the emerging fungal pathogen Hymenoscyphus fraxineus is impacting people across Europe. This poses challenges to: public safety; productivity of commercial forestry; green spaces and human wellbeing; and ecosystem services and carbon sequestration. Here, we seek to quantify the impact of ash dieback on tree mortality by analyzing surveys counting the proportion of trees that have died in sites across Europe. However, more and better data are needed to inform policy makers, foresters, conservationists, and other stakeholders as they plan for a long‐term future with ash dieback. Summary: The ash dieback epidemic, caused by the fungus Hymenoscyphus fraxineus, has been present in Europe for over 20 years and caused widespread damage and mortality in ash tree (Fraxinus excelsior) populations. Ash is a major natural capital asset and plays an important role in nature's contribution to people in Europe.Here, we present a meta‐analysis of surveys of ash mortality due to ash dieback, and a time‐dependent model to estimate longer term mortality. In plantations established previous to the arrival of the epidemic, we analyze 12 surveys, finding a maximum recorded mortality of ~85%. In woodlands with exposure to ash dieback of between 4 and 20 years, we analyze 36 surveys, finding a maximum recorded mortality (which may have missed some dead trees) of ~70%. We also analyze 10 surveys of naturally regenerated saplings, finding maximum recorded mortality of ~82%. We apply logistic models to these data sets to seek longer term predictions.More data are needed before our models can be relied upon for policy decisions. If survival found so far in woodlands is due in part to heritable resistance, natural selection or a breeding program may allow future recovery of ash populations in Europe. We review surveys of ash tree death due to ash dieback in Europe and find a maximum mortality so far of 70% of trees in woodlands and 85% of trees in plantations established before the epidemic. It is still hard to predict what the long‐term outcomes will be, and more data are needed. [ABSTRACT FROM AUTHOR]

Ash dieback caused by a non-native pathogenic fungus Hymenoscyphus fraxineus has been decimating populations of European Fraxinus species for over 30 years. Nevertheless, there is still a considerable amount of valuable ashes of this species in European cities and landscapes. Although it has been shown in many studies that the effect of H. fraxineus on adult and senescent trees tends to be rather chronic, the infection by this fungus also causes a substantial decrease in the lifespan of these trees. This research is dealing with the control of ashes using the tree injection technology. It is focused on the development of bark necroses, as this symptom results in the highest damage to a tree. Trees were first inoculated with two different strains of H. fraxineus. Trees with proven positive inoculation were injected with six different fungicides. Subsequently, over the course of nine months, the area of developing bark necroses was measured using image analysis methods. After the field part of the experiment was finished, the extent of the infection by H. fraxineus in tissues was determined and confirmed by PCR. Three out of six tested variants (Tr. 1: 7.1% propiconazole, Tr. 3: 7% propamocarb and 2.7% fosetyl, and Tr. 6: 0.015% sodium selenite) displayed significant deceleration of the growth of bark necroses compared to control. Tr. 3 demonstrated the highest effectiveness with only a very small increment of the necrosis area during the first three months after the injection treatment and a follow-up complete cessation of growth. Reisolations confirmed high mortality of H. fraxineus after this treatment. Hence the presented study can contribute to substantial prolongation of the lifespan of ashes in Europe. • Tree injection is an effective protection of the ash against Hymenoscyphus fraxineus. • Three preparations showed to be effective, greatly decelerating the lesion growth. • A mixture of 7% propamocarb and 2.7% fosetyl can eliminate the pathogen in tissues. [ABSTRACT FROM AUTHOR]

An undescribed disubstituted dihydrofuranone, named diplofuranoxin, was isolated, together with the six well known metabolites sphaeropsidins A and C, epi -sphaeropsidone, mellein and cis - and trans -4-hydroxymelleins, from the fungal species Diplodia subglobosa , an emerging pathogen involved in the ash dieback aetiology in Europe. Currently, the disease represents the main threat to European ash heritage and the wood associated industry. Diplofuranoxin, was characterized essentially by NMR and HRESIMS spectra as (3Z)-3-(2,3-dihydroxybutylidene)-5-methyldihydrofuran-2(3 H)-one. Its relative and absolute configuration was determined by joining NOESY NMR experiments and computational analysis of electronic circular dichroism spectrum. All the metabolites were screened for phytotoxic, antioomycetes and zootoxic activities and only sphaeropsidin A and epi -sphaeropsidone were active in two out of three bioassays performed. In addition, sphaeropsidin A completely inhibited mycelium growth of Phytophthora cambivora , whereas the inhibition rate of epi -sphaeropsidone was less than 50% at the higher concentration used. Both metabolites were inactive in the Artemia salina assay. Results obtained in this study have allowed to characterize for the first time the main metabolites produced in vitro by D. subglobosa and to increase the knowledge on the metabolic profile of Botryosphaeriaceae for a correct taxonomic classification of the strains belonging to this family. [Display omitted] • Diplodia subglobosa is the causal agent of ash diseases. • A disubstituted dihydrofuranone was isolated from its culture filtrates. • It was characterized by NMR and HRESIMS spectra and named diplofuranoxin. • Other six phytotoxins were isolated and identified. [ABSTRACT FROM AUTHOR]

English Ivy (Herdera helix , Araliaceae) is an evergreen woody climbing plant occurring naturally in temperate forests of Europe and as introduced ornamental plant in many parts of the world, becoming invasive in some. Over the last decades an increase of ivy occurrence in middle Europe was observed in forests as well as in urban or rural areas. As a consequence, ivy might also play a larger role in food webs and nutrient and carbon cycling in the future. We followed the decomposition of litter of ivy and compared it to litter of four common host trees species (sycamore, ash, oak, beech) using mesh bags with and without mesofauna access over a period of 10 months. Experiments were set up in nine gardens spread over Germany, involving citizen scientists. In the presence of mesofauna, ivy litter decomposed faster than litter of three of the tree species. The contribution of mesofauna to decomposition followed a similar pattern. In the absence of mesofauna, ivy decomposed faster than two of the tree species. In a changing world where ivy becomes more abundant, and tree species face population decline due to disease or climatic stress, litter composition will also change. Ivy adds medium to fast decaying litter to the ecosystem, and in other seasons than tree species. Conducting experiments with citizen scientist offers the possibility for wide-spread multi-site experiments generating robust data and overcoming local restrictions with low budgets. • In the presence of mesofauna, ivy litter decomposed faster than litter of oak, beech and sycamore. • In the absence of mesofauna, ivy litter decomposed faster than oak and beech litter. • Experiments done by citizen scientist can generate robust data with low budgets. • We did not detect a citizen scientist bias in our data. [ABSTRACT FROM AUTHOR]

Chalara fraxinea, the cause of ash dieback, is present in many European countries and constitutes a severe threat to Fraxinus spp. in forests, urban areas and nurseries. This study investigated tissue colonisation strategies of the parasite in Common ash at ultrastructural level and demonstrated that C. fraxinea develops intracellularly, moving through the cells and easily colonising the phloem, paratracheal parenchyma and parenchymatic rays. However, damage to either the plasmalemma or cell walls was not observed under our experimental conditions. Furthermore, the presence of intrahyphal hyphae was frequently detected, which are likely associated with a regeneration mechanism. [ABSTRACT FROM AUTHOR]

Dieback of Fraxinus excelsior L. associated with Chalara fraxinea is observed in Europe, and in some areas dying trees exhibit symptoms of root and butt rot. Our study was conducted (1) to estimate the impact of the rot on F. excelsior dieback severity; (2) to identify fungi colonizing roots of dieback-affected trees; (3) to check their pathogenicity to F. excelsior; and (4) to estimate sprouting incidence and sprout health condition in relation to presence/absence of rot. The extent of rot was measured in 33 trees with different dieback intensity, 150 fungal isolations were attempted from roots of 50 trees, 26 fungi were tested for pathogenicity to 286 saplings, and sprouting was evaluated for 328 stumps on three clear-felled sites. Root rot was mainly caused by Armillaria cepistipes, and the extent of rot correlated positively with dieback severity although it played a secondary role in tree decline. Four years after tree felling, root rot had a negative impact on sprouting frequency, yet rot did not enter sprouts from stumps, and when experimentally inoculated, fungi associated with root rot in mature ash had no visible impact on tree vigor, showing that after formation of sprouts, rot does not affect the subsequent phytosanitary condition. Sprouts on investigated sites exhibited Chalara dieback symptoms on leaves and bark irrespectively of presence/absence of rot, indicating that vigorous natural regeneration of F. excelsior in dieback-affected areas could not be expected. [ABSTRACT FROM AUTHOR]

Abstract: In Europe, remnants of formerly widespread natural mixed forests are rare. We analyzed an exceptionally tall tree stand with a very high wood volume in Hrončokovský grúň reserve, covering 55.2ha in Slovenské Rudohorie Mountains in central Slovakia (48°43′N and 19°35′E) between 730 and 1050m a.s.l. We compared our data to other natural stands to see if the growing stock and tree height were higher in Hrončokovský grúň. Fifty-four circular plots of 0.05ha each and spaced at 100m×100m distance sampled the entire reserve. Within each plot, live and dead trees were measured. Number, basal area and volume of live trees and volume of CWD were calculated. Moreover, measurements of the tallest trees were conducted in the whole reserve area. Total volume of live trees and CWD in the studied reserve was 1030m3 per hectare. It is the second highest value ever recorded in an area of several hectares or larger in European forests. The feature that makes the stand in Hrončokovský grúň a unique one is the height of trees. In the whole reserve, about 600 trees exceeded the height of 45m. Individuals of three deciduous species (Acer pseudoplatanus, Fraxinus excelsior, Ulmus glabra) are the tallest ones found in unmanaged forests in the whole Europe. Those of Fagus sylvatica reached height near the maximum value ever recorded. In Europe, stands of average total live and dead wood volume above 1000m3 ha−1 with numerous trees above 40m tall were probably very rare even several centuries ago, when virgin forests were widespread. Most likely, they were confined to rather small mountain areas, where rich soils supporting high stand productivity occurred along with wind-protected slopes, where natural disturbances were rare and stand turnover rate was low. [Copyright &y& Elsevier]

We used chloroplast polymerase chain reaction-restriction-fragment length polymorphism (PCR-RFLP) and chloroplast microsatellites to assess the structure of genetic variation and postglacial history across the entire natural range of the common ash (Fraxinus excelsiorL.), a broad-leaved wind-pollinated and wind-dispersed European forest tree. A low level of polymorphism was observed, with only 12 haplotypes at four polymorphic microsatellites in 201 populations, and two PCR-RFLP haplotypes in a subset of 62 populations. The clear geographical pattern displayed by the five most common haplotypes was in agreement with glacial refugia for ash being located in Iberia, Italy, the eastern Alps and the Balkan Peninsula, as had been suggested from fossil pollen data. A low chloroplast DNA mutation rate, a low effective population size in glacial refugia related to ash's life history traits, as well as features of postglacial expansion were put forward to explain the low level of polymorphism. Differentiation among populations was high (GST = 0.89), reflecting poor mixing among recolonizing lineages. Therefore, the responsible factor for the highly homogeneous genetic pattern previously identified at nuclear microsatellites throughout western and central Europe () must have been efficient postglacial pollen flow. Further comparison of variation patterns at both marker systems revealed that nuclear microsatellites identified complex differentiation patterns in south-eastern Europe which remained undetected with chloroplast microsatellites. The results suggest that data from different markers should be combined in order to capture the most important genetic patterns in a species. [ABSTRACT FROM AUTHOR]

Understanding the genetic diversity and structure of invasive pathogens in source and in introduced areas is crucial to the revelation of hidden biological features of an organism, to the reconstruction of the course of invasions and to the establishment of effective control measures. Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) is an invasive and highly destructive fungal pathogen found on common ash Fraxinus excelsior in Europe and is native to East Asia. To gain insights into its dispersal mechanisms and history of invasion, we used microsatellite markers and characterized the genetic structure and diversity of H. pseudoalbidus populations at three spatial levels: (i) between Europe and Japan, (ii) in Europe and (iii) at the epidemic's front in Switzerland. Phylogenetic and network analysis demonstrated that individuals from both regions are conspecific. However, populations from Japan harboured a higher genetic diversity and were genetically differentiated from European ones. No evident population structure was found among the 1208 European strains using Bayesian and multivariate clustering analysis. Only the distribution of genetic diversity in space, pairwise population differentiation (GST) and the spatial analysis of principal components revealed a faint geographical pattern around Europe. A significant allele deficiency in most European populations pointed to a recent genetic bottleneck, whereas no pattern of isolation by distance was found. Our data suggest that H. pseudoalbidus was introduced just once by at least two individuals. The potential source region of H. pseudoalbidus is vast, and further investigations are required for a more accurate localization of the source population., (© 2014 John Wiley & Sons Ltd.)