Fungi Detected in the Previous Year’s Leaf Petioles of Fraxinus excelsior and Their Antagonistic Potential against Hymenoscyphus fraxineus

Schauerella fraxinea gen. nov., sp. nov., a bacterial species that colonises ash trees tolerant to dieback caused by Hymenoscyphus fraxineus

The tolerance of ash trees against the pathogen Hymenoscyphus fraxineus seems to be associated with the occurrence of specific microbial taxa on leaves. A group of bacterial isolates, primarily identified on tolerant trees, was investigated with regard to their taxonomic classification and their potential to suppress the ash dieback pathogen. Examination of OGRI values revealed a separate species position. A phylogenomic analysis, based on orthologous and marker genes, indicated a separate genus position along with the species Achromobacter aestuarii. Furthermore, analysis of the ratio of average nucleotide identities and genome alignment fractions demonstrated genomic dissimilarities typically observed for inter-genera comparisons within this family. As a result of these investigations, the strains are considered to represent a separate species within a new genus, for which the name Schauerella fraxinea gen. nov., sp. nov. is proposed, with the type strain B3P038T (=LMG 33092 T = DSM 115926 T). Additionally, a reclassification of the species Achromobacter aestuarii as Schauerella aestuarii comb. nov. is proposed. In a co-cultivation assay, the strains were able to inhibit the growth of a H. fraxineus strain. Accordingly, a functional analysis of the genome of S. fraxinea B3P038T revealed genes mediating the production of antifungal substances. This potential, combined with the prevalent presence in the phyllosphere of tolerant ash trees, makes this group interesting for an inoculation experiment with the aim of controlling the pathogen in an integrative approach. For future field trials, a strain-specific qPCR system was developed to establish an efficient method for monitoring the inoculation success.

Bioactive Compounds from an Endophytic Pezicula sp. Showing Antagonistic Effects against the Ash Dieback Pathogen

A fungal endophyte originating from the Canary Islands was identified as a potent antagonist against the fungal phytopathogen Hymenoscyphus fraxineus, which causes the devastating ash dieback disease. This endophyte was tentatively identified as Pezicula cf. ericae, using molecular barcoding. Isolation of secondary metabolites by preparative high-performance liquid chromatography (HPLC) yielded the known compounds CJ-17,572 (1), mycorrhizin A (3) and cryptosporioptides A-C (4-6), besides a new N-acetylated dihydroxyphenylalanin derivative 2, named peziculastatin. Planar structures were elucidated by NMR and HRMS data, while the relative stereochemistry of 2 was assigned by H,H and C,H coupling constants. The assignment of the unknown stereochemistry of CJ-17,572 (1) was hampered by the broadening of NMR signals. Nevertheless, after semisynthetic conversion of 1 into its methyl derivatives 7 and 8, presumably preventing tautomeric effects, the relative configuration could be assigned, whereas comparison of ECD data to those of related compounds determined the absolute configuration. Metabolites 1 and 3 showed significant antifungal effects in vitro against H. fraxineus. Furthermore, 4-6 exhibited significant dispersive effects on preformed biofilms of S. aureus at concentrations up to 2 µg/mL, while the biofilm formation of C. albicans was also inhibited. Thus, cryptosporioptides might constitute a potential source for the development of novel antibiofilm agents.

Fungal succession in decomposing ash leaves colonized by the ash dieback pathogen Hymenoscyphus fraxineus or its harmless relative Hymenoscyphus albidus

Introduction

The ascomycete Hymenoscyphus fraxineus, originating from Asia, is currently threatening common ash (Fraxinus excelsior) in Europe, massive ascospore production from the saprotrophic phase being a key determinant of its invasiveness.

Methods

To consider whether fungal diversity and succession in decomposing leaf litter are affected by this invader, we used ITS-1 metabarcoding to profile changes in fungal community composition during overwintering. The subjected ash leaf petioles, collected from a diseased forest and a healthy ash stand hosting the harmless ash endophyte Hymenoscyphus albidus, were incubated in the forest floor of the diseased stand between October 2017 and June 2018 and harvested at 2-3-month intervals.

Results

Total fungal DNA level showed a 3-fold increase during overwintering as estimated by FungiQuant qPCR. Petioles from the healthy site showed pronounced changes during overwintering; ascomycetes of the class Dothideomycetes were predominant after leaf shed, but the basidiomycete genus Mycena (class Agaricomycetes) became predominant by April, whereas H. albidus showed low prevalence. Petioles from the diseased site showed little change during overwintering; H. fraxineus was predominant, while Mycena spp. showed increased read proportion by June.

Discussion

The low species richness and evenness in petioles from the diseased site in comparison to petioles from the healthy site were obviously related to tremendous infection pressure of H. fraxineus in diseased forests. Changes in leaf litter quality, owing to accumulation of host defense phenolics in the pathogen challenged leaves, and strong saprophytic competence of H. fraxineus are other factors that probably influence fungal succession. For additional comparison, we examined fungal community structure in petioles collected in the healthy stand in August 2013 and showing H. albidus ascomata. This species was similarly predominant in these petioles as H. fraxineus was in petioles from the diseased site, suggesting that both fungi have similar suppressive effects on fungal richness in petiole/rachis segments they have secured for completion of their life cycle. However, the ability of H. fraxineus to secure the entire leaf nerve system in diseased forests, in opposite to H. albidus, impacts the general diversity and successional trajectory of fungi in decomposing ash petioles.

Fungi associated with woody tissues of Acer pseudoplatanus in forest stands with different health status concerning sooty bark disease (Cryptostroma corticale)

From 2018 to 2020, Germany experienced periods of exceptional weather conditions. Extremely high summer temperatures and precipitation deficits induced stress and mortality in forest trees. Acer pseudoplatanus (sycamore) was one of the affected tree species. Symptoms of sooty bark disease (SBD) and severe damage of entire stands, both caused by the fungal species Cryptostroma corticale, were reported more frequently. To explore the non-symptomatic distribution of C. corticale, wood cores from visibly healthy sycamore stems were sampled and all outgrowing fungi were identified and recorded. In total, 50 trees, aged 30–65 years, were sampled at five different forest stands, from which 91 endophytic filamentous morphotypes could be isolated. The fungal endophytic community in the woody tissue of the sycamore trees varied greatly at the different sites and between the trees. The number of isolated morphotypes at the different sites ranged from 13 to 44 and no morphotype was found at all sites. At 1.20-m stem height, 3.3 fungi could be isolated from woody tissue per tree on average. The most abundant species isolated from visibly healthy sycamore in regard to both occurrence at the studied sites and continuity was C. corticale. It was recorded at four of the studied forest stands, from 26% of all studied sycamore trees, and had a frequency of 7.85% relative to the 293 isolated filamentous strains that were isolated. The second most abundant species was Xylaria longipes followed by Lopadostoma turgidum. In this study clear evidence for the endophytic lifestyle of C. corticale is presented which thus appears to be spread further than expected based on visible SBD symptoms.

Fungicolous Fungi on Pseudosclerotial Plates and Apothecia of Hymenoscyphus fraxineus and Their Biocontrol Potential

In the present work, research tasks were carried out in the search for fungi with potential biocontrol possibilities in relation to the ash dieback pathogen, Hymenoscyphus fraxineus. In the years 2012–2021, dead petioles of F. excelsior and F. mandshurica were collected, on which morphological structures of H. fraxineus showed unusual symptoms of dying (apothecia) and signs of colonization by other fungi (pseudosclerotial plates). Based on morphological and molecular phylogenetic data, 18 fungal taxa were identified. Thirteen of them belong to Ascomycota: Clonostachys rosea, Cl. solani, Cordyceps sp., Minimidochium sp., Nemania diffusa, Fusarium sp., Pestalotiopsis sp., Trichoderma atroviride, T. harzianum, T. polysporum, T. rodmanii, T. tomentosum, Trichoderma sp., and five other taxa are represented by Basidiomycota: Corticiales sp., Cyathus olla, Efibula sp., Gymnopus sp. and Polyporales sp. In 108 dual cultures in vitro, three different types of interactions were distinguished: (i) physical colony contact (5.6%), (ii) presence of an inhibition zone between the colonies (0.9%), and (iii) copartner overgrowth of H. fraxineus colonies and partial or complete replacement of the pathogen (93.5%). In the dual cultures, various morphological deformations of H. fraxineus hyphae were observed: the development of apical or intercalary cytoplasmic extrusions, development of internal hyphae of the test fungi in pathogens’ hyphae, the deformation and disruption of significant sections of H. fraxineus hyphae via lysis and mycoparasitism, complete desolation of H. fraxineus cells and breakdown of hyphae into short fragments, and disappearing of pigment in the affected hyphae of H. fraxineus. The inoculation tests performed in vivo or in glass Petrie dishes showed that all the identified taxa were able to lead to pathological changes in H. fraxineus apothecia, and the mycelium of some of them completely covered pseudosclerotial plates of H. fraxineus. It was emphasized in the discussion that such activity of these fungi in forest stands may contribute to the reduction in the H. fraxineus inoculum reservoir.

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

Fifty-nine fungal taxa, isolated from re-emerging Fraxinus excelsior sites in Lithuania, were in vitro tested against three strains of Hymenoscyphus fraxineus on agar media to establish their biocontrol properties. All tested fungi were isolated from leaves and shoots of relatively healthy Fraxinus excelsior trees (<30% defoliation), which were affected by ash dieback but their phytosanitary condition has not worsened during the last decade. The inhibition of H. fraxineus growth by tested fungal taxa ranged between 16−87%. Occasionally isolated fungal taxa such as Neonectria coccinea, Nothophorma quercina, and Phaeosphaeria caricis were among the most effective fungi inhibiting the growth of H. fraxineus cultures. Among the more commonly isolated fungal taxa, Cladosporium sp., Fusarium sp., Malassezia sp., and Aureobasidium pullulans showed a strong growth inhibition of H. fraxineus.

Pyrenochaeta fraxinina as colonizer of ash and sycamore petioles, its morphology, ecology, and phylogenetic connections

Pyrenochaeta fraxinina was first described in 1913 from the state of New York (USA) on petioles of Fraxinus sp. Since then, the species has not been reported from North America and reports from the other regions of the world are very sparse. The results of this study on P. fraxinina are based on the material collected in various regions of Poland from 2012 to 2019. The material comprised 2700 previous year’s leaf petioles of Fraxinus excelsior and 1970 petioles or leaf residues of eight other deciduous tree species. As a result, the occurrence of pycnidial conidiomata of P. fraxinina was confirmed on F. excelsior (3.4% of petioles), F. mandshurica (1.5%), F. pennsylvanica (3.2%), and Acer pseudoplatanus (2.0%). The morphology of the microstructures was described based on the fresh material and compared with the holotype of P. fraxinina. The optimal temperature for the growth of the fungus in vitro was estimated as 20 °C. The analyses based on ITS-LSU rDNA sequences and a protein coding sequence of TUB2 and RPB2 genes showed that P. fraxinina isolates form a well-supported clade in the phylogenetic trees. The species proved to be closely related to Nematostoma parasiticum (asexual morph Pyrenochaeta parasitica), a species occurring on Abies alba in connection with needle browning disease. Interactions between P. fraxinina and the ash dieback pathogen, Hymenoscyphus fraxineus, were analyzed in vivo on ash petioles and in vitro in dual cultures. Among 93 petioles of F. excelsior, for which P. fraxinina conidiomata were detected, 26 were also colonized by H. fraxineus. Mostly, these two fungi occurred separately, colonizing different sections of a petiole. For all dual cultures, both fungi, P. fraxinina and H. fraxineus, showed growth inhibition toward the counterpartner. The role of P. fraxinina as a saprotrophic competitor toward H. fraxineus in ash petioles is discussed.

Use of Secondary Metabolites of Wood-Decaying Fungi to Reduce Damping off Disease

Phytopathogenic fungi can cause plant diseases that are difficult to control, including mass mortality of some tree species. The Fusarium oxysporum complex (sensu lato) is one of the most dangerous groups of phytopathogenic fungi, causing the death of conifer species, including Pinus sylvestris seedlings in forest and ornamental nurseries. Recently, non-chemical methods of plant protection have become the basis of integrated pest management (IPM) in the European Union (EC Directive). The possibility of protection of pine seedlings against the pathogen F. oxysporum using active substances from wood-destroying fungi commonly found in forests was examined. Methanolic extracts of Fomitopsis pinicola, Ganoderma applanatum, and Trametes versicolor were found to contain substances effective in both prevention and treatment of infected seedlings. G. applanatum and T. versicolor showed particular biological activity in increasing plant resistance. Efficacy, especially of the extract of F. pinicola, increased with concentration. Further field trials are needed to confirm the results obtained in laboratory tests on plant protection.

Culturable Endophytic Fungi in Fraxinus excelsior and Their Interactions with Hymenoscyphus fraxineus

The species diversity of culturable endophytic fungi was studied in the leaves and twigs of symptomatic and asymptomatic Fraxinus excelsior trees. Endophytic mycobiota was dominated by Ascomycota species, with Pleosporales (44.17%) and Diaporthales (23.79%) endophytes being the most frequently observed in the tree samples. The number of endophytic isolates and species richness varied depending on the sampling date (May and October) and tissue location. Of the 54 species identified based on ITS sequences, 14 were classified as dominant. The most frequently isolated species were Diaporthe eres, followed by Alternaria alternata, Dothiorella gregaria, and Fraxinicola fraxini. The inhibitory effect of 41 species (75 isolates) of endophytes on the radial growth of a Hymenoscyphus fraxineus isolate was studied under in vitro conditions (dual cultures). The radial growth of H. fraxineus was the most inhibited by four endophytic fungi from twigs (Fusarium lateritium, Didymella aliena, Didymella macrostoma, and Dothiorella gregaria). The inhibitory effect of the four isolates was also studied under in planta conditions. The isolates artificially inoculated into the trunks of ash trees reduced the length of necroses formed by H. fraxineus co-inoculated in the same trunks. This effect depended on the isolate, and the inhibition was most prominent only on trunks inoculated with F. lateritium and D. aliena. Although the total length of necrotic lesions formed by the H. fraxineus infection was shorter in the ash trunks co-inoculated with the endophytes, the difference was not significant.

The Impact of Biotic and Abiotic Stress Factors on Development of European Ash Tissue Cultures

Fraxinus excelsior L. is threatened by a variety of environmental factors causing a decline of the species. The most important biotic factors negatively affecting the condition of the F. excelsior population are fungi such as the pathogen Hymenoscyphus fraxineus. Abiotic factors with potentially harmful effect to the F. excelsior population are the accumulation of heavy metals and salinity in soils. Thus, the aim of this study was to investigate the impact of selected biotic and abiotic stress factors to determine which of them pose a threat to European ash. The study was conducted using in vitro techniques based on callus and seedlings regenerated via indirect organogenesis. Tissue cultures exclude the influence of other factors, including the environmental impact on ash extinction. The results confirmed very strong pathogenic potential of H. fraxineus in which after 14 days the callus tissue cells died as the tissue failed to activate its defense mechanisms. Experiments showed the high toxicity of cadmium in concentration of 0.027 mmol/L. Salinity caused the activity of oxidation enzymes to vary among seedlings and calluses in the control suggesting the enzymes play a role in controlling the morphogenetic development of tissue cultures.

The Occurrence of Apiognomonia hystrix and Its Pathogenicity towards Acer pseudoplatanus and Fraxinus excelsior under Field Conditions

Apiognomonia hystrix is an ascomycetous fungus within Diaporthales that is found on maples and to a lesser extent on other hardwood trees in Europe, Northern America and Asia. To date, varying opinions on the species’ status as a cause of plant diseases have been expressed. In this study, we present the results of analyses conducted from 2012–2017 at forest sites in Poland on the occurrence of A. hystrix on Acer pseudoplatanus and Fraxinus excelsior and the pathogenicity of this fungus towards both tree species. For the sycamore leaves, A. hystrix conidiomata were detected in connection with 19.2% of galls caused by Dasineura vitrina, 20.4% of galls caused by Drisina glutinosa and 67.9% of extensive vein-associated necroses. The A. hystrix colonization of galls caused by both midge species resulted in statistically significantly larger necroses. On European ash leaves, conidiomata of A. hystrix occurred in connection with 0.8% of Dasineura fraxinea galls. Perithecia of A. hystrix were detected on overwintered leaf petioles in 8.1% of A. pseudoplatanus and 1.2% of F. excelsior samples. Twelve representative cultures were characterized molecularly by barcoding three marker genes (ITS, ACT, CAL). Results of phylogenetic analyses indicate that A. hystrix isolates are genetically variable, and three lineages are distinguishable. Eight isolates, including four originating from sycamore and four from European ash, were used to determine A. hystrix pathogenicity. Among the 48 A. pseudoplatanus petioles inoculated with A. hystrix, 41 developed necrotic lesions after 8 weeks, with the average necrosis length caused by particular isolates ranging from 14.5 to 67.2 mm. None of the 48 inoculated F. excelsior petioles developed necrotic lesions. Finally, selected aspects of A. hystrix morphology on natural substrates and in vitro are discussed in this paper, as well as the species’ potential to cause disease symptoms.

Priming of Resistance-Related Phenolics: A Study of Plant-Associated Bacteria and Hymenoscyphus fraxineus

European ash (Fraxinus excelsior) is highly affected by the pathogenic fungus Hymenoscyphus fraxineus in all of Europe. Increases in plant’s secondary metabolite (SM) production is often linked tol enhanced resistance to stress, both biotic and abiotic. Moreover, plant-associated bacteria have been shown to enhance SM production in inoculated plants. Thus, our hypothesis is that bacteria may boost ash SM production, hence priming the tree’s metabolism and facilitating higher levels of resilience to H. fraxineus. We tested three different ash genotypes and used Paenibacillus sp. and Pseudomonas sp. for inoculation in vitro. Total phenol (TPC), total flavonoid (TFC) and carotenoid contents were measured, as well as the chlorophyll a/b ratio and morphometric growth parameters, in a two-stage trial, whereby seedlings were inoculated with the bacteria during the first stage and with H. fraxineus during the second stage. While the tested bacteria did not positively affect the morphometric growth parameters of ash seedlings, they had a statistically significant effect on TPC, TFC, the chlorophyll a/b ratio and carotenoid content in both stages, thus confirming our hypothesis. Specifically, in ash genotype 64, both bacteria elicited an increase in carotenoid content, TPC and TFC during both stages. Additionally, Pseudomonas sp. inoculated seedlings demonstrated an increase in phenolics after infection with the fungus in both genotypes 64 and 87. Our results indicate that next to genetic selection of the most resilient planting material for ash reforestation, plant-associated bacteria could also be used to boost ash SM production.