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Elms (Ulmus spp.) are widely spread in the forest, shelter belts, and urban landscaping. This research aimed to reveal the trends of Ulmus sp. health in Ukraine under biotic damage. The tasks included: (i) analyzing the presence of Ulmus sp. in the forests; (ii) studying the dynamics of Ulmus sp. health for 2001–2015 in the monitoring plots in the frame of the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP-Forests); (iii) assessing the prevalence of the dominant biotic factors affecting elm health and the probability of tree death or recovery. As a result of research, elms were found in 3.58% of the area in the stands with other main forest-forming species in the forests subordinated to the State Specialized Forest Enterprise «Forests of Ukraine». Four elm species are present in the forests of all regions of Ukraine. In the Forest zone, U. minor predominates, U. glabra is more common in the western part of the country, and U. pumila in the southern and eastern regions. In the ICP-Forests monitoring plots for 2001–2015, a trend of elm deterioration in 2007–2012 was found. The highest incidence of trees with disease symptoms was recorded for U. pumila. In the sample plots for 2023–2024, the health of three elm species tended to deteriorate. In 2024, mortality occurred among all elm species with symptoms of Dutch elm disease (DED) and among U. pumila trees with symptoms of wetwood. However, several trees have recovered. The results show the gaps in our knowledge that need to be filled, particularly in identifying resistant individuals and using their progeny to create resistant stands.

In Europe as in North America, elms are devastated by Dutch elm disease (DED), caused by the alien ascomycete Ophiostoma novo-ulmi. Pathogen dispersal and transmission are ensured by local species of bark beetles, which established a novel association with the fungus. Elm bark beetles also transport the Geosmithia fungi genus that is found in scolytids’galleries colonized by O. novo-ulmi. Widespread horizontal gene transfer between O. novo-ulmi and Geosmithia was recently observed. In order to define the relation between these two fungi in the DED pathosystem, O. novo-ulmi and Geosmithia species from elm, including a GFP-tagged strain, were grown in dual culture and mycelial interactions were observed by light and fluorescence microscopy. Growth and sporulation of O. novo-ulmi in the absence or presence of Geosmithia were compared. The impact of Geosmithia onDEDseveritywastestedinvivobycoinoculating Geosmithia and O. novo-ulmi in elms. A close and stable relation was observed between the two fungi, which may be classified as mycoparasitism by Geosmithia on O. novo-ulmi. These results prove the existence of a new component in the complex of organisms involved in DED, which might be capable of reducing the disease impact.

The mortality of a young elm Ulmus minor in 2014 in Geneva prompted a search for the microorganisms potentially involved. Symptoms included foliar chlorosis and wilting followed by defoliation of branches. Wood symptoms included a brown streaking of sap wood and brown stains in trunk and branches. The comparison of the resulting ITS rDNA sequences to the NCBI Nucleotide database allowed to identify 10 different organisms. The genus Geosmithia represented 48% of the isolates belonging to three species: Geosmithia langdonii (7 isolates) and 2 unknown morphologically and genetically different Geosmithia sp. 1 and sp. 2 (4 isolates). Geosmithia species are very little known ascomycetes, which have been recently shown to be opportunistic pathogens on broadleaved trees and conifers, living as saprobes in galleries of many bark beetle species. In the case described here, Geosmithia langdonii, and the unknown Geosmithia species were found in symptomatic wood while bark beetle galleries were found in close regions of the symptomatic wood. Geosmithia langdonii was the major fungus retrieved from the symptomatic wood and could have contributed, along with other identified fungal species, to a pathogenic complex producing symptoms similar to the ones of the Dutch Elm Disease and led to the dieback of this elm tree. Geosmithia langdonii and 2 yet unknown Geosmithia species (sp. 1 and sp. 2), different from any other reported Geosmithia species are reported from an elm tree in Switzerland for the first time.

Dutch elm disease (DED) is a vascular wilt disease caused by the pathogens Ophiostoma ulmi and Ophiostoma novo-ulmi with multiple ecological phases including pathogenic (xylem), saprotrophic (bark) and vector (beetle flight and beetle feeding wound) phases. Due to the two DED pandemics during the twentieth century the use of elms in landscape and forest restoration has declined significantly. However new initiatives for elm breeding and restoration are now underway in Europe and North America. Here we discuss complexities in the DED ‘system’ that can lead to unintended consequences during elm breeding and some of the wider options for obtaining durability or ‘field resistance’ in released material, including (1) the phenotypic plasticity of disease levels in resistant cultivars infected by O. novo-ulmi; (2) shortcomings in test methods when selecting for resistance; (3) the implications of rapid evolutionary changes in current O. novo-ulmi populations for the choice of pathogen inoculum when screening; (4) the possibility of using active resistance to the pathogen in the beetle feeding wound, and low attractiveness of elm cultivars to feeding beetles, in addition to resistance in the xylem; (5) the risk that genes from susceptible and exotic elms be introgressed into resistant cultivars; (6) risks posed by unintentional changes in the host microbiome; and (7) the biosecurity risks posed by resistant elm deployment. In addition, attention needs to be paid to the disease pressures within which resistant elms will be released. In the future, biotechnology may further enhance our understanding of the various resistance processes in elms and our potential to deploy trees with highly durable resistance in elm restoration. Hopefully the different elm resistance processes will prove to be largely under durable, additive, multigenic control. Elm breeding programmes cannot afford to get into the host–pathogen arms races that characterise some agricultural host–pathogen systems.

Vascular wilts are among the most destructive plant diseases that occur in annual crops as well as in woody perennials. These diseases are generally caused by soil-borne bacteria, fungi, and oomycetes that infect through the roots and enter the water-conducting xylem vessels where they proliferate and obstruct the transportation of water and minerals. As a consequence, leaves wilt and die, which may lead to impairment of the whole plant and eventually to death of the plant. Cultural, chemical, and biological measures to control this group of plant pathogens are generally ineffective, and the most effective control strategy is the use of genetic resistance. Owing to the fact that vascular wilt pathogens live deep in the interior of their host plants, studies into the biology of vascular pathogens are complicated. However, to design novel strategies to combat vascular wilt diseases, understanding the (molecular) biology of vascular pathogens and the molecular mechanisms underlying plant defense against these pathogens is crucial. In this review, we discuss the current knowledge on interactions of vascular wilt pathogens with their host plants, with emphasis on host defense responses against this group of pathogens.

This review illuminates key findings in our understanding of grapevine xylem resistance to fungal vascular wilt diseases. Grapevine (Vitis spp.) vascular diseases such as esca, botryosphaeria dieback, and eutypa dieback, are caused by a set of taxonomically unrelated ascomycete fungi. Fungal colonization of the vascular system leads to a decline of the plant host because of a loss of the xylem function and subsequent decrease in hydraulic conductivity. Fungal vascular pathogens use different colonization strategies to invade and kill their host. Vitis vinifera cultivars display different levels of tolerance toward vascular diseases caused by fungi, but the plant defense mechanisms underlying those observations have not been completely elucidated. In this review, we establish a parallel between two vascular diseases, grapevine esca disease and Dutch elm disease, and argue that the former should be viewed as a vascular wilt disease. Plant genotypes exhibit differences in xylem morphology and resistance to fungal pathogens causing vascular wilt diseases. We provide evidence that the susceptibility of three commercial V. vinifera cultivars to esca disease is correlated to large vessel diameter. Additionally, we explore how xylem morphological traits related to water transport are influenced by abiotic factors, and how these might impact host tolerance of vascular wilt fungi. Finally, we explore the utility of this concept for predicting which V. vinifera cultivars are most vulnerable of fungal vascular wilt diseases and propose new strategies for disease management.

Species distribution modelling (SDM) is a valuable tool for predicting the potential distribution of invasive species across space and time. Maximum entropy modelling (MaxEnt) is a popular choice for SDM, but questions have been raised about how these models are developed. Without biologically informed baseline assumptions, complex default SDM models could be selected, even though alternative settings may be more appropriate. Here we explored the effects of various SDM design strategies on distribution mapping of four forest invasive species (FIS) in Canada. We found that if we ignored the underlying FIS biology such as use of biologically relevant predictors, appropriate feature selection and inclusion of dispersal and biotic interactions when we developed our SDMs, we obtained complex SDMs (default) that provided an incomplete picture of the potential FIS invasion. We recommend simplifying SDM complexity and including biologically informed assumptions to achieve more accurate dispersal predictions, particularly when projecting FIS spread across time. We strongly encourage SDM users to perform species-specific tuning when modeling FIS distributions with MaxEnt to determine the best SDM design.

Annual monitoring of wilt pathogens on elm trees in Alberta is part of a provincially-regulated prevention and control program for Dutch elm disease. Over the past eight years (2016–2023), twig samples with wilt symptoms from 200 elm trees across Alberta were tested for the presence of wilt pathogens. Plenodomus tracheiphilus, the causal agent of Mal secco disease of citrus trees, was isolated from 116 out of the 200 elm trees. The identification of this fungus was confirmed morphologically by comparison with the type culture, and sequencing the internal transcribed spacer region, the β-tubulin gene, the DNA-directed RNA polymerase II second largest subunit gene, the actin gene and eight protein-coding genes exclusively present in the P. tracheiphilus genome. The pathogenicity of P. tracheiphilus isolated from Alberta was tested by artificial inoculation on elm trees to fulfill Koch's postulates based on symptom observation and fungal re-isolation. Our data indicated that P. tracheiphilus is commonly present in Alberta’s elm trees especially in the Edmonton area and that the previously described Dothiorella elm wilt is actually caused by P. tracheiphilus and not Dothiorella ulmi.

The lack of a high-quality Ulmus parvifolia genome assembly has impeded research on disease resistance and hindered breeding programs for resilient elm cultivars. In this study, we presented a chromosome-level genome assembly of U. parvifolia using integrated sequencing technologies. LTR retrotransposons, predominantly the Ty3/Gypsy Retand subfamily, underwent recent large-scale amplification, potentially contributing to the genome size of U. parvifolia . Phylogenetically, U. parvifolia is sister to a clade containing Moraceae , Cannabaceae , and Urticaceae . Notably, gene families associated with disease resistance and immune response were significantly expanded in U. parvifolia , pointing to an adaptive evolution to various biotic and abiotic stresses. To dissect differential susceptibility to Dutch elm disease, we inoculated U. parvifolia (resistant) and U. americana (susceptible) with Ceratocystis ulmi and performed comparative transcriptomics at 48, 96, and 144 h post-inoculation (hpi). Several plant defense and immune response pathways were markedly upregulated in U. parvifolia at 48/96 hpi, revealing a genetic basis for its enhanced resistance. This study advances genomic resources for elms, elucidates molecular mechanisms of disease resilience, and provides a foundation for breeding and conservation efforts.

Dutch elm disease (DED), caused by the pathogenic ascomycete Ophiostoma novo-ulmi , has devastated natural elm ( Ulmus spp.) populations in Europe and North America. Elm species vary in their susceptibility to this vascular disease, which may partly reflect differences in their associated mycobiomes. To investigate the diversity and composition of fungal endophyte communities in relation to host genotype, health status, and environment, we analyzed twig-associated fungi in symptomatic and asymptomatic individuals of highly susceptible U. glabra , less susceptible U. laevis , and hybrid elms growing in Estonia and Russia. Fungal communities were analyzed using PacBio long-read amplicon sequencing of the ITS1-5.8S-ITS2 gene region. Tree species exhibited distinct fungal community profiles. Ophiostoma novo-ulmi was detected exclusively in symptomatic trees and was dominant in U. glabra ; it was absent in symptomatic hybrid elms. In contrast, the canker-associated pathogen Sphaeropsis ulmicola occurred in both symptomatic and asymptomatic trees, was dominant in symptomatic U. laevis , and common in symptomatic and healthy U. glabra , though less prevalent in symptomatic hybrid elms. Remarkably, S. ulmicola was associated with the highest level of damage in U. laevis while being present also in hybrid elms. While O. novo-ulmi ’s presence did not affect overall fungal richness, S. ulmicola was linked to higher fungal diversity. Additionally, fungal species richness was significantly greater in urban compared to rural environments. This was the first comparable analysis of fungal diversity and composition on three different Ulmus species shoots.