Alien Pathogens Research Articles

A Marine Bacterium with Animal-Pathogen-Like Type III Secretion Elicits the Nonhost Hypersensitive Response in a Land Plant

Active plant immune response involving programmed cell death called the hypersensitive response (HR) is elicited by microbial effectors delivered through the type III secretion system (T3SS). The marine bacterium <i>Hahella chejuensis</i> contains two T3SSs that are similar to those of animal pathogens, but it was able to elicit HR-like cell death in the land plant <i>Nicotiana benthamiana</i>. The cell death was comparable with the transcriptional patterns of <i>H. chejuensis</i> T3SS-1 genes, was mediated by SGT1, a general regulator of plant resistance, and was suppressed by AvrPto1, a type III-secreted effector of a plant pathogen that inhibits HR. Thus, type III-secreted effectors of a marine bacterium are capable of inducing the nonhost HR in a land plant it has never encountered before. This suggests that plants may have evolved to cope with a potential threat posed by alien pathogen effectors. Our work documents an exceptional case of nonhost HR and provides an expanded perspective for studying plant nonhost resistance.

The immune response mechanism of Nilaparvata lugens against a combined infection of rice ragged stunt virus and Metarhizium anisopliae.

Previous studies of brown planthopper (BPH), Nilaparvata lugens, showed that carrying the plant pathogenic virus, rice ragged stunt virus (RRSV), enhanced the lethality of the entomopathogenic fungus, Metarhizium anisopliae (YTTR). The underlying mechanism for this was not established but a serine protease cascade was hypothesized to be involved. Two immune response genes, NlKPI and NlVenomase, were identified and shown to be involved. The synthesized double-strand RNA (dsRNA)techniques used in this study to explore gene function revealed that treatment with dsRNA to silence either gene led to a higher BPH mortality from M. anisopliae infection than the dsRNA control treatment. NlKPI and NlVenomase play vital roles in BPH immunity to defend against alien pathogens. Both genes participate in the immune response process of BPH against co-infection with RRSV and M. anisopliae YTTR by regulating the expression of antimicrobial peptides and phenoloxidase activity. Our study provided new targets for BPH biocontrol and laid a solid foundation for further research on the interaction of virus-insect-EPF (entomopathogenic fungus). © 2023 Society of Chemical Industry.

Nanoplate digital PCR assays for detection and quantification of Xylella fastidiosa

Xylella fastidiosa is a fastidious Gram-negative bacterium that is associated with several important plant diseases, and is regulated as a quarantine pest in many countries where strategies are implemented to prevent its introduction and spread. To enact efficient quarantine measures, effective and early detection of the pathogen are essential, especially because global trade of goods increases the risks of introduction of alien pathogens. this study aimed to adapt two qPCR-based diagnostic methods (SYBR Green and Probe based qPCR), already in use to detect X. fastidiosa, for use with a nanoplate based digital PCR assay. Detection of the pathogen using the two digital PCR assays (EvaGreen- and Probe-based) was similar to standard qPCR, giving 100% sensitivity, specificity, and accuracy, while providing accurate absolute quantification of the pathogen when using experimental samples that had low concentrations of host DNA. Using undiluted plant DNA added with low concentrations of X. fastidiosa, only the TaqMan method maintained satisfactory performance and quantification, and is therefore preferred. These results are a first step demonstrating the usefulness of nanoplate-based digital PCR for detection of plant pathogens, which allows greater throughput than qPCR, reducing the time and cost of diagnostic assays.

Cypress canker: An important disease discovered for the first time on a native South African tree

Cypress canker is a branch and stem canker disease of Cupressaceae trees, particularly those in the genera Cupressus and Hesperocyparis. These trees have been planted in many parts of the world as ornamentals and the Seiridium species that cause the disease, consequently, also have an almost global distribution. The taxonomy of Seiridium species causing cypress canker has recently been revised and numerous species are now believed to cause the disease. This study describes, for the first time, cypress canker on the native South African Cupressaceae tree, Widdringtonia nodiflora. The aim was to identify the causal agent and confirm its pathogenicity. Phylogenetic analyses of sequence data for four regions identified the fungus as Seiridium neocupressi, a species previously known only from Australia, New Zealand and Italy. Field inoculations of W. nodiflora branches resulted in distinct cankers within 6 weeks and the fungus could be reisolated from the treated trees. Cypress canker has been known in South Africa for many decades, where it causes a serious disease on nonnative species of Cupressus, but it has never been found on native Cupressaceae. The newly discovered disease caused by a probable alien pathogen is of particular concern because only three species of Widdringtonia occur in South Africa and are important components of the native flora. The two other species, W. wallichii and W. schwartzii, occur in small endemic and threatened populations. The origin of S. neocupressi in South Africa and the relative susceptibility of the three Widdringtonia species, consequently, requires urgent attention.

Drivers of Forest Pathogen Invasions

In the past 25 to 30 years, a marked increase has occurred in numbers of invasive alien pests and pathogens recorded, damaging agriculture, horticulture and forest environments. The reasons for this upsurge are clear: all involve human-driven factors, with, arguably, global trade as the most important component in the complex. Climate change is another major factor. Each plant transported through trade could carry cryptic pests or pathogens, which may be introduced into previously naïve environments and lead to major health issues on previously unaffected plants. Global trade in plants is a complex system, driven by a desire to use large plants in landscapes and demand for varied species for gardens. Examples of invasive alien pathogens causing wide-spread problems on a global scale currently include many Phytophthora species, Dothistroma needle blight of pines, and Xylella fastidiosa on a wide range of trees. A striking example recently was the spread of the Dutch elm disease pathogen, Ophiostoma novo-ulmi, in North America and Western Europe. Many invasive pathogens cause problems in localized regions, including Ceratocystis platani, killing Platanus in some European countries. Other examples include the threat of pine wilt nematode (Bursaphelenchus xylophilus), damage to Cupressus in Mediterranean landscapes caused by Seiridium cardinale, dieback of sweet chestnut (Castanea) caused by the canker pathogen Cryphonectria parasitica, and many others. In addition to global trade, climate change appears to be exacerbating the problems, with major influences on establishment and spread of alien invasive species. Hope lies in the enormous genetic ‘power’ of plants: humans could establish selection and breeding programmes that will ultimately enable us to plant trees with greater tolerance of changing climate and pests and pathogens, whether native or invasive.

Gastrointestinal Parasite Community and Phenotypic Plasticity in Native and Introduced Alien Lagomorpha.

Simple SummaryThe introduction of alien species outside their native range represents one of the main threats to biodiversity conservation and it is mainly driven by human activities. In this work, we used the introduction of an alien lagomorph (Sylvilagus floridanus) outside its natural range as a case study to describe and evaluate the long-term effects on the native lagomorph community. In particular, S. floridanus was host to different exotic parasites, which adapted to new biotic and abiotic conditions and, as a consequence, invaded and largely altered the parasite community of native Lagomorpha species. This work represents the first exploratory study on the ecological macro-community formed by native and alien host species and their parasitofauna. Further efforts are needed to fully understand the strategies and factors conditioning the evolution of these specific parasite communities, and more in general the factors influencing the effect and evolution of alien pathogens in a new geographic area.The Convention on Biological Diversity classifies “Invasive Alien Species” as those whose introduction and spread represents a threat for biodiversity. Introduction of alien pathogens, including parasites, is one of the main consequences of the introduction of invasive alien species. The objective of this work was to assess the parasite community composition in native lagomorphs (Lepus europaeus and Lepus timidus varronis) in sympatric and non-sympatric conditions with an alien lagomorph (Sylvilagus floridanus), and to evaluate the phenotypic traits of exotic parasites in such conditions. We firstly describe the characteristics of the parasite community in the different host species (richness, prevalence, abundance and intensity), and, secondly, the phenotypic traits of the observed parasite species in each host. Nine helminths were reported on: eight nematodes (Obeliscoides cuniculi, Trichostrongylus calcaratus, Trichostrongylus retortaeformis, Trichostrongylus affinis, Trichuris leporis, Trichostrongylus colubriformis, Passalurus ambiguus, and Nematodirus sp.) and one unidentified cestode. In addition, exotic parasites showed significantly different phenotypic plasticity after spillover from S. floridanus to L. europaeus, whereas endemic parasite species were not isolated in the alien S. floridanus. Our results highlight that the community of autochthonous and allochthonous Lagomorpha in northwestern Italy represents an extremely interesting system for modelling ecological and evolutionary interactions between parasites and their hosts.

The alien invasive forest pathogen Heterobasidion irregulare is replacing the native Heterobasidion annosum

Invasions by alien pathogens are a major threat to forest conservation. The North American fungal pathogen of conifers Heterobasidion irregulare, inadvertently introduced in Central Italy in the 1940s, has been spreading causing high mortality of Italian stone pine (Pinus pinea). While invading newfound niches, H. irregulare has established itself in the current range of the native congener H. annosum. The aims of this study were to determine whether in time: (I) H. irregulare populations may be increasing in size; (II) H. irregulare may be replacing H. annosum, rather than simply coexisting with it; and, (III) H. annosum may disappear in forests infested by H. irregulare. The presence, abundance and distribution of H. annosum and H. irregulare were assessed through an aerobiological assay replicated ten years apart in a forest in which both species have been coexisting. Replacement index (RI), Markov chains and geometric progressions were used to model the interspecific interaction between the two species and to assess the invasiveness of H. irregulare. Results showed that, in 10 years, the incidence of H. annosum dropped from 39.4 to 6.1%, while that of H. irregulare increased from 57.6 to 81.8%, with the alien pathogen replacing the native species (RI = 84.6%) and spreading at a maximum rate of 139 ha/year. Although our models show that the extinction of H. annosum may be unlikely, the ability of H. irregulare to replace it suggests the alien pathogen may also readily colonize those parts of Europe where H. annosum is more abundant than in Central Italy.

Domestic quarantine: An introspection and future perspectives on biosecurity interventions to contain pathogen spread in vegetatively propagated spices in India

Safeguarding biodiversity poses a major challenge that warrants preventive legislative and regulatory frameworks to ensure biosecurity, preserve intrinsic biodiversity and mitigate risks from the invasion of exotic pathogenic microbes thereby sustaining agricultural productivity and food safety. The anthropogenic activities contribute enormously towards transboundary movement of invasive alien pathogens through trade and transport of seeds/propagation materials that need to be regulated by enacting appropriate laws and policy guidelines. Black pepper, cardamom, ginger, turmeric and vanilla are the major vegetatively propagated spices cultivated in India. Since primary spread of the major pathogens in these spices are mediated through planting materials, it is imperative to formulate guidelines for flawless and meticulous implementation of quarantine measures which are otherwise not adopted. This review discusses the significance of domestic quarantine in major vegetatively propagated spices with a comprehensive background on plant biosecurity and invasive plant pathogens which could pose threat to spice cultivation in India. It further illustrates the constraints and possible strategies to annihilate the cryptically disseminated pathogens through vegetative planting materials ultimately to safeguard our self-sustaining agricultural systems.

Alien Invasive Pathogens and Pests Harming Trees, Forests, and Plantations: Pathways, Global Consequences and Management

Forest health worldwide is impacted by many invasive alien pathogens and pests (IAPPs) that cause significant harm, with severe economic losses and environmental alterations. Destructive tree pathogens and pests have in the past devastated our forests, natural landscapes and cityscapes and still continue to represent a serious threat. The main driver of pathogen and pest invasions is human activities, above all global trade, which allows these invasive species to overstep their natural distribution ranges. While natural transport occurs according to a regular, expected colonization pattern (based on the dispersive capacity of the organism), human-mediated transport takes place on a larger, unpredictable scale. In order for a pathogen or pest species to become invasive in a new territory it must overcome distinct stages (barriers) that strongly affect the outcome of the invasion. Early detection is crucial to enabling successful eradication and containment. Although sophisticated diagnostic techniques are now available for disease and pest surveillance and monitoring, few control and mitigation options are usable in forestry; of these, biological control is one of the most frequently adopted. Since invasion by pathogens and pests is an economic and ecological problem of supranational relevance, governments should endorse all necessary preventive and corrective actions. To this end, establishing and harmonizing measures among countries is essential, both for preventing new introductions and for diminishing the eventual range expansion of IAPPs present at a local scale. Research is fundamental for: (i) developing effective and rapid diagnostic tools; (ii) investigating the epidemiology and ecology of IAPPs in newly introduced areas; and (iii) supporting policymakers in the implementation of quarantine regulations.

Alien pathogens and parasites impacting native freshwater fish of southern Australia: a scientific and historical review

ABSTRACT Native freshwater fish of Australia have a diverse but largely undescribed endemic pathogen and parasite fauna. However, due to long-shared evolutionary histories and virulence/transmissibility trade-offs, effects of these endemic pathogens and parasites appear to be subtle: significant impacts are rarely observed and epizootics have not been recorded. In contrast, a number of alien pathogens and parasites are now established across southern Australia, causing manifestly harmful effects to native fish species and known or suspected epizootics in native fish populations. Undetected and/or undescribed alien viral pathogens are also suspected of being present. Alien pathogens and parasites were introduced to Australia with imports of live alien fish or their fertilised eggs. A review of the scientific and historical evidence indicates that they have had, and continue to have, greater impacts on native fish species than previously realised—especially for freshwater species. This review also documents a previously unknown, Murray-Darling-Basin-wide epizootic of Murray cod Maccullochella peelii in 1929–30, which may have contributed to strong declines in the related eastern freshwater cod Maccullochella ikei. A serious Chilodonella epizootic of M. peelii in 1982 is also examined. In addition, a possible role for alien viruses of the family Iridoviridae (Ranavirus and Megalocytivirus) in the general decline of the critically endangered silver perch Bidyanus bidyanus, and the rapid collapse of two specific native fish populations—upper Murrumbidgee River B. bidyanus and Shoalhaven River Macquarie perch Macquaria australasica—is suggested. It is argued that the severity of the impact of the virulent alien oomycete Saprolegnia parasitica, both historical and present day, has been underestimated. Finally, action is recommended against emerging new pathogen and parasite threats, and the extreme risk current alien fish importations pose in introducing them. These will further threaten already stressed native fish populations in southern Australia, particularly across the Murray-Darling Basin.

First report of Erysiphe corylacearum on Corylus avellana and C. colurna in Austria

In September 2020, prominent powdery mildew disease symptoms were seen on leaves of Corylus avellana (hazelnut) and C. colurna (Turkish hazelnut) at several localities in eastern and southern Austria (Burgenland, Carinthia, Lower Austria, Styria, Vienna). The symptoms differed from those previously observed (caused by Phyllactinia guttata) by mycelia on the upper leaf surface and by smaller chasmothecia with branched appendices. The disease was observed on Corylus spp. growing in mixed deciduous forests, hedges as well as city gardens and parks. Disease symptoms started as small spots of white, amphigenous surface mycelium, that radially enlarged to form effuse patches reaching up to 2 cm in diameter, eventually becoming confluent and then covering the entire leaf surface (Figs. 1, 2). Infections on the lower leaf surface caused chlorotic spots on the upside (Fig. 1). Upon severe infection, leaves were distorted (Figs. 1, 2). On surface mycelium, ellipsoid to doliiform conidia (27-36 × 18-23 μm) were produced singly on conidiophores (Fig. 3). In addition, dark brown chasmothecia, 70-120 μm in diameter were abundantly produced (Fig. 4), which had up to 15, 60-90 μm long, stiff, hyaline appendages with repeatedly dichotomously branched tips. They contained 2-6 broadly ellipsoid, mostly (3-)7-8 spored asci of 51-70 × 38-57 μm, with hyaline ascospores 19-28 × 13-18 μm (Fig. 5). Based on these characters, the powdery mildew was identified as Erysiphe corylacearum (Braun & Cook, 3). To confirm the morphological identification, the ITS-LSU of sample WU 43287 (Lower Austria, Gießhübl, host: Corylus avellana) was sequenced. DNA was extracted from chasmothecia using the innuPREP DNA Micro Kit (Analytik Jena, Germany) according to the manufacturer's instructions, with a lysis time of 16 hours. The ITS-partial LSU was amplified as a single stretch with primers V9G and LR5 and sequenced according to Voglmayr et al. (6). The obtained sequence was deposited in GenBank (Accession No. MW031866), and representative voucher specimens were deposited in the fungarium of the University of Vienna (WU 43287, WU 43421, WU 43423, WU 43424, WU 43514, WU 43515, WU 43520, WU 43525). A nBLAST analysis revealed 99-100% identity to sequences of E. corylacearum, with 100% identity to sequences from Azerbaijan (LC270863), Iran (MF668615, MH047243), Japan (MT095104), Switzerland (MN822721-MN822723) and Turkey (KY082910), confirming the species identification. Native to East Asia and probably North America, E. corylacearum was first observed in Turkey in 2013 and has since rapidly extended its distribution range to West Asia as well as Eastern and Central Europe (Beenken et al., 2; Heluta & Fokshei, 4). Erysiphe corylacearum has been reported as a severe invasive alien pathogen, causing significant yield losses in hazelnut orchards in Turkey, Iran and Georgia (Arzanlou et al., 1; Meparishvili et al., 5). The authors thank Christian Scheuer for information on a record from Styria.

New invasive specie of <i>Mycosphaerella dearnessii</i> in the composition of mycobiota of pine needles in the territory of Belarus

Different pines species are heavily infected with fungal pathogens all over the world, some of them are alien species for European region. The potential invasion of alien species into forests, forest nurseries, urban greeneries causes great concern among Belarusian phytopathologists. In this regard, in recent years local scientists have been monitoring the phytosanitary state of natural and managed plantations to identify alien species. The aim of this investigation was to monitor the phytosanitary state of conifers in botanical and dendrological gardens, urban plantations and tree nurseries in the Republic of Belarus, and assessment of the incidence of alien pathogens of pines. There were used light microscopy and molecular genetic techniques. As a result of our research the brown spot needle blight fungus, Mycosphaerella dearnessii was for the first time noted in the region of the Republic of Belarus in a consignment of imported plants of pines Pinus mugo, Pinus nigra, Pinus ponderosa. The disease was revealed both on young and adult plants in botanical gardens and nurseries. In the forests, urban greeneries Mycosphaerella dearnessii is currently not identified. The fungus affects not only the old, but also young needles, strongly inhibiting the growth of trees. Based on the investigation results we suppose that this disease can be dangerous for pinetums in our country.

Interrelations between Alien and Native Foliar Fungal Pathogens and Woody Plants in Siberia

This article presents the results of 15-year observations of alien and native foliar fungal pathogens and woody plants in Siberian arboreta and urban ecosystems and discusses the possibility of using these data to determine how the pathogenic mycobiota of woody species was formed under Siberian conditions. In total, 121 species of pathogenic micromycetes affecting the leaves of woody plants in botanical gardens and city and belonging to 2 divisions, 14 orders, and 46 genera have been revealed. Most of these species belong to the Ascomycota division (86.8%). The Basidiomycota division is represented by fungi from the order Pucciniales. Among all observed pathogens, 21 species belong to powdery mildew fungi, 15 species belong to rust fungi, and 84 species represent anamorphic fungi. A study of 108 woody plant species growing in Siberian cities has revealed 101 species of leaf pathogens, while 158 woody plant species growing in arboreta have been inhabited by 105 pathogen species. All fungal species revealed during this study, except for three species (Mycopappus alni, Venturia acerina, and Phyllosticta westendorpii), have been observed in Siberia earlier. However, some fungus–host plant associations are apparently new to science, suggesting that complexes of cryptic species differing in their host range and geographic range may occur. The main sources for tree mycobiota formation in Siberia include the invasion of pathogens together with alien plant species, the transition of pathogens from native to alien plant species, the enhancement of a pathogenicity of saprotrophic fungi, and the expansion of fungi onto new hosts. The majority of pathogens (65−75% of the total number of revealed fungi) locate on native plants. Alien plants are infected mainly by (native) pathogens. Only eight pathogen species have been introduced in Siberia with alien plants; three of them originate from the Far East (Cercospora gotoana, Phyllosticta phellodendricola, and Erysiphe palczewskii), while five are of European origin (Cladosporium syringae, Phyllosticta vincae-minoris, Erysiphe syringae, Erysiphe alphitoides, and Erysiphe berberidis). In Siberia, the lateness of phenological phases of plants originating from mild climatic zones, may be favorable for the development of fungal pathogens, such as Erysiphe alphitoides, causing the powdery mildew of oak trees.

Novel interactions between alien pathogens and native plants increase plant–pathogen network connectance and decrease specialization

Abstract Alien plant pathogens are a threat to native plants and are increasingly integrating into native plant–pathogen networks, but how these novel plant–pathogen networks are structured remains unclear. Theory predicts that novel antagonists are likely to be generalists, resulting in interaction networks with greater nestedness as well as lower modularity and specialization than native networks. We tested these predictions by quantifying associations between native plants and their native and alien pathogens using a comprehensive database of plant–fungal associations in New Zealand. We compared the host ranges of alien and native pathogens and the structure of native and alien pathogen subnetworks. As predicted, alien pathogens associated with a greater number and diversity of native plant host species than native pathogens. The alien pathogen subnetwork was more nested and connected, but less modular and less specialized than the native pathogen subnetwork, consistent with expectations for novel interactions. Alien pathogens altered the overall native plant–pathogen network structure, making the full network more connected and less specialized than the native network. Modules in the native and alien subnetworks were clustered by host phylogeny but did not show a clear signal associated with host habitat or region. Synthesis. Our study provides some of the first empirical insights into the structure of novel plant–pathogen networks and the changes that occur when alien pathogens invade a native network. Because alien pathogens interacted with more hosts than native pathogens, alien pathogens have an increased risk of adverse indirect effects, including pathogen spillover, host jumps and network destabilization.

Identification of Phytophthora tolerance in the Anemone sylvestris tetraploid

The introduction and naturalization of alien pathogens and pests pose one of the biggest challenges in contemporary plant pathology. Chromosome doubling can be an effective method to obtain plants that are tolerant to these hazards and to reduce the impact of these hazards in both artificial and natural environments. The susceptibility of the induced tetraploid of Anemone sylvestris to artificial Phytophthora plurivora root infection was compared with the susceptibility of the diploid. The proportion of surviving plants and plants with disease symptoms was evaluated in a greenhouse experiment. Phytophthora infection decreased survival in the diploids, whereas no mortality was found in the tetraploids. Moreover, the surviving diploids were in poor health and showed unbalanced development, while the tetraploids were usually vital, and uniform, with compact growth regardless of phytophthora infection. Induced tolerance to P. plurivora was unknown to date in A. sylvestris.

The economic cost of an epidemic

Plant Pathology Native tree populations in many parts of the world are threatened by alien pathogens, which are often imported inadvertently via the trade in living plants. Hill et al. estimate the likely economic cost of the current epidemic affecting ash ( Fraxinus excelsior ) in Britain. They find that ash dieback, caused by the fungal pathogen Hymenoscyphus fraxineus imported from continental Europe in ash saplings, may cost £14.8 billion over the next 100 years, with half of that amount accrued over the coming decade. Most of the cost is in lost economic services such as recreation, avoided runoff, and carbon sequestration, although there are also substantial costs incurred by felling dead trees and replanting. The authors point out that these potential costs dwarf the value of the plant trade. Curr. Biol. 29 , R315 (2019).

Mitotic Recombination and Rapid Genome Evolution in the Invasive Forest Pathogen Phytophthora ramorum.

Invasive alien species often have reduced genetic diversity and must adapt to new environments. Given the success of many invasions, this is sometimes called the genetic paradox of invasion. Phytophthora ramorum is invasive, limited to asexual reproduction within four lineages, and presumed clonal. It is responsible for sudden oak death in the United States, sudden larch death in Europe, and ramorum blight in North America and Europe. We sequenced the genomes of 107 isolates to determine how this pathogen can overcome the invasion paradox. Mitotic recombination (MR) associated with transposons and low gene density has generated runs of homozygosity (ROH) affecting 2,698 genes, resulting in novel genotypic diversity within the lineages. One ROH enriched in effectors was fixed in the NA1 lineage. An independent ROH affected the same scaffold in the EU1 lineage, suggesting an MR hot spot and a selection target. Differences in host infection between EU1 isolates with and without the ROH suggest that they may differ in aggressiveness. Non-core regions (not shared by all lineages) had signatures of accelerated evolution and were enriched in putative pathogenicity genes and transposons. There was a striking pattern of gene loss, including all effectors, in the non-core EU2 genome. Positive selection was observed in 8.0% of RxLR and 18.8% of Crinkler effector genes compared with 0.9% of the core eukaryotic gene set. We conclude that the P. ramorum lineages are diverging via a rapidly evolving non-core genome and that the invasive asexual lineages are not clonal, but display genotypic diversity caused by MR.IMPORTANCE Alien species are often successful invaders in new environments, despite the introduction of a few isolates with a reduced genetic pool. This is called the genetic paradox of invasion. We found two mechanisms by which the invasive forest pathogen causing sudden oak and sudden larch death can evolve. Extensive mitotic recombination producing runs of homozygosity generates genotypic diversity even in the absence of sexual reproduction, and rapid turnover of genes in the non-core, or nonessential portion of genome not shared by all isolates, allows pathogenicity genes to evolve rapidly or be eliminated while retaining essential genes. Mitotic recombination events occur in genomic hot spots, resulting in similar ROH patterns in different isolates or groups; one ROH, independently generated in two different groups, was enriched in pathogenicity genes and may be a target for selection. This provides important insights into the evolution of invasive alien pathogens and their potential for adaptation and future persistence.

MARYBLYT Study for Potential Spread and Prediction of Future Infection Risk of Fire Blight on Blossom of Singo Pear in Korea

Since fire blight (Erwinia amylovora) firstly broke out at mid-Korea in 2015, it is necessary to investigate potential spread of the invasive pathogen. To speculate environmental factors of fireblight epidemic based on disease triangle, a fire blight predicting program, MARYBLYT, was run with the measured meteorological data in 2014-2017 and the projecting future data under RCP8.5 scenario for 2020-2100. After calculating blossom period of Singo pear from phenology, MARYBLYT was run for blossom blight during the blossom period. MARYBLYT warned “Infection” blossom blight in 2014-15 at Anseong and Cheonan as well as Pyungtak and Asan. In addition, it warned “Infection” in 2016-17 at Naju. More than 80% of Korean areas were covered “Infection” or “High”, therefore Korea was suitable for fire blight recently. Blossom blight for 2020-2100 was predicted to be highly fluctuate depending on the year. For 80 years of the future, 20 years were serious with “Infection” covered more than 50% of areas in Korea, whereas 8 years were not serious covered less than 10%. By comparisons between 50% and 10% of the year, temperature and amount of precipitation were significantly different. The results of this study are informative for policy makers to manage the alien pathogen.

Armillaria root rot spreading into a natural woody ecosystem in South Africa

Signs and symptoms of a disease similar to those of armillaria root rot have recently been observed on various native woody plants on the foothills of Table Mountain in South Africa, one of the most botanically diverse natural environments globally. This is of concern because the root rot fungus Armillaria mellea has previously been shown to be an alien pathogen of European origin in planted gardens in the City of Cape Town. An aim of this study was to identify the cause of the root rot disease on infected plants. Based on DNA‐sequence phylogeny, it was shown that isolates collected from at least 16 native tree and woody shrub species represented the non‐native A. mellea. Microsatellite markers were then used to determine the genetic diversity and population structure of the A. mellea isolates from Table Mountain and two planted gardens where the pathogen has previously been found. Population genetic analyses revealed low levels of gene diversity and no population differentiation amongst the three populations. The results provide the first firm evidence that A. mellea has escaped the planted environment and invaded a sensitive and ecologically important natural woody environment in South Africa. This is only the second definitive case of a non‐native tree pathogen invading a natural ecosystem in the country.

Population structure of the ash dieback pathogen, Hymenoscyphus fraxineus, in relation to its mode of arrival in the UK.

The ash dieback fungus, Hymenoscyphus fraxineus, a destructive, alien pathogen of common ash (Fraxinus excelsior), has spread across Europe over the past 25 years and was first observed in the UK in 2012. To investigate the relationship of the pathogen's population structure to its mode of arrival, isolates were obtained from locations in England and Wales, either where established natural populations of ash had been infected by wind‐dispersed ascospores or where the fungus had been introduced on imported planting stock. Population structure was determined by tests for vegetative compatibility (VC), mating type and single‐nucleotide polymorphisms (SNPs). VC heterogeneity was high at all locations, with 96% of isolate pairings being incompatible. Frequencies of the MAT1‐1‐1 and MAT1‐2‐1 idiomorphs were approximately equal, consistent with H. fraxineus being an obligate outbreeder. Most SNP variation occurred within study location and there was little genetic differentiation between the two types of location in the UK, or between pathogen populations in the UK and continental Europe. There was modest differentiation between UK subpopulations, consistent with genetic variation between source populations in continental Europe. However, there was no evidence of strong founder effects, indicating that numerous individuals of H. fraxineus initiated infection at each location, regardless of the route of pathogen transmission. The ssRNA virus HfMV1 was present at moderate to high frequencies in all UK subpopulations. The results imply that management of an introduced plant pathogen requires action against its spread at the continental level involving coordinated efforts by European countries.