Forest pathogens have repeatedly restructured ecosystems in the Northern Hemisphere, where epidemics such as chestnut blight, Dutch elm disease, and sudden oak death eliminated foundation species and reshaped landscapes. In contrast, the Southern Hemisphere, buffered by geographic isolation, unique floras, and historically lower trade volumes, experienced comparatively few incursions until recent decades. These barriers are rapidly eroding due to globalization, climate change, and the expansion of industrial plantation forestry dominated by exotic species of Pinus and Eucalyptus. These vast monocultures with low genetic diversity have created highly susceptible production systems that can amplify epidemic spread and act as sources of novel pathogen emergence. Drawing on lessons from the Northern Hemisphere, this review highlights the escalating risk of forest epidemics in the Southern Hemisphere and calls for proactive, coordinated responses. Stringent phytosanitary standards, advanced diagnostic techniques, and risk-optimized surveillance must be paired with collaborative breeding programs that prioritize resistance in high-risk hosts. At the same time, international cooperation across science, policy, industry, and communities is essential to strengthen prevention, support early detection, and enable rapid response. The resilience of Southern Hemisphere forests, both native and plantations, will depend on shifting from reactive management toward integrated, globally coordinated strategies that recognize the shared nature of pathogen threats.

The European Commission requested the EFSA Panel on Plant Health (PLH) to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as 'High risk plants, plant products and other objects'. This Scientific Opinion covers plant health risks posed by rooted plants in pots of Robinia pseudoacacia 'Frisia' imported from the United Kingdom (UK). The assessment was performed considering the available scientific information, including the technical information provided by the UK. All pests potentially associated with the commodity were evaluated against specific criteria. Two EU-regulated pests (Bemisia tabaci and Phytophthora ramorum non-EU isolates), present in the UK and potentially associated with the commodity, were considered as relevant for this Opinion. No non-regulated pests in the European Union were identified to be selected for further evaluation.

The spread of Phytophthora ramorum, the causal agent of sudden oak death and sudden larch death, has resulted in a destructive loss of trees, woody shrubs, and ornamentals in nurseries and forests in the United States, Canada, and Europe since the late 1990s. Twelve lineages of P.ramorum are described that vary in global distribution and virulence. Herein, we present a maximum likelihood phylogeny for P.ramorum inferred using IQ-TREE and the Tree-Based Alignment Selector (T-BAS) toolkit. The phylogeny was generated based on six loci (avh120, avh121, btub, gweuk.30.30.1, hsp90, and trp1). This phylogeny of P.ramorum improves on previous phylogenies because it is dynamic and interactive and incorporates a diverse set of all known global lineages from the United States and Europe (NA1, NA2, EU1, and EU2) and ancestral lineages from the putative native range in East Asia. The phylogenetic relationships inferred in the T-BAS tree support lineages NP1 and NP2 of P.ramorum as ancestral to NA1 and NA2 lineages found in North America. In addition, East Asian IC1, IC2, IC3, and IC4 lineages are ancestral to EU1 and EU2 lineages found in Europe. We used sequence data generated from isolates of P.ramorum collected from Ireland and Northern Ireland and placed them accurately in the tree. The P.ramorum phylogeny is available through T-BAS within the DeCIFR platform. This "interactive phylogeny" can be used by the research community to rapidly update and better reflect the evolutionary relationships of new lineages of P.ramorum.

Phytophthora ramorum is a plant pathogen that causes disease in nurseries and forests in the United States, Canada, Asia, South America, and Europe. Since 2002, disease caused by P. ramorum has led to the destruction of horticultural plants and trees in Ireland and Northern Ireland. Two lineages, EU1 and EU2, have been reported in both countries; EU1 is more prevalent in Ireland, whereas EU2 is more prevalent in Northern Ireland. We examined the genetic structure of P. ramorum populations in Ireland and Northern Ireland from 2005 to 2022 using microsatellite genotyping. Populations were sorted by country, county, lineage, year, host, and source (nursery or forest). Twenty-nine EU1 multilocus genotypes (MLGs) and six EU2 MLGs were observed. EU1MG1, the most prevalent MLG in both countries (64% Ireland, 57% Northern Ireland), was first observed in nurseries on rhododendron and later found in forests on larch. EU2MG1 was more common in Northern Ireland, whereas EU2MG3 was more common in Ireland. By 2006, four EU1 MLGs were identified from nurseries and forests in Ireland, suggesting that P. ramorum had already been introduced multiple times, likely via the plant trade. Minimum spanning networks revealed similar EU1 MLGs in Ireland nurseries and forests and in forests in Ireland, the United Kingdom, and Europe. A T-BAS SSR interactive phylogenetic tool was created to query the EU1 and EU2 lineages. Our data place Ireland and Northern Ireland P. ramorum populations into a broader European context and reveal novel genotypes of P. ramorum EU1 and EU2 in Ireland and Northern Ireland.

Phytophthora ramorum causes Sudden Oak Death, threatening global forests and nurseries. We modelled its current and future global distribution using an optimized MaxEnt model (mean AUC = 0.978). Temperature seasonality was the key driver (29.4%). Currently, suitable habitats (~42.48 million km2) concentrate in mid-latitude coastal regions (e.g., North America, Europe, East Asia). Future projections (SSP126-585) show no systematic contraction but rather significant spatial shifts: suitable areas expand poleward and upward in elevation while contracting in some current regions. High-suitability areas (5.52–6.40 million km2) change by −3% to +12%, with the centroid shifting northwestward, notably into the African Sahel under high emissions. Our findings indicate that, within the framework of the current modeling approach and the emission scenarios applied, climate warming is likely to lead to a net increase in the suitable habitat for this pathogen at the global scale, providing a critical scientific basis for the development of international phytosanitary strategies.

Nerium oleander L. (Apocynaceae; common oleander) is a woody plant native to the Palearctic and planted as an ornamental worldwide. In May 2024, small brown necrotic spots were detected on leaves of a residential oleanders, an area with many California Bay Laurel (Umbellularia californica) infested by P. ramorum, in Mill Valley, Marin County, CA (37.90481ºN, 122.55282ºW). Symptomatic leaves tested positive using a genus-specific lateral flow assay (LFA) for Phytophthora (Agdia Inc, Elkhart, IN, USA). Leaves were surface sterilized using 70% ethanol, plated on selective PARPH media (Jeffers et al. 1986) and incubated at 20ºC. After 6 days, a single culture was isolated (NORS074) from the necrotic spots and produced morphological structures typical of P. ramorum, including coralloid mycelium, chlamydospores and semi-papillate sporangia (Fig. 1) (Werres, S., et al. 2001). The internal transcribed spacer (ITS) and beta tubulin regions were sequenced using the primers ITS1/ITS4 (White et al. 1990; accession no. PX270955) and TUBUF2/TUBUR1 (Kroon et al. 2004; accession no. PX551880), respectively. A BLAST search revealed 99.50% identity for ITS and 99.68% for the beta tubulin with P. ramorum ex-type strain CBS 101553 (accession no. NR_147877.1 and LC595884.1, respectively). Ten unwounded and ten wounded leaves of N. oleander were inoculated with strain NORS074 using two methods: i) mycelial plugs and ii) zoospore solution (1.72 x 105 zoospores/ml). All four treatments induced necrotic leaf spots; and P. ramorum was re-isolated from the leaves. Sporulation of the isolate on inoculated leaves was observed. Pathogenicity tests were also performed on whole plants of N. oleander variety Austin City Limits (N = 10; 40 cm tall and 35 cm wide) using two methods: i) a zoospore solution (1.9 × 105 zoospores/ml) was sprayed on the plants until runoff; ii) leaf tips were immersed in 1.5 ml of the zoospore solution in a 15 ml conical tube attached to the plant during entire experiment. Ten leaves per plant were wounded for each assay. The inoculations were carried out in an open-field environment to simulate natural condition. Plants were misted with water and kept individually in plastic bags for 72 hours to maintain humidity level. Leaves inoculated with all four different treatments developed symptoms 12 dpi consisting of small black necrotic spots and lesions along the midrib vein (Fig. 2). Symptomatic leaves tested positive using the same Phytophthora LFA, and P. ramorum was re-isolated on PARPH media. The identity of the re-isolates was confirmed morphologically and by ITS sequencing. No symptoms were detected on untreated control plants. To our knowledge, this is the first report of P. ramorum occurring on N. oleander in the United States, and the first time ever, that Koch’s postulates have been confirmed for this host-pathogen combination. Previously, P. ramorum was detected on N. oleander in Ireland, but no inoculation studies were reported (O'Hanlon et al. 2016). This study provides the evidence to reclassify N. oleander from an APHIS 'associated host' (APHIS 2022) to a proven host. An ‘associate host’ is a plant from which a pathogen was isolated; a ‘proven host’ is a plant, for which Koch’s Postulates were confirmed. The original infection, occurring during the exceptionally wet 2023/24 winter in a shaded area, was condition-dependent; failure to re-isolate P. ramorum in Spring 2025 suggests the occurrence was transient and driven by extreme environmental pressures.

Species of the genus Phytophthora are among the most detrimental plant pathogens globally, representing a significant threat to global agriculture, horticulture, and forestry. These zoosporic oomycetes have historically caused devastating outbreaks, including, just to mention a few, late blight of potato in Ireland; jarrah dieback of eucalyptus in Western Australia; ink disease of chestnut in Europe; sudden oak death and sudden larch death of coast live oak and tanoak in the Western US, and of Japanese larch in the UK. The environmental and ecological impacts of the diseases they cause result in significant economic costs that often have social repercussions. With the acceleration of globalization, enhancing the movement of plant material, in particular with the global live plant trade, the spread of Phytophthora to new, uncontaminated territories has intensified. Nurseries play a key role in the movement of these pathogens, the trade of contaminated stocks representing their major dissemination route. However valuable, conventional detection techniques, including baiting and direct isolation, are too slow and labour-intensive to meet current diagnostic requirements, particularly given the huge volumes of plants traded globally. This problem becomes even more acute when large volumes of potentially infectious plant material need to be processed in a short time frame, as it is often necessary to provide accurate and timely responses to interested parties. Early and precise detection is thus vital to avert outbreaks and mitigate long-term consequences. This review evaluates and contrasts the efficacy of novel detection methods against traditional approaches, emphasizing their significance in managing the escalating threat posed by Phytophthora spp. worldwide. Despite technological advances, critical challenges remain that limit the reliability and large-scale adoption of new diagnostic methods. Research still needs to bridge the gap between the laboratory and the field in terms of accuracy, sensitivity and diagnostic costs. Recent innovations focus on sensor technology and point-of-care (POC) devices for faster, more sensitive, and low-cost specific detection of Phytophthora spp. in plant matrices, water and soil. Enhancing diagnostic capabilities through these tools is crucial for protecting agricultural productivity, local economies, and natural ecosystems.

Phytophthora ramorum is an invasive generalist plant pathogen introduced to North America and Europe in the mid-1990s and is now established in forests and the nursery industry. It causes sudden oak death in the western US and sudden larch death in Western Europe, leading to extensive forest decline and mortality. While well studied in California and Oregon, no quantitative assessment exists for its potential economic impact on European forestry. We assessed the potential direct economic impact of P. ramorum on larch and beech in Europe under a “no-control” scenario. Climatically optimal areas for disease expression were derived using the CLIMEX niche model with refined parameter values, updated climate data, and P. ramorum occurrence records from symptomatic forest trees. These areas were overlaid with host distribution data to identify assets at risk. We then applied a radial range expansion model and a partial budgeting method to quantify annualized average damage costs. Our results indicate that 10 % of the study area is climatically optimal for disease expression. Within that area, 4 223 km² of larch and 2 577 km² of beech are at risk. Under worst-case spread and mortality assumptions, annual direct damage costs could exceed €117 million for larch and €130 million for beech. Countries such as the UK, Italy, Austria, and Germany face the highest risks, while potential impacts in Southern Europe are negligible. This study provides an updated risk assessment of the current post-invasion state of P. ramorum in Europe, facilitating informed decision-making and the development of appropriate management strategies. • Phytopthora ramorum remains a major invasive threat to European forests. • 10 % of Europe is identified as climatically optimal for disease expression. • Larch stands most at risk in the United Kingdom, Austria, and Italy. • Beech stands most at risk in Italy, Germany, and Austria. • Need for stronger focus on nursery biosecurity and forest resilience.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as 'High risk plants, plant products and other objects'. This Scientific Opinion covers plant health risks posed by rooted plants in pots of Hamamelis mollis imported from the United Kingdom (UK). The assessment was performed considering the available scientific information, including the technical information provided by the UK. All pests associated with the commodities were evaluated against specific criteria. Two EU regulated pests (Phytophthora ramorum non-EU isolates and Scirtothrips dorsalis), present in the UK and potentially associated with the commodity, were considered as relevant for this Opinion. No pests non-regulated in the EU were identified to be selected for further evaluation.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as 'High risk plants, plant products and other objects'. This Scientific Opinion covers plant health risks posed by unrooted cuttings of Salix species (S. aegyptiaca, S. eriocephala, S. gmelinii, S. miyabeana, S. purpurea, S. rehderiana, S. schwerinii, S. udensis, S. viminalis) imported from the United Kingdom (UK). The assessment was performed considering the available scientific information, including the technical information provided by the UK. All pests potentially associated with the commodities were evaluated against specific criteria. Only two EU Regulated pests (the fungus Entoleuca mammata and the oomycete Phytophthora ramorum (non-EU isolates)), present in the UK and potentially associated with the commodity, were considered as relevant for this Opinion. No pests non-regulated in the EU were identified to be selected for further evaluation.

Traditional plant pathogen detection often relies on molecular technologies, which allow species-level detection but are often time-consuming. Plant volatile organic compounds (VOCs) have recently been harnessed to assist in disease detection and plant health monitoring. However, current VOC detection methods are unsuitable for field use due to the need for expensive laboratory equipment and slow processing times. To address this, we developed a portable paper-based colorimetric sensing technology for early detection of ramorum blight in rhododendron caused by Phytophthora ramorum. This colorimetric sensor array, which includes nanomaterials and organic dyes, was optimized to detect alcohol, terpene, and ester, key VOC biomarkers emitted by infected rhododendron leaves. Color quantification was done quickly by smartphone imaging. Principal component analysis (PCA) was used to cluster and classify individual plant volatiles. Our VOC sensing platform detected ramorum blight 2 days after inoculation, aligning with real-time loop-mediated isothermal amplification (LAMP) analysis. Moreover, the platform distinguished pathogen-induced VOCs from those produced by nonbiological stresses such as drought and mechanical damage. This noninvasive diagnostic technology demonstrates significant potential for disease detection in the field.

Invasive species can cause environmental impacts through various mechanisms. Assessing their impact can inform management decisions and illuminate risks to non-invaded areas. Research on the environmental impacts of invasive insects is heavily focused on a few well-known examples, with agricultural pests in particular receiving little attention. We aimed to investigate whether evidence for environmental impacts of insect pests of agriculture may be overlooked. We conducted in-depth literature reviews of three globally relevant insect agricultural pests– Halyomorpha halys , Helicoverpa armigera , and Spodoptera frugiperda . For comparison, we reviewed two forest pathogens known for their environmental impacts– Bursaphelenchus xylophilus and Phytophthora ramorum . We identified many published articles containing evidence of environmental impacts among the three insect agricultural pests that were not captured by existing reviews on invasive insects, with some demonstrating high levels of impact severity. Crucially, a preponderance of the identified articles did not directly address the findings in relation to environmental impacts. As expected, we recorded more conspicuous examples of environmental impacts among the case-study forest pathogens, though we also identified underappreciated impact mechanisms. We further provide evidence that supports the importance of considering management interventions as a key mechanism of non-target environmental impacts. This review raises awareness about the underreported environmental impacts of agricultural insect pests and specifies knowledge gaps that should guide future research.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as ‘High risk plants, plant products and other objects’. This Scientific Opinion covers plant health risks posed by graftwood, whips, bare root plants and potted plants of Castanea sativa imported from the United Kingdom. The assessment was performed considering the available scientific information, including the technical information provided by the UK. All pests associated with the commodities were evaluated against specific criteria. Three EU regulated pests (Cryphonectria parasitica, Dryocosmus kuriphilus, Phytophthora ramorum), present in the UK and associated with the commodity, were considered relevant for this opinion. One pest that is not regulated in the EU (Phytophthora kernoviae) fulfilled all relevant criteria and was selected for further evaluation. For the selected pest, the risk mitigation measures described in the submitted technical dossier were evaluated. An expert judgement was given on the likelihood of pest freedom taking into consideration the risk mitigation measures acting on the pest, including uncertainties associated with the assessment. The degree of pest freedom varied among the commodities evaluated, with P. kernoviae being most frequently expected on the imported bare root and potted plants. The Expert Knowledge Elicitation indicated with 95% certainty that 9060 or more units per 10,000 will be free from P. kernoviae.

Abstract This document provides the conclusions of the pest survey card that was prepared in the context of the EFSA mandate on plant pest surveillance (M‐2020‐0114) at the request of the European Commission. The full pest survey card for the non‐EU isolates of Phytophthora ramorum is published and available online in the EFSA Pest Survey Card gallery at the following link and will be updated whenever new information becomes available: https://efsa.europa.eu/plants/planthealth/monitoring/surveillance/phytophthora-ramorum

ABSTRACT Larch ( Larix ) and, to a lesser extent, rhododendron ( Rhododendron ponticum ) have emerged as key drivers of the Phytophthora ramorum epidemic in Britain because of the copious numbers of sporangia that are released from their foliage following infection; the inoculum then initiates stem and foliar infections on nearby broadleaf and conifer species. This has raised concerns that other conifer species could play a similar role to larch, with most concern centred on Sitka spruce which currently comprises > 50% of all commercial conifer plantations in Britain. To address this, we assessed the potential of the EU1 lineage of P. ramorum to sporulate on foliage of conifers already known to have some susceptibility (Sitka spruce ( Picea sitchensis ), grand fir ( Abies gradis ), Douglas fir ( Pseudotsuga menziesii ), western hemlock ( Tsuga heteophyllum ) and coastal redwood ( Sequoia sempervirens )). Our findings suggested these conifers are much less effective at sustaining sporulation (means ranging from ~16 to ~49 sporangia per cm 2 of needle) compared with larch (nearly 600 per cm 2 ). Lesions formed by both EU1 and EU2 lineages of P. ramorum in Sitka spruce bark were significantly smaller than those in larch, indicating that spruce is a less susceptible host. However, substantial zoospore concentrations (50,000 to 500,000 mL −1 ) were required to cause infections through intact bark of both larch and spruce. Outcomes of these comparisons are discussed in the context of the frequency with which larch and Sitka spruce were found to be naturally infected during surveys of P. ramorum in Britain. Overall, we conclude that in contrast to larch, Sitka spruce and several other conifers grown in Britain do not have potential to sustain an epidemic of P. ramorum , although they may possibly act as low‐level reservoirs of infection.

The European Commission requested the EFSA Panel on Plant Health to prepare and deliver risk assessments for commodities listed in Commission Implementing Regulation (EU) 2018/2019 as ‘high risk plants, plant products and other objects’. Taking into account the available scientific information, including the technical information provided by the applicant country, this Scientific Opinion covers the plant health risks posed by the following commodities: Berberis thunbergii, bare root plants (up to 3 years old), whips (up to 2 years old) and rooted plants in pots (up to 4 years old) imported into the EU from the UK. A list of pests potentially associated with the commodities was compiled. The relevance of each pest was assessed based on evidence following defined criteria. One EU quarantine pest (Phytophthora ramorum (non‐EU isolates)), one protected zone quarantine pest (Bemisia tabaci (European populations)) and one non‐quarantine pest (Phytophthora kernoviae) were selected for further evaluation. For the selected pests, the risk mitigation measures implemented in the UK and specified in the technical dossier were evaluated taking into account the factors reducing their efficacy. For these pests, an expert judgement is given on the likelihood of pest freedom taking into consideration the risk mitigation measures acting on the pest, including uncertainties associated with the assessment. The degree of pest freedom varies between the pests evaluated, with P. ramorum being the pest most frequently expected on the evaluated imported commodities. Expert Knowledge Elicitation indicated, with 95% certainty, that between 9975 and 10,000 per 10,000 B. thunbergii rooted plants in pots would be free from P. ramorum.

The perils of naïve use of open-source data: A comment on “Spatiotemporal distribution of sudden oak death in the US and Europe”

The invasive pathogen Phytophthora ramorum persists within nurseries, potentially within buried, infested leaf debris. To determine how the environment-notably soil temperature and moisture levels-affects the epidemiological risk of soil inoculum reserves, we performed laboratory assays assessing how variable conditions impact pathogen survival and its capacity to sporulate. We first established that incubating the inoculum at 4°C increases the number of sporangia produced from infested rhododendron leaf disks. In a second experiment, the inoculum was incubated in soil at a range of temperature (6.7, 14, 20, or 28°C) and soil moisture (approximately 0, -40, or -400 kPa) conditions for up to 18 weeks. Our ability to culture P. ramorum was only negatively affected by the warmest and driest regimes. In contrast, the capacity to sporulate was affected over a much wider range of conditions, whereby declines in sporulation potential were observed over time from the inoculum incubated at both 20 and 28°C in all soil moisture conditions. However, subsequent incubation of this inoculum at 4°C for an additional 7 weeks restored sporulation potential, at times exceeding pre-incubation levels. These results are consistent with field observations that P. ramorum becomes more biologically active after exposure to cooler temperatures and highlights the risk a soilborne inoculum poses during some times of the year. Disinfestation of soils through artificially high heat is likely required to prevent recurrent infections within nurseries from soilborne sources and thus prevent the further spread of this invasive pathogen.

Upon introduction, clonal pathogen populations are expected to go through a genetic bottleneck followed by gradual clonal divergence. Two distinct and purely clonal lineages of the sudden oak death pathogen Phytophthora ramorum recently emerged in forests in the Western United States, providing the unique opportunity to study a naturally replicated invasion into the same ecosystem. We characterized population genomic patterns during early invasion using whole-genome sequencing of two P. ramorum clonal lineages sampled in the first 5 years following their detection. We re-sequenced genomes from populations of two dominant clonal lineages, NA1 (n = 134; 2001 to 2005) and EU1 (n = 160; 2015 to 2019), and obtained 106,070 high-quality single-nucleotide polymorphisms in genic regions. Our results are consistent with the hypothesis of one introduction for each lineage. The NA1 population had a wider distribution of pairwise genetic distances than EU1 and higher genetic diversity, though neither NA1 nor EU1 populations clustered clearly by year. There was significant correlation between genetic distance and geographic distance for NA1 (P = 0.042), but not for EU1 (P = 0.402). The genetic diversity in NA1 is strongly driven by loss of heterozygous positions, which impacted more than one-third of the sampled NA1 population. However, loss of heterozygosity was rare in EU1. This work provides novel insights into the invasion biology and dynamics of clonal plant pathogens in natural ecosystems.

Phytophthora ramorum is an invasive, federally regulated pathogen in the United States, with at least 75 confirmed hosts and over 100 associated hosts. The movement of ornamental plants infected with P. ramorum, as a regulated pathogen, across the United States resulted in a surge of samples experienced by diagnostic laboratories, underscoring the need for a specific and rapid screening assay. This work adapted and validated a previously published P. ramorum recombinase polymerase amplification (RPA) diagnostic tool for commercial application. This assay was validated through inclusivity testing for the four P. ramorum lineages and exclusivity testing on a panel of Phytophthora species and other commonly detected oomycetes. The reformulated assay demonstrated sensitive and specific detection of P. ramorum down to 1 fg for each lineage, without cross-reacting with over 100 different oomycetes across at least 30 hosts. The RPA assay underwent three tiers of validation. In Tier 1, a single-lab, multi-operator validation, it achieved diagnostic specificity and sensitivity of 100 and 92%, respectively, with 100% reproducibility and repeatability among operators. Tier 2 validation, performed on a blind sample set at two independent labs, resulted in 100% diagnostic specificity and sensitivity. Tier 3 involved a blind performance study across five independent labs using multiple fluorescent cycler platforms and operators. The reformulated RPA assay offers a rapid and simple approach for the molecular detection and identification of P. ramorum. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .