Pest Risk Analysis for Hymenoscyphus Pseudoalbidus for the UK and the Republic of Ireland Pest Risk Analysis
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Pest Risk Analysis for Hymenoscyphus pseudoalbidus for the UK and the Republic of Ireland Pest Risk Analysis Pest Risk Analysis for Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea ) for the UK and the Republic of Ireland 1 STAGE 1: PRA INITIATION 1. What is the name of the pest? Hymenoscyphus pseudoalbidus (teleomorph – sexual stage). This was first described by Queloz et al . (2011) (published online March 2010). Synonyms: Chalara fraxinea , first described by Kowalski (2006), is the anamorph (asexual) stage of H. pseudoalbidus , and, until recently, this has been the name ascribed to the disease-causing organism. However, as described below, it has become clear that the teleomorph H. pseudoalbidus is the epidemiologically important stage of the fungus, producing the infective sexually-produced ascospores. H. pseudoalbidus should, therefore, be the name now used for the pathogen responsible for ash dieback. In this PRA, irrespective of whether C. fraxinea is the term used in cited publications many of which pre-date the description of H. pseudoalbidus , the pathogen is referred to as H. pseudoalbidus in addition to or as an alternative to C. fraxinea. Common names of the pest: The pest (also referred to as the fungus or the pathogen in this PRA) does not have a common name. The disease caused by the fungus is known as ash dieback, but it is not the same as the ‘ ash dieback’ syndrome which has previously been investigated in the UK (Hull and Gibbs, 1991). It is also commonly referred to as Chalara dieback. Taxonomic position: Kingdom - Fungi; Phylum - Ascomycota; Class - Leotiomycetes; Order - Helotiales; Family – Helotiaceae. Special notes on nomenclature or taxonomy: The anamorph Chalara fraxinea was first described as a new species of fungus, isolated from shoots and twigs of Fraxinus excelsior (European or common ash) exhibiting symptoms of dieback in Poland (Kowalski, 2006). Although not proven to be the cause of the disease at that time, it was suspected. No teleomorph stage was reported at that time. Subsequently, the fungus was found to be pathogenic to young ash trees when they were artificially inoculated under field conditions (Kowalski and Holdenrieder, 2009). 1 Please cite this document as: Sansford, CE (2013). Pest Risk Analysis for Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea ) for the UK and the Republic of Ireland. Forestry Commission. 2 PRA for Hymenoscyphus pseudoalbidus C.E. Sansford 23 rd May 2013 Pest Risk Analysis In 2009, C. fraxinea was considered to be the anamorph (asexual stage) of Hymenoscyphus albidus, a species first described in 1850 (assigned then to Peziza albida ) and considered to be non-pathogenic, native and widespread in Europe (Kowalski and Holdenrieder 2009a). Because of the disparity between the emergence of the disease in Europe (in Poland in the early 1990s without an identifiable cause at that time) and the apparent non-pathogenic status of the teleomorph, the authors commented that further investigation was needed in case the organism they described was a ‘ new mutant, a hybrid of H. albidus with an unknown introduced species or an exotic invasive species, which is indistinguishable morphologically from H. albidus’. Following on from this, molecular studies of Swiss isolates concluded that the teleomorph of C. fraxinea was not H. albidus but a new species, Hymenoscyphus pseudoalbidus (online paper March 2010, published as Queloz et al., 2011; Husson et al ., 2011), morphologically similar to but genetically distinct from H. albidus . Evidence from recent research (e.g. Gross et al ., 2012; Bengtsson et al ., 2012) supports the view that H. pseudoalbidus is the pathogen responsible for the current ash dieback epidemic in Europe including the recent findings in the UK and Ireland. 2. What is the pest’s status in the EC Plant Health Directive (Council Directive 2000/29/EC) (Anon., 2000)? H. pseudoalbidus /C. fraxinea is not listed in the EC Plant Health Directive. There is no emergency legislation in place in the EU. However, there is emergency legislation at the national level (Great Britain, Northern Ireland and Ireland – see 13. and Table 6) as well as for Jersey, Guernsey and the Isle of Man (see 6.). 3. What is the recommended quarantine status of the pest in the lists of the European and Mediterranean Plant Protection Organisation (EPPO)? Chalara fraxinea was added to the EPPO Alert List in September 2007 (EPPO, 2007). It is still listed. See: http://www.eppo.int/QUARANTINE/Alert_List/alert_list.htm At the time of listing, although C. fraxinea was suspected to be the main cause of ash dieback in Europe, many other fungi had been isolated from diseased ash trees in European countries, and it was not known which abiotic factors (e.g. frost, drought) were involved in the disease. EPPO felt it appropriate to add the fungus to the Alert List because they were concerned that the disease might pose a threat to ash trees growing in European forests, parks, and nurseries. 3 PRA for Hymenoscyphus pseudoalbidus C.E. Sansford 23 rd May 2013 Pest Risk Analysis EPPO (2007) considered that ‘ plants for planting and wood of F. excelsior could be pathways for spreading the disease over long distances’ and warned ‘that there may be a risk in moving diseased F. excelsior plants across the region without any precaution ’. However, the fungus has never been moved to the lists of organisms recommended for regulation by EPPO. Such a recommendation has to be based on a PRA. By the time H. pseudoalbidus was eventually confirmed as the causal pathogen (online paper March 2010, published as Queloz et al ., 2011) the disease was already quite widespread within the EPPO region. 4. What is the reason for the PRA? At the request of the Forestry Commission (FC) this PRA was initiated in January 2013 to update the Rapid Assessment (RA) produced by Forest Research (FR) in August 2012 (Webber and Hendry, 2012). At the time of the RA there had been a low number of interceptions of ash saplings infected with the fungus in UK nurseries and in recent plantings. Since the RA was published, surveillance has shown that the disease is present in the wider environment in parts of Great Britain (GB). Northern Ireland (NI) and the Republic of Ireland (ROI) (not considered in the 2012 RA) have found infected material in nurseries and recent plantings (See 6. and 7.). This PRA reflects the current status of the pathogen from surveillance in the PRA area. New findings of infected trees and nursery plants have been reported since the RA. Studies of the distribution and value of ash and the possible economic impact of the disease have been undertaken. Risk management activities have been carried out in the UK and the ROI. The PRA also examines additional pathways of entry (seeds, airborne ascospores) and all potential pathways (plants, wood, seeds, soil/growing media, airborne ascospores) are examined in depth. A more detailed consideration of pest risk management options has been undertaken. Suggestions for future controls and a list of uncertainties with associated research needs are provided. 5. What is the PRA area? The PRA area is the UK and the Republic of Ireland (ROI). For the UK, the Crown Dependencies of Guernsey, Jersey and the Isle of Man are not included except with respect to comments on the status of the pest. STAGE 2: PEST RISK ASSESSMENT 6. What is the pest’s present geographical distribution? H. pseudoalbidus /C. fraxinea is currently known to occur in at least 24 European countries outside of the UK and the ROI (see 7. for pest status). It has also been detected in imported material in the Crown Dependencies 4 PRA for Hymenoscyphus pseudoalbidus C.E. Sansford 23 rd May 2013 Pest Risk Analysis of Jersey and Guernsey. It has not been found on the Isle of Man. It is considered to be an exotic introduction in Europe, possibly from Asia where it has been reported from Japan (Zhao et al ., 2012). More detail is given below. Ash trees with symptoms of ash dieback were first observed in the early 1990s in Poland; however, the anamorph form of the causal organism of the disease was not named until 2006 (Kowalski, 2006). The fungus has spread across Europe since the early 1990s, and it was found causing disease, for the first time, in the UK and the ROI in 2012. Nearly all European disease records have been attributed (some retrospectively) to the anamorph C. fraxinea but it is now known that these should be attributed to H. pseudoalbidus. Dates of publication of records do not necessarily reflect the date at which the pest entered the affected country. For early records this is partly because the cause of the disease was unknown. For all records, this is because the detection of symptoms at an early stage of introduction of the fungus is unlikely. Because of this, no attempt has been made to document a geographical pattern of perceived spread over the period since diseased trees were first observed in Europe in this PRA. Table 1 below lists all known countries where the pest has been recorded. The EPPO Plant Quarantine Retrieval System (a database of pests of quarantine concern) confirms the status of ‘ No record ’ for the Americas, the Caribbean and Oceania (EPPO, 2013). A record in Japan has recently been published (Zhao et al ., 2012). Further commentary on the status of the pathogen in the UK and the ROI is given under 7. Whilst not considered further in this PRA, Jersey, Guernsey and the Isle of Man have all undertaken surveys for the disease, followed-up by diagnostic tests on symptomatic material; this is ongoing. Jersey began casual surveillance in 2011 with official surveillance of the whole island commencing in November 2012.