Scientific Opinion on the Pest Categorisation of Elm Phloem Necrosis Mycoplasm

Scientific Opinion on the Pest Categorisation of Elm Phloem Necrosis Mycoplasm. Baker, R., Bragard, C., Candresse, T., Gilioli, G., Grégoire, J., Holb, I., Jeger, M. J., Karadjova, O. E., Magnusson, C., Makowski, D., Manceau, C., Navajas, M., Rafoss, T., Rossi, V., Schans, J., Schrader, G., Urek, G., Vloutoglou, I., Winter, S., & van der Werf, W. 12(7):3773+.

Paper doi abstract bibtex

The Panel on Plant Health performed a pest categorisation of Elm phloem necrosis mycoplasma, now renamed Candidatus Phytoplasma ulmi (CPu), for the European Union (EU) territory. CPu is a well-defined phytoplasma species of the genus Candidatus Phytoplasma, for which molecular detection assays are available. CPu is transmitted by grafting and vegetative propagation material as well as by insect vectors. CPu is reported from North America and is present in at least four EU Member States: the Czech Republic, France, Germany and Italy. CPu distribution in Europe is suspected to be underestimated, with high uncertainty since no systematic surveys are carried out. CPu has a host range restricted to Ulmaceae species, and especially to the genus Ulmus, with some variations in susceptibility to the disease. It is listed in Annex IAI of Directive 2000/29/EC. CPu is not expected to be affected by EU ecoclimatic conditions wherever its hosts are present and has the potential to establish largely within the EU territory. Two insect vectors, Macropsis glandacea and Philaenus spumarius, are widely distributed in Europe. The uncertainty about other potential vector species, in which the phytoplasma has been detected, is considered as high. There is a lack of data to fully assess the potential consequences of the disease, with regards to the susceptibility of European elm species and virulence of European CPu strains. Data are not sufficient to reach a conclusion on pest categorisation of CPu and a full risk assessment can be conducted but is unlikely to bring any additional value unless the key additional data gaps on distribution, insect vectors, elm species susceptibility and potential consequences of the pest are filled. [Excerpt:Biology of Candidatus Phytoplasma ulmi] Elm phloem necrosis was first described by Swingle (1938) in the USA. Symptoms of the disease develop often in mid- to late summer (Sinclair et al., 1987), with leaf yellowing, epinasty and witches' brooms followed by a death of the affected branches (Figure 1). Symptoms are often present on some branches only. They vary according to the host species. Symptoms are often confused with leaf senescence at the end of the growing season. [...] [\n] Briefly, phytoplasmas are acquired by insects whilst feeding on phloem sap, pass into the alimentary canal and cross the midgut epithelium, thus reaching the haemocoel. They actively multiply in the haemolymph before reaching the salivary glands. Finally, they are injected into another host plant via the saliva. The transmission mode is defined as persistent and propagative, since after the acquisition the vector is persistently and systemically infected (even through moulting) and, after the completion of a latent period, it is infectious for life. Even though phytoplasmas actively multiply in the insect vectors, transovarial transmission has been reported for few phytoplasma-vector associations (Kawakita et al., 2000; Tedeschi et al., 2006). There is a temperature-dependent latency period between the phytoplasmas acquisition by the insect and its transmission to another host plant, between 12 days and well over a month (Weintraub and Beanland, 2006). [\n] Transmission is also possible through vegetative propagation techniques including bark patch grafts and root grafts (Sinclair et al., 2000; Braun and Sinclair, 1979; Carter and Carter, 1974). CPu is not transmissible by mechanical inoculation. [\n] Transmission by detached bark or wood is considered as not possible (Webber, 2014) [\n] [...] [Uncertainty] The taxonomy of CPu is well defined, but as symptoms may be difficult to recognise, early detection of diseased plants in the field (surveys, first alert, etc.) remains uncertain. Detailed identification requires specific laboratory techniques and reagents that are not available everywhere; this may jeopardise the early detection of outbreaks. No dedicated surveys are organised in EU MSs to check for the presence or absence of CPu. For those reasons, the global distribution of CPu within the EU remains uncertain. [\n] Although this does not affect the conclusions of this pest categorisation, the status of some potential insect vectors still needs to be confirmed. [\n] The host range is apparently limited to the genus Ulmus, but uncertainties remain regarding the level of susceptibility of the different species and cultivars. [\n] Limited information is available on the potential impact of CPu on European species and cultivars, strains occurring in the USA as well as cultivars may be different from those in the EU, that makes uncertain the potential impact of CPu in the EU context. [Conclusions] [\n] [...] Candidatus Phytoplasma ulmi (CPu) is reported in a limited part of the PRA area (four countries), but no systematic surveys have been conducted. CPu has also been reported in other countries either through detection or based on limited scientific evidence. The data available are not considered complete enough for a detailed absence/presence statement in the PRA area. [...] [\n] [...] Elm trees and other Ulmaceae species are widely distributed in Europe and CPu is unlikely to be affected by EU ecoclimatic conditions. Although some reported vectors are absent from the PRA area, vectors are widely present in Europe. CPu is efficiently spread by the movement of plant for planting. [...] CPu is graft transmissible and is efficiently transmitted through plant propagation material, which is widely used by nurseries [...] [\n] [...] CPu is causing mortality of elm trees, with reduction of biodiversity. The impact of the disease was documented in USA, but probably underestimated because of the concomitant occurrence of Dutch elm disease. Elm trees may suffer more or less of the disease depending on species and cultivar susceptibilities, which are still largely unknown [...] CPu has been reported in nurseries, but there is a lack of data on the level of impacts of the pest as a result of intended use of the plants for planting. [...] [\n] [...] Data are not sufficient to reach a conclusion on pest categorisation of CPu and a full risk assessment cannot be conducted unless the key data gaps on CPu distribution, insect vector, elm species' susceptibility and potential consequences are filled. [...] [\n] [...]

@article{bakerScientificOpinionPest2014a,
  title = {Scientific {{Opinion}} on the Pest Categorisation of {{Elm}} Phloem Necrosis Mycoplasm},
  author = {Baker, Richard and Bragard, Claude and Candresse, Thierry and Gilioli, Gianni and Grégoire, Jean-Claude and Holb, Imre and Jeger, Michael J. and Karadjova, Olia E. and Magnusson, Christer and Makowski, David and Manceau, Charles and Navajas, Maria and Rafoss, Trond and Rossi, Vittorio and Schans, Jan and Schrader, Gritta and Urek, Gregor and Vloutoglou, Irene and Winter, Stephan and van der Werf, Wopke},
  date = {2014-07},
  journaltitle = {EFSA Journal},
  volume = {12},
  pages = {3773+},
  doi = {10.2903/j.efsa.2014.3773},
  url = {https://doi.org/10.2903/j.efsa.2014.3773},
  abstract = {The Panel on Plant Health performed a pest categorisation of Elm phloem necrosis mycoplasma, now renamed Candidatus Phytoplasma ulmi (CPu), for the European Union (EU) territory. CPu is a well-defined phytoplasma species of the genus Candidatus Phytoplasma, for which molecular detection assays are available. CPu is transmitted by grafting and vegetative propagation material as well as by insect vectors. CPu is reported from North America and is present in at least four EU Member States: the Czech Republic, France, Germany and Italy. CPu distribution in Europe is suspected to be underestimated, with high uncertainty since no systematic surveys are carried out. CPu has a host range restricted to Ulmaceae species, and especially to the genus Ulmus, with some variations in susceptibility to the disease. It is listed in Annex IAI of Directive 2000/29/EC. CPu is not expected to be affected by EU ecoclimatic conditions wherever its hosts are present and has the potential to establish largely within the EU territory. Two insect vectors, Macropsis glandacea and Philaenus spumarius, are widely distributed in Europe. The uncertainty about other potential vector species, in which the phytoplasma has been detected, is considered as high. There is a lack of data to fully assess the potential consequences of the disease, with regards to the susceptibility of European elm species and virulence of European CPu strains. Data are not sufficient to reach a conclusion on pest categorisation of CPu and a full risk assessment can be conducted but is unlikely to bring any additional value unless the key additional data gaps on distribution, insect vectors, elm species susceptibility and potential consequences of the pest are filled.

[Excerpt:Biology of Candidatus Phytoplasma ulmi] Elm phloem necrosis was first described by Swingle (1938) in the USA. Symptoms of the disease develop often in mid- to late summer (Sinclair et al., 1987), with leaf yellowing, epinasty and witches' brooms followed by a death of the affected branches (Figure 1). Symptoms are often present on some branches only. They vary according to the host species. Symptoms are often confused with leaf senescence at the end of the growing season. [...]

[\textbackslash n] Briefly, phytoplasmas are acquired by insects whilst feeding on phloem sap, pass into the alimentary canal and cross the midgut epithelium, thus reaching the haemocoel. They actively multiply in the haemolymph before reaching the salivary glands. Finally, they are injected into another host plant via the saliva. The transmission mode is defined as persistent and propagative, since after the acquisition the vector is persistently and systemically infected (even through moulting) and, after the completion of a latent period, it is infectious for life. Even though phytoplasmas actively multiply in the insect vectors, transovarial transmission has been reported for few phytoplasma-vector associations (Kawakita et al., 2000; Tedeschi et al., 2006). There is a temperature-dependent latency period between the phytoplasmas acquisition by the insect and its transmission to another host plant, between 12 days and well over a month (Weintraub and Beanland, 2006).

[\textbackslash n] Transmission is also possible through vegetative propagation techniques including bark patch grafts and root grafts (Sinclair et al., 2000; Braun and Sinclair, 1979; Carter and Carter, 1974). CPu is not transmissible by mechanical inoculation.

[\textbackslash n] Transmission by detached bark or wood is considered as not possible (Webber, 2014) 

[\textbackslash n] [...]

[Uncertainty] The taxonomy of CPu is well defined, but as symptoms may be difficult to recognise, early detection of diseased plants in the field (surveys, first alert, etc.) remains uncertain. Detailed identification requires specific laboratory techniques and reagents that are not available everywhere; this may jeopardise the early detection of outbreaks. No dedicated surveys are organised in EU MSs to check for the presence or absence of CPu. For those reasons, the global distribution of CPu within the EU remains uncertain.

[\textbackslash n] Although this does not affect the conclusions of this pest categorisation, the status of some potential insect vectors still needs to be confirmed.

[\textbackslash n] The host range is apparently limited to the genus Ulmus, but uncertainties remain regarding the level of susceptibility of the different species and cultivars.

[\textbackslash n] Limited information is available on the potential impact of CPu on European species and cultivars, strains occurring in the USA as well as cultivars may be different from those in the EU, that makes uncertain the potential impact of CPu in the EU context. 

[Conclusions]

[\textbackslash n] [...] Candidatus Phytoplasma ulmi (CPu) is reported in a limited part of the PRA area (four countries), but no systematic surveys have been conducted. CPu has also been reported in other countries either through detection or based on limited scientific evidence. The data available are not considered complete enough for a detailed absence/presence statement in the PRA area. [...]

[\textbackslash n] [...] Elm trees and other Ulmaceae species are widely distributed in Europe and CPu is unlikely to be affected by EU ecoclimatic conditions. Although some reported vectors are absent from the PRA area, vectors are widely present in Europe. CPu is efficiently spread by the movement of plant for planting. [...] CPu is graft transmissible and is efficiently transmitted through plant propagation material, which is widely used by nurseries [...]

[\textbackslash n] [...] CPu is causing mortality of elm trees, with reduction of biodiversity. The impact of the disease was documented in USA, but probably underestimated because of the concomitant occurrence of Dutch elm disease. Elm trees may suffer more or less of the disease depending on species and cultivar susceptibilities, which are still largely unknown [...] CPu has been reported in nurseries, but there is a lack of data on the level of impacts of the pest as a result of intended use of the plants for planting. [...]

[\textbackslash n] [...] Data are not sufficient to reach a conclusion on pest categorisation of CPu and a full risk assessment cannot be conducted unless the key data gaps on CPu distribution, insect vector, elm species' susceptibility and potential consequences are filled. [...]

[\textbackslash n] [...]},
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  number = {7},
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