Scientific Opinion on the pest categorisation of Elm phloem necrosis mycoplasm

Widespread occurrence of 'Candidatus Phytoplasma ulmi' in elm species in Germany

BACKGROUND

'Candidatus Phytoplasma ulmi' is the agent associated with elm yellows and has been categorised in the European Union as a quarantine pathogen. For central and northern European countries, information on the occurrence and distribution of the pathogen and its impact on elms is scarce, so a survey of native elm trees has been conducted in Germany.

RESULTS

About 6500 samples from Ulmus minor, Ulmus laevis and Ulmus glabra, were collected nationwide. Phytoplasma detection was performed by applying a universal 16Sr DNA-based quantitative PCR (qPCR) assay and a novel 'Ca. P. ulmi' specific qPCR assay targeting the 16S-23S spacer region. Both assays revealed that 28% of the samples were infected by 'Ca. P. ulmi', but infection rates of the elm species and regional incidences differed. The phytoplasma presence in the trees was not correlated to disease-specific symptoms. The survey identified a regional disparity of infection which was high in east, south and central Germany, whereas only a few infected sites were found in the western and northern parts of the country. Monitoring the seasonal titre of 'Ca. P. ulmi' in an infected tree by qPCR revealed a high colonisation in all parts of the tree throughout the year.

CONCLUSIONS

'Ca. P. ulmi' is widely present in elms in Germany. The rare occurrence of symptoms indicates either a high degree of tolerance in elm populations or a low virulence of pathogen strains enabling high infection rates in a long-living host.

Pest categorisation of Scaphoideus luteolus

The Panel on Plant Health performed a pest categorisation of Scaphoideus luteolus, a well-defined phloem sap-feeding insect species in the family Cicadellidae (Insecta: Hemiptera). It can be identified using taxonomic keys. S. luteolus is only present in the eastern part of the USA. The main host plants of S. luteolus are species of the genus Ulmus (U. americana, U. alata, U. bergmanianna, U. szechuanica, U. rubra), but specimens have also been collected on Vitis sp., Salix sp. and Populus sp. The species does not cause damage by itself, but it is the only confirmed vector of the phytoplasma Candidatus Phytoplasma ulmi (CPu), which is present in North America where it causes heavy damage to the local elms, as well as in some European countries where the local elms are considered less susceptible. S. luteolus has three developmental stages (egg, nymph, adult). It overwinters in the egg stage, takes 36-42 days to complete nymphal stage, and adults are found throughout the summer period. Both nymphs and adults are capable of transmitting CPu and, after acquiring the pathogen, remain infective for the rest of their life. The main pathways are cut branches and plants for planting. These pathways are not regulated for the main host, Ulmus, though requirements are in place in relation to other pests on Ulmus. These pathways are also not regulated for Salix. Establishment would be favoured by the wide coverage of Ulmus spp. in the EU territory and by climatic conditions comparable to those of the pest's native range. S. luteolus meets all the criteria assessed by EFSA for consideration as potential Union quarantine pest. The criteria for considering it as a potential Union regulated non-quarantine pest are not met since the species is absent from the EU.

Pest categorisation of Stegophora ulmea

Following a request from the European Commission, the EFSA Panel on Plant Health (PLH) performed a pest categorisation of Stegophora ulmea, a well‐defined and distinguishable fungal species of the family Sydowiellaceae. S. ulmea causes a tree disease known as black spot of elm (Ulmus spp.). The pathogen is reported from North America (native range) and Asia (Far‐East Russia and China), but not from the EU. S. ulmea is regulated in Council Directive 2000/29/EC (Annex IIAI) as a harmful organism whose introduction into the EU is banned on plants of Ulmus L. and Zelkova L., intended for planting, other than seeds. The pathogen has been occasionally intercepted on imported bonsai plants (and then destroyed) in the Netherlands and the UK. It could enter the EU and spread within it via plants for planting (including bonsai) and cut branches. Hosts and favourable climatic conditions are common in the EU. The European native elm species Ulmus glabra and Ulmus laevis were found to be more susceptible to the disease than North American elm species, but information is lacking on Ulmus minor. The disease is rarely fatal, but S. ulmea can cause considerable damage, particularly in wet summers. Reduction of inoculum by the removal of leaf debris and avoiding overhead watering in nurseries can reduce the risk of spread of the pathogen. The main knowledge gaps concern (i) the distribution of the pest in Asian countries, (ii) the relative role of the means of entry/spread and (iii) the potential consequences in mature tree plantations and native woodland. The criteria assessed by the Panel for consideration as potential quarantine pest are met. For regulated non‐quarantine pests, the criterion on the pest presence in the EU is not met.