Phytophthora Ramorum is a threat to European forests, parks and gardens, says EFSA

A fungal-like pathogen that causes “sudden oak death” in California and blight in a number of common plant species is a growing threat to forests, parks and gardens across the European Union, according to the European Food Safety Authority (EFSA)[1].

Following large-scale outbreaks of Phytophthora ramorum in Japanese larch (Larix kaempferi) plantations in the UK and Ireland[2], the European Commission asked EFSA to deliver a scientific Opinion on a pest risk analysis of the disease published in 2009 by RAPRA (acronym for Risk Analysis of Phytophthora ramorum), the EU-funded Sixth Framework Programme research project[3]. 

EFSA’s Panel on Plant Health has published its Opinion, in which it agrees with RAPRA’s conclusion that there is “a risk of further entry and establishment” of P.ramorum in the EU and supports the risk reduction options proposed.

However, after considering comments from Member States and information that became available only after the publication of the RAPRA report, EFSA’s Panel of independent scientists goes further. It concludes that the recent outbreaks – which have affected an estimated 1,900 hectares of Japanese larch, or 500,000 trees, in England and Wales alone – represent a “major step change” in the epidemiology of P.ramorum and the associated risk assessment and management issues. It suggests additional options to reduce the likelihood of further spread.

The EFSA Opinion points out that there are large regions across Europe that are climatically suitable for the spread of P.ramorum and where susceptible host plants are present. There are many common species among potential hosts, such as European beech (Fagus sylvatica), rhododendron, camellia and viburnum.

The Japanese larch P.ramorum outbreak in the UK and Ireland raises the possibility of a threat to European larch across the EU.

In addition, there is growing evidence that some forms of P. ramorum are more aggressive than others. Allowing new, potentially more virulent lineages to enter the EU could lead to P.ramorum becoming more widely established in the region.

P.ramorum is not listed as a harmful organism in the EU but in 2002 the Commission adopted emergency measures to prevent its introduction and spread. A decision will be taken as to whether permanent measures are needed – and, if so, what type of measures – based on the RAPRA research and EFSA’s subsequent Opinion.

The emergency measures, which are based on specific import requirements, the “plant passport” certification system and phytosanitary measures at the place of production, seem to have been partially successful in removing P. ramorum from plant nurseries, although it is uncertain to what extent the reduction in outbreaks is causal. Plant traders in many Member States are still reporting the presence of the pathogen in stocks.

EFSA’s experts also express reservations about the effectiveness of the three-month quarantine period that follows eradication measures, given uncertainties about how long P.ramorum can lie dormant.

The measures have not been successful in reducing the occurrence of the disease outside plant nurseries, the Panel says. As well as the recent major outbreaks in Japanese larch in the UK and Ireland, P.ramorum has been found on a range of host species in parks, gardens, woodlands and forests in The Netherlands, the UK, Belgium, Denmark, France, Germany, Ireland, Luxembourg, Norway, Serbia, Slovenia, Spain and Switzerland.

Controlling the spread of the disease outside nurseries is therefore a significant challenge for risk managers. Large-scale felling is taking place in the affected Japanese larch plantations in the UK and Ireland. However, Rhododendron ponticum, the most widespread host species in the UK, regrows from rootstock and requires root-and-branch removal. Previous attempts to remove it from British woodlands have been unsuccessful.

EFSA’s Panel suggests that management could focus on protecting trees that experts deem worthy of conservation by clearing the surrounding area of healthy plants belonging to host species.

The experts also highlight continuing uncertainties surrounding P.ramorum such as: a lack of data related to the origin of the pathogen; the efficacy of detection methods; the breadth of the host range; and the virulence of the pathogen’s progeny.

[1] In the United States P.ramorum has caused substantial mortality in several oak species but in the European context the term “sudden oak death” is inaccurate because although the disease affects trees such as Japanese larch and garden plants including rhododendron, viburnum and camellia, there is little evidence to date that it affects European oak species
[2] An estimated 1,900 hectares of Japanese larch plantations show symptoms of P.ramorum in England and Wales (Brasier and Webber 2010). The area covered by Japanese larch in England and Wales is about 60,000 hectares
[3] Question No EFSA-Q-2010-00841, adopted on 18/05/2011

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