Phytophthora ramorum: monitor its spread and eradicate contaminated sites

First detected in France in the Finistère region in May 2017, the plant pathogen Phytophthora ramorum poses a significant risk for many ornamental and forest-growing plants (rhododendrons, vibernum and larch trees). ANSES issues recommendations to monitor the spread of this fungus-like microorganism, prevent the risk of its introduction and eradicate outbreaks where it is already found.

Known in Europe since the early 2000s, Phytophthora ramorum is a plant pathogen that can cause foliage dieback on ornamental plants, mainly rhododendrons and larch trees. Responsible for the “Sudden Oak Death” phenomenon in California, where the disease causes cankers to form on the trunks of certain highly vulnerable tree species before it kills them, Phytophthora ramorum has also been responsible, since 2009, for an epidemic on Japanese Larch plantations in Great Britain, with symptoms including needle loss, branch mortality, resinous cankers and finally massive tree mortality.

Health authorities have concentrated their efforts on Phytophthora ramorum since it was first detected in France on Japanese larch trees in the Finistère region in May 2017. At the request of the Ministry of Agriculture, ANSES carried out an expert assessment to identify the climatic and anthropogenic factors conducive to the establishment and spread of this oomycete (“fungus-like microorganism”), as well as to identify forest species potentially threatened by its spread.   

Measures to monitor and eradicate Phytophthora ramorum

Due to its wide host range, many forest species other than the larch could be threatened by Phytophthora ramorum when climatic conditions are favourable for its development. In this context, ANSES recommends intensive surveillance, as a priority in Brittany to detect possible new outbreaks of Phytophthora ramorum, as well as in other regions climatically favourable to its development (Normandy, Limousin, etc.) and where high densities of vulnerable species capable of transmitting the disease are planted (particularly larch and chestnut trees). In areas climatically favourable to the development of Phytophthora ramorum, ANSES also recommends that planting of the three larch species should be avoided due to their vulnerability.

When Phytophthora ramorum is detected in forests, the Agency recommends that eradication measures be taken for infected forest species, including those growing in the understory, such as Rhododendron ponticum.

In order to prevent the spread of Phytophthora ramorum from ornamental host species in France and the European Union, the Agency recommends that control of ornamental nurseries and garden centres be intensified.

The ANSES expert assessment also highlights the need to provide information to park and garden owners in order to raise their awareness of the risk of uncontrolled introduction of Phytophthora ramorum host plant material from infected areas (Brittany and Great Britain). In parallel, the experts also point out the importance of conducting research work to resolve uncertainties, in particular on the epidemic role of the chestnut tree (very widespread in favourable climates), and to assess the sensitivity to climate change of evolutions in Phytophthora ramorum pathogenicity.