Plant Health

Plant pest Phytophthora ramorum


Report no: 2023: 19

Published: 06.07.2023

Key message:

It is highly likely that the plant pest responsible for Ramorum dieback and Ramorum blight (Phytophthora ramorum) will continue to be introduced into Norway. The level of uncertainty is low.

This is the key message in a risk assessment of the plant pest that VKM (Norwegian Scientific Committee for Food and Environment) has carried out for the Norwegian Food Safety Authority.


VKM carried out a risk assessment of Ramorum dieback/Ramorum blight in 2009. Since then, the pathogen has been detected repeatedly in Norway, mainly in southern and southwestern parts of the country.

The only genetic line of the pathogen that has been confirmed in Norway is EU1, which is of mating type A1. Rhododendron is the most important host plant in Norway. In Europe, other genera of ornamental plants are also considered as main hosts, such as Viburnum, Pieris and Kalmia. The pathogen has been detected at least once in Norway on plants of all these genera.

The major route for the continued introduction of the P. ramorum into Norway is through import of plants from nurseries in the EU.


VKM has carried out a systematic search of the literature. Besides integrating information from Norwegian reports, the risk assessment is largely based on literature from other countries. VKM has carried out a quantitative risk assessment that describes the degree of confidence in conclusions and identifies uncertainties and data gaps.


According to the assessment, Ramorum dieback and blight has great potential to spread and establish itself along the coast of Norway in the south and southwest. Here, the climatic conditions are favourable, and rhododendrons and other host plants are commonly found.

"If efforts in preventing the import of infected plants and eradicating attack by the pathogen were to cease in Norway, we consider it very likely that the pathogen would spread and establish itself in new areas," explains Iben Thomsen, Scientific Leader of the project team.

The probability that the pathogen would spread to other parts of the country is assessed to be medium, with medium uncertainty.

"Despite the fact that Ramorum dieback and blight has repeatedly been detected in certain places, outbreaks in new areas have been rare. It is difficult to determine why. However, we cannot ignore the fact that a more conducive climate in certain parts of Norway could change this," says Thomsen.

The most effective risk-reducing measures against Ramorum dieback and blight are to continue to monitor host plants for symptoms and test for the presence of pathogen, especially in imported plants. Measures such as removing and destroying diseased ornamentals, especially rhododendrons, and not replanting with susceptible plants, will also be effective.

The present risk assessment has been approved by the Panel for Plant Health at VKM.

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