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Phytophthora ramorum

Phytophthora ramorum in the New Forest

Questions & Answers

1. What is Phytophthora ramorum?

It is a fungus-like pathogen from the large Phytophthora group of organisms. It causes 'bleeding cankers', or oozing lesions, on the trunks of infected trees, necrosis (dieback) of leaf tips, and stem wilt and stem lesions on infected shrubs and plants, and some trees. It is spread through the air, probably in rain-splash and mist-laden winds, or via watercourses. It can also be spread through human and animal movements.

2. Where exactly in the New Forest is this incidence of the disease?

An initial outbreak of the disease was found on rhododendron bushes growing near the Tall Trees Trail, on the Rhinefield Drive at Brockenhurst (26 November 2008). The infected bushes were identified during a routine plant health inspection of the area. Previous inspections going back to 2003 had failed to find any evidence of infection

3. What are the authorities doing about it?

A survey of the immediate area was carried out, during which further samples from symptomatic plants were taken for analysis. Routine monitoring surveys are being carried out by Food & Environment Research Agency (Fera) officials on rhododendron in the area, and survey zones are being extended outwards until we have a 3-kilometre infection-free zone.

The infected rhododendron bushes, along with plants in precautionary buffer zones, have been cut down, and the resulting material has been burnt on site. Herbicides have been applied to the cut stumps to kill the root system of the infected plants. Maintenance work over the next three years will entail herbicide application to any re-growth to ensure the plants are eradicated.

A survey of Vaccinium myrtillus (bilberry) was also carried out across a number of areas of the New Forest in the Spring of 2010 (see Q9). Whilst initial observations recorded that there were some possible symptoms of die back on some of the Vaccinium, all samples have resulted in a negative response for the Phytopthora ramorum pathogen. Further monitoring is being carried out.

4. How does this affect people visiting the New Forest?

The public may continue to enjoy the majority of New Forest's attractions as usual, although small areas will be cordoned off for safety/ bio-security reasons during plant removal and containment operations. Currently the western side of the Tall Trees Trail is partially closed to the public. There is a diversion in place and notices are displayed to advise the public to:

  • stay on the main paths;
  • not take plant cuttings; and
  • keep dogs on leads.

It is important that visitors and animals are kept out of the fenced or closed-off areas, and heed any warnings. This is a precaution to help prevent the spread of the pathogen. Phytophthora ramorum can be found in leaf litter and in soil up to a depth of 15cm, so it can be moved about on the footwear of humans and possibly on the feet of animals, and potentially by vehicles. All staff carry out strict decontamination procedures before leaving the site of infection.

There is no risk to human health from this disease. These simple precautions will need to stay in place until the incidence of the disease is reduced, which might be several months.

5. How could the disease affect the trees and woods of the New Forest?

It is still too early to tell. Although the risk to native oaks is small, there is evidence that other native species such as ash and beech trees, and some non-native species which occur in the New Forest, are susceptible to P. ramorum. Nevertheless, there are still no confirmed cases of P. ramorum on trees in the New Forest and Dorset.

However, a significant number of Japanese larch trees (Larix kaempferi) and small numbers of western hemlock (both of which are conifer species) have been found with P. ramorum infection in Devon, Cornwall and Somerset. Although these species are not present in significant numbers in the New Forest, we are taking the precaution of surveying larch crops in Hampshire and Dorset, especially those which have rhododendron shrubs. Further information about the South West England outbreak is available at www.forestry.gov.uk/news.

In the New Forest context we are also concerned that the pathogen can infect bilberry (Vaccinium myrtillus), an important heathland plant in the New Forest. (See also Q.9)

Meanwhile, we and our partners in Fera and the Scottish Government are monitoring for outbreaks throughout Britain. As well as working to contain or eradicate any outbreaks we find, we are urgently researching all we can about the pathogen and its behaviour so that we can formulate advice and guidance for its management. 

The key to preventing the spread of the disease is the removal of Rhododendron ponticum host plants, and the timely eradication of infected plants capable of producing inoculum. This reduces the number of spores produced that could be spread to surrounding trees by rain-splash or the movement of infected plants.

6. Will this mean the end of the rhododendrons of the Rhinefield Drive?

All infected rhododendron bushes, and others in the immediate vicinity, have been destroyed to get rid of the infection and prevent the organism from spreading. Clearance of the west side of the trail is complete. This, as well as continued surveillance and follow up work on individual bushes and spraying of regrowth should help to ensure the long-term future of the trees on Rhinefield Drive.

7. How did P. ramorum get into the New Forest?

It is still not known how the pathogen reached the New Forest, or when. We do know that its spread is usually facilitated by the movement of infected planting stock or animal and people movements, but we are confident that the plants which have been removed are unlikely to have been infected when they were first planted in 2001. Further research into P. ramorum is continuing nationally.

8. Are there any impacts from the disease on commoning or the free-roaming livestock of the New Forest?

No. The disease has been found within one of the New Forest woodland inclosures (an area not grazed by commoning stock), so the practice of commoning is not affected.

9. Are there any potential impacts from the disease for the New Forest’s heathlands?

Yes. Among the non-tree species that have been affected in other parts of Britain are vaccinium (bilberry) species, which are reasonably common on the New Forest's heaths. This is another reason why we are anxious to control this infection before it spreads.

10. Why is this disease sometimes called "sudden oak death"?

P. ramorum got its nickname in the United States because it has killed significant numbers of North American native oak (Quercus species) and tanoak trees (Lithocarpus densiflorus, which are not true oaks at all). However, Britain's two native oak species have proved to be much less susceptible than their American cousins. Our native oaks usually only become infected if they are standing very close to heavily infected shrubs such as Rhododendron ponticum.

Britain’s native oak species are sessile oak (Quercus petraea) and pedunculate oak (Q. robur), which is also known as ‘English’ or ‘common’ oak.

11. What should people do if they suspect a tree or shrub is infected by Phytophthora ramorum?

A number of other plant diseases and disorders can cause similar symptoms to those caused by P. ramorum, so the public should not jump to the conclusion that an unhealthy-looking tree or plant is infected with it.

If they are concerned about a particular plant or plants, we recommend they first consult the on-line information at www.fera.defra.gov.uk/plants/plantHealth/pestsDiseases/pRamorum.cfm

If they continue to be concerned, they should contact the relevant authorities as follows:


There are a number of sources of further information about P. ramorum:

Forestry Commission
16 December 2009