Phytophthora ramorum FAQs

What is Phytophthora ramorum?

Phytophthora ramorum is an exotic fungus-like pathogen of plants. Since the mid 1990's it has caused widespread death of millions of trees in forest environments in coastal California and Oregon in the USA. Because the most commonly affected trees that have been killed are tanoaks (not true oaks) as well as several true oak species, this extensive phenomenon is commonly known there as 'Sudden Oak Death'. The pathogen was first found in the nursery trade in the USA/Canada in 2001.

P. ramorum has a very wide natural host range. In Great Britain the majority of nursery hosts are species of rhododendron, viburnum and camellia. Trees confirmed exhibiting bleeding cankers in GB are mainly European beech, but individual trees of horse chestnut, sweet chestnut, several oak species, sycamore, southern beech and Schima have also become affected.

Why is Phytophthora ramorum described as a fungus-like pathogen?

P. ramorum belongs to a group of organisms known as the oomycetes. Until recently oomycetes were believed to be fungi as they have many characteristics, such as spores and hyphae, which are also shared by fungal species. However, DNA sequence analysis in the 1990s indicated that the oomycetes are actually more closely related to algal groups, particularly the diatoms and brown algae. Consequently oomycetes were placed in a separate taxonomic kingdom, the algal kingdom Chromista as opposed to kingdom Fungi.

When was the disease first found in GB?

P. ramorum was first found on viburnum at a nursery in Sussex in February 2002.

What are its hosts?

A 'host' in biological terms is something which a pathogen can infect and colonise. Infected plants typically display symptoms of disease.

P. ramorum has a very wide natural host range. Currently numerous species in over 70 host genera are affected, representing at least 33 different plant families. The types of hosts that are affected vary between countries. In GB, the majority of nursery hosts are species of rhododendron, viburnum and camellia causing infection of shoots and leaves. Other species affected include bay laurel (Laurus nobilis), Californian bay laurel (Umbellularia californica), Griselinia, ironwood (Parrotia persica), Kalmia, lilac (Syringa), Leucothoe, Magnolia spp., Pieris, yew (Taxus), witch hazel (Hamamelis virginiana), Chinese witch hazel (Hamamelis mollis), and Winter's bark (Drimys winteri).

The pathogen has also been detected in rivers and streams near some outbreak sites.

The first finding on a tree in GB occurred in November 2003 on an American southern red oak (Quercus falcata). Since then 26 other trees have been confirmed exhibiting bleeding cankers. These are mainly European beech, but individual trees of horse chestnut, sweet chestnut, several oak species, sycamore, southern beech and Schima have also become affected.

In January 2009 the first finding in the wild of Phytophthora ramorum on Bilberry, Vaccinium myrtillus, was confirmed at a site in Staffordshire. This is the first finding of P. ramorum on Vaccinium myrtillus. It is significant because it is outside of the main disease areas and re-emphasises the potential of the disease to spread beyond the south and west of the country.

An Exotic Pest Alert and further information about the disease on trees can be found on the Forestry Commission website.

Information is also available on those plants reported as natural hosts of P. ramorum.

What are its symptoms?

P. ramorum causes three main types of disease.

• 'Ramorum bleeding canker' refers to cankers (discoloured lesions) on trunks of trees which emit a dark ooze. As they increase in size they can lead to tree death. This is referred to as 'Sudden Oak Death' in the USA.
• 'Ramorum leaf blight' refers to infection of the foliage of trees, shrubs and some herbaceous plants that leads to discoloured lesions on the leaves.
• 'Ramorum dieback' refers to leaf and shoot/stem infections which result in wilting and dieback of affected parts.

An illustrated information leaflet is available which provides a detailed breakdown of symptoms specific to species.

How is it spread?

P. ramorum has an optimum temperature for growth of 20°C and a requirement for moisture; it is therefore well suited to a cool-temperate climate. It produces sporangia on the leaves and shoots of a wide range of plants; these are known as sporulating hosts. These sporangia are mostly spread locally over short distances during rain. P. ramorum can be found in soil up to a depth of 15cm and leaf litter and can be moved on the footwear of humans and possibly on the feet of other animals, and potentially by vehicles. It is also found to contaminate and persist in watercourses at infected sites but it is not known whether this can lead to new infections of plants. Long-distance spread is primarily by movement of infected plant material, e.g. in trade.

Tree hosts only produce infective sporangia from infected foliage. Some tree species only develop bleeding cankers; these do not produce sporangia and so are not sources of infection for themselves or for other host species. These trees become infected as a result of being in the proximity of sporulating hosts. In GB, all of the trees that have developed bleeding cankers have been adjacent to, or very close to, infected rhododendron, particularly Rhododendron ponticum.

Examination of bleeding cankers has shown that P. ramorum can be found extending up to 25mm into the wood of some tree species and can survive there for at least 27 months. This appears to be a dead end for the pathogen but it may be possible for this to lead to further spread via movement of infected timber. Currently no wood has been harvested from known infected trees in GB.

Why are Defra and the Forestry Commission so concerned about Phytophthora ramorum?

In GB and across the EU, P. ramorum is subject to an eradication/containment programme and so its full potential in terms of impact on the environment has not been realised. An epidemic on the scale of California has not yet occurred. The geography, climate, hosts and mixture of hosts in California and Oregon are different to GB and favours disease development.

In California, California bay laurel and tanoak are the main sporulating hosts in affected woodlands. In GB, evergreen rhododendron (especially R. ponticum)is the main sporulating host that will drive woodland epidemics, with beech and some oak species being particularly threatened. In California, sporulation is seasonal with little or no sporulation in hot dry summers. Under GB conditions, the pathogen can produce spores all year round on rhododendron. P. ramorum may be particularly damaging in GB woodlands infested with rhododendron and in historic gardens with susceptible plants, especially in the west or in other parts with favourable microclimates.

P. ramorum has the potential to affect GB heathland environments but is yet to be found there; species of Vaccinium, a common heathland plant, could be at risk as laboratory experiments have shown these to be susceptible and V. myrtillus (bilberry) was recently found infected with another quarantine species (P. kernoviae) in woodland and heathland in Cornwall. Vaccinium ovatum (evergreen huckleberry) is susceptible in forests in North America.

The longer-term risk may increase if additional non-European isolates are introduced or if climatic conditions become more favourable for the pathogen. If the pathogen was not controlled it is not yet clear when or whether the whole of GB would become affected. However, in the absence of existing measures, potential spread into and within the environment is predicted to increase; the scale of environmental damage is uncertain but the maximum development of the epidemic in GB habitats is likely to take decades.

What damage is being caused by Phytophthora ramorum in GB?

In England and Wales, between April 2002 and February 2008, there have been 217 outbreaks at 198 sites in locations other than nurseries, of which 65 have been eradicated, leaving 152 ongoing cases, most of which are subject to containment measures. These woodland or garden/park sites are fairly widely distributed but the highest incidence and severity of disease has been in the south and west of England and in south Wales; these western areas of the country appear more favourable for the disease since they are mild and wet. Although the number of trees that have developed bleeding cankers is relatively low it is increasing; a few trees have also been infected outside of the south west.

Ornamental plants in historic gardens involved in tourism have been badly affected by the pathogen and some rare or historically-important specimens or collections are now considered to be at risk. Visitors to some historic gardens have complained about the appearance of the plants; gardens which rely on spring-flowering rhododendrons and camellias to attract visitors have been most affected.

Nurseries have mainly been affected by the phytosanitary measures that have been implemented to try to prevent spread to the environment. Between April 2002 and January 2008 in England and Wales there were 576 nursery outbreaks at 488 sites of which 464 have been eradicated. Controls are still in place at 112 sites.

Between October 2003 and December 2007, 27 trees have become affected with bleeding cankers in GB. One of the affected trees (the first) was in the south-east, with most of the remainder in the south-west of England. In October 2007 a beech tree was been found with a bleeding canker in a historic garden in West Yorkshire, and around the same time a red oak was diagnosed with the disease in Northern Ireland. All the diseased trees have been close to large numbers of infected Rhododendron.

What was done previously to control P. ramorum?

The previous policy towards the disease was one of containment and eradication, on a precautionary basis whilst more evidence was gathered as to the extent of its likely impact.

Why change from the current position?

After executing this policy since the first confirmations, P. ramorum has continued to spread, albeit slowly and mainly in the southern and western parts of GB. Following a review of the historic and current situation, available scientific and economic evidence and public consultation on options for management of the pathogen in the future Ministers have concluded that more needs to be done to contain and eradicate P. ramorum.

How will the disease be managed in the future?

A new 5 year programme includes research and development, an awareness programme and disease control through funding clearance of host plants in high risk areas. This will involve an increased level of activity, aimed at reducing the level of inoculum to epidemiologically insignificant levels; by removal of infected sporulating hosts in woodlands and the wider environment; combined with enhanced containment and eradication measures in infected gardens and nursery sites, as well as the identification and control of any new outbreaks.

Research has shown that the eradication of rhododendron is the most effective control measure to reduce disease spread in the wider environment (woodland, gardens and parks). Rhododendron ponticum is the principal species of rhododendron growing in the wild in GB. At a selected number of woodland gardens and areas of semi-managed or unmanaged woodland, clearance of all Rhododendron ponticum, whether infected or not, has proved effective at reducing inoculum levels and appears to have prevented further infection of trees on those sites.

Will stakeholders be expected to share responsibility for managing the risk from these diseases?

The management of plant diseases is a shared responsibility. There is a broad range of stakeholders; each stakeholder group should recognise their responsibilities and how they can contribute to successful disease management. Defra and the Forestry Commission recognise the contributions already made by stakeholders in managing these diseases.

Defra and the horticulture industry are engaged in a separate programme of activity on responsibility and cost sharing, which is examining the longer term strategic position.

The government accepts it has a major role in funding the programme of work to manage P. ramorum. But it will seek contributions from other key stakeholders where this is appropriate and will accelerate the programme activity.

Stakeholders will be required to adhere to responsible biosecurity and disease management practices which enhance and support disease control and management.

The longstanding policy of successive Governments is that compensation is not paid when plants have to be destroyed to eradicate or contain outbreaks of plant disease. It is felt that the limited resources of the Plant Health Service are better employed in the detection and identification of outbreaks, and research into risk and risk-management measures. We do keep that policy under review and consider each case on its merits.

What happens when the disease is confirmed on a nursery?

Notice is served on the nursery owner, requiring:

• the destruction of infected plants and all susceptible hosts within 2 metres
• the disinfection of the surface on which containers have been standing or, if soil-grown, sterilisation, or removal and destruction, of soil
• movement restrictions for at least 3 months on all known susceptible plants grown within 10 metres of the infected plants, and any remaining plants from the affected lot
• a prohibition on the use of anti-Phytophthora fungicides on plants under hold in order to prevent fungicidal suppression of symptoms
• the provision of information on the origin of infected material and a distribution list of any plants from the infected lot which have been sold on

The costs of carrying out such activities are normally borne by the person on whom notice is served.

Why do you destroy infected plants?

The threat P. ramorum poses to trees and the environment, and the function of infected plants as a pathway for infection, necessitates a policy of destruction as the only guaranteed means of achieving eradication in a diseased specimen.

What is the legal basis for the measures you are taking?

Following the first finding of P. ramorum in February 2002, emergency measures were introduced in Great Britain. The measures were notified to the EU Standing Committee on Plant Health, which agreed EU-wide emergency measures in November 2002, based largely on GB's action. Those measures are still in place. A review of the EU measures in December 2003 confirmed that they should continue.

The latest relevant EU legislation can be found at:

• COMMISSION DECISION (2007/201/EC) - Amending Decision 2002/757/EC on provisional emergency phytosanitary measures to prevent the introduction into and the spread within the Community of Phytophthora ramorum

The Plant Health (Phytophthora ramorum) (England) Order 2004 came into force on 29 October 2004 and was amended in 2007 by the Plant Health (Phytophthora ramorum) (England) (Amendment) Order 2007.

The latest legislation in England relating to P. ramorum is:

• The Plant Health (Phytophthora ramorum) (England) (Amendment) Order 2007

Similar legislation applies in Scotland, Wales and Northern Ireland.

Some commentators suggest that the trees which have become infected were particularly weak specimens?

Promoters of this point of view have cited heavy metals present in the soil at outbreak locations and have speculated that planted trees, possibly enduring less compatible soil conditions, might be less robust than native specimens in oak woodland. However, we know from laboratory tests that, regardless of site and growth conditions, some trees are intrinsically more susceptible to P. ramorum and P. kernoviae. European beech, Turkey oak and red oak are particularly vulnerable to P. ramorum. In the case of P. kernoviae, beech is more susceptible to infection. In laboratory tests, the pathogen can infect unwounded bark of several tree species. Another key factor is the proximity of trees to infected rhododendrons. Of all the trees which have become diseased, most are only a few metres away from infected rhododendrons and many are actually touching them.

What do you have to say to those employed in the horticulture industry who are directly affected by this disease and the measures you taking are against it?

Our aim is to minimise any adverse impact upon individuals and we appreciate the cooperation and support offered by many affected parties. Fera Plant Health and Seeds Inspectors work closely with nursery, garden centre and historic garden owners to this end.

Our actions against Phytophthora ramorum arise from our grave concerns about the potential impact of the disease on horticulture, biodiversity and the landscape. A major epidemic amongst native trees would have debilitating knock-on effects across many aspects of local economies, especially in relation to revenues from tourism.

Have you undertaken any research into Phytophthora ramorum?

A wide-ranging research and development programme has been implemented by Defra and the Forestry Commission, with contributions from the Scottish Government and the Horticulture Development Council. Areas covered include: analysing the means by which the pathogen spreads; development of diagnostic methods to detect and identify the pathogen; determination of the risk posed to native flora; and disease management strategies. The results of this research are used to underpin policy and to determine the most effective eradication measures. For example, epidemiological investigations have helped us to understand the pathways by which Phytophthora ramorum spreads. Research into diagnostic methods has already generated better detection techniques, including the development of an on-site diagnostic methods that are being used by PHSI and Forestry Commission field staff. Studies into the control of the disease have earmarked disinfectants appropriate for use in decontamination procedures as well as nursery practices that can reduce risk. Similarly, fungicides which a) could be used to protect valuable plants and b) should be prohibited in an eradication situation have both been identified.

There is also a European project to assess the risk posed by P. ramorum which is led by the UK.

A summary of all the UK-funded research on both P. ramorum and P. kernoviae can be found on the Defra website.

Hasn't the disease been present in Great Britain for a long time?

The widespread death of trees in California, and problems with dieback in established Rhododendrons in Germany and the Netherlands, were first recognised in the mid 1990's. The pathogen causing the dieback in Europe was only named as a new species in 2001 and, during that year, it was recognised that the same organism was causing 'Sudden Oak Death' in the USA.

We cannot determine when the pathogen first entered GB, or where it came from, but it is thought to have arrived within the last ten to twenty years. (The initial introductions to Europe and the USA appear to be separate, both of unknown origin). Surveys have not found evidence of widespread infection in the natural environment; over 70% of confirmed cases to date have been on nursery or other retail premises. Genetic analyses support the hypothesis that P. ramorum is a recently introduced pathogen since the genetic diversity of the pathogen's population in the UK and Europe is low.

Some growers insist that the disease is endemic?

Surveys have not found evidence of widespread infection in the natural environment; over 70% of confirmed cases to date have been on nursery or other retail premises.

Forestry Commission surveys of woodland areas conducted between 2004 and 2007 did not find any signs of the disease.

It is true that growers have always known symptoms of dieback on Viburnum and Rhododendron, but these symptoms are not specific to Phytophthora ramorum. Dieback on Viburnum can be caused by Phoma exigua var. viburni, an entirely unrelated fungus. Similarly, other Phytophthora species cause leaf and shoot symptoms on Rhododendron, although they are generally less severe.

If the infection has come from abroad, why haven't you introduced an import ban on susceptible hosts?

We do not know how P. ramorum first entered Europe or where it came from. Plant Health and Seeds inspectors have found a higher proportion of plants of GB origin to be infected than plants of continental origin. We therefore believe that a ban on imports would be a disproportionate response. Import controls have, however, been introduced on all susceptible plants from the USA. Susceptible plants are only eligible for export from the US to the EU if they originate from a pest-free area or from a place of production which has been inspected by the US authorities and found to be free from signs of the disease.

Within the EU, rhododendrons (other than R. simsii), viburnums and (since 2004) camellias can only be moved from nurseries which have been officially inspected and found free from the disease, or where appropriate eradication measures have been taken. Consignments of these plants must be accompanied by plant passports (used already in the EU to manage risks from a number of other plant pests and diseases). This aims to ensure that plants moved in trade are free of the pathogen; if symptoms develop after movement, the infection can be traced back to the originating nursery and follow-up inspections carried out at sites which have received plants from the same batch.

What has been the effect of measures taken so far?

Our records indicate that both the total number of outbreaks, and percentage rates of confirmed infection on nurseries, have fallen year-on-year 2003 to 2007. This is despite the number of inspections carried out having increased significantly.

What should I do if I think I have observed an instance of Phytophthora ramorum?

If you continue to be concerned please contact your local Fera Plant Health and Seeds Inspector.

For suspect symptoms on trees contact the Forestry Commission Plant Health Service on 0131 314 6414.