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Phytophthora ramorum in larch trees - Update

Many readers will be aware of the significance of the recent infection of larch trees by Phytophthora ramorum.  In the following article bringing the national situation up to date, we summarise the developments and current situation, and look ahead to the implications for British forestry.

30 July 2012

Phytophthora ramorum (P. ramorum) was first discovered in Great Britain in 2002 infecting a viburnum plant in a garden centre in West Sussex. This discovery resulted from surveillance for the pathogen in England and Wales by the Plant Health & Seeds Inspectorate (PHSI), which had begun in 2001 in response to the identified risk posed by this Phytophthora species, which was then unknown in Britain.

It has since been found infecting a wide range of plants and trees in all four countries of the United Kingdom and the Isle of Man. Until recently P. ramorum was mostly found affecting shrubs such as rhododendron, camellia and viburnum, and only a limited number of trees. Since late 2008 the environmentally important bilberry plant (known as blaeberry in Scotland and winberry in Wales) has also become infected on a few sites. Globally (Europe and North America), P. ramorum is known to have more than 150 host species.

It is impossible to ascertain when P. ramorum entered Britain before the 2002 detection, or where it came from. However, contrary to some views, research has shown that it did not arrive here from the USA, where different forms of the pathogen occur. In fact four different genetic lineages have been identified – two present in North America (NA1 and NA2) and two in Europe 1 and 2 (EU1 and EU2). Both of the European lineages are known to be present in the UK. (See press release). The evidence suggests that P. ramorum is native to another part of the world, possibly Asia. Other European countries (Germany and The Netherlands) are now known to have had findings of the pathogen (as a then unknown Phytophthora) on shrubs dating back to 1993, but these are also likely to have been introduced.

Until 2009, very few trees of any species had been affected in the UK, and in most cases they were growing close to infected “wild” rhododendron, a plant that is abundant in some woodlands and which produces P. ramorum spores from infected foliage. Rhododendron ponticum was, and still is, a cause for concern because it is one of the hosts which produce these infective spores, it is invasive, and it is common in woodland. Under the five-year, Defra-funded, Phytophthora Disease Management Programme led by the Food & Environment Research Agency (Fera), which started in April 2009, great efforts have been made to remove this species and other sporulating hosts from affected woodlands and other affected areas. These efforts particularly target areas known to be at high risk from P. ramorum, building on action taken against P. ramorum since 2002.

In 2009 P. ramorum was unexpectedly found infecting and killing large numbers of Japanese larch trees (Larix kaempferi) in south-west England. This was the first time it had been found causing lethal infection (in the form of stem cankers) on a commercially important conifer species anywhere in the world. Only a few ornamental conifers had previously been found affected in Britain, The Netherlands and France before 2009, and these were all different types of yew, and were all located in nurseries. In addition, a single coastal redwood was also found infected in a garden. In the western states of the USA lethal ramorum infection had only been observed on a non-commercial conifer species, Pacific yew, while non-lethal infection is not uncommon on other conifer species in the USA, such as Douglas fir, grand fir and coastal redwood. In Britain we have also found Douglas fir to be a host but, unlike the situation in the USA where the impact has been minor, a number of younger Douglas fir trees of about 5 – 10 years of age have been killed in south-west England.

P. ramorum also occasionally infects European larch (Larix decidua) and hybrid larch (Larix x eurolepis). It has only once been confirmed on Sitka spruce (Picea sitchensis), another commercially important conifer species widely grown in the British Isles. In this instance, the finding was made in the Republic of Ireland and consisted of a small tree growing under a heavily infected Rhododendron bush, and was therefore exposed to heavy inoculum pressure. However, Sitka spruce had previously been shown in laboratory trials to be moderately susceptible to P. ramorum. See "P. ramorum on Sitka spruce" below for further advice.

The major impact on trees in the USA has been focused on broadleaf species. The American strain of P. ramorum has killed millions of North American native oak and tan oak trees, giving rise to the common name of “sudden oak death”. However, that name is a misnomer in the UK. Our two native species of oak, sessile and pedunculate oak (Quercus petraea and Q. robur), have been little affected in the wild, with only a tiny number of cases recorded. Laboratory tests have also shown them to be more resistant than their American cousins. The preferred generic term in the UK for infection by P. ramorum is therefore “Ramorum disease”.

The discovery of P. ramorum-related mortality of Japanese larch in England was followed in 2010 by similar developments in Wales and one small site in western Scotland, as well as in Northern Ireland, The Isle of Man and the Republic of Ireland. Further outbreaks were identified in 2011 in the Peak District of Derbyshire in central England, Lancashire and Cumbria in North West England, and the island of Mull in western Scotland, and Galloway in South-West Scotland.

The number and area of outbreaks identified in 2011 were significantly lower than those recorded in the previous two years, but to date in 2012, further significant areas of infection have been confirmed in Wales and South West Scotland, with a lower incidence of new infection in England.

Current knowledge

P. ramorum is a fungus-like pathogen, one of about 100 in the Phytophthora genus. It can infect a wide range of trees and other plants, but it does not sporulate (produce reproductive spores) on all the species it infects. One of the reasons why the infection of Japanese larch is of such concern is that it sporulates particularly heavily on Japanese larch needles – much more so than on rhododendron. This means it releases very high levels of inoculum (spores) into the environment, and these are capable of being borne by the wind over considerable distances, especially from tall, mature trees. Apart from infecting Japanese larch needles, it can also infect and kill mature Japanese larch bark, leading to the outward signs of branch dieback and crown death.

Entire trees can be killed by “girdling” as the pathogen invades and destroys the cambium layer, which is the regenerative layer of tissue immediately under the bark. The pathogen appears to be able to kill trees within one growing season after its presence is first detectable, although it might have been present in the tree for some time before that. However, compared with other tree diseases, this is very fast acting.

Phytophthora species in general - and P. ramorum is no exception - thrive in damp environments, which is partly why larch trees only on the milder, moister, western side of Britain have been found affected so far. It spreads easily in moisture and water, so moist air currents, mists, fog and watercourses all provide the potential for movement. It can also be spread as contaminating spores by animals and possibly also by birds, as well as on footwear, vehicle wheels, and machinery and equipment used in forests or other habitats where it is present. This means we cannot afford to be complacent by presuming it will stay confined to larch trees in the west.

It has so far not been confirmed on hybrid larch, but we are closely monitoring and researching this species for any change in this situation.

There is no evidence that it harms Japanese larch’s timber quality, and the pathogen does not appear to be capable of being spread from the timber sawn from infected trees. However, the bark can harbour P. ramorum without producing spores, and it can also transport infected needles and soil as a contaminant trapped in its fissures, and must be destroyed or treated in ways that kill the pathogen.

Maps and other information about all sites found with P. ramorum infection in England and Wales are available on the Fera website at www.fera.defra.gov.uk .

Symptoms

There are two main forms of symptom of P. ramorum infection of Japanese larch:

  • wilted, withered shoot tips with blackened needles, with the infected shoots shedding their needles prematurely; and
  • bleeding cankers exuding resin on the upper trunk and branches.

The foliar symptoms (symptoms on the needles) are not readily detectable during the winter, when larch trees, unlike most other conifers, have shed their needles. However, cankers can be visible all year round, although the resin bleeding becomes less visible once it dries and hardens.

Diagnosis

Diagnosis uses a combination of:

  • visual inspection by trained observers;
  • field tests of symptomatic bark and needles with test kits known as lateral flow devices (LFDs), which indicate Phytophthora infection to genus level; and
  • laboratory tests of bark and foliage samples to isolate the pathogen and confirm its species.

Laboratory tests to confirm the presence of P. ramorum in larch bark and foliage present a number of technical challenges, and return a conclusive result in only about one-third of all symptomatic material sent in for testing, so we cannot rely solely on laboratory tests.

Sometimes other factors can also point to P. ramorum, even when a laboratory test result is inconclusive. Such other factors might, for example, be findings of P. ramorum in larch litter from the forest floor, or infection on Rhododendron ponticum in the under-storey of the forest in adjacent woodland. If the results of these diagnostic methods point to P. ramorum, it will be treated as a confirmed case.

If, however, laboratory analysis confirms the presence of another causal agent, the site will not be classed as infected, but will be kept under surveillance.

Treatment

No cure has been found, so the objective of any control approach must be to prevent or minimise any further spread of P. ramorum and the damage it causes. The best available scientific advice is to remove and kill the living plant tissue on which the organism depends for reproduction. In the case of infected larch, this means affected trees should be felled or otherwise killed as quickly as possible after detection of the disease and before the next spring or autumn period of sporulation begins on the needles.

Action

The Forestry Commission has suffered extensive infection within its own Japanese larch woodlands, and is undertaking felling on all of its infected sites. We are also serving statutory Plant Health Notices on other woodland owners requiring their infected trees to be felled.

We are ensuring that private-sector interests are addressed and taken into consideration as we develop our strategy for dealing with this threat, through representation on our Outbreak Management Team from the Confederation of Forest Industries (ConFor) and the UK Forest Products Association (UKFPA), and through meetings with key sector representatives. We recognise the burden that this is placing on some forest owners and industry representatives, and want to ensure that, as far as possible, we minimise this.

We have also defined three risk zones in Great Britain, based on the risk of pathogen spread in each from ‘high’ in Risk Zone 1 to ‘low’ in Risk Zone 3. A map showing the risk zones is available in the ‘Advice for Owners and Agents’ section of the information at  www.forestry.gov.uk/pramorum. The boundaries of the risk zones are kept under review as the situation develops.

A system has been established for recording the rate of spread of the pathogen based on notifications and aerial survey, and regular updates of the map showing sites can be found at the “Outbreak map” link on our website.

Legal basis

P. ramorum is a quarantine organism listed under EU emergency measures (Commission Decision 2002/757/EC, as amended), and its presence or suspected presence on plants of any sort in Great Britain must be notified to the relevant authority (the Forestry Commission in the case of forest and woodland). National implementing legislation1 gives Forestry Commission plant health inspectors powers to require the remedial actions that, on the best available scientific advice, are deemed to be appropriate and proportionate to the risk posed by the pathogen. This also enables the Forestry Commission to help ensure the Government discharges its obligations under the EU emergency measures to take action to prevent the spread of the disease.

1   The Plant Health (Forestry) (Phytophthora ramorum) (Great Britain) Order 2004 (SI 2004/3213), as amended by SI 2007/3450. The Plant Health (Forestry) Order 2005 (SI 2005/2517), as amended by SI 2006/2696, SI 2008/644, SI 2009/594 and SI 2009/3020

Owners’ obligations

Anyone who suspects the presence of P. ramorum on trees anywhere in Great Britain (England, Scotland and Wales) must immediately notify the Forestry Commission at the following contact points:

or to the Tree Health Diagnostic & Advisory Service by:

The contact details for reporting suspected P. ramorum on plants other than trees are:

  • ENGLAND & WALES - Anyone suspecting the presence of P. ramorum on plants other than trees in England and Wales should immediately contact their local Fera Plant Health and Seeds Inspector or:
    Tel: 01904 465625
    Email: planthealth.info@fera.gsi.gov.uk 

 

 

  • NORTHERN IRELAND - Anyone suspecting the presence of P. ramorum on any plants, including trees, in Northern Ireland must contact the DARDNI helpline:
    Tel: 028 9052 4999
    Web: www.dardni.gov.uk

Statutory Notices which require tree felling or other action for disease management purposes have the force of law, and the Forestry Commission may initiate proceedings against anyone who fails to comply without reasonable excuse. Plant health inspectors may also visit premises where they have reason to suspect that a quarantine organism is present.

Although we have power to do this without permission (except in the case of a private dwelling, where a warrant is required), we will always try to work co-operatively with owners or occupiers. We have published our internal guidance to plant health inspectorson our website so that those whose properties might need to be inspected can find out what to expect.

The vast majority of owners have recognised the necessity of rapid tree felling, and have willingly complied with Plant Health Notices, or have begun taking steps towards compliance. However, any delays in taking action will increase the risk of spread and threaten the livelihoods of other owners and staff of woodlands, nurseries, retail plant outlets and ornamental gardens. We encourage anyone who is experiencing difficulty in complying to contact us to discuss possible ways forward.

Winter felling licence moratorium

It is difficult to detect P. ramorum infection in larch during the winter, when the trees have no needles and the symptoms are much less obvious. Although cankers on the trunks and branches can still be visible, they can be difficult to spot, and result in too many false negatives to be a practicable means of identification.

This means we have to wait until the spring flush of new foliage before we can see whether P. ramorum is infecting any new woodland areas. We want to avoid a situation where we unknowingly permit infected larch to be felled and enter the supply chain, circumventing the biosecurity precautions designed to prevent pathogen spread during timber movements. We are therefore not issuing any new felling licences for larch crops, or crops with a component of larch, until the spring in Risk Zone 1 if their disease status is unknown. Inspections and processing of new felling licence applications for larch will resumed in the spring. With new foliage on the trees in spring, it is easier to distinguish between uninfected stands, which can be felled and marketed in the usual way, and infected stands, to which the biosecurity measures must apply.

Financial assistance

Two grants have been set up to support the felling of infected larch in England and Wales. The first pays towards the cost of employing a professional advisor to plan the work required by a Plant Health Notice, and the second helps cover the costs of felling young trees less than 26 years of age, where there is no possibility of selling the timber. 

Both grants have been extended until March 31, 2012. For further information see the “Support for Owners and Managers” advice under www.forestry.gov.uk/pramorum.

Enhanced Forestry Commission restocking (replanting) grants are also available in England to help  owners with the costs of re-establishing trees on sites that have been cleared after P. ramorum infection.

There are similar packages available in Scotland.

Processing grant applications

Landowners in south-west England have been seriously affected, and Forestry Commission woodland officers have more than doubled their engagement with the private sector there. Officers are working alongside the private sector, helping owners and their agents to identify the disease, take samples for diagnosis and execute timely felling. This work has been managed on top of a sustained growth in demand for grants from the Forestry Commission’s England Woodland Grant Scheme.

Timber market

The prospect of a greater than usual volume of larch logs entering the timber market has depressed prices. Well known for its ability to perform well as fencing, larch (known as a ‘redwood’ by the industry because of its high resin content) can and has readily substituted for spruce logs (known as a ‘whitewood’ because of its lower resin content) in the sawmill yard. Larch is also well suited to decking and other garden timber products, and higher-quality larch timber can penetrate markets for exterior building cladding.

However, the challenge is greater for utilising large-diameter logs, because the UK construction industry has little previous experience of using redwood in carcassing for, for example, structural framework in building construction. Acoustic testing of larch in the woodland demonstrates that the strength properties of larch logs are second to none. However, if ‘red’ logs are to find a niche in traditional ‘white’-log markets, other factors such as reliability, consistency, and sustainability of supply have to be guaranteed. 

In conjunction with the private sector, we are supporting some rapid work into additional grading standards for larch to open up new market opportunities in the construction sector. This should enable the larger volumes of material coming on to the market to find higher-value outlets.

We are also aiming to minimise the additional volume of timber that is being harvested in our forests, largely by substituting infected timber for uninfected material that had been due for harvest, and deferring harvesting of the uninfected trees. In Wales in particular we are working to market material from infected stands within current sales plans and through existing contracts as much as possible. Material that is not possible to market through existing contracts will be offered to the open market from spring 2011 in a planned and phased way designed to minimise market distortion as much as possible.

We have also established a Great Britain marketing group, with representation from private grower organisations and the processing industry, to look at all aspects of the supply chain in relation to current and future development of timber flows resulting from the outbreak.

Timber movement authorisation system

It is important that we prevent the pathogen being transmitted through the movement of roundwood (logs) from infected stands. We are therefore requiring a number of biosecurity measures to be taken, such as sweeping off vehicles and machinery between movements, and destroying or treating bark, which can harbour infective spores. This means that timber hauliers and timber processing companies must be authorised to handle material from infected stands.

In order to gain movement or processing approval the operator must demonstrate to a Forestry Commission plant health inspector’s satisfaction that they have put in place, and are carrying out, the measures required. Approved hauliers and processing plants will be inspected from time to time to ensure that the measures are working. We will withdraw approval from operators found failing to comply with the terms of their approvals.

Full details and application forms for approvals are available for downloading from the www.forestry.gov.uk/pramorum area of the Commission’s website.

Replanting/restocking

The choice of ‘successor’ tree species to plant on a felled site previously infected with P. ramorum is one for woodland owners and managers to make. However, we are recommending at present that affected sites are not restocked with any species of larch or other known susceptible species for at least five years. This is because the pathogen can remain viable in the soil for some time after clearance of infected plants, including trees, from the site. Owners might therefore wish to consider other, non-susceptible species. More-detailed advice is available in the document “Guidance on replanting affected sites”, also available from the 'Advice to owners, agents and industry' page.

P. ramorum on Sitka spruce

In March 2011 the Forest Service of the Irish Department of Agriculture, Fisheries & Food reported the first finding of a Sitka spruce (Picea sitchensis) in the wild infected with Phytophthora ramorum. Sitka spruce is the most widely grown timber-producing tree species in the United Kingdom.

However, we do not believe the finding indicates that the threat from P. ramorum to Sitka spruce crops in Britain has increased, as Dr Joan Webber, principal plant pathologist at Forest Research, advises:

“This was a small tree growing beneath the canopy of a large, infected Rhododendron ponticum shrub. No other infected Sitka spruce have been found in the vicinity.

“Its infection is consistent with our experience with several other conifer species in the UK. High ‘inoculum pressure’ (i.e. large quantities of infective spores) are more likely to lead to trees in the vicinity becoming infected and showing disease symptoms, especially if they are very close to a heavily sporulating plant, which is what happened in this case.

“Previous laboratory test findings by Forest Research scientists indicate that Sitka spruce is moderately susceptible to P. ramorum infection if it is wound inoculated, but intact bark is usually effective at preventing infection.

“Over the past year a small number of spruce samples had already been collected from survey sites in Britain and sent for diagnosis, and all have been found to be negative. 

“Therefore, given the close association of the infected tree with a heavily infected rhododendron shrub, and the lack of confirmed cases on Sitka spruce in Britain, we do not believe the finding indicates that the threat from P. ramorum to Sitka spruce crops in Britain has increased.

“We continue to urge constant vigilance by all woodland owners for signs of disease in their trees. The key to getting on top of ramorum disease is to detect and remove sporulating plants as quickly as possible, because this limits the risk to other plants and prevents the spread of the pathogen.”

The Irish authorities are supplying further information on this case to us as it becomes available. This information will be passed on to our surveyors and scientists to assist with the detection and diagnosis of this disease on Sitka spruce.

Future research

We and our partners in Fera will continue to carry out research into this pathogen, or commission it from other expert bodies, to gain as full an understanding of it as possible. Research will focus on its behaviour, means of spread, evolution, the further risks it poses, and its management. This will enable us to draft sound advice and guidance on its management and means of preventing or limiting its impact and spread.

One particular piece of research we have commissioned is looking into the possibility that migratory birds can provide a pathway for spreading P. ramorum.

The prospects

About 3000 hectares of larch trees in infected or suspected infected stands have been felled or are to be felled in the UK. Following the emergence of  new needles on larch in the spring, we can be clearer about the progress of the disease, and how much more effort might be required. Overall, the number and area of new sites of infection were down in 2011 on the previous two years, and most were close to or contiguous with previously known sites. Wales and Scotland appear to be suffering the highest incidence of new infection in 2012. Continued felling of infected trees in time to further limit sporulation in the autumn is essential if we are to get to grips with this disease.

Infection of Japanese larch by P. ramorum ranks alongside Red Band Needle Blight of pine in terms of the number of trees it is affecting and its potential significance to the forestry sector. The pathogen has behaved unexpectedly by infecting Japanese larch, meaning the situation demands a lot of time and attention. Other species of tree have become infected for the first time as a result of the outbreaks in Japanese larch and, based upon experimental work, there are more forestry species which could be at risk from P. ramorum.

The future remains unclear, given the ability of the pathogen to spread to previously unaffected species, and the challenges of dealing with outbreaks of this magnitude in forest situations. However, as an industry, we might be reaching a point when it is no longer sensible to plant Japanese larch in some parts of Britain. Vigilance by all in the forestry and woodland sector is essential, and forest managers should regularly inspect their woodlands for signs of declining condition and poor tree health, and immediately report any concerns to us.

Our research effort and work with partners in government, Europe and globally will continue as we strive to ensure that the best scientific expertise is being brought to bear on this issue. However, until such time as a cure or other treatment can be found, felling infected trees remains the most practical course of action.

Keeping up to date

Up-to-date information is available from the Forestry Commission website’s pages dedicated to P. ramorum topics at www.forestry.gov.uk/pramorum.

It is also possible to subscribe to receive Forestry Commission press releases about tree pests and diseases directly by email soon after they are issued. Go to www.forestry.gov.uk/subscriptions and follow the on-screen instructions.

An archive of previous press releases about P. ramorum is available by going to www.forestry.gov.uk/news and using the search facility on the keyword ‘ramorum’.

We also "tweet" significant pest and disease developments. To receive "tweets" from us, go to @treepestnews at twitter.com.

More information about the Phytophthora Disease Management Programme and P. ramorum in rhododendron can be found on Fera’s website at www.fera.defra.gov.uk/plants/plantHealth/pestsDiseases/phytophthora/pRamorum/.

Finally, this situation report will be updated as new developments occur. Suggestions for updates and corrections can be made to:

Charlton Clark
Senior Communications Officer
Forestry Commission
E-mail: charlton.clark@forestry.gsi.gov.uk

Last updated: 11/26/2012
Bark sampling - Phytophthora ramorum