"Sudden Oak Death"

Introduction: Phytophthora ramorum,
 a potential threat to our trees, woodland and heathland

Subsequent surveys have shown it to be present in at least nine European countries, including the UK. and in 2003 it has occurred a few hundred instances in Great Britain, mostly in nursery plants.
In November and December 2003 we had the first confirmations that the disease had struck trees growing in the UK. A Southern Red Oak, Beech,  Horse  Chestnut  and  Holm  Oak were affected.
We are still hoping that the English Oak will be resistant to the fungus, but it is early days yet.

We aim to bring you an overview on this page of the Defra and FC information so far available. 
You can either click on the links above (under 'Content') or just scroll through the page.

An  intensive  survey  programme  of retail outlets, nurseries, gardens and other  established planting areas has been underway since the first finding was in Britain was confirmed in April 2002.  Precautionary eradication and containment action  will continue, and surveillance will now be increased to reduce the risk of spread of this pathogen into the wider natural environment.
Buyers of host species – mainly rhododendrons, camellias and viburnum – are advised  to check plants before purchase for symptoms of the disease and to contact their garden centres if these subsequently develop.

Emergency measures against the introduction and spread of P. ramorum were  introduced  throughout  the  EU  in November 2002, replacing the UK's emergency  legislation  put  in place in May 2002. The EU will be reviewing its measures in December 2003, and will be studying survey reports from all member states.

The Forestry Commission is undertaking a survey of 1,000 woodland sites across Britain, due to be completed by the end of March, as part of efforts to determine the scale of the problem.

Return to Contents

What is it and why the concern?

Phytophthora ramorum is a serious fungal pathogen causing death of oaks and damage to a range of native plants in California, Oregon and Washington, USA and British Columbia, Canada . The disease, commonly known as 'sudden oak death', has reached epidemic proportions along the Californian coast. Death of tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), Californian black oak (Q. kellogii), interior live oak (Q. parvula var. shrevei) has occurred. P. ramorum has also been found to be damaging to a wide range of other native Californian plants causing 'ramorum dieback' (e.g. madrone (Arbutus sp.)) and 'ramorum leaf blight' (e.g. bay laurel (Umbellularia sp.)). In addition, the pathogen causes a needle and shoot dieback of Douglas fir (Pseudotsuga menziesii) and coastal redwood (Sequoia sempervirens).

Confirmation of three new findings of Phytophthora ramorum in tree species (December 2003) has raised concerns about the recent spread of the disease to trees in Britain. Phytophthora ramorum is the pathogen causing death of oak trees on the west coast of America.  The  new  cases,  at  two locations in the south west of England, have been found  in  beech,  horse  chestnut  and  holm  oak  and are associated with previous adjacent findings of the disease in rhododendron. This follows the pattern set by the first confirmation of Phytophthora ramorum in a southern red  oak  in  Sussex  earlier  in November. All the sites are being closely monitored.
On 2 Feb.o4 it was confirmed that a further six trees in Cornwall have the fungus. In another worrying development, officials revealed at the same time that rhododendrons and another tree at a site in Cornwall have been infected with a previously unknown strain of the phytophthora fungus. It is thought that at each of the three sites in Cornwall – two very close to one another – where trees have been infected, the disease has spread from nearby rhododendrons. The first indications, and it is early days, are that it is at least as aggressive if not more so than phytophtora ramorum. There is a lot of work being done to make sure it hasn’t been recorded elsewhere in the world but our scientists are sure they have not found anything like it.

It has been found causing twig and leaf dieback, principally on rhododendron and viburnum. It has also been found on Camellia, Kalmia, Leucothoe, Pieris, Syringa (lilac), and Taxus (yew) in the UK and on Vaccinium vitis-idaea and Arbutus unedo (strawberry tree) in other parts of Europe. The pathogen may also pose a threat to heathland, as it is causes damage to wild plants of Vaccinium and Arctostaphylos in California. The susceptibility of European tree species has not been determined fully, but preliminary results indicate that native European oaks may be more resistant than American oaks. Research suggests that at greatest risk are red oak (Quercus rubra), Turkey oak (Q. cerris), holm oak (Q. ilex), European beech (Fagus sylvatica), sweet chestnut (Castanea sativa), Sitka spruce (Picea sitchensis), Douglas fir (Pseudotsuga menziesii) and Lawson's cypress (Chamaecyparis lawsoniana).

Return to Contents

What are its symptoms?

Forestry Commission advice on diagnosis:
Sudden Oak Death is often characterised by the presence of dark red to black sap oozing from the trunk (‘bleeding cankers’ or ‘tarry spots’) (Fig. 3). This often occurs on the lower portion of the trunk, but may also occur several metres up the stem. Sunken or flattened cankers may occur beneath the bleeding areas. When outer bark is removed from the bleeding cankers, mottled areas of necrotic, dead and discoloured inner-bark tissue may be seen  (NB. This can be confused with the normal oxidative reddening of the phloem tissues). Black ‘zone lines’ are often present within and around the edges of the necrotic areas. On young or thinner barked trees, a distinct edge between necrotic and healthy tissues may be visible. Wood boring beetles such as Agrilus spp., Ambrosia beetles, or bark beetles such as Scolytus spp. may attack the wounded areas, sometimes leaving visible frass associated with entry wounds.
Foliage: Girdling necroses often result in sudden death of the tree. As a result, foliage may change colour rapidly and uniformly throughout the crown, with the leaves still remaining on the branches after death.
Spread and distribution: In California spread is probably aerial. It is therefore often without a clear pattern. Dying trees may be either widely scattered or in broadening patches (Fig.1), but their distribution does not usually follow any specific site factor.

Other advice mostly from Defra:

• On oaks in California the most consistent and diagnostic symptom of the disease on larger trees is the development of cankers that have brown to black discoloured outer bark on the lower trunk and that seep dark-red sap (commonly called 'bleeding cankers' or 'tarry spots'). These cankers often occur on the lower portion of the trunk. When the outer bark is removed mottled areas of necrotic, dead and discoloured inner-bark tissue with black 'zone lines' around the edges may be seen. Wounded areas may become colonised by beetles. Damage to the trunk often results in sudden death of the tree resulting in a rapid change in the colour of the foliage. Cankers do not extend below the soil line and do not damage the roots. P. ramorum may infect all aerial parts of tanoaks and death is usually rapid when infection occurs on the main stem. Whilst for Quercus species, only basal cankers are seen and although death may be rapid it can be much slower taking several years to kill trees or the tree may even recover.
• On conifers in California, the pathogen causes a needle blight and dieback of young shoots of Douglas fir and coastal redwood.
• On other native plants in California the symptoms are quite variable but are usually associated with leaf spotting and branch dieback.
• On rhododendron, P. ramorum causes a twig and leaf blight. Affected twigs develop a brown to black discoloration that spreads along the twig and can move into the leaves. Characteristic symptoms include blackening of the petiole (leaf stalk), leaf base and leaf tip and may extend along the mid-rib. Twig cankers can lead to wilting of shoots; in such cases leaves remain attached but may not develop any spots. Leaf infection can also occur without twig infection. Roots are unaffected. Symptoms are similar to those caused by other Phytophthora spp. on rhododendron, but the development of symptoms is more rapid in P. ramorum.
• On viburnum, infection commonly begins at the stem base causing wilting and ultimately death. It may also cause leaf symptoms similar to those on rhododendron.
• On Pieris and Vaccinium, the pathogen causes brown stem lesions that lead to aerial dieback and leaf symptoms as described on rhododendron.
• On Camellia, Kalmia, Syringa (lilac) and Leucothoe, the pathogen causes leaf blight with large brown to black lesions typically occurring at the tip or edges of the leaves.
• On Taxus (yew), P. ramorum causes a needle-blight affecting young foliage leading to an aerial dieback.

Return to Contents

Similar Symptoms

British oaks are already subject to a widespread but local mortality and dieback of complex cause, known as ‘oak decline’. In some cases, this is associated with infection by other, root-infecting Phytophthoras. It also involves recurrent droughts, root disease fungi such as Collybia, recurrent insect defoliation (e.g. by Tortrix spp.), and scale insect attacks. Oak decline symptoms could therefore be confused with those of Sudden Oak Death. However Q. robur and Q. petraea trees with oak decline do not usually exhibit bleeding cankers of the main stem. Occasionally, declining trees exhibit dark, watery fluxes from bark cracks. These tend to run freely down the tree. Decline mainly affects mature oaks; and the severity of foliar symptoms often increases gradually, affected trees becoming ‘stagheaded’. Oak decline also tends to be in pockets, with the longer dead and dying trees in the centre. However, scattered individual trees exhibiting decline symptoms are common, especially in open situations.

Disease Status: Since it is not yet known whether European oaks are susceptible to the Sudden Oak Death Phytophthora or whether roots of European oaks could become infected in addition to their aerial parts, caution is needed in interpreting symptoms that could indicate Sudden Oak Death.

Return to Contents

Where is it found?

P. ramorum is widespread in coastal California, USA and is under eradication in Oregon, USA. It has also been found on ornamental plants in nurseries in Washington State, USA and British Columbia in Canada. In Europe, it has been reported from several countries, principally from container-grown rhododendron and viburnum plants in nurseries and garden centres but also on a few occasions on bushes in public gardens. Currently, there are no reports of the pathogen outside Europe or North America.

Phytophthora ramorum has now been found at more than 211 sites in England and Wales (November 2003). This total includes some on established plants growing outside in soil.
The first finding in the UK was recorded in April 2002.  Rhododendron and Viburnum are still the main hosts, but it has also been found on Camellia, Pieris, Kalmia, Syringa and, inJune 2003 in 10 Taxus baccata  (yew) trees in a Lancashire nursery.

The first case on a tree in the UK was a  bleeding canker on a 100-year-old Southern Red Oak in Sussex in early November 2003.Infected rhododendrons on the same estate could be to blame.

Corfirmation followed in early Dec 2003 of the diseases at  two locations in the south west of England, where it was found  in  beech, horse chestnut and holm oak and are associated with previous adjacent findings of the disease in rhododendron. In early February a third site was found in Cornwall.

Return to Contents

How does it develop and spread?

As P. ramorum has only recently been described our knowledge of disease development and spread is limited. The fungus is regarded as a cool-temperate organism with an optimum temperature for growth of around 20°C; but has a minimum temperature for growth of 2°C and a maximum of 26-30°C. The fungus may be spread locally by rain splash, wind-driven rain or irrigation water. Long distance spread may be by movement of contaminated plant material (including diseased wood), growing media, and probably in soil carried on vehicles, machinery, footwear or animals.

Two different types of asexual 'spores' called sporangia and chlamydospores are formed. In the USA, these are produced abundantly on leaves of susceptible under-storey hosts (but not directly on canker surfaces of Quercus and Lithocarpus). The pathogen may also form long-lived resting sexual spores (oospores) if the two 'opposite' mating types (A1 and A2) are present. Until recently, only the A2 mating type was known in the USA and the A1 mating type in Europe. However, a single isolate of the A2 mating type has now been found in Belgium and the A1 mating type in an Oregon nursery although sexual spores have not been found in nature. Research has demonstrated that American and European isolates of the pathogen are the same species but have some differences. There are also differences in the growth rate of American and European isolates as well as their aggressiveness in laboratory studies when assessing tree susceptibility.

Tell-tale signs are a dark sap, red to black in colour, oozing from the trunk. When the outer bark is removed mottled areas of dead and discoloured tissue may be seen.

When the first Southern Red Oak tree in Sussex was spotted by Defra Health official s, the following appeared in the The Times: "The Phytophthora microbe, an alga-like a fungus, thrives in moist conditions. Its spores can be carried in water or in mud on shoes or vehicle tyres and may travel for short distances through the air.

Roddie Burgess, head of plant health at the Forestry Commission, said the likely cause of spread was rain splash or irrigation water. It was also possible that the bark beetle could be a carrier, as could a bird, wild mammal or even a person. He confirmed, however, that there were diseased rhododendrons in the vicinity. "

Return to Contents

List of known and suspected hosts

Known Hosts In forest and urban plantings in the USA:

• - stems of Quercus species and related genera, particularly:

  • tanoak (Lithocarpus densiflorus)

  • coast live oak (Q. agrifolia)

  • black oak (Q. kellogii)

  • interior live oak (Q. parvula var shrevei)

  • canyon live oak (Q. chrysolepsis)

• - small branches and leaves of

  • madrone (Arbutus sp.)

  • bay laurel (Umbellularia sp.)

• - needles and shoots of

  • Douglas fir (Pseudotsuga menziesii) (Fig. 2)

  • coastal redwood (Sequoia sempervirens).

• Other forest trees affected in the USA include

  • big leaf maple (Acer macrophyllum) and

  • Californian buckeye (Aesculus californica).

• Numerous understorey shrubs are also hosts:

  • evergreen huckleberry (Vaccinium ovatum),

  • coffeeberry (Rhamnus californica)

  • cascara (Rhamnus purshiana)

  • salmonberry (Rubusspectabalis)

  • poison oak (Rhus diversiloba) and

  • beaked hazlenut (Corylus cornuta).

• In both the USA and Europe 

  • stems and leaves of Rhododendron spp.

• In Europe -

  • stem bases of Viburnum spp. areaffected.

  • It has also been occasionally recorded on Pieris and Camellia spp. (UK)

  • Vaccinium vitis-idaea (Poland - interceptions only).

Possible Additional Hosts In UK and Europe

• oak (Quercus) spp

• sweet chestnut (Castanea)

• beech (Fagus)

• other Fagaceae

• Ericaceae

• Lauraceae.

Return to Contents

Some Legal info

Emergency legislation against the threat was introduced in England and Wales in May and was replaced by legislation implementing EC-wide emergency measures on 1st November.

Under the EC measures, as under the emergency national measures susceptible plants will only be eligible for export from the US to the EC if they originate from a pest free area or from a place of production which has been inspected by the US authorities and found free from signs of the disease. The list of susceptible plants, which is slightly different from that in the national measures, is as follows: Acer macrophyllum, Aesculus californica, Arbutus menziesii, Arctostaphylos spp., Heteromeles arbutifolia, Lithocarpus densiflorus, Lonicera hispidula, Quercus spp., Rhamnus californica, Rhododendron spp. other than R simsii (pot azaleas - otherwise known as Azalea indica), Umbellularia californica, Vaccinium ovatum and Viburnum spp. This list is based on the plants which have been found to be infected in the US. The EC legislation requires all member states to carry out surveys for the presence of the disease.

The obligation in the emergency national measures to notify movements of these plants has been replaced by an extension of the plant passport regime to cover Viburnum and Rhododendron, except Rhododendron simsii. The arrangements will operate in a similar way to those which already apply to host plants of fireblight disease, for example. All deliveries to commercial growers, retailers and landscapers will have to be accompanied by a plant passport issued by growers who are registered and authorised for the purpose.

Registration will be free with no time limit. Authorisation to issue passports will be granted annually on the basis of an official inspection of the plants during the growing season and a check on record keeping. These official inspections are chargeable at the standard PHSI rate of £81 per hour on site.

Phytophthora ramona is a notifiable disease (please see below).
If results are positive any known host within 10 metres of infected plants have to be destroyed and there is no compensation available.
Sterilising etc of tools used on infected plants and footwear worn in the infected area should be carried out as Phytophthora pathogens can be passed on from plant material and in soil.

Please keep yourself informed if this disease could affects your land, because further measures and legislation may be introduced depending on developments in the nearby future.

Return to Contents

What to do if you suspect this disease on your premises

If you suspect the presence of this disease on your premises you should immediately contact 

• Your local DEFRA  Plant Health and Seeds Inspector 
  or the PHSI HQ, York (Tel: 01904 455174, Fax: 01904 455197 Email:planthealth.info@defra.gsi.gov.uk ).
  Website: www.defra.gov.uk/planth/ph.htm

• FC Pathology Disease and Diagnosis Service
  Tel: 01420 23000. Fax: 01420 23653

• FC Plant Health Service
  Tel: 0131 314 6414, Fax: 0131 314 6148

Return to Contents

A few illustrations

Oak bark bleeding from Phytophthora ramorum	Phytophthora ramorum - Camelia  Leaf blight
Phytophthora ramorum - Rhododendron  Shoot Canker & Leaf blight	Phytophthora ramorum - Douglas Fir Needle and Shoot Dieback
Phytophthora ramorum - Viburnum Wilting and Dieback	Phytophthora ramorum - Yew Needle Blight & Young Shoot Dieback
Mottled Necotic Inner Bark  © Forestry Commision	Sudden Browning of foliage © Forestry Commision

Return to Contents