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Sudden Oak Death (Phytophthora ramorum)
- A potential threat to our trees


  • Phytophthora ramorum is a fungus which has caused the deaths of oak trees in epidemic proportions throughout California and Oregon, USA.
  • P. ramorum is also known to affect a range of other native plants.
  • The disease has been found in parts of Europe causing dieback in rhododendrons and viburnums, and may threaten other species.
  • Symptoms encountered on rhododendrons include brown/black discolouration of shoots, aerial dieback and leaf spotting.
  • Symptoms encountered on viburnums include a spread of the disease from the stem base upwards resulting in wilting and dieback.
  • 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.

Phytophthora ramorum is a fungal pathogen that causes damage to a range of native plants in California (USA) and the death of oak trees. Deaths as a result of Phytophthora ramorum have reached epidemic proportions within coast live oak (Quercus agrifolia), Californian black oak ( Q. kellogii) interior live oak (Q. parvula) and tanoak ( Lithocarpous densiflorus).

P. ramorum has also been found to cause damage to a wide range of Amercian native plants including: California huckleberry (Vaccinium ovatum); mandrone (Arbutus menziesii); Californian bay laurel (Umbellularia californica); Californian honeysuckle (Lonicera hispidula); Californian buckeye (Aesculus calfornica); toyon (Heteromeles arbutifolia); big leaf maple (Acer macrophyllum); manzanita ( arctostaphylos manzanita); and Californian coffeeberry (Rhamnus californica).

Why the concern?

Although P. ramorum has not been found on oaks in Europe, in parts of Europe it has been found on rhododendron and viburnum causing dieback. P. ramorum could threaten other species as in California, USA.

Although research is in progress, species susceptibility of European trees has not been fully determined. Preliminary results indicate that the European oak and chestnut may have a greater resilience than the American oaks. However, there is concern that other broad-leaved species, such as European beech might be susceptible initiating tests on other potential tree and ornamental hosts.


<< Dying oaks in California

What are the symptoms?

American Oak trees

Bark Bleeding

The most consistent and diagnostic symptom of P. ramorum on larger oak trees in California is the development of cankers that have brown to black discoloured outer bark on the lower trunk and seep a dark red sap commonly called 'bleeding cankers' or 'tarry spots'.

  • The cankers often occur on the lower portion of the trunk.
  • Lesions may also occur up to 20 metres up the stem.
  • Upon removal of the outer bark, 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 the sudden death of the tree, characterised by a rapid change in the colour of foliage.
  • Cankers do not extend below the soil line and do not damage the roots.
  • Other plants native to California

    Although the symptoms are quite variable, they are usually associated with leaf spotting and branch dieback.

    On Rhododendron

    • Twig and leaf blight. Symptoms are similar to those caused by other Phytophthora spp on rhododendron, however in the case of P. ramorum the development of symptoms is more rapid.
    • Affected twigs develop a brown to black discoloration that spreads along the twig. The symptom can move into the leaves.
    • Dark brown blotches may be seen on infected leaves.
    • Leaf infection may occur without twig infection.
    • Roots are unaffected
    << Rhododendron - shoot canker

    On Viburnum

    • Infection begins at the stem.
    • Spreading upward the disease causes wilting and ultimately death

    << Viburnum - wilting and dieback

    Where is it found?

    P. ramorum is found to be widespread in coastal California, and is under eradication in Oregon, USA. Up until April 2002 there has been localised findings of the fungus on rhododendron plants in the Netherlands, Germany and the UK. In each case eradication measures were been taken.

    How does it develop?

    Knowledge of the development of the disease is limited, as P. ramorum has only been recently described. Infection begins in the outer bark and progresses toward the cambium and eventually through to the xylem. Regarded as a cool temperate organism, P. ramorum with an optimum growth temperature of 20oC. (Minimum growth temperature = 2 o C; Maximum growth temperature = 26-30 oC).

    How does it spread?

    P. ramorum may spread locally by:

    • rain splash
    • wind-driven rain
    • irrigation water
    • also, possibly aerially or by insects.

    The movement of contaminated plant material may facilitate long distance transport of the disease via:

    • Diseased wood
    • Growing media
    • Analogy with other Phytpphthora tree pathogens

    The disease may also be transported through soil that may be carried on vehicles, machinery, footwear or livestock.

    What is being done in the UK?

    To check for the presence of the organism, the Plant Health and Seeds Inspectorate are conducting an extensive survey. Wherever the pathogen is found eradication measures, including the destruction of affected plants and the tracing of related stocks, are being taken.

    Although no evidence of P. ramorum has yet been found, the Forestry Commission is also investigating oak trees that are showing symptoms of Oak Dieback.

    Research to investigate P. ramorum is also being commissioned by DEFRA and the Forestry Commission.

    What should you do?

    Phytophtora rmorum is a notifiable pathogen that is not yet established within the UK. The aim is to avoid its entry and establishment within the UK ecology.

    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

    See also:

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