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Sudden Oak Death Phytophthora ramorum

What is it and why the concern?

The disease has reached epidemic proportions along the Californian coast resulting in death of coast live oak (Quercus agrifolia), Californian black oak (Q. kellogii), interior live oak (Q. parvula var. shrevei) and tanoak (Lithocarpus densiflorus). P. ramorum has also been found causing damage to a wide range of other native Californian plants (e.g. Californian huckleberry (Vaccinium ovatum), madrone (Arbutus menziesii), Californian bay laurel (Umbellularia californica), Californian honeysuckle (Lonicera hispidula), Californian buckeye (Aesculus californica), toyon (Heteromeles arbutifolia), big leaf maple (Acer macrophyllum), manzanita (Arctostaphylos manzanita), and Californian coffeeberry (Rhamnus californica)).

The susceptibility of European tree species has not been determined fully but research is in progress. Preliminary results, which require confirmation, indicate that European oaks and chestnut may be more resistant than American oaks.

Phytophthora ramorum is a serious fungal pathogen causing death of oaks and damage to a range of native plants in California, USA.

In Europe it has not been reported on oaks. However in parts of Europe it has been found causing dieback of Rhododendron and Viburnum and could threaten other species.

There are indications that American and European isolates of P. ramorum have some genetic differences. For example, the fungus requires two 'opposite' mating types (A1 and A2) to produce long-lived resting sexual spores (oospores). Currently, only the A2 mating type is known in the USA, whereas only the A1 mating type in known in Europe.

What are its symptoms?

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, but lesions may also occur up to 20 metres up the stem. 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.

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. Leaf infection can also occur without twig infection. Infected leaves show dark brown blotches. Roots are unaffected. Symptoms are similar to those caused by other Phytophthora spp, on Rhododendron, but the development of symptoms is more rapid.

On Viburnum, infection begins at the stem base and spreads upwards causing wilting and ultimately death.

Where is it found?

P. ramorum is widespread in coastal California, USA and is under eradication in Oregon, USA. By April 2002 there had been localised findings of the fungus in the Netherlands, Germany, Poland and UK, but eradication measures had been taken in each case.

What is being done in the UK?

The Plant Health and Seeds Inspectorate are carrying out an extensive survey to check for the presence of the organism. Eradication measures are being taken whenever the pathogen is found. These measures include destruction of affected plants, and tracing of related stocks. The Forestry Commission is also investigating oak trees which are showing symptoms of the phenomenon known as Oak Dieback, but no evidence of P.ramorum has been found. DEFRA and the Forestry Commission are commissioning research to investigate P.ramorum.

What should you do?

Phytophthora ramorum is a notifiable pathogen. It is not established in the UK and the aim is to avoid its entry and establishment in the UK.

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

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