Article by Harry Harrington. All images and information courtesy of Defra. For more information and updates visit www.defra.gov.uk/planth/oak.htm

Information correct at the time of writing (28/02/04)

In the 1970's, over 30 million English Elms were wiped put in the UK by Dutch Elm Disease. Entire areas of countryside were changed forever as the Elm population was devastated by a small beetle. Nearly 30 years later, the Elm population in the UK has yet to recover.

 

With Sudden Oak Death there is even more at stake; over 200 million Oaks are at risk from an organism called Phytophthora ramorum.

Worse still, it has now been discovered that P. ramorum also infects and kills Beech trees, threatening a further 100 million trees. 30 million Horse Chestnuts and Sweet Chestnuts have also been found to be at threat.

In the mid-1990's, large numbers of Tan Oaks were found to be dying in California. At first thought to be as a result of drought, mortality rose to epidemic proportions and forests became scarred by swathes of browning trees. Closer scrutiny revealed that the bark of many of the trees was infected with large bleeding cankers. In some coastal areas over 80% of Oaks were affected.

By 2000, Phytophthora ("plant destroyer") ramorum, a previously unknown pathogen, was found to be the cause. Related to the blight that caused the potato famine in Ireland, P ramorum was also found to have infected Rhododendron and Viburnum in Germany and the Netherlands.

By April 2002, P. ramorum had been found on a Rhododendron in the UK. Since then there have been more than 300 cases, mostly on Rhododendrons but also Camellia and Viburnum.

 

The difficulty in fighting Sudden Oak Death is that it is the source of many unknowns. First of these is where it came from in the first place. Contrary to popular belief, the UK strain is probably not from the US; the strain that has ravaged forests in California and Oregon is different than the one that has emerged in Europe.

It is thought that the two strains originated from one common pathogen, possibly in South East Asia. At some time, possibly as long as a decade ago, an infected plant (probably a Rhododendron), arrived in the UK and began spreading the disease.

As a water borne disease, it has now been found that P ramorum is capable of permeating and infecting healthy bark and not just weak or damaged trees with open wounds as originally thought.

Left: Bleeding Canker and Right: Discolouration of dead cambium

Once the spores have made their way under the bark, the fungus spreads like a cancer. Externally the tree develops cankers which bleed sap and this disintegrates the bark and cambium tissue. As the canker spreads, it girdles the entire trunk and the tree is unable to support itself. No-one is certain how quickly this takes but it is thought the time period is around a year from first infection.

At present there is no known cure. The only recourse is to chop down and burn infected trees in an effort to halt the spread of infection to other trees.

 

How to Spot Phytophthora ramorum

In the UK the disease has been found on Red Oak, Turkey Oak, Holm Oak, Horse Chestnut, Sweet Chestnut and Beech. Some conifers such as Sitka Spruce and Douglas Fir may also be susceptible.

Rhododendron and Viburnum are thought to be the main hosts but it has also been found on Camellia, Hamamelis, Leucothoe spp, Pieris, Syringa and Ornamental Yews.

On Rhododendron, P. ramorum causes twig and leaf blight. Twigs develop dark discolouration that can move into the leaves. Symptoms can also include blackening of the leaf stalk, base and tip. Leaf infection can also occur without twig infection. Roots are unaffected.

On Viburnum, infection begins at the stem base, causing wilting and eventually death. It may also cause similar symptoms to those found on Rhododendron.

On Pieris, the pathogen causes brown stem lesions that lead to aeriel dieback and leaf blackening. On Camellia, Syringa and Leucthoe, the pathogen causes leaf blight with large brown to black lesions typically occurring at the tip or edges of leaves. On Yew (Taxus) it causes needle-blight affecting young foliage to dieback.

Implications to Bonsai

There is no reason to think that Phytophthora ramorum will react any differently with Bonsai. With increased cases of  P. ramorum now being found in the UK and the discovery of infections in species that are often used as Bonsai, it is worth keeping a close eye for signs of infection on your Bonsai, garden shrubs and trees. In particular on Rhododendron (Azalea), Quercus/ Oaks, Taxus/ Yew and Fagus/ Beech.

If an infection is suspected, quarantine the tree from the rest of your collection and do not use the same tools on both (suspected) infected trees and healthy trees. If P. ramorum is confirmed, the tree cannot be saved and should be disposed of by burning. Delay increases the possibility of spreading infection.

Oaks, Beech and Yew are often collected in the UK as yamadori and these should be quarantined until such time that the absence of P. ramorum can be confirmed.

An over-reaction? Not if 10, 20 or more years of a bonsai collection is at risk from one infected tree.