Fighting the SOD that kills our national tree

 

ANOTHER LIFE: In the heart of Brackloon Wood, outside Westport, there's a sapling of American red oak that lights up in October in shades of crimson and carmine, its leaves glowing in the sun like shards of stained glass. As the wood is cleared of alien trees to restore its native ecosystem, I hope they'll turn a blind - still better, an enraptured - eye to this lone outrider of the Fall, writes Michael Viney

This special tree comes to mind with the fact that, from November 1st, American oaks and their timber from much of California and Oregon will be banned from entry to Ireland, along with a whole range of other trees and shrubs that share their west coast forest ecosystems: beech and maple, laurel and honeysuckle, dogwood and huckleberry (Vaccinium ovatum).

Anything, in fact, that might carry the spores of Phytophthora ramorum, the fungus that has already brought Sudden Oak Death (SOD) to tens of thousands of trees along the Pacific coast.

SOD was first noticed in California in 1995, when oaks began to wilt, their trunks scarred by weeping wounds of stem canker. The pathogen was finally identified as a fungus of the same family that brought potato blight, Phytophthora infestans, to Ireland. Spreading with special speed and vigour in cool and moist weather, it attacks the bark of the oaks, opening the tree to secondary invasion by tunnelling beetles and sapwood fungi. When the damage has girdled the tree, cutting off the flow of food through the conduits beneath the bark, it begins to wither and die.

California's controls to quarantine the outbreaks recall those for foot-and-mouth: closure of campsites, washing of tyres and hikers' boots, a ban on transporting leaves, wood and soil. In Oregon, there have been pyres of sick trees. This is all very distant from the Fall glory of New England, but America's eastern red oaks have been shown to be as potentially susceptible to the disease as California's four oak species.

SOD has already arrived in Germany and the Netherlands, probably in rhododendron and viburnum plants imported from the US. Its discovery this spring in plants in an English garden centre brought Plant Health Orders in the UK and Northern Ireland that bar imports of the whole range of susceptible plants from the US without clearance certificates. Our own Department of Agriculture has followed suit.

In American experience, the pathogen is a forest disease, rarely leaping to isolated oaks in urban parks or back gardens.

But the international trade in plants still seems firmly implicated. Early suspicion fell on rhododendrons grown from stock brought from the Himalayas and China. But it is clear that many more plants carry the fungus while sometimes suffering nothing worse than black spots on the leaves.

Ireland's cool, moist weather would seem to make oakwoods such as Killarney specially vulnerable to Sudden Oak Death: not only rhododendron and honeysuckle but the Californian form of arbutus - Killarney's famous "strawberry tree" - is a carrier of the fungus. The garden centres that receive the department's new import regulations should take them very seriously.

Fungal attacks on plants are, of course, entirely natural, but are often much more destructive than the teeming parasites of animals, birds and fish..

There is something almost predatory in the way that different species of fungi can seem to team up and take over from each other in the killing of a live tree and the breaking down of its dead wood - all part of the great natural merry-go-round of growth, death and recycling.

Dutch elm disease (first identified in Holland in 1919) is caused by a fungus, Ceratocystis ulmi, that is carried around by the elm bark beetle. It is a very old organism - some version of it probably wiped out Ireland's elms 5,100 years ago at the same time as Neolithic farmers were making their clearances in the forests.

The virulent modern strain, which has killed elms by the million on both sides of the Atlantic, seems to have arrived in Britain with beetles in Canadian logs.

Phytophthora ramorum is not the only new killer in its family. In 1993, a lethal stem disease in alders was identified in Britain, where it has since killed many thousands of Alnus glutinosa, most of them growing along the banks of streams and rivers in southern England. The cause is a new hybrid of the fungus Phytophthora ambivora, well-known as a cause of root and stem disease in other broadleaf trees but never recorded on alder. Since then, a destructive epidemic has hit the alders of France, the disease has been found in Austria, Germany, the Netherlands and Sweden, and the EU has begun "concerted action" in research.

Mutations and hybridisations are, again, natural processes, but fungi that would never normally meet, so to speak, are being flung together in a million plant nurseries and garden centres, as well as being subjected to all manner of chemical stresses.

When I look at the - relatively harmless - silver mildew, a fungus that dusts our young oak trees from August until leaf-fall, I remember Anthony Huxley's information that one square centimetre of a mildewed rose leaf can produce half a million spores.

If SOD should reach our shores, our national tree could face a blight that brings its own kind of sorrow.