Oak and Horse Chestnut Trees (Hansard, 25 October 2001)

§ Mr. Morley

We have collected information about two diseases that affect oak trees, neither of which is thought to pose a significant risk to trees in Britain.

The first is known as Oak Dieback. The Forestry Commission has been investigating this over the last decade, and a similar decline in the health of oak trees has been reported from many other countries in Europe. The cause remains unclear but our experts consider that a strain of fungus may be thriving as a result of climate change. Drought and insect defoliation are also thought to be playing a part. The species affected is the Common oak (Quercus robur) while our native, the Sessile oak (Q. petraea), appears to be immune.

The second is known as Sudden Oak Death. This new disease, recently named Phytophthora ramorum, was identified in parts of California last year. The principal host is Tanoak (Lithocarpus densiflorus) although some species of true oak (Quercus spp.), which do not occur in Europe, and certain other plants including rhododendron and viburnum have also been affected. Similar symptoms were recorded on rhododendron and viburnum plants in Germany and the Netherlands as long ago as 1993 although it was only earlier this year that it was established that P. ramorum was the causal agent. The Forestry Commission and DEFRA are working together to establish whether any British oak trees have been affected or are, indeed, even susceptible to this disease. Early indications are that European oaks may not be susceptible. Current quarantine controls on plants exported from the USA, aimed at other pests and diseases, are considered sufficient to provide protection against accidental import of the disease. This disease and controls against it are being considered by the EU Standing Committee on Plant Health.

There are no specific pests or diseases affecting horse chestnut trees in Britain, although we are monitoring the progress of the Horse Chestnut Leaf Miner (Cameraria ohridella) in continental Europe. This pest was first detected in Austria in 1989 and has since spread to many parts of Central and Eastern Europe. It is only found in leaves and leaf litter, and is dispersed when leaves are moved to new areas. Unfortunately, cars, lorries and trains often carry leaves long distances, and this is probably how it has spread across Europe. Leaves are, of course, also dispersed by the wind. Attacks are not fatal and trees may survive repeated attacks by successive generations of larvae. Our scientists are closely involved in research into control methods. The Forestry Commission is also planning to publish an illustrated leaflet describing the biology and symptoms of infestation of the moth next spring, to coincide with the period of the moth's activity.