Oak Trees (Hansard, 22 January 1992)

§ Mr. Curry

Under plant health legislation administered by the Forestry Commission, imports of oak plants and oak wood to Great Britain are strictly controlled to guard against the introduction of new pests and diseases, particularly oak wilt disease from north America. Imports of oak plants are prohibited from areas where such pests or diseases are known to occur, while oak plants from other areas and all oak wood are subject to inspection and have to be free of these pests and diseases.

In addition, the Forestry Commission investigates instances of ill health in oak trees in Britain, and it is currently conducting research into the local decline of oak at several places, mostly in the east midlands.

Over the past decade there have been many reports of death and dieback of oaks—Quercus robur and Q petraea—in various European countries. In Britain only a few such cases had been reported prior to 1988, but in 1989 the 248W Forestry Commission pathology branch received several inquiries about dieback in woodland and parkland oak and initiated an investigation into the problems.

There are about 30 sites in the southern half of England where the problem is known to occur. These are mostly in the east midlands but are also to be found as far apart as Devon and Kent. The trees range from 40 to 200 years old and occur on a range of soil types. About half are in woodlands and the remainder in parkland. Although the onset of symptoms—small leaves, yellow pale green foliage, dieback of branches—mainly dates to 1987–88, growth of many of the affected trees started to decline around 1984–85.

At present a variety of factors appear to be involved in the decline syndrome but the same factors may not be involved on each site. The onset of growth decline in the mid eighties suggests that some event such as the droughts in the summers of 1983 and 1984 or the severe winters of 1984, 1985 and 1986 may have acted as a trigger for the problem. The role of defoliating caterpillars, such as those of the oak leaf roller moth and the winter moth is unclear. Recent years have not been marked by particularly serious outbreaks of these insects. On some sites the trees appear to be suffering from lime-induced chlorosis. Evidence from the investigation of the root systems of trees on three sites indicates that root damage is not a major factor. No evidence of a vascular wilt disease, similar to Dutch elm disease, has been found. Leaves from severely affected trees with conspicuous yellow foliage, have been tested for viruses and related pathogens with negative results.