Fomes root rot in Thetford Forest, East Anglia: Past, present and future

Abstract

Fomes root rot caused by the pathogenic wood-rotting basidiomycete, Heterobasidion annosum, has caused much mortality in the pine plantations of Thetford Forest, East Anglia, which were first established in the period between the two World Wars. This paper presents an evaluation of over 50 years of research into the disease, and includes much previously unpublished data. Special emphasis is given to the position of Corsican pine, Pinus nigra ssp. laricio, as this is now the principal tree used in the plantations. Freshly cut stumps of both Corsican and Scots pine (P. sylvestris) are equally susceptible to colonization by air-borne basidiospores of H. annosum and, in both species, this colonization can be effectively prevented by treatment with spores of Phlebiopsis gigantea. It is well known that on first-rotation sites, a high soil pH favours the development of disease from thinning stumps. Data from one experiment show that on a site with a pH of 7.5-8.0, 1095 trees per hectare had been killed by the time the crop of Scots pine was 36 years old; representing 25 per cent of the original planting. Losses tend to fall as the crop reaches the end of the rotation. There are some data to show that pure Corsican pine is less vulnerable than Scots pine to disease developing after thinning. When a new crop of pine is planted on a site carrying H. annosum-colonized stumps, losses averaging 30 per cent can occur during the first 10 years on soils with a pH >6.0. Even on soils with a pH <6.0, losses average nearly 20 per cent. At this early age, Corsican pine is as susceptible to the disease as Scots pine. A series of experiments has shown that it is only through stump removal that adequate control can be achieved, and for 30 years, this process has been standard practice as the first-rotation crops have been progressively felled. Experiments with a wide range of trees on different soil types have shown that very few species are resistant both to killing and to buttrot caused by H. annosum. Those that have performed best - most notably Picea omorika, Abies grandis and Fagus sylvatica - have major limitations for use in the forest. Nevertheless it is concluded that, while the justification for using Corsican pine as the principal species is well founded, there are reasons for making some continued use of other species, most notably Scots pine, on certain sites. A continued commitment to good quality stump treatment is required in order to minimize H. annosum problems in future rotations.