An Analysis of Peridermium Stem Rust of Scots Pine in Thetford Forest in 1984 and 1985

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Abstract
The status of stem rust ( Peridermium pini or (Pers.) Lev.) in Scots pine ( Pinus sylvestris L.) in the central block of Thetford forest, East Anglia was determined by means of detailed analysis of freshly felled trees. A total of 935 trees was examined, these comprising c. 100 from each of nine compartments planted between 1922 and 1933. Lesions in all stages of development were found; from those in their first year of sporulation on young twigs to those which had been established for many years on the trunk and main branches. Death of cambium occurred typically on lesions on shoots that were more than five years of age. Downward growth of the fungus was estimated to average between 4.7 and 5.6 cm per year, when based on the length of sporulating tissue on young twigs, and 6.3 cm per year when based on cambial killing in trunk lesions. The average rate of circumferential killing of trunk lesions averaged 4.3 crn per year (i.e. 2.2 cm in each direction). There was no evidence that wounds provided a significant infection court. A strong association was found between the presence of the ‘purple mould’ ( Tuberculina maxima ) and an absence of sporulation by the rust fungus. However, there was no evidence for lesion inactivation by T. maxima . Data are presented on the distribution of disease between sites and between trees. Five per cent of the total sample of trees were dead and an additional 12 per cent had crown symptoms associated with girdling trunk lesions. Among the 775 trees with predominantly healthy crowns, it is estimated that 9 per cent would have died within 5 years, 15 per cent within 10 years and 22 per cent within 20 years. By contrast 66 per cent had no disease at all, probably for genetic reasons. It is concluded that, although the disease situation is serious, there is no requirement for a major change to be made in the felling programme for the pre-war Scots pine in Thetford.

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