Epidemiology of acute oak decline in Great Britain

Abstract

Oak has long been affected by episodes of decline, an interaction of multiple factors that reduces host vigour. A wide range of abiotic, insect and pathogenic agents may play causal roles in an outbreak; acting independently, together, or consecutively in a complex process.

In Britain a novel form of Acute Oak Decline (AOD) is increasingly reported. Typified by symptoms of stem “bleeding”, bark-splits that weep dark exudate, the stem symptoms overlay patches of necrotic cambial tissue concealed beneath the bark. An initial investigation by Forest Research identified a suite of bacterial species as putative causal agents. Signs of insect activity are frequently found in association with the stem lesions. Galleries are usually present in the conductive tissue and some of the affected trees have distinctively D-shaped exit holes created by the buprestid beetle Agrilus biguttatus. Although declines are typified by complex causes, the dominance of these two organisms (bacteria and beetle) makes understanding their interaction an important step in defining the epidemiology of the syndrome.

The current study aims to provide initial investigations into the epidemiology of AOD. By mapping and monitoring trees within eight study sites symptom development was accurately recorded from a reliable baseline. These data give insights into disease development in terms of rate of spread, tree mortality and disease distribution. Repeated monitoring at multiple time points documents the progression of symptoms, within and among trees.

Site monitoring is complemented by preliminary studies investigating the role of the buprestid beetle Agrilus biguttatus in the decline complex. By developing trapping methods populations of Agrilus beetles can be monitored in terms of flight period and species composition. The focus on A. biguttatus seeks to confirm whether the larvae present in galleries are A. biguttatus and to investigate the link between beetle, larvae and bacteria.