Acute Oak Decline and the Decline Disease Spiral Model: the State of the Game in the UK

In 1981 US forest pathologist Paul Manion published an account of his Decline Disease Spiral Model to help understanding of decline diseases in trees (Manion 1981). The spiral begins with predisposing factors such as climatic change, pollution, or genetic lack of fitness that cause the tree to be stressed. These enable low levels of infection by pathogens that further challenge the tree’s defenses, facilitating attack and injury by insects or other animals. Those injuries may provide a route for invasion further into the deeper plant tissues, resulting in serious infection and death at the end of the spiral of decline. A particular characteristic of this model is that no single causative agent can be held responsible for the progress and final result of the disease.

British researchers have been using this model to provide insights into the epidemiology of acute oak decline (AOD), an emerging disease of pedunculate oak (Quercus robur) and sessile oak (Q. petraea) that was first observed in England in the late 20th century. Thousands of trees have been infected to date and in some cases death will occur within six years of the onset of symptoms. Affected trees display vertical cracks between bark plates, from which a black fluid oozes and runs down the trunk. The tissue below these bleed sites becomes necrotic (Forest Research 2025a).

The key predisposition factor for AOD appears to be soil moisture deficit—the disease is currently restricted mainly to the low rainfall areas of south-east England, the Midlands and the Welsh borders—but relatively high temperatures and atmospheric nitrogen pollution from vehicle exhausts and/or intensive agriculture are also probably implicated.

Four species of bacteria have been isolated from stem lesions (Gathercol et al. 2021), of which a newly described species, Brenneria goodwinii, has the potential to cause tissue necrosis and is the most damaging member of the quartet. Research to date has shown that this species can be isolated from leaf surfaces of both healthy and infected trees and at sites both affected and unaffected by AOD (Action Oak 2023). This suggests, tentatively, that B. goodwinii may be primarily a relatively non-pathogenic component of the oak’s above-ground external microbiome until such time that it can breach the external defenses and make contact with the below-bark tissue.

Many affected trees have been found to harbor populations of the native two-spotted oak buprestid, also known as the oak jewel beetle (Agrilus biguttatus). They have a preference for stressed and declining oaks. They lay eggs in bark crevices and the emerging larvae tunnel further into the tree and create extensive galleries in the inner bark and cambium. A serious infestation can cause the death of the tree, irrespective of any bacterial infection.

Although further research is required and is under way, it appears likely that larvae carry AOD-related bacteria through the bark and into the oak’s vascular tissue, facilitating infection and the resulting necrosis. It’s known that B. goodwinii bacteria can survive in the gut and ovipositor of the beetle, which produces chemical compounds that render the bacteria more pathogenic. Adult beetles emerging in the spring fly into the canopies of neighboring oaks to feed on fresh leaves, possibly carrying those bacteria with them and contributing to the spread of AOD.

In the UK, A. biguttatus is currently confined to central and southern England, because relatively warm summer temperatures are required to complete the development of the larvae, so this might be thought to be a uniquely English problem. However, climate change could lead to range expansion, both in the UK and on the continent: the beetle is currently found in much of Europe, as far north as southern Sweden, in Morocco and Algeria in North Africa, and in Turkey, Syria, and Iran in the Middle East. Moreover, AOD-associated bacteria have been isolated from Holm oak (Q.ilex) and Pyrenean oak (Q. pyrenaica) in Spain, and from chestnut-leaved oak (Q. castaneifolia) and Brant’s oak (Q. brantii) in Iran.

And the disease is not confined to these six oak species. Others that are known to have been infected include downy oak (Q. pubescens), oriental white oak (Quercus aliena var. acuteserrata), Turkey oak (Q. cerris), cork oak (Q. suber), pin oak (Q. palustris), red oak (Q. rubra), scarlet oak (Q. coccinea) and water oak (Q. nigra).

AOD has some prominence because it has attracted considerable research attention in Britain, but it’s only one of many oak decline diseases. There have been frequent episodes of a chronic oak decline (COD) in Britain and Europe (Forest Research 2025b); Central European oaks are at risk from both bacterial leaf streak and drippy nut disease (Tkaczyk 2023); Iranian oak declines have been linked to a diverse range of abiotic factors, pathogens and insects (Jamali and Haack 2024); and populations of many oak species in North and Central America have been linked to various decline syndromes. Despite the bewildering range of causative factors, they all have in common a predisposing stressor, fungal, bacterial or viral pathogens, and animal vectors.

These oak declines will continue to be a major worry for woodland and forest managers, with no obvious remedies. For the custodian of specimen oaks there is more hope, as the vector-pathogen chain of events will not be initiated if the predisposing stress can be eliminated. In many cases that may point towards a regime of regular mulching.

Works cited

Action Oak. 2023. Action Oak Report 2022-2023. [link]

Forest Research. 2025a. “Tools and Resources: Acute oak decline.” Forest Research [link]

Forest Research. 2025b. “Tools and Resources: Chronic oak decline / dieback.” Forest Research [link]

Gathercole, L.A., G. Nocchi, N. Brown, T.L. Coker, W.J. Plumb, J.J. Stocks, R.A. Nichols, S. Denman, and R.J. Buggs. 2021. Evidence for the Widespread Occurrence of Bacteria Implicated in Acute Oak Decline from Incidental Genetic Sampling. Forests 12(12): 1683 [link]

Jamali, S., and R.A. Haack. 2024. From Glory to Decline: Uncovering Causes of Oak Decline in Iran. Forest Pathology 54(5): e12898 [link]

Manion, P. 1981. Tree Disease Concepts. Englewood Cliffs: Prentice Hall.

Tkaczyk, M. 2023. Worldwide review of bacterial diseases of oaks (Quercus sp.) and their potential threat to trees in Central Europe. Forestry: An International Journal of Forest Research 96(4): 425–433. [link]