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Oak decline, with focus on Acute Oak Decline
(AOD)
Sandra Denman, Joan WebberTree Health Forestry & Climate Change Centre
Risks to trees, London 24th January 2013
Oak decline is a complex disorder - typically several damaging agents interact
agents can be abiotic (eg drought) and biotic (eg insects and fungi)
simultaneous &/or sequential
the outcome can be a serious decline in tree health
may be terminal, but trees can also recover
not a new disorder but recently we have come to recognise chronic oak decline and acute oak decline
What is oak decline?
Episodes of oak decline have been documented for almost 100 years
1920s acute dieback of mature woodland oak caused by sequential years of defoliation by Tortrix roller moth caterpillars and mildew, which led to the death of “alarming” numbers of trees and only abated when Tortrix attacked ceased
1950s death of young oaks apparently triggered by drought and exposure combined with defoliation (insects and mildew)
1980-90s death in woodland and parkland oak, possibly associated with root attacking pathogens
Oak Decline over time
Trees with ‘Decline’
• Trees are in less than optimum health and lose vigour • Canopy dieback and /or thinning. By this time the decline
is usually fairly advanced• In some cases bleeds may be evident on tree stems as a
first indication• Tree ring width may be reduced
Types of decline on oakTwo types of oak decline:• Chronic Oak Decline (COD) (slow effect about 10-50 years)
focus on roots• Acute Oak Decline (AOD) (fast effect approximately 3-5
years) focus on above ground parts• Based on rate of decline – process rather than cause• Both can occur together, or one can lead to the other
Acute Oak Decline (AOD)
Acute Oak Decline is the term given to oak trees that develop symptoms over a short time and high levels of mortality occur
AOD can occur in two forms:
Triggered by cycles of foliage destruction weakening the tree
Triggered by bark damage on the tree stem, in which phloem and cambium is destroyed
Foliage type: Key agents are defoliating insects and powdery mildew
Stem type: (Probably) bacteria and other agents including insects
What do we mean by AOD?
Stem Form
Acute Oak Decline – Stem form (1990s?) • Identified by patches of profuse stem weeping• In UK, two key agents associated with disorder - bacteria and
Agrilus biguttatus• Relationship between these agents is not yet fully understood• Their roles in AOD are under investigation • Conditions that make the trees susceptible to invasion by these
organisms is not yet known or researched in Britain
Symptoms of Acute Oak Decline
• Affected trees have symptoms of stem bleeding emanating from small splits in the stem
• Mostly mature trees affected (50+ years old)• Both species of native oak affected (Q. petraea and Q. robur)
Symptoms of AOD
• Close up inspection shows cracks occur between bark plates and are about 5-10cm long
• Bleeding may be active or dried up
Beneath the bleeding point bark necrosis is present, inner bark is broken down and a cavity formed
Symptoms of AOD
Beneath the bleeding point bark necrosis is present, inner bark is broken down and a cavity forms
Symptoms of AOD
Symptoms and players
Agrilus biguttatus
Bacterial lesions
In most instances the galleries of Agrilus biguttatus can be seen in close proximity to the lesions
AOD reports in Britain
1/24/2013
Known distribution of AOD in Britain
Comparison AOD and Agrilus
DDAS
NBN
Agrilus distribution Britain
AOD reports up to end of 2010
Conclusions
• Cause of AOD and impact are not yet resolved.
• Problem is fairly recent. Flagged up in the 1980s-1990s in Britain, but not present prior to 1950s.
• Not caused uniquely by Agrilus which is a native and first reported in 1874.
• Condition is spreading, suggesting microbial involvement. Epidemiological statistics and modelling will give an indication of the nature of the spread.
• Hypothesised that Agrilus and bacteria play roles, but need to be sure we have identified all the players – metagenomics! Testing is difficult (Agrilus are elusive; inducing pathogenicity with bacteria is tricky, also use genomic technology).
• In the interim manage the situation following a precautionary approach (sanitation; firewood; monitoring; informed)
• Mapping and monitoring is essential. It yields information on proportion of affected trees, annual increase, number of deaths and remissions.
Acknowledgements
Funding
• Forestry Commission
• Woodland Heritage
• Rufford
• Memorial Trust
• Woodland Trust
• National Trust
FR Scientific Team
• Susan Kirk• Nathan Brown• Gavin Hunter• Carrie Brady• Andy Jeeves