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