Update on Pests and
Diseases of Trees
Joan Webber, Principal Pathologist
Tree Health
Forestry & Climate Change Centre
AA Annual Conference, Reading 4th September 2012
Defra-FC Tree Health Action Plan
Arrival of damaging pests and
pathogens in Britain
Weather impacts – pests and
diseases
• Cold February with frosts and snowfall
• Third warmest March since records
began
• Wettest April for over 100 years
• Twice the usual amount of rain in June
• Summer fourth wettest since records
began in 1727
• September warmer than average?
Impact of weather
Cristulariella on sycamore
Oak mildew
Prolonged
surface wetness
on leaves
Flooding or
water-logging of
soil
Alternating rain
and sun
Anthracnose, plane
Scab on Sorbus
Verticillium wilt
Recent tree disorders

Oak processionary moth
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Acute oak decline (AOD)
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Thaumetopoea processionea
Single agent disorder?
Phytophthora lateralis

Lawson cypress, outbreaks apparently
geographically scattered
Oak Processionary
Moth
• In 2006, oak
processionary moth,
Thaumetopoea
processionea was found
at several London
locations
• It is a major defoliator
of oak in Europe
• Caterpillars feed on the
foliage of many species
of oaks, including
English, sessile and
Turkey oak
• Arrived on 4-8 m tall
imported oak for
street landscape
plantings
OPM caterpillars carry thousands of tiny irritating
hairs that are dislodged on contact. They pose a
considerable hazard to
public health
0.1 mm
Ssssssspp
Kitzingen, Germany
sssssss
June 2010
Affected Quercus species
Nests
2007
Nests
2008
Nests
2009
Nests
2010*
Nests
2011
708
508
2450
2176
4410
Quercus species affected
Q. cerris
% of total
144
11
Q. robur v. fastigiata
2
0.2
Q. frainetto
1
0.1
Q. x hispanica
1
0.1
Q. x robur
6
0.5
Q. ilex
9
0.7
Q. petraea
4
0.3
1141
87.2
1
0.1
Q. robur
Q. turnerii
sssssss
Infested trees
** 44 OPM nests also found at Pangbourne, Berkshire, in August
2010 (arising from a separate introduction)
Hhhh
OPM outbreak
area in
London,
2006–2011
• on average,
the population
has spread at
a rate of 0.9 km
per year
2008
2010
• the outbreak in
London now
covers an area
of 99 km2
2011
5 km
sssssss
Pppppp
Controlling OPM is difficult
and costly
ppp
Pppppp
ppp
Acute Oak Decline (AOD)


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
Increase in reports of oak
dieback from 2002
2006 & 2007, first reports
from Charnwood Forest,
Leicestershire
Hatchlands (NT), Guildford,
many trees affected in 2007
Charnwood reported rapid
tree mortality: 2 trees in
2004, many by 2007/8
Reports of oak mortality in
Spain with similar
symptoms
What is it?




Acute Oak
is the
term given to oak trees
Acute
OakDecline
Decline
(AOD)
that develop symptoms over a short time and
high levels of mortality occur
In Britain we catagorise two types of AOD:
 Foliage
 Stem
Foliage type: Key agents are defoliating insects
and powdery mildew
Stem type: (Probably) bacteria and other agents
including insects
Symptoms
Type of AOD





A new episode of acute oak decline is taking
place in Britain – stem attacking
Affected trees are identified by symptoms of
extensive stem bleeding, and rapid decline
and death (3-5 years)
Sometimes trees also recover
At this stage organisms causing the ‘stem
type’ of AOD probably include bacteria –
newly described genera and species, as well
as other insects, root health, drought
Distribution – mainly midlands and south east
Reports of AOD – 2006 and 2012
2006
2011
Symptoms and players
Bacterial lesions
Agrilus biguttatus
Role of insect pests and Agrilus



Evidence of a key role for bacteria is
growing, and now unravelling how the
decline works
Insects are likely to play a role in tree
mortality, possibly also transmission of
the bacteria
The insect commonly associated with
symptomatic trees is Agrilus biguttatus


D-shaped
holes
Our results so far show there
is a significant co-occurrence
of AOD symptoms with exit
holes of Agrilus biguttatus
Why now!
Photos by Gyorgy Csoka, Hungary Forest Research Institute; LouisMichel Nageleisen,Département de la Santé des Forêts - France.
Another Phytophthora on the block
• Phytophthora lateralis
• Infects Lawson cypress
• Previously known in the USA,
origins in Taiwan
• Recent outbreaks in France &
Netherlands
• Found in Scotland in November
2010
• Now 6+ outbreak areas known
(mainly 30 mile radius Glasgow)
• Mature trees killed
• Two infected sites in England
(2011) and one in Wales (2012)
• England outbreaks: one in
Devon, other in Yorkshire
New outbreak in Wales
• Since 2008 FR part of a
collaboration to find origin of
Phytophthoras such as P.
ramorum
• Discovered P. lateralis in Taiwan
in 2008, foliar pathogen in 2010
• Origin probably Taiwan/ Japan
• With IT/US/Tai/F partners
looking at genetic differences
between US- TaiwaneseEuropean populations
• Potential for hybridisation with
P. ramorum
• Pathogenicity, some host testing
to determine any wider risk
Photo: C. Robin INRA, Bordeaux
More recent arrivals

Chestnut blight


Ash dieback


Cryphonectria parasitica
Chalara fraxinea (= Hymenoscyphus
pseudoalbidus)
Asian Longhorn Beetle (ALB)

Anaplophora glabripennis
Chestnut blight






Causal agent – fungus
Cryphonectria parasitica
Quarantine pathogen that has
long been regarded as high
risk
Now known to be native to
eastern Asia, it was
accidentally introduced into
the USA more than a 100
years ago and caused the
demise of the American
chestnut (4 billion trees)
It can also infect and
sporulate on some species of
oak in the USA
First seen in Italy in 1938
Only countries free of disease
NL, EI and UK
Spread of chestnut blight in Europe
Robin and Heiniger (2001)
Symptoms
Images
courtesy D.
Rigling
Potential for spread of chestnut blight in GB?


Cryphonectria parasitica – good
controls in place to prevent
spread in wood (debarking/
inspection)
But…

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Spread of disease almost
throughout all EU MS
Demand for plants for nut
production, more imports
Lack of awareness about the
risks
Apparently a long latent period
before infection shows up
Three outbreaks in recent
plantings (5 or less years)
Centralised distribution
Evidence suggests not
widespread
Prognosis?



Not all doom and gloom!
In Europe the disease is largely
managed through a natural
biological control
Hypovirulence

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
Virus which debilitates the
pathogen C. parasitica
Spreads from hypovirulent (virus
infected genotypes, to virulent
genotypes, converting them to
less pathogenic forms
Providing there is not too much
variation in the C. parasitica
population, these viruses will
spread and take effect
With hypovirulence, cankers go
into remission and ‘heal’, dieback
is arrested
Spread of Chalara fraxinea in Europe

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
Symptoms of ash dieback first documented in eastern
Europe in 1990s
In 2006, first formal publication that a new pathogen was
involved
Origins unknown
Kiristits (2011)
Chalara fraxinea symptoms
Dieback so severe in some
countries, 60-90% of trees are
affected in some locations
eg Poland, Denmark, Lithuania
Chalara fraxinea symptoms
• Not to be confused with the ash
dieback that was first characterised
in Britain in the 1980s (Pawsey; Hull
& Gibbs)
• Confusingly, also known as
Hymenoscyphus pseudoalbidus
• Currently only found in nurseries or
recently planted areas
• One recent outplanting in Scotland
(3-4 years); one in England
(2011/12 winter)
• Fraxinus excelsior apparently highly
susceptible, F. nigra and F.
angustifolia slightly less so, F.
americana much less
• Sporulates on fallen leaves
Currently many questions

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Spread beyond recent plantings in the UK?
Host range? Is it only the genus Fraxinus?
Where has its come from? Its closest relative is a
considered a native (both in GB and wider Europe)
and is a non-damaging agent coloniser of fallen
ash leaves
Natural spread - how far, how fast?
Resistant genotypes in the ash population?
Assessment of the risk posed by this pathogen has
been undertaken by Forest Research pathologists
and is now available for consultation via Fera
website – have your say
Asian Longhorn Beetle
• Affects a wide range of broadleaf tree
species
• Typically arrives in untreated packaging
• Now epidemic in parts of USA, causing
damage valued at $3.5 billion each year
• With its ‘cousin’ CLB, starting to have an
impact in some European countries
Asian Longhorn Beetle Outbreak in
Kent – March 2012
ALB eradication: March – August
Most frequently affected trees:
sycamore, poplar
and willow
More than 100 trees infested, and
many larvae and pupae
End note

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Challenges ahead!
Control and eradication can also be difficult
and expensive
Invasive pests and pathogens need ongoing
management, but don’t ignore the old foes

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Dutch elm disease, horse chestnut bleeding
canker and leaf miner are still there and still
important
Vigilance and your help to counter new
introductions
Better awareness about changes in the health
of our trees
Acknowledgements

Forest Research, Centre for
Forestry & Climate Change

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
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Sandra Denman (oak decline): Alice Holt
Nigel Straw, Dave Williams (OPM, ALB, horse
chestnut leaf miner, ALB): Alice Holt
Joan Webber, Steven Hendry, Gavin Hunter (ash
dieback and chestnut blight); Alice Holt and NRS
Sarah Green (Phytophthora lateralis); NRS
Pest & Disease Diagnostic Advisory Service
 Christine Tilbury, David Rose, Steven Hendry
ddas.ah@forestry.gsi.gov
ddas.nrs@forestry.gsi.gov