Download IF YOU GO DOWN TO THE WOODS TODAY…

If you go
down to
the woods
today…
Ann Fullick
…you’re sure of a big surprise!
The invasion of alien species and
the effects of climate change are
threatening our British woodlands.
The battle to overcome this threat to
our woodland biodiversity involves
some fascinating biology.
Woodlands have a special attraction.
Many childhood stories are set in the
woods - Winnie-the-Pooh, Goldilocks,
Little Red Riding Hood – or peopled by
woodland creatures such as mice, owls
and squirrels. Some of our best loved
native flowers are found in woodlands in
early spring, with plants such as primroses
and bluebells famed for their delicate
beauty. But woodlands are also places
of work involving heavy machinery and
millions of pounds of income in the
timber industry.
The threat of disease
The idea that human infectious diseases
such as flu and chickenpox are caused by
pathogens - microorganisms including
viruses, bacteria and fungi - is a familiar
one. However, not everyone is aware that
plants can also come under attack by
pathogens, even large, long-lived plants
such as trees.
People in the UK tend to love plants, from
the familiar oaks and beech trees of the
countryside to exotic species for the
garden or home. The problem is that
some of the exotic species – and some
more ordinary things like logs from other
countries - carry with them new plant
pathogens or pests. Many of the familiar
species of trees in British woodlands are
coming under attack from these invading
organisms.
When considering the problems of
disease in plants, people look at the plant
disease triangle. This shows that the
intensity and impact of a disease depends
on both the plant host, the nature of the
pathogen and the environment. This in
turn means that when you are looking at
what causes plant diseases – and how
they might be controlled – all three
aspects of the disease process need
to be considered.
Host
Disease
Pathogen
Environment
Caption fig 1: The plant disease triangle
Often the pests and pathogens which
cause new plant diseases in the UK don’t
cause major problems in the countries
they have come from. This is because in
their natural habitat they have been
around for a long time. By a process of
natural selection, the plants which are
most resistant to the disease will have
survived long term to breed. There are
natural predators for any pest insects and so an equilibrium is reached in the
ecosystem. However, once an organism is
introduced into a new environment there
are no checks and balances in place.
They may become fast spreading and
virulent, causing widespread damage.
One of the best known historical examples
of this is Dutch Elm disease.
Dutch Elm disaster
Brought into the UK from Europe on
imported elm logs, Dutch Elm Disease is
one of the most serious tree diseases in
the world. It has had a devastating effect
on elm populations across Europe
including the UK. The disease is caused
by one of two different fungi (genus
Ophiostoma) which is carried under the
bark to infect the tree by various different
elm bark beetles. The first wave of the
disease in the UK was seen in the early
20th century and it was not as devastating
as people feared. But the second wave,
which began in the 1970s, was caused by
a different and more deadly pathogen –
a different species of the same genus of
fungus. This succeeded in wiping out
many of our British yew trees. And just
as the populations started to recover,
regrowing from underground suckers,
they were attacked again – the beetles
only bore into the bark once the trees
get reasonably big!
Caption Fig 2: By the 1990s over 25 million UK elm
trees out of an estimated population of around
30 million had been killed by Dutch Elm Disease
New pests and problems
People are not only bringing more exotic
species into the country, they are also
importing large, semi-mature trees for
instant landscaping and gardening.
Many of these grow in warm, sunny
countries so they can carry out the
maximum amount of photosynthesis and
grow as fast as possible. Although people
at airports and ports are vigilant, many
plant diseases are carried in the soil or
under the bark of the plants and insect
pests may arrive on vehicles or as eggs.
The numbers of these invaders are
increasing all the time. And as a result of
climate change the conditions in many
places are more favourable to exotic
species, so plants survive in the wild that
would previously only been kept in
glasshouses, and insect pests can survive
our milder winters. For example, the oak
processionary moth (which originates in
the warmth of Southern and Central
Europe) is thought to have entered the
country as eggs on some semi-mature
oak trees imported for a landscaping
project in London. The moths have
survived, bred and spread in the capital.
Not only do the caterpillars totally destroy
the leaves of oak trees, devastating the
foliage, they also shed tiny irritating hairs
which can cause skin and throat irritation
in people.
Climate change and changing
distributions
There is a growing body of scientific evidence
showing that the surf ace temperature of the Earth is
going up (global warming), and that this is probably
at least in part as a result of our actions in burning
ev er more f ossil fuels, deforestation and f arming ever
more rice and cattle.
Rising temperatures affect rainfall and weather
patterns and statistical evidence suggests that the
rise in global temperatures may be linked to the
observ ed rise in extreme weather ev ents around the
world.
Changes in global climate seem to affecting the
distribution of many different organisms. Alpine
plants in mountainous areas of the UK are getting
rarer as mean temperatures rise. The range of ov er
60% of non-migratory European butterflies had
shif ted northwards by up to 240km in the last 100
y ears – only one species had shifted south!
Changing distributions mean new pests and
pathogens unexpectedly survive if they are carried
into a new area by accident. And old pathogens –
and the insects that often act as vectors – are less
likely to be killed of f if winters become milder. This
will af f ect people – f or example if the range of the
malaria-carry ing Anopheles mosquito spreads into
more of Europe – and plants of all ty pes. For
example, the warmer the spring in the UK and
Europe, the easier it is f or potato and tomato blight to
inf ect and destroy crops.
Finding solutions
The idea of ever more plant pests and
diseases arriving in the UK is a daunting
one but the future is not completely bleak.
Sometimes scientists and forestry experts
can overcome the problem. Sometimes
the problem is simply with a particular
pathogen. Sometimes it is a pest species
alone, and sometimes the problem is a
pest species which introduces a pathogen
– such as the elm bark beetles which give
the fungal spores of Dutch Elm Disease
access to the tissues under the bark.
The method used for tackling plant
disease will depend in part on how the
problem is caused.
So when the great spruce bark beetle was
accidentally brought into the country from
continental Europe, people thought our
commercial conifer forests would be at
risk. However, by banning the movement
of logs covered with bark and introducing
the tiny predator beetle Rhizophagus
grandis as a form of biological pest
control, the sitka spruces have been
protected and the beetle kept at bay. This
tackles the pathogen aspect of the plant
disease triangle.
A fungal disease, Dothistroma
septosporum, is attacking pine trees in the
UK including many Christmas trees. Like
many fungi, it thrives in damp conditions
so forestry managers are working to thin
the trees so more air can move between
them so it is less damp and the fungi find
it harder to spread from tree to tree. In this
case, controlling the environment helps to
keep the disease at bay.
Caption fig 3: These are some of the effects of
climate change predicted by the IPCC (International
Panel on Climate Change) in 2007
Careful import controls and inspections of
plants and plant material brought into the
UK help to reduce the risk of imported
pathogens and pests. And the people
working in the woodlands can help
prevent the spread of many diseases.
They do this by being observant and
making sure the trees are healthy, by
cleaning and disinfecting tools and boots
before leaving a site where there are
infected trees and by being very careful to
avoid introducing new diseases with new
plantings. With increased knowledge and
awareness, many more diseases can be
controlled or prevented completely.
Biological pest control
Biological pest control involves using
one living organism to control the
activities of another. This may involve
using natural predators, developing
biological pesticides or importing exotic
species as a control mechanism –
although this can go terribly wrong if the
exotic species then becomes a pest or
pathogen in its own right! However if
the biological control agent is chosen
carefully, biological pest control can be
very effective. A good example is the
use of R. grandis to control the great
spruce bark beetle, because it only eats
that beetle and therefore poses little or
no risk to our main native species.
Fig 4: Images of the great spruce b ark
b eetle and the control b eetle
Rhizophagus grandis eg these two from
http://www.forestry.gov.uk/images/Dend
roctonus_mican.jpg/$FILE/Dendroctonu
s_mican.jpg and
http://www.forestry.gov.uk/images/Rhiz
ophagus_grandis.jpg/$FILE/Rhizophag
us_grandis.jpg
The great spruce b ark b eetle is actually
only a few millimetres long. R.grandis
is even smaller. It finds its prey b y
picking up chemical signals with the
sensitive hairs on its antennae. They
are so sensitive that it will find the great
spruce b ark b eetle in a wood even if
only a few trees are infected, making it
an excellent method of b iological
control.
Detection and isolation
An important aspect of dealing with plant
diseases once the trees (or other plants)
are infected is to identify and isolate the
infection quickly. Ever more sophisticated
diagnostic tools are being developed to
identify plant diseases as soon as
possible when they appear in woodlands
or crops. Instant field tests using
monoclonal antibodies are being
developed to detect both old and new
plant diseases. So for example, a
monoclonal antibody based diagnostic test
has been developed for the old problem of
fungal blight Phytophthora cinnamoni. As
a result this devastating pathogen, which
affects around a thousand plant species
worldwide and causes the loss of much
valuable timber woodland and crops such
as avocados and pineapples, is becoming
much easier to control. Scientists are
working on similar tests for Phytophthora
ramorum, a very new pathogen in the UK
which is causing severe disease in the
commercially important Japanese larch
and in species of rhododendrons. It is
known as ‘sudden oak death’ disease
because it has killed millions of oak trees
in the USA – but fortunately our native
British oak trees seem to be mainly
resistant to the disease. Once the disease
is identified infected trees and an area
around them can be felled to prevent the
spread of the fungus – but early diagnosis
is key.
The more we know about plant diseases,
the way they are spreading and how we
can detect them, the more chance there is
that we can control and prevent infection.
Plant biologists are key in the struggle to
make sure that our woodlands and forests
are here for generations to come.
Further reading:
Christmas trees under fire
The Biologist December 2011
Climate Change
Effects on Insects and Pathogens
http://www.climateandfarming.org/pdfs/
FactSheets/III.2Insects.Pathogens.pdf
Dutch Elm disease in Britain
http://www.forestry.gov.uk/fr/HCOU4U4JCL
Management of great spruce bark
beetle
http://www.forestry.gov.uk/fr/INFD6XPC8D