Pests & diseases information sheet:
Phytophthora disease
of alder
What is Phytophthora disease
of alder?
Phytophthora is a genus of plant-damaging water
moulds (oomycetes). Phytophthora is sometimes
referred to as being fungus-like but, though
similar, they are from a different biological group.
Phytophthora disease of alder affects most of the
alder species commonly regenerating or planted
in Britain.
Indicators of Phytophthora disease of
alder
Infected trees have abnormally small, yellow and
sparse leaves which frequently fall prematurely.
The crowns of trees which have been infected for
many years have dead twigs and branches. The
disease causes death of the phloem and tarry
or rusty spots upon the trunk or eventually lethal
root and collar rot.
Example of an infected tree. (© Forestry Commission)
Cause of Phytophthora disease of
alder
First identified in the UK in 1993, it is believed
that this disease is a hybrid between two other
species of phytophthora and that it has spread
in the wild mainly by mobile spores (zoospores)
that are dispersed through water and in the soil.
Infection of alder via zoospores in water could
explain the high incidence of the disease on alder
in riparian zones.
Root and collar
rot caused by
Phytophthora
disease of alder.
(© Thomas Jung,
Bugwood.org)
The course of the disease is varied, with many
trees dying rapidly once symptoms appear.
However, others may deteriorate slowly over
many years. The sight of a series of defoliated,
dying alder trees along a river course is a typical
indicator of Phytophthora disease of alder.
The National Forest Company
Bath Yard, Moira, Swadlincote, Derbyshire DE12 6BA
Tel: 01283 551211 email: [email protected]
www.nationalforest.org
Distribution of Phytophthora disease
of alder
Research has shown that it is now widespread in
the riparian ecosystems where alder commonly
grow. The principal information on the scale of
the disease comes from a series of riparian plots
established in 1994 on rivers over 8m wide in the
southern half of England and east Wales. The
percentage of trees affected by the disease has
increased steadily each year. In 2003, the survey
indicated that around 16% were diseased or
dead.
On average, the disease incidence is highest in
southeast England. However, heavy losses are
occurring in some of the large alder populations
that occur along western rivers – for example, in
the Marches and parts of Wales. The disease is
also causing damage to alders on Scottish river
systems.
Managing Phytophthora disease of
alder
All species of alder that occur commonly in
Britain are susceptible to the disease to varying
degrees. Although the disease is usually seen
along river systems, it has been found in sites
well away from riverbanks or other water
courses, for example in orchard shelter belts and
in new woodland plantings. This suggests that
the alder plants may have been infected prior to
planting. If infected plants are planted out, it may
only become apparent after several years that the
trees were infected.
It is known that the planting of alder on sites
liable to flooding by rivers, on the banks of rivers
where diseased alders are known to occur,
presents a high risk. Although alder is often the
most suitable genus for a variety of reasons, take
account of the threat of disease and consider
other flood-tolerant species, such as willow, as
replacements or in mixture.
Care should be taken not to introduce the
fungus to remote riparian sites. Instead,
natural regeneration of alder from seed can be
encouraged as it occurs readily, although the
young plants may need to be protected with a
stock-proof enclosure. For further advice see the
Forestry Commission’s Information Note FCIN6
‘Phytophthora disease of alder’.
Who to contact
If you want more advice and are situated in The
National Forest, the National Forest Company is
a useful first point of contact (tel: 01283 551211).
If you are outside of The National Forest, your
local Forestry Commission office may be able
to help (tel: 0845 3673787). If you want to be
sure of any suspected tree health concern,
the Forestry Commission has a Tree Health
Diagnostic and Advisory Service (www.forestry.
gov.uk/fr/INFD-5V6L5N#advisory). Though free
advice is dispensed, any further investigation can
be on a chargeable basis.
References:
Forestry Commission’s Information Note FCIN6
‘Phytophthora disease of alder’:
www.forestresearch.gov.uk/pdf/fcin6.pdf/$FILE/
fcin6.pdf
Forestry Commission information page:
www.forestry.gov.uk/fr/infd-737hun
Sullivan, M. 2010. CPHST Pest Datasheet for
Phytophthora alni. USDA-APHIS-PPQCPHST
Note:
Web addresses correct as of August 2011