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MICROBIO LOGY TODAY VOL 30/AUG03 107
●The dry-rot fungus, Serpula lacrymans, is a
major cause of timber failure in the built
environment in many parts of the world, most
notably northern Europe, parts of eastern Europe and
Asia, and Australia. This basidiomycete fungus may
cause spectacular decay of damp building timbers,
resulting in what is ironically known as ‘Dry Rot’. Often
referred to as a plague or malignant cancer of our
buildings today, dry rot has apparently instilled fear and
dread for centuries; its destructive nature was even
recognized by the Royal Navy in the 18th–19th
centuries. So much so that the Royal Society of Arts
offered prizes for the discovery of cures for dry rot.
Currently estimated to cause at least £200 million of
damage per year in UK buildings alone, the dry-rot
fungus may also irrevocably harm the fabric of our
important historic buildings.
Conventional methods for dealing with dry rot are
often dramatic, involving the removal of infected timber
and application of chemicals to prevent re-infection.
Popular perceptions are that eradication of dry rot is
extremely difficult and that the organism is particularly
resistant to treatment. These beliefs are unjustified
and arise mainly from two specific abilities of the
organism. First, it can survive for extended time periods
in masonry; second, it can transport water and nutrients
over long distances. So, simple drying of localized
infected timbers may be insufficient to achieve
eradication of the organism. Because of these abilities,
there is no doubt that control of S. lacrymans can be a
challenge, but this may easily be overcome by removing
moisture from the entire environment in which the
fungus is growing. Timber used in correctly constructed
and well-maintained buildings is therefore safe from the
threat of dry rot. S. lacrymans is not an unusually resilient
species, indeed it appears to display a fastidious
environmental sensitivity. Consequently, the organism is
apparently an extremely rare occupant of the natural
woodland environment, with only a few reliably
recorded isolations made from even fewer geographical
locations. Appreciation of the organism’s physiology and
sensitivity has more recently informed subtle eradication
and control strategies.
● Growth and decay
S. lacrymans growth normally requires wood and a source
of certain metallic elements, a minimum moisture
content of 30% for spore germination and 20% for
mycelial growth, moderate temperature, low ventilation
(in concealed spaces) with ensuing accumulation of
humidity. Once established the organism may produce
foraging mycelial cords which develop and extend from
a previously colonized resource, behind plaster or
over brickwork, to exploit distant available wood
reserves. Thus, S. lacrymans may spread through a
building by transporting water and other substances,
via cord systems, to support
exploitation of timbers
hitherto inappropriate
for decay. Wood cell-wall
cellulose and hemi-
celluloses act as growth
substrates, the removal
of which results in brown
rot due to the colour
contributed by the
remaining lignin. On
drying, decayed timber
shrinks and splits into
large cubical pieces by deep
cross-cracking. Cultivation
of S. lacrymans in a wood/
mineral microcosm results in the formation of an
extensive mycelium outwith the wood and over the
surface of the mineral. Rapid coating of the individual
hyphae with crystals of calcium oxalate occurs, as a
result of the production of oxalic acid by S. lacrymans.
In addition, with minerals rich in iron, red/brown
discolouration of the growth front of the mycelium
occurs. This is caused by ferric ions and demonstrates the
ability of the fungus both to remove iron from minerals
and to transport it to the growth front. Since iron is a
probable source of radicals used in the wood-degrading
Fenton’s reaction (Fe2++H2O2→Fe3++OH–+OH.),
and since wood contains low levels of iron, the ability of
the fungus to mobilize and transport the element from
iron-rich minerals to iron-poor wood makes S. lacrymans
a particularly effective degrading agent. Whether the
oxalic acid that the organism produces during meta-
bolism is used to initiate depolymerization of cellulose,
or its primary role is to degrade minerals and release iron
has yet to be conclusively demonstrated. However, the
degradation of mineral sources releases many metal ions,
including those of iron and, in much greater amounts,
calcium. Since calcium is toxic in high levels, and iron is
crucial to timber degradation, oxalic acid is a particularly
suitable agent as calcium oxalate is highly insoluble,
whereas ferric oxalate is relatively soluble.
● Detecting surreptitious dry rot
Obvious signs of S. lacrymans usually signify massive,
but concealed, timber decay. The organism tends to
reside and is active within humid, poorly ventilated
building spaces. Airflow and desiccation tend to
promote the development of flat rust-red sporophores
from which millions of air-borne spores may emerge
and subsequently germinate to exploit new woody
resources. Thus, reliable detection methods are vital in
achieving the early control of dry rot. To this end, the
borescope and trained sniffer dogs have been successfully
employed, and a hand-held ‘electronic nose’ sensor
system is currently under development.
Everything you wanted to know
about the dry-rot fungus but were
afraid to ask
John W. Palfreyman & Nia A. White
Dry rot of timber,
caused by the
fungus
Serpula
lacrymans
, is a
serious problem
not only in domestic
and commercial
buildings worldwide,
but also in important
historic edifices.
John Palfreyman
and Nia White
describe the dry-rot
fungus and the
conditions
necessary for its
growth, how it
causes timber to
decay and what can
be done to prevent
and control its
effects.
ABOVE:
Growth of the dry-rot fungus
S. lacrymans
mycelium in a floor
space.
COURTESY J.W. PALFREYMAN
MICROBIOLOGYTODAY VO L 30/AUG03
108
● Controlling the growth of S. lacrymans
Experimentation with full-scale models of parts of
buildings confirms observations from the woodland
environment that the biomass of the dry-rot fungus is
distributed between woody and non-woody resources. In
the built environment, a key to controlling the dry-rot
fungus is to break the connection between wood and
sources of both moisture and the mineral components of
masonry. In addition, the fungus can be disabled rapidly
by ventilation, but quickly revives if stagnant air
conditions reoccur. Controlling dry rot by such methods
is now termed ‘environmental control’. Such a strategy
demands regular and careful building maintenance and
inspection, the latter sometimes aided by the use of
remote sensing moisture meters. Chemicals have
traditionally been used when environmental control has
not been considered or is deemed impracticable. An
alternative, which has been occasionally used, depends
upon the heat sensitivity of S. lacrymans and in some
situations is feasible. Developed in Denmark, the
procedure involves heating the whole of a tented
building, or isolated parts of a building, to a temperature
of 50 °C using moist heat. A final, as yet still
experimental method of controlling dry rot involves the
use of competitor fungi, notably isolates of Trichoderma.
To date such biocontrol works well in microcosms, but is
disappointing in the field. Reasons for this probably
relate to issues such as inoculum potential, the
undoubted fit between the needs of S. lacrymans and the
environment which may be found in badly maintained or
poorly designed buildings and the inappropriate source
of the Trichoderma isolates used until recently. The
enhanced performance of Trichoderma isolates from the
natural woodland environment of S. lacrymans holds
promise for the future. For example, new isolates from
northern California in the USA have demonstrated an
ability to produce volatiles that can kill extended S.
lacrymans mycelial systems even without contact
between the two organisms. This has not been reported
before.
● The origins of dry rot?
Despite its widespread occurrence in the built
environment, S. lacrymans has rarely been found
colonizing woodland environments. Fruit bodies have
been found in northern India (Himalayan foothills), the
Czech Republic and the USA (Mount Shasta, northern
California), although the identity has been verified for
only the first two using molecular methods (SDS-PAGE
and RAPD and rDNA-ITS PCR). Investigation of the
Day 21 Day 27 Day 52 Day 59 Day 73
Day 43 Day 52 Day 73
Experimental microcosms with (a) a common airspace and (b) the
biocontrol interaction airspace isolated from the remainder of the
extended
S. lacrymans
mycelium, where
S. lacrymans
-colonized wood
blocks (RHS) are baited with fresh uncolonized wood blocks (LHS) to
stimulate connecting mycelial cord formation. Colonized baits are then
treated with a
Trichoderma
isolate (USA1; lower LHS) to achieve
biocontrol. Death of
S. lacrymans
mycelium is achieved at a distance
even when the airspace of treated mycelium is isolated from the rest
of the colony.
COURTESY N.A.WHITE
(a)
(b)
S. himantioides
BB24
S. himantioides
Belgium
S. himantioides
MD5
S. himantioides
BF15
S. lacrymans
H67
S. lacrymans
DIT101
S. lacrymans
888HARM
S. lacrymans
H28
S. lacrymans
FORFAR
S. lacrymans
BF50
S. lacrymans
BF25
S. lacrymans
MIL26
S. lacrymans
17B
S. lacrymans
18A
S. lacrymans
7B
S. lacrymans
12C
S. lacrymans
MDR2
Basidiomycete H79
RIGHT:
RAPD fingerprints of strains of
building and Himalayan woodland
(H67 and H28)
S. lacrymans
isolates and its close relative
S. himantioides
. Similarities
between building and woodland
isolates of
S. lacrymans
may
support suggestions that strains
currently affecting buildings are
descended from those accidentally
imported from woodlands such as
those in the Himalayas several
centuries ago.
COURTESY N.A. WHITE
LHS RHS
BB24
USA94b
Czech95
H67
DIT101
BF-050
H28
FPRL12c
molecular variation and phylogeography of building and
woodland S. lacrymans isolates and its closest relatives,
informs a demographic database and may indicate how
the current geographic distribution of the dry-rot fungus
has developed. From studying the organism in the wild
we may also develop ideas for better control of the
organism in the built environment. However, there is no
doubt that the internal environment of buildings should
not be conducive to the growth of S. lacrymans and that
good maintenance, careful design and appropriate usage
will prevent damage by the dry-rot fungus. Indeed
prevention of dry rot could be considered more of an
educational issue than a technological or scientific one.
However, in circumstances where lack of finance, past
mistakes or lack of understanding have allowed
problems to develop, we still need better science-based
solutions.
● Prof. J.W. Palfreyman is Head of the School of
Contemporary Sciences, University of Abertay
Dundee, UK, and was instrumental in setting up
the Dry Rot Research Group at Abertay. With an
interest in historic buildings much of his research
on dry rot has centred around developing less
destructive ways of treating the fungus and,
perhaps more importantly, preventing its
occurrence within the built environment.
For further information see
http://scieng.tay.ac.uk/dry_rot/
email j.palfreyman@tay.ac.uk
● Dr N.A. White is a lecturer in microbiology in
the School of Contemporary Sciences, University
of Abertay Dundee, UK, and a researcher within
the Dry Rot Research Group and SIMBIOS. Her
research activities centre on the physiology,
ecology and phylogeography of S. lacrymans,
and on modelling fungal growth and community
dynamics.
email n.a.white@abertay.ac.uk
BioSciences Federation
(in collaboration with the LTSN Centre
for Bioscience)
Education Colloquium
Changes and Challenges
The Changing Face of the Bioscience
Undergraduate
1000–1700, Monday 6 October 2003
Hamilton House (NUT HQ), Mabledon Place,
London WC1H 9BD
This one day event aims to consider the mismatch
between what science students learn at school and
what they are expected to know when they begin
their university courses. It will bring together school
teachers, careers advisers and admissions tutors,
providing them with information about recent changes
in the school science curriculum and discussing
ways of meeting the resultant challenges.
Key issues include:
●How can we attract the best students into
science?
●How is the school science curriculum going to
change?
●How will it affect the quality of our future science
undergraduates?
The meeting will be chaired by Reverend Professor
Michael Reiss (Head of School, Institute of Education,
University of London), who has been a driving force
behind the new Salters’ Nuffield Biology ‘A’ level.
There will be two talks:
●
Changes to school science
(Rebecca Edwards,
QCA)
●
The mismatch between school science and
university expectations
(Peter Cotgreave, SBS)
And three roundtables on:
●
New challenges faced by first year undergraduates
●
Student perception of university science courses
●
The link between universities and schools
The colloquium will be followed by a drinks reception.
Lunch and refreshments will be provided.
Admission is free, but pre-registration is essential as
places will be limited.
For further information and to register online see
www.bsf.ac.uk
Closing date for registrations: 30 September
2003.
Sponsored by:
The Biochemical Society, British
Society for Immunology, Institute of Biology, The
Physiological Society, Society for Experimental
Biology, Society for General Microbiology
.
MICROBIO LOGY TODAY VOL 30/AUG03 109
Further reading
Palfreyman, J.W. & Low, G.
(2002). The Environmental
Control of Dry Rot. Technical
Advice Note for Historic
Scotland. TAN 24. Edinburgh:
Historic Scotland.
Ridout, B. (2000). Timber
Decay in Buildings. The
Conservation Approach to
Treatment. New York: E. &
F.N. Spon.
White, N.A., Dehal, P.K.,
Duncan, J.M., Williams,
N.A., Gartland, J.S.,
Palfreyman, J.W. & Cooke,
D.E.L. (2001). Molecular
analysis of intraspecific
variation between building
and ‘wild’ isolates of Serpula
lacrymans and their relatedness
to S. himantioides. Mycol Res
105, 447–452.
LEFT:
Dendrogram based on a
distance-based analysis of the
ITS1, 5.8S and ITS2 sequences of
Serpula
species. The European
‘building’ and ‘wild’ (H67, H28 and
Czech95) isolates were identical.
Woodland isolate USA94b lies
between all other
Serpula
isolates
studied and
S. himantioides
(BB24)
which may indicate that much of
the evolutionary history of the
species occurred in Northern
America.
COURTESY N.A. WHITE
Education
Colloquium
