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The Himalayas - probable home of the dry rot fungus Serpula lacrymans - where a biolgical control organism may lie? |
The causative organism of dry rot, the
basidiomycete fungus Serpula lacrymans, is estimated to cause up to £200
million damage to buildings in the UK and comparable amounts in other countries
where the organism is active in buildings. In addition incalculable damage is
caused to heritage buildings where the actual fabric of the building is itself
of great historic importance. Traditional methods for treatment of outbreaks of
dry rot have taken little account of the highly specialised nature of the
organism which, as has often been commented upon by research scientists, is not
easy to grow in the laboratory and is very rare in nature. Why then should a
relatively sensitive organism be the cause of such distress in the building
situation? Firstly, given the correct environmental conditions, the organism can
be spectacularly destructive, much more so than other comparable fungi.
Secondly, unlike other wood decay basidiomyctes found commonly in buildings
simple drying of the area of wood infected by S. lacrymans is not
sufficient to kill the organism. It can survive localised drying by colonising/gaining
access to neighbouring areas that retain elevated moisture levels. This
combination of the aggressive nature of S. lacrymans and its
refractoriness to simple environmental control measures has lead to a reliance
on the use of toxic chemicals.
Chemicals used to inhibit, or destroy, wood decay fungi have
certain characteristics which may make them seem environmentally suspect. They
need to be persistent, and also highly toxic to organisms which are capable of
initiating complex degradative processes. Also, because wood is in many
instances a relatively low value primary resource, preservatives need to be
cheap which itself imposes constraints on the types of chemical which can be
used.
Concepts and principles from the field of wood preservation
have very definitely had an affect on operations in the remedial treatment
industry. However the needs of the two industries are, or at least should be
rather different. The preservative industry needs persistent and toxic chemicals
specifically where timbers are to be kept in high hazard class situations.
However timber in correctly constructed and properly maintained buildings should
not need preservation since the conditions which favour decay are generally not
desirable in buildings because of adverse effects on health, comfort and other
non-wood structural elements of buildings. In practice buildings are not always
constructed properly, are sometimes modified inappropriately, not always
maintained correctly and often used in ways that favour decay. These factors
make the remediation of timber decay in buildings, and particularly the
eradication of dry rot, a more difficult task than it would seem to be in
theory.
Environmental control
S. lacrymans, like any other biological entity,
needs a specific environment in which to live. Indeed the environmental
requirements of S. lacrymans are rather specific. It needs wood with a
moisture content around the fibre saturation point (i.e. when there is free
moisture in the timber), it cannot survive mildly elevated temperatures
(temperatures as low as 30oC have been reported as being lethal), it is
sensitive to air movements and requires a relatively high humidity environment
and seems to have an absolute requirement for certain metallic elements, the
lack of which can effectively prevent it from decaying wood. If any of these
requirements are not met the organism will not decay timber and therefore
eradication of the organism should be a relatively easy task. Indeed specialist
companies in the UK and abroad are now offering services to correct dry rot
infestations which rely solely on environmental control; keeping moisture levels
in wood low and increasing ventilation to both favour low moisture levels and
prevent the build up of high humidity. By inserting remote sensing moisture
meters into at-risk timbers, e.g. beneath valley gutters or at the beam ends or
in wall plates, it is possible to keep a check on a building without the need
for complex and expensive inspection procedures.
This type of strategy is relevant in many situations and
could theoretically be used in every situation. However it requires perfect
building maintenance, co-operative users and a check on any structural
modifications. As such it would appear likely that there are situations where
the efficacy of environmental control could not be guaranteed despite the
obvious appropriateness of the strategy. On account of this additional controls
are necessary and these may well include the use of chemicals.
Additional strategies
Chemical
Traditional wood preservatives have usually been
highly toxic chemicals. However in recent years there has been an increased use
of relatively non-toxic boron compounds to protect wood from both insects and
fungi(1,2). The increased use of boron compounds has been driven partly by the
need to develop less toxic preservatives and partly by the development of new
formulations such as glass rods and monoethylene glycol based solutions.
Currently there is much activity in wood preservation
research to find other biocides of low mammalian toxicity. Such chemicals as
IPBC (3-iodo-2-propynyl butyl carbamate) and chlorothalonil (a widely used
agricultural chemical) may have a role to play in the treatment of dry rot
though use of such chemicals and indeed boron compounds, should remain as a
secondary line of defence.
Heat treatment
It has been known for many years that the dry rot
fungus is peculiarly sensitive to relatively small increases in temperature.
This lack of thermo-tolerance, which is reported as a range of different lethal
temperature/ period combinations (e.g. any time period between 1 and 6 hours at
40oC), has resulted in the use of whole building heat treatments to eradicate
dry rot(3). Total killing of organisms has been shown in this system though it
is only the mycelial form of the fungus which is inactivated, active spores
still persist. However given an appropriate drying regime for a building these
spores are unlikely to germinate and produce destructive mycelia and indeed
there are no reported recurrences of dry rot in buildings which have been heat
treated. This system has so far only been used in Denmark though its
applicability in buildings in other countries seems likely and should be tested.
It could be argued that environmental control alone is
sufficient to kill the dry rot organism and that heat treatment is therefore
unnecessary. Heat treatment ensures the organism is rapidly killed at the start
of the control procedure and also that any further degradation which might occur
during drying is prevented. Of course, as in the case of environmental control
more effective building management practices will have to be installed in order
to prevent recurrences of dry rot infestations. The failure to maintain rigorous
building management practices as a follow-up to heat treatment remains a major
concern in the long-term protection of buildings. Currently there is no
effective alternative to the use of chemical preservatives in poorly managed
buildings or in buildings where constructional properties render it difficult to
ensure that wetting of timbers does not occur.
New research on dry rot Serpula lacrymans
In 1989 Thornton, from CSIRO in Melbourne, Australia,
published a paper at the International Research Group on Wood Preservation
meeting asking why more people do not work with S. lacrymans nowadays(4).
Whilst it may be debated whether Thornton was correct in assuming that few
people are undertaking research on S. lacrymans it is undoubtedly true
that for an organism which causes so much damage relatively few studies had been
reported at that time which could assist in the understanding of the growth of
the organism and its ability to decay wood. In many areas the major information
about S. lacrymans owed much to the work of Richard Falck in the early
part of this century(5). Whilst much important work has been undertaken
subsequently the application of new techniques and methodologies, which have
transformed the biological sciences in the last 20 years, was lacking in
research on S. lacrymans.
As part of the overall research programme of the Scottish
Institute for Wood Technology our group initiated studies on S. lacrymans
around 1987 and a brief description of some of our studies, and related studies
from other groups will indicate that whilst there are still many important areas
of research to be initiated some questions about the organism have been answered
and new control strategies are being assessed.
Biological control
The concept of using a predator fungus to destroy a
detrimental fungus is not new, however it has not been applied to remedial
treatment of dry rot until recently. Studies by Doi and Yamada(6) and Score and
Palfreyman(7) have indicated that Trichoderma spp. are highly effective
at antagonising S. lacrymans in laboratory based systems and field trials
are currently underway. Whilst it might seem inappropriate to use a sporulating
organism such as Trichoderma to control dry rot there are a number of
reasons why such research is important. First biological control might be a
highly effective, and cheap, way of preserving buildings which are falling into
disrepair due to lack of use and maintenance. Second, by fully understanding the
way that Trichoderma antagonises S. lacrymans new control methods
may well be identified either based on live organisms (wild type, mutants, or
perhaps genetically engineered) metabolites from such organisms or specific
chemicals which can induce or mimic effects which occur during lethal
interactions.
The potential biocontrol organisms tested so far have one
major disadvantage when it comes to competing with S. lacrymans in the
field: they have not been isolated from the same environmental niche as the
basidiomycete. To overcome this difficulty, efforts are now being made to
isolate potential antagonistic organisms from the Himalayas, the probable
natural home of S. lacrymans(8).
Other factors affecting growth and decay
For many years it has been reported by operatives in
the field that the dry rot fungus requires certain ions for its activity, in
particular calcium ions(9). The reason for this requirement are not fully
understood though they may be due to a need for the organism to switch its
metabolism from the breakdown of cellulose (in the early stages of colonisation)
to the utilisation of the sugars released from the cellulose (later on in the
decay process). Recent studies(10) have shown, for the first time, that the
requirements reported in the field are also applicable to the decay of wood
samples in the laboratory and new control methods may be developed by further
analysis of such requirements.
Conclusions
Despite the development of new treatment regimes there
is still much information that is required about S. lacrymans if such
regimes are to be optimised. Whilst environmental control may be the ultimate
objective the limits of such control need to be more fully defined and those
parameters which have most lethal effects on the growth of the organism more
fully understood. Fortunately the idea that there is only one standard method
for treatment of dry rot is becoming understood to be a fallacy and the
development of specific treatment regimes for specific situations is now
becoming more widespread. To further process a better understanding of S.
lacrymans is required and the widespread dissemination of that understanding
is essential.