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Sapstain fungi in wood: identification, prevention and treatment
Introduction
Sapstain or blue stain, often seen
as dark or blue colour streaks and
patches through timber, is caused
by a class of fungi that can rapidly
infect sapwood of green wood.. Sapwood is the living part of a tree
used for transport and storage of
nutrients. Sapstain fungi digest the
stored labile nutrients in the wood
cells of sapwood only. They are not
able to attack the cellulose or lignin
structural contents of cell walls and
hence do not structurally weaken
wood. Both softwoods and hardwoods are susceptible to sapstain
fungi in appropriate conditions.
Similar to moulds and decay fungi,
sapstain fungi thrive in moderate
conditions with access to moisture,
air and suitable nutrients. However
whereas moulds mainly affect the
wood surface, sapstain discolouration can extend throughout the
wood. In the right conditions, sapstain can infect and start to discolour wood within a few days of logging or sawing. Although it doesn’t
cause structural damage to wood,
sapstain discolouration can affect
the appearance and marketability
of wood products. Hence control of
sapstain is economically important for most wood processing
operations.
For the purposes of dealing with
sapstain in wood products, an important distinction needs to be
made between these staining only
fungi and more damaging wood
decay fungi. Sapstain fungi do not
materially change the wood structure so do not affect strength and
stiffness of the wood products.
Wood decay fungi which generally
take a longer time to establish can
degrade the wood structure by
enzymatically breaking down the
cellulose and/or lignin structural
components of wood. This results
in loss of structural strength and
other physical changes in wood
products.
Appearance of sapstain
Sapstain fungi operate only in
sapwood and not in heartwood or
truewood which does not contain
suitable free nutrients.
Early onset of sapstain in Radiata
pine is often seen as dark grey to
blue radial streaks as shown below.
Sapstain infection can be seen in this image of a young Radiata pine log with the
dark blue/black radial streaks.
http://
www.nzffa.org.nz/farm-forestry-model/the
-essentials/forest-health-pests-anddiseases/diseases/storm-damage-andsapstain-estimating-the-salvage-period-in
-pine-plantations/
Once established, it is practically
impossible to correct the colour of
sapstain affected wood. In extreme
cases the colour of the whole sapwood can affected to the extent
that it becomes a feature as shown
over page with example of beetle
kill pine in the USA.
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these should be isolated so that
they don’t infect other stock and
processed separately from stain
free logs where possible.
Beetle kill pine in North America often displays seep sapstain discolouration
through 100% of the sapwood which ironically has become a selling feature http://
www.houzz.com/photos/9832621/BeetleKill-pine-hardwood-flooring-other
Some wood end uses such as landscaping timber, posts and poles are
not sensitive to the presence of
sapstain particularly when treated
with copper based preservatives
which also cause a colour change.
However many clear and appearance grade timbers may be downgraded in value by the presence of
sapstain.
Example of a board cut from a log that was
pre-infected with sapstain. Unfortunately
anti-sapstain treatment of this board will
not fix the stain already present.
It is important to frequently check log
stocks for development of sapstain in the
end grain.
Chemical treatment: Surface application of suitable chemicals by dip
or spray is highly effective in inhibiting sapstain fungi and other
fungi from infecting freshly sawn
timber during processing, storage
and transport.
Prevention of sapstain
As discussed, once sapstain infection has discoloured the wood it is
not practically possible to remove
the stain. Hence the only practical
treatment for sapstain fungi is prevention. Prevention is achieved by
rapid log procurement, preventative
chemical treatment and/or kiln drying.
Log management: The most common source of sapstain in wood
processing sites is from preinfected logs. Thus it is important
that logs are brought to the mill as
quickly as possible after felling,
processed quickly after delivery
and stocks are frequently checked
for sapstain pre-infection. If preinfected logs are encountered,
Contact Lonza for details of antisapstain chemical treatments suitable for various circumstances.
Kiln drying: As with virtually all fungi, sapstain fungi cannot infect and
are not viable in wood with less
than approximately 20% moisture
content. So kiln drying immediately
after sawing to less than 20% moisture content will prevent sapstain
from developing. Note that sapstain
and other fungi can establish in kiln
dried wood if it is allowed to become
wet again such as by being left outside uncovered.
Additional considerations
Anti-sapstain chemical treatments are
effective in preventing infection providing
that the wood is not pre-infected and
other general risk reduction measures are
taken.
Protection periods provided by
such treatments will depend on
the local environmental factors,
the type of chemical applied and
the concentration or loading of
chemical applied to the wood.
Note that surface chemical treatment of sawn timber will not stop
sapstain organisms that are preinfected in the wood from the log.
Similar to mould prevention, site
hygiene is an important factor in
minimising the risk of sapstain infection. Freshly sawn timber stocks
should be kept well away from wood
waste and redundant pieces that
may be a source of infection.
Insects such as pine bark beetles
(Hylastes ater) and burnt pine beetles (Arhopalus ferus) can help
spread sapstain infection so an insect management plan may also be
required particularly during spring
and summer seasons.
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QUESTIONS & ANSWERS
Does the appearance of sapstain mean that the timber will rot?
Not necessarily. As discussed, sapstain fungi do not cause structural
decay of timber. However, the conditions that promote sapstain are
also conducive to the development of moulds and decay fungi in time.
Does sapstain affect the structural grade of timber?
No, sapstain fungi do not digest the cellulose or lignin content of wood
and hence do not cause structural weakening.
Mould or sapstain infected fillet boards will
spread infection to fresh wood.
Pre-infected timber bearers and
spacers are also another common
source of infection so ensure that
all wood packing items used are
also free of stain. Use anti-sapstain
chemical treatments at the loading
as recommended.
Sapstain infection may increase
the porosity of wood and result in
significantly higher absorption of
preservative fluid with vacuum
pressure treatment. For example,
even only moderately sapstain affected wood can result in double
the normal uptake of LOSP (Light
Organic Solvent Preservative) compared to clean, uninfected wood.
What is the best way to deal with sapstain infection?
Like many related issues, prevention is the best and only practical
cure. For wood processing sites, site hygiene, good log and timber
stock management and anti-sapstain pre-treatments where appropriate are the best ways to prevent sapstain development in wood products. Dry wood should be kept dry (<20% moisture content) in which
case the risk of sapstain developing is dramatically reduced.
If wood does become sapstain infected, then unfortunately there are
no practical ways to remove the colour stain although the infected
wood may still be suitable for other uses such as for full pressure
treatment.
Are there health hazards from exposure to sapstain fungi?
Unlike moulds, there does not appear to be any evidence of adverse
health effects in humans due to exposure to sapstain infected wood.
New Zealand
Tel: +64 9 276 3646
Zelam
Tel: +64 6 755 9234
Australia
Tel: +61 3 9339 8915
Email: [email protected]
Email: [email protected]
Email: [email protected]
www. lonza.com
www.zelam.com
www.lonza.com
Disclaimer: The information contained herein is provided in good faith for general guidance on preventing sapstain in freshly sawn or
processed timber. However, no warranty is made, either express or implied, regarding the information or recommendations provided by
Lonza, the accuracy of the information contained herein, or the results to be obtained from the use of such information. Any user must
make their own determination and satisfy themselves that the products, information, and recommendations given by Lonza are (i) suitable for intended process and purpose and (ii) in compliance with environmental, health and safety regulations. © Lonza Wood Protection December 2016. Sentry® trademark is the property of Chemcolour Industries