|
Wood Decaying
Fungi – An Insight on Types and Roles in Wood Decay
Fungi can grow on almost any
surface provided there is a source of nutrient present. All fungi require glucose for energy.
This simple sugar is usually present in the environment in complex molecules that must be digested before
the glucose becomes available for use in producing energy in the fungus. Nutrient sources (food) for
some fungi must contain simple sugars or starches that are easily digested. Other fungi produce
enzymes that allow the digestion of more complex sugar sources. These complex sugar sources include
cellulose, hemicellulose, and lignin. In order to cause wood rot, a fungus must be able to degrade
one or more of these compounds.
There are three major types of
fungal induced wood decay: soft rot, white rot, and brown rot. The fungi producing these rots
can produce airborne spores and influence air quality. However, they are primarily of concern because
of their capacity to destroy the structural integrity of wood.
The soft rot fungi can degrade
all three of the structural wood components, although the major food sources are cellulose and
hemicellulose. A wide variety of fungi can cause soft rot, including the ubiquitous
Chaetomium. For soft rot to occur, liquid water must be continuously present. Wood
with soft rot is spongy. The most common sites for soft rot in buildings are wooden windowsills, and
areas where roof drips continually wet wood materials.
The white rot fungi degrade
primarily lignin. These fungi remove the lignin and leave the white cellulose. Wood that has
been decayed by a white rot fungus is whitish, and stringy. Most of the fungi that cause white rot
are Basidiomycetes and Phellinus (one of the Basidiomycetes) is a common white rot fungus.
While the white rot fungi may form rhizoid-like strands, they are usually fine and fragile.
Phellinus produces flat fruiting bodies on the surface of colonized wood, and can release
basidiospores into the occupied space.
Brown rot fungi degrade only
the cellulose in wood. In the process they modify the lignin, but do not use it for food. The
rotted wood is brownish and both longitudinal and transverse cracks appear, giving the wood a cubed
appearance. Only a few fungi can produce brown rot, and all are Basidiomycetes.
Coniophora is a brown rot fungus that produces spore-bearing structures flat on the surface of
continuously damp wood. This rot usually occurs on wood in contact with damp soil or other continuous
water sources. Control of wet brown rot involves removing all of the rotted wood, and controlling the
water source before the wood is replaced.
Some brown rot fungi cause what
is incorrectly termed “dry rot”. Poria incrassata and Merulius
lacrymans are two Basidiomycetes that cause dry rot. These fungi form resistant rhizoids that
can transport water for long distances. These can often be seen on wood surfaces as root-like strands
that are somewhat rubbery when moist. If brown rot appears in a place where no obvious sources of
water can be found, it is useful to test the wood’s moisture content, and if moisture content is
high, then try to trace the fungus to the moisture source. Control of dry rot involves removing all
of the rotted wood, and interrupting the source of water on which the fungi depend.
Other fungi will grow on wood,
but generally do not cause structural damage. The sap stain fungi, for example, grow in the sugary
sap of living trees or cut lumber. They may cause discoloration of the wood, but do not cause any
other damage. While possibly unattractive, these fungi do not present an air quality concern, nor do
they lead to rot. Many fungi will use wood exudates (sap) for food, since this is a source for
soluble sugar. Also the common molds will grow on the surface of wood, possibly causing discoloration
and affecting air quality, but not damaging the wood itself. These fungi can be removed from
surfaces, and as long as water does not remain, will not return.
White and brown rot fungi can
form fruiting bodies on the surface of colonized wood, and can produce massive numbers of
basidiospores. If basidiospores are higher indoors than out on air samples, the possibility of wood
rot should be considered, and the type of basidiospores explored more carefully.
In
summary:
-
If windowsills or other
continuously wet wood is spongy, suspect wood rot.
-
If wood is white and stringy,
suspect white rot.
-
If wood is brown, crumbly,
and cracked, suspect brown rot.
-
If no obvious water source is
present, suspect dry (brown) rot.
-
If fungal growth is
superficial and can easily be wiped away, suspect common mold.
-
If basidiospore
concentrations are unusually high indoors consider the possibility of white or brown rot somewhere in the
space.
References
Identification Manual for
Fungi from Utility Poles in the United States, Edited by C.J.K. Wang, and R.A. Zabel. 1990. ATCC,
12301 Parklawn Drive, Rockville, Maryland 20852.
Micro-Morphological Studies
of Soft Rot Fungi in Wood, A.S. Zainal. 1975. Strauss & Cramer GmbH, 6845 Hirschberg,
Germany.
Microorganisms in Home and
Indoor Work Environments, Edited by Brian Flannigan, Robert A. Samson, and J. David Miller. 2001. Taylor
and Francis Inc., 29 West 35th Street, New York, NY 100001.
|
|
Fungi of the
month (II): Lumberyard molds- Ceratocystis /Ophiostoma (C/O) group:
Dr. Payam Fallah, Dr. Kamash
Ramanathan
The ascomycete genera
Ceratocystis and Ophiostoma include numerous species that can appear on both angiosperm
and conifer woods. Each genus is distinct and contains many species of its own. The reason we
lump the two genera together in a group is due to the extreme morphological similarities that exist between
the two genera. While many species are considered plant pathogens, few species, such as C.
coerulescens, C. piliferum, and C. virescens, merely cause dark discolorations of
the wood, known as sapstain or blue stain. Because of this discoloration, the lumber becomes less
visually appealing. Importantly, there are no reports indicating that the group causes structural
damage.
The C/O group can be found on
almost any kind of lumber, especially pine, and it may have been there long before the lumber was brought
to the lumberyard. The dark coloration on wood can often be mistaken with that of
Stachybotrys species, and consequently cause concern for some individuals. It is important
to note that growth of Stachybotrys species on wood is extremely poor and its occurrence very
rare.
The primary source of food for
the C/O group is sugar as well as certain carbohydrates found in cells located in the sapwood area of the
lumber. Sapwood basically refers to the area near the bark. Once the mycelium of the fungi
appears on the surface, it may go through the sexual phase, and if environmental conditions (e.g. moisture/
temperature) are suitable, it produces dark microscopic fruiting structures (ascomata) containing numerous
ascospores. Occasionally, we report another fungus with this group, known as Gonatobotryum
species. Gonatobotryum is a mycoparasitic mold (a fungus parasitizing another fungus) that
is often found growing on C/O group. The spores of C/O group can not be identified on spore traps nor
can these fungi be cultured easily. However, the Gonatobotryum spores occasionally can be
detected in the air and easily identified on spore traps. Its presence may strongly suggest that the
C/O group may also be present in the vicinity. The best way to sample for the C/O group is by a tape
lift, preferably, or by swabbing the colonized surfaces. Culturing the C/O group is not
necessary.
In general, fungal spores can
be disseminated by variety of ways, most commonly by air. The spores of C/O group, however, have
evolved to be disseminated by insects and as a result the spores are produced in a sticky mass on the
lumber. Since the spores are produced in a sticky mass; the airborne exposure potential of C/O group
is greatly reduced. Currently the C/O group is not reported to infect humans or animals. A connection
between Ophiostoma and the human pathogen Sporothrix schenckii has been proposed but not
confirmed. People should realize that the C/O group is probably present in a very large number,
arguably a majority, of wood framed buildings. It could therefore be inferred that the health risks
are very low and the issue, for this group of fungi, is largely cosmetic.
|
