appressoria to initiate the bore hole production
is evident in blue staining hyphae. Daniel
points out that wood species or types that are
less lignified, such as tension wood in hardwoods, can be more readily attacked by stain
fungi, and hemicellulose is more readily
removed by stain fungi in this type of wood
(Encinas and Daniel 1995; Daniel 2016).
Fungal stain genera commonly include Ceratocystiopsis, Ophiostoma, and Grosmannia,
with genomic information increasingly separating and reclassifying some stain fungi into new
genera (De Beer et al. 2013a, b). In species that
produce a sexual stage, these fungi produce
dark colored, flask-shaped, spore-bearing
structures known as perithecia that are produced on the surface of wood. Spore-containing
asci are produced in the base of the perithecia,
and as the perithecia matures, the asci and
spores are exuded through the neck of the perithecia for dissemination. Typically, the spore
mass is discharged in a moist viscous fluid
droplet which rests at the top of the opening
in the perithecia neck (Fig. 15.7b). Although
spores can be dispersed by wind and rain
water, many species of fungal stain that produce perithecia have adapted to be
disseminated by beetles. The height of the perithecia neck for particular fungal species has
evolved to match the height and anatomy of
particular beetles or other insects and arachnids (particularly mites). The viscous fluid
mass of spores is often described as a sticky
slime, which adheres to the body or legs of the
insects/arachnids as they feed upon the fungal
mycelium and spores. The spores are then
disseminated as the vector travels to other
sites. An excellent review of wood staining by
fungi and other agents (Uzunovic et al. 2008)
also reports that log harvesting equipment can
be a major factor in fungal stain dissemination.
Spores will germinate on suitable wood surfaces, but the wood must be at or above the
fiber saturation point, at least in localized surface areas, for stain fungi to grow and penetrate.
Because these fungi must first gain a foothold in wood by accessing simple monomeric
or oligosaccharide substrates in the sapwood,
they typically do not penetrate deeply into
heartwood, as simple sugars and starches have
been depleted in the heartwood, and fungistatic
or fungitoxic phenolic extractives are present
instead. For this reason, these fungi are also
sometimes known as “sapstain” fungi. As the
fungal hyphae mature in wood, they produce
melanin in their cell walls, which is thought to
be useful in the protection of fungal hyphae
from sunlight UV exposure and resulting oxygen radical damage. This melanization also
occurs in some sporophores and is presumed
also to protect against sunlight. These fungi
may cause the wood to take on the appearance
of different colors, and in some cases the wood
can appear blue, green, yellow, red, or other
shades of the spectrum. Blue staining is perhaps
most noticeable, and it is caused when fungal
hyphae grow in the parenchyma, especially the
ray parenchyma, just under the surface of the
wood. As the fungal hyphae melanize, they take
on a dark brown or black coloration. This pigmentation is reflected through the crystalline
layers of cellulose, and as the light is scattered
by the crystalline cellulose, different colors may
be apparent. Most often a blue or gray color is
apparent to the eye, and for this reason this
type of wood is often described as “bluestained.” The appearance of different colors is
an optical illusion, much in the way that dark
soot particles in smoke can appear blue, or
other colors, as they scatter and reflect light.
Stain discoloration can occur under favorable conditions in as little as 3 days in lumber. Some softwood
genera with wide sapwood bands and large resin canals
such as pines (Pinus) will stain more readily than wood
species that have more heartwood and smaller/absent
resin canals such as spruce (Picea) or fir (Abies). Staining typically occurs at temperatures above 15
C, and
moving wood to a cold environment to arrest stain
development is a potential remediation method in
some cases. However staining at lower temperatures
has been reported in some areas where the fungi have
apparently adapted to colder climates (Miller and
Goodell 1981). Prevention of stain development can
be achieved by keeping logs wet to exclude oxygen
needed by the fungi for growth. In mill yards, water
sprinklers above log decks are sometimes used for this
purpose. Biocidal treatments to protect against wood
staining can also be used, although rapid processing to
get logs sawn and the lumber then dried to 19% MC or
less is preferred from both an environmental and
expense standpoint whenever possible.
388
B. Goodell
is evident in blue staining hyphae. Daniel
points out that wood species or types that are
less lignified, such as tension wood in hardwoods, can be more readily attacked by stain
fungi, and hemicellulose is more readily
removed by stain fungi in this type of wood
(Encinas and Daniel 1995; Daniel 2016).
Fungal stain genera commonly include Ceratocystiopsis, Ophiostoma, and Grosmannia,
with genomic information increasingly separating and reclassifying some stain fungi into new
genera (De Beer et al. 2013a, b). In species that
produce a sexual stage, these fungi produce
dark colored, flask-shaped, spore-bearing
structures known as perithecia that are produced on the surface of wood. Spore-containing
asci are produced in the base of the perithecia,
and as the perithecia matures, the asci and
spores are exuded through the neck of the perithecia for dissemination. Typically, the spore
mass is discharged in a moist viscous fluid
droplet which rests at the top of the opening
in the perithecia neck (Fig. 15.7b). Although
spores can be dispersed by wind and rain
water, many species of fungal stain that produce perithecia have adapted to be
disseminated by beetles. The height of the perithecia neck for particular fungal species has
evolved to match the height and anatomy of
particular beetles or other insects and arachnids (particularly mites). The viscous fluid
mass of spores is often described as a sticky
slime, which adheres to the body or legs of the
insects/arachnids as they feed upon the fungal
mycelium and spores. The spores are then
disseminated as the vector travels to other
sites. An excellent review of wood staining by
fungi and other agents (Uzunovic et al. 2008)
also reports that log harvesting equipment can
be a major factor in fungal stain dissemination.
Spores will germinate on suitable wood surfaces, but the wood must be at or above the
fiber saturation point, at least in localized surface areas, for stain fungi to grow and penetrate.
Because these fungi must first gain a foothold in wood by accessing simple monomeric
or oligosaccharide substrates in the sapwood,
they typically do not penetrate deeply into
heartwood, as simple sugars and starches have
been depleted in the heartwood, and fungistatic
or fungitoxic phenolic extractives are present
instead. For this reason, these fungi are also
sometimes known as “sapstain” fungi. As the
fungal hyphae mature in wood, they produce
melanin in their cell walls, which is thought to
be useful in the protection of fungal hyphae
from sunlight UV exposure and resulting oxygen radical damage. This melanization also
occurs in some sporophores and is presumed
also to protect against sunlight. These fungi
may cause the wood to take on the appearance
of different colors, and in some cases the wood
can appear blue, green, yellow, red, or other
shades of the spectrum. Blue staining is perhaps
most noticeable, and it is caused when fungal
hyphae grow in the parenchyma, especially the
ray parenchyma, just under the surface of the
wood. As the fungal hyphae melanize, they take
on a dark brown or black coloration. This pigmentation is reflected through the crystalline
layers of cellulose, and as the light is scattered
by the crystalline cellulose, different colors may
be apparent. Most often a blue or gray color is
apparent to the eye, and for this reason this
type of wood is often described as “bluestained.” The appearance of different colors is
an optical illusion, much in the way that dark
soot particles in smoke can appear blue, or
other colors, as they scatter and reflect light.
Stain discoloration can occur under favorable conditions in as little as 3 days in lumber. Some softwood
genera with wide sapwood bands and large resin canals
such as pines (Pinus) will stain more readily than wood
species that have more heartwood and smaller/absent
resin canals such as spruce (Picea) or fir (Abies). Staining typically occurs at temperatures above 15
C, and
moving wood to a cold environment to arrest stain
development is a potential remediation method in
some cases. However staining at lower temperatures
has been reported in some areas where the fungi have
apparently adapted to colder climates (Miller and
Goodell 1981). Prevention of stain development can
be achieved by keeping logs wet to exclude oxygen
needed by the fungi for growth. In mill yards, water
sprinklers above log decks are sometimes used for this
purpose. Biocidal treatments to protect against wood
staining can also be used, although rapid processing to
get logs sawn and the lumber then dried to 19% MC or
less is preferred from both an environmental and
expense standpoint whenever possible.
388
B. Goodell