lesser amounts of total lignin in the cell walls, more
readily attacked. Most species of soft rot fungi isolated
from softwood poles are capable of producing both
Type I and Type II soft rot attack with one, Phialophora
fastigiata, producing Type II attack followed sporadically by Type I attack with longer exposure in laboratory testing (Morrell and Zabel 1985).
III. Staining and Mold Fungal
Degradation of Wood
This section reviews fungi that inhabit the surface of wood or that penetrate primarily into
the parenchyma of sapwood and cause only
limited amounts of degradation to the nonstructural wood components. It is important
to recognize that other causes of discoloration
and surface coloration of wood and wood products occur, and casual observation by laypersons is often not enough to distinguish fungal
stain and mold from these other causes.
A. Mold Fungi
Mold fungi typically are classified in the Ascomycota, but where sequencing has not been
done and the sexual stage of the mold fungus
is unknown, many are still classified as fungi
imperfecti. Mold fungi inhabit the surface of
wood and feed on simple surface sugars and
starches. Typically, the mycelial growth is hyaline (translucent), and the spores are pigmented, which discolors the surface of wood
resulting in a degraded appearance in finished
lumber or other appearance-graded wood products. Most mold fungi grow on a variety of
surfaces, and they require only surface moisture and a nutrient source for growth. The same
species can often grow on inert objects like
glass or plastic when those materials are coated
or spotted with a thin layer of plant or animal
residues that contain enough nutrient to support growth of the mold. Mold fungi such as
Trichoderma/Hypocrea are commonly used in
bioprocessing and have been engineered to
produce special enzymes for biorefinery and
other industrial applications.
When growing on the surface of wood,
sugars from parenchyma will substitute for the
sugar residues from other plant or animal residues to enable fungal mold growth. The presence of mold on structural lumber does not
cause any structural damage, and in many
areas of the world, structural lumber with mold
growth is marketed and classified with no defect;
the same as un-molded wood. However, for
esthetic reasons and also because of growing
concern about mold spores from a human and
animal health perspective (World-HealthOrganization 2009), it may be desirable to
prevent mold from occurring on wood or
remediating any mold growth. As reviewed
earlier in the introduction of this chapter,
some architectural manuals indicate that
fungi—including mold—will grow on wood
at only 20% MC. Technically, this is incorrect
as the mold fungi require liquid moisture to
survive and sporulate; however, at 20% MC
with the cycling of temperature, water can
condense on the surface of wood to permit
mold growth. In these localized zones of the
wood surface, liquid water in the surface capillaries would occur, permitting mold growth.
B. Staining of Wood by Fungi
As noted in the section on soft rot fungi, some
stain fungi can intergrade with, and cause, soft
rot under certain conditions. Stain fungi can
also intergrade with mold fungi, and in early
stages of stain fungal growth, when these fungi
may grow just on the surface of wood and
produce spores, they may be considered to be
mold fungi. However, true stain fungi have the
ability to penetrate into wood to seek out
sugars, starches, but also wood resins and
waxes (Ma ¨kela ¨ et al. 2015) in the parenchyma
cells. Some stain fungi also have the ability to
remove the more amorphous polysaccharide
components such as pectins and hemicellulose.
Staining fungi typically produce fine bore holes
in wood (Fig. 15.7a) although these are smaller
diameter bore holes than seen with wood decay
386
B. Goodell
readily attacked. Most species of soft rot fungi isolated
from softwood poles are capable of producing both
Type I and Type II soft rot attack with one, Phialophora
fastigiata, producing Type II attack followed sporadically by Type I attack with longer exposure in laboratory testing (Morrell and Zabel 1985).
III. Staining and Mold Fungal
Degradation of Wood
This section reviews fungi that inhabit the surface of wood or that penetrate primarily into
the parenchyma of sapwood and cause only
limited amounts of degradation to the nonstructural wood components. It is important
to recognize that other causes of discoloration
and surface coloration of wood and wood products occur, and casual observation by laypersons is often not enough to distinguish fungal
stain and mold from these other causes.
A. Mold Fungi
Mold fungi typically are classified in the Ascomycota, but where sequencing has not been
done and the sexual stage of the mold fungus
is unknown, many are still classified as fungi
imperfecti. Mold fungi inhabit the surface of
wood and feed on simple surface sugars and
starches. Typically, the mycelial growth is hyaline (translucent), and the spores are pigmented, which discolors the surface of wood
resulting in a degraded appearance in finished
lumber or other appearance-graded wood products. Most mold fungi grow on a variety of
surfaces, and they require only surface moisture and a nutrient source for growth. The same
species can often grow on inert objects like
glass or plastic when those materials are coated
or spotted with a thin layer of plant or animal
residues that contain enough nutrient to support growth of the mold. Mold fungi such as
Trichoderma/Hypocrea are commonly used in
bioprocessing and have been engineered to
produce special enzymes for biorefinery and
other industrial applications.
When growing on the surface of wood,
sugars from parenchyma will substitute for the
sugar residues from other plant or animal residues to enable fungal mold growth. The presence of mold on structural lumber does not
cause any structural damage, and in many
areas of the world, structural lumber with mold
growth is marketed and classified with no defect;
the same as un-molded wood. However, for
esthetic reasons and also because of growing
concern about mold spores from a human and
animal health perspective (World-HealthOrganization 2009), it may be desirable to
prevent mold from occurring on wood or
remediating any mold growth. As reviewed
earlier in the introduction of this chapter,
some architectural manuals indicate that
fungi—including mold—will grow on wood
at only 20% MC. Technically, this is incorrect
as the mold fungi require liquid moisture to
survive and sporulate; however, at 20% MC
with the cycling of temperature, water can
condense on the surface of wood to permit
mold growth. In these localized zones of the
wood surface, liquid water in the surface capillaries would occur, permitting mold growth.
B. Staining of Wood by Fungi
As noted in the section on soft rot fungi, some
stain fungi can intergrade with, and cause, soft
rot under certain conditions. Stain fungi can
also intergrade with mold fungi, and in early
stages of stain fungal growth, when these fungi
may grow just on the surface of wood and
produce spores, they may be considered to be
mold fungi. However, true stain fungi have the
ability to penetrate into wood to seek out
sugars, starches, but also wood resins and
waxes (Ma ¨kela ¨ et al. 2015) in the parenchyma
cells. Some stain fungi also have the ability to
remove the more amorphous polysaccharide
components such as pectins and hemicellulose.
Staining fungi typically produce fine bore holes
in wood (Fig. 15.7a) although these are smaller
diameter bore holes than seen with wood decay
386
B. Goodell