promote the activity of lytic polysaccharide
monooxygenases (LPMOs) for potential
enhancement of biorefinery activities (Brenelli
et al. 2018).
D. Soft Rot/Ascomycota Fungi
Soft rot fungal degradation is traditionally considered to be a type of decay where the outer
few millimeters of wood surface wood are
attacked in relatively wet environments by
Ascomycota genera. In some cases, the fungal
species causing soft rot attack of wood may still
informally be classified as fungi imperfecti,
when these fungi have not yet been examined
using molecular systematics. The surface attack
by soft rot often occurs when the wood is in a
waterlogged or partially submerged condition
such as in driftwood or industrial cooling tower
slats, resulting in a loss of surface density
(hardness). The traditional perspective (since
the 1950s) of how soft rot fungi attack wood is
changing however, as it is now known that soft
rot fungal attack can occur in both wet and dry
environments. Further, it has also been
observed that these fungi can penetrate several
centimeters deep into the wood, particularly
when the wood has been buried, as observed
in field tests and in-service poles (Greaves
1977).
Researchers have pointed out that in wet
environments, the exclusion of oxygen deep in
wood probably prevents growth of soft rot
fungi other than at the surface of wood (Worrall
et al. 1991). Soft rot fungi seem to have a more
selective requirement for oxygen than Basidiomycota species and some bacteria, and therefore decay fungi or bacteria that attack wood
would outcompete soft rot fungi in many environments.
All wood undergoing decay ultimately has
reduced density, so to some degree the term
soft rot is misnomer. Further, previous reports
(Daniel and Nilsson 1998) have also noted that
soft rotted wood, particularly in chromated
copper arsenate (CCA wood preservative)-treated poles, is quite hard at the surface. Upon
drying, soft rotted wood also becomes brown
and develops surface checks across the grain as
the wood shrinks. Although the decay may be
superficial, the surface appearance may be similar to brown rot decay. The wood may assume
a weathered appearance, similar to unpainted
“barn board” (Zabel et al. 1985). Several common genera of soft rot include Chaetomium,
Humicola, and Lecythophora. Soft rot in North
America is less commonly reported as a structurally degrading microorganism compared to
brown rot, and it is more widely reported in
other environments, ranging from tropical
to temperate and even polar environments
(Blanchette et al. 2004).
Two different types of attack on the wood
cell wall can be produced by soft rot fungi. Type
I soft rot is characterized by cavity formation,
which typically occurs in the S 2 layer of the
wood cell wall (the thickest layer of the secondary cell wall of lignified woody tracheids or
fibers—Fig. 15.6). These cavities may also be
formed in some cases in the adjacent S 1 layers,
often in the same cell. Type II soft rot is a
general erosion of the wood cell wall layers
similar to that seen in white rot, and similarly,
starting from the S 3 -lumen interface and working outward. Often, particularly in hardwoods,
both Type I and Type II attack can be produced
by the same fungus in the same sample. Soft rot
fungi are sensitive to high levels of lignin and
preferentially grow in low-lignin hardwoods
such as Populus or Betula. Lignin levels in the
wood also impact the pattern of attack which
can occur within the wood cell wall (Daniel and
Nilsson 1998).
Type I Soft Rot:
Soft rot fungi typically initiate Type I attack using
fine microhyphae, to penetrate from the lumen and
bore perpendicularly into the wood cell wall (Unligil
and Chafe 1974; Daniel 2014; Hale and Eaton 1985). The
fungal microhyphae reorient and realign in the direction of the S 2 cellulose microfibrils when they reach the
S 2 layer. T-branching of the hyphae also occurs once
the hyphae are within the S 2 layer, with the two hyphal
tips of the branches then growing in opposite directions, spiraling in the long direction of the fiber. At the
start of cavity formation, the microhyphae extend longitudinally into the S 2 cell wall via development of what
384
B. Goodell
monooxygenases (LPMOs) for potential
enhancement of biorefinery activities (Brenelli
et al. 2018).
D. Soft Rot/Ascomycota Fungi
Soft rot fungal degradation is traditionally considered to be a type of decay where the outer
few millimeters of wood surface wood are
attacked in relatively wet environments by
Ascomycota genera. In some cases, the fungal
species causing soft rot attack of wood may still
informally be classified as fungi imperfecti,
when these fungi have not yet been examined
using molecular systematics. The surface attack
by soft rot often occurs when the wood is in a
waterlogged or partially submerged condition
such as in driftwood or industrial cooling tower
slats, resulting in a loss of surface density
(hardness). The traditional perspective (since
the 1950s) of how soft rot fungi attack wood is
changing however, as it is now known that soft
rot fungal attack can occur in both wet and dry
environments. Further, it has also been
observed that these fungi can penetrate several
centimeters deep into the wood, particularly
when the wood has been buried, as observed
in field tests and in-service poles (Greaves
1977).
Researchers have pointed out that in wet
environments, the exclusion of oxygen deep in
wood probably prevents growth of soft rot
fungi other than at the surface of wood (Worrall
et al. 1991). Soft rot fungi seem to have a more
selective requirement for oxygen than Basidiomycota species and some bacteria, and therefore decay fungi or bacteria that attack wood
would outcompete soft rot fungi in many environments.
All wood undergoing decay ultimately has
reduced density, so to some degree the term
soft rot is misnomer. Further, previous reports
(Daniel and Nilsson 1998) have also noted that
soft rotted wood, particularly in chromated
copper arsenate (CCA wood preservative)-treated poles, is quite hard at the surface. Upon
drying, soft rotted wood also becomes brown
and develops surface checks across the grain as
the wood shrinks. Although the decay may be
superficial, the surface appearance may be similar to brown rot decay. The wood may assume
a weathered appearance, similar to unpainted
“barn board” (Zabel et al. 1985). Several common genera of soft rot include Chaetomium,
Humicola, and Lecythophora. Soft rot in North
America is less commonly reported as a structurally degrading microorganism compared to
brown rot, and it is more widely reported in
other environments, ranging from tropical
to temperate and even polar environments
(Blanchette et al. 2004).
Two different types of attack on the wood
cell wall can be produced by soft rot fungi. Type
I soft rot is characterized by cavity formation,
which typically occurs in the S 2 layer of the
wood cell wall (the thickest layer of the secondary cell wall of lignified woody tracheids or
fibers—Fig. 15.6). These cavities may also be
formed in some cases in the adjacent S 1 layers,
often in the same cell. Type II soft rot is a
general erosion of the wood cell wall layers
similar to that seen in white rot, and similarly,
starting from the S 3 -lumen interface and working outward. Often, particularly in hardwoods,
both Type I and Type II attack can be produced
by the same fungus in the same sample. Soft rot
fungi are sensitive to high levels of lignin and
preferentially grow in low-lignin hardwoods
such as Populus or Betula. Lignin levels in the
wood also impact the pattern of attack which
can occur within the wood cell wall (Daniel and
Nilsson 1998).
Type I Soft Rot:
Soft rot fungi typically initiate Type I attack using
fine microhyphae, to penetrate from the lumen and
bore perpendicularly into the wood cell wall (Unligil
and Chafe 1974; Daniel 2014; Hale and Eaton 1985). The
fungal microhyphae reorient and realign in the direction of the S 2 cellulose microfibrils when they reach the
S 2 layer. T-branching of the hyphae also occurs once
the hyphae are within the S 2 layer, with the two hyphal
tips of the branches then growing in opposite directions, spiraling in the long direction of the fiber. At the
start of cavity formation, the microhyphae extend longitudinally into the S 2 cell wall via development of what
384
B. Goodell