Tree diseases are important components in healthy forest ecosystems
but they can also have a detrimental impact on forest productivity
and a devastating impact on urban tree populations. We continually
are plagued with exotic pests that have been introduced into the
United States, such as white pine blister rust and Dutch elm disease.
We also have epidemics occurring from native diseases because of
different environmental conditions, reduced levels of resistance
in the host and disturbances in the forest from urban development
and other activities that have changed the landscape. In forests,
diseases are responsible for more than 65% of the wood volume lost
each year and in urban areas diseases continue to be the major cause
of tree decline and death. One of the reasons diseases cause such
drastic losses is due to an incomplete understanding of basic biological
and ecological processes that take place. We investigate how microbes,
woody plants and the environment interact so a better understanding
of these complex associations can be obtained and more appropriate,
environmentally safe integrated control procedures developed.
Wood decay fungi
Wood decay fungi cause billions of dollars in losses each
year by destroying wood in forest trees that could be used for timber,
by attacking urban shade trees and by causing decay in buildings
and other wood in service. Our research is providing new information
on how these microbes function and finding better ways to control
them. Although the negative impact of decay fungi can cause huge
economic losses, new investigations on some forest microbes indicate
that they can be used beneficially in a variety of new technologies.
We have found that many microorganisms can be used for biotechnological
processes in the wood products, pharmaceutical, and agricultural
industries and we have selected and evaluated superior microbes
for biological control, biopulping, biobleaching, depitching of
wood chips before paper production, bioremediation of toxic substances,
etc. These investigations will contribute significantly to more
environmentally safe processes for industry, an improved way of
life, and new economies from industries using these new biotechnological
Microbes that degrade wood produce
extracellular enzymes that break down the woody cell wall. Growth
characteristics of the microorganism in wood and the type of degradative
system produced results in different decay patterns being produced.
Depending on the type of decay, different physical, chemical and
morphological changes occur in wood. These decay processes have
been well characterized and provide useful insights to elucidate
deterioration in living trees and wood in service.
An examination of decay patterns produced by different fungi suggests
that three categories can be used to separate the types of decay
produced in wood. Names for these categories are based on visual
characteristics of the advanced decay. Two major groups of decay
produced by fungi taxonomically classified in the subdivision Basidiomycota,
are white and brown rot fungi.
White rot fungi
White rot fungi can degrade all cell wall components, including
lignin. They often cause a bleaching of normal wood coloration.
Their ability to metabolize large amounts of lignin in wood is unique
among microorganisms. The thousands of species that cause white
rots are a heterogeneous group that may degrade greater or lesser
amounts of a specific cell wall component. Some species preferentially
remove lignin from wood leaving pockets of white, degraded cells
that consist entirely of cellulose, while others degrade lignin
and cellulose simultaneously. Degradation is usually localized to
cells colonized by fungal hyphae and substantial amounts of undecayed
wood remains even after advanced decay has occurred. A progressive
erosion of the cell wall occurs when components are degraded simultaneously
or a diffuse attack of lignin may occur by species that preferentially
remove lignin . Strength losses are not significant until late stages
of decay. White-rot fungi are common parasites of heartwood in living
trees and are aggressive decomposers of woody debris in forest ecosystems.
Cross section of an oak tree with white rot. The fungus has
decayed the sapwood and dark heartwood turning it white. This
white rot fungus attacked all cell wall components.
Scanning electron micrograph showing the hypha of a white
rot fungus in the cell lumen of a wood cell. Extracellular
enzymes are degrading all of the cell wall components simultaneously
causing erosion troughs to form in the cell wall
A cross section of wood with white rot showing the fungus
has degraded some cells completely but not others.
A split section of a pine tree with white-pocket rot caused
by Phellinus pini. The white areas are delignified
zones where the fungus has removed lignin but not the cellulose.
White-pocket rot fungi cause a selective attack on lignin
and hemicellulose in wood.
A mottled white rot in wood decayed by Ganoderma applanatum.
This fungus causes a combination of delignification and a
simultaneous white rot attack in the wood. White areas with
black spots containing manganese (deposited by the fungus)
are delignified while the tan areas have a simultaneous white
rot. In the tan areas large degraded zones form and these
holes fill with white mycelium of the fungus.
A cross section of wood from a white-pocket area of decayed
wood showing delignified wood cells. These cells have no middle
lamella (this is the area between cells that has high lignin
concentration). Only the cellulose-rich secondary walls remains
after advanced decay.
Brown rot fungi
Brown-rot fungi depolymerase cellulose rapidly during incipient
stages of wood colonization. Considerable losses in wood strength
occur very early in the decay process, often before decay
characteristics are visually evident. Cell wall carbohydrates
are degraded extensively during decay leaving a modified,
lignin-rich substrate . The residual wood is brown and often
cracks into cubical pieces when dry. Brown-rot fungi commonly
cause decay in living trees, downed timber and wood used in
buildings. Since large losses of wood strength result from
brown rot, living trees with this decay can be hazardous and
wood in service may fail. Commonly, this type of decay has
been referred to as dry rot. This term, apparently first used
to describe any deterioration of dead wood or wood in service
is misleading because moisture must be present for the decay
An urban tree with brown rot. The large branch failed
and broke off due to the presence of decay. Brown rot
has little structural integrity and large losses of
wood strength occurs early in the decay process. Urban
trees with decay can be very hazardous.
A closer view of brown- rooted wood is shown in this
photo. In advanced stages of decay the wood cracks and
checks into cubicle pieces. Little to no integrity remains
in this decayed wood.
Scanning electron micrograph of brown-rotted wood. Only
slight pressure causes the wood cell walls to crumble
into minute fragments.
|Soft rot fungi
Fungi that cause soft-rot are taxonomically classified
in the subdivisions, Ascomycota and Deuteromycota. Soft
rot was first characterized as a soft, decayed surface
of wood in contact with excessive moisture. However, soft
rots can occur in dry environments and may be macroscopically
similar to brown rot. Two distinct types of soft rot are
currently recognized. Type 1 is characterized by longitudinal
cavities formed within the secondary wall of wood cells
and Type 2 used to describe an erosion of the entire secondary
wall. The middle lamella is not degraded (in contrast
to cell wall erosion by white-rot fungi), but may be modified
in advanced stages of decay. Large strength losses in
wood can be associated with soft rot attack. Cavities
formed in the wood as well as extensive cellulose degradation
can result in extremely poor strength characteristics
when soft-rot wood is visually evident. As decay progresses,
extensive carbohydrate loss occurs and lignin concentrations
increase in the residual wood.
Soft rot in wood often appears brown and can
be confused with decay caused by brown rot fungi.
Soft rot is different from other types of wood
decay. Chains of cavities are produced inside
the cell wall. This micrograph taken of a section
from soft-rotted wood and viewed with a light
microscope shows cavities within the cell walls.
For more information on this topic see:
Eriksson, K.-E. L., R. A. Blanchette and P. Ander.
1990. Microbial and enzymatic degradation of wood and
wood componenets. Springer-Verlag. Berlin, New York.
Blanchette, R. A. and A. R. Biggs. Defense mechanisms
of woody plants against fungi. Springer-Verlag, Berlin,
New York, 458 pages.
University of Minnesota class web site for Diseases
of Forest and Shade Trees