Wood decay of southern forest trees is responsible for nearly 80 percent
of all loss attributed to disease. This decay is caused by fungi which
mainly attack heartwood in the central portion of stems, branches, and
roots. Wood-rotting fungi gain entrance into the tree through broken
branches, wounds, and damaged or exposed roots. Spores, which land at
these damaged areas, germinate and produce a microscopic mycelium which
attacks and spreads throughout the heartwood. The decay is caused by the
action of the mycelium, which penetrate the cell walls and dissolve or
alter the wood in various ways. Fungus development within the tree may
continue for many years without any apparent effect on the growth of the
host. Eventually the mycelium will aggregate and break through the bark
to form the reproductive stage, either before or after the death of the
host. The fruiting body (sporophore, conk) produces vast amounts of
spores which are capable of spreading the fungus to other trees.
Heartrots may be separated into broad classes on the basis of the host
portion attacked, such as root rots, root and butt rots, stem rots, and
top rots. Decay fungi may be further separated into two broad classes
based on their effect on wood. The first class causes white rots,
decomposing all components of the wood and reducing it to a spongy mass
with white pockets or streaks separated by firm wood. The second class,
causing brown rots, utilize the cellulose, leaving the lignin more or
less unaffected. This usually results in a rot which appears as some
shade of brown.
The separation of wood decay fungi on the basis of their host range, the
portion of the host attacked, and the type of rot produced are useful
aids to a pathologist in determining a tentative identification of the
fungus responsible for a particular type of rot. However, there are
numerous fungi which cause decay, many of which are exceptions to the
various methods of classification. This forces the pathologist to use
microscopic examination and various artificial keys to arrive at the
proper identification of a given rot-producing fungus.
[Illustration: Sectioned stump showing rot and decay in heartwood.]