_Polyporus schweinitzii_ is a common cause of root and butt rot of
conifers throughout North America. The primary hosts of the fungus are
Douglas fir, spruce, and pine. All southern pines are susceptible to
attack by _P. schweinitzii_. Common names of the rot are: red-brown butt
rot and brown cubical butt rot.
[Illustration: Fruiting body of brown cubical root and butt rot of
pine.]
The fungus enters living hosts through damaged roots, fire scars, and
other wounds near the tree base. The initial stage of decay appears as a
light yellow stain. In the advanced stage, the heartwood becomes brittle
and breaks into large yellow-brown to reddish-brown cubes. The fungus
develops primarily in the roots and butt and seldom extends more than 15
or 20 feet up into the stem. Diseased trees are subject to wind-throw
and wind breakage. Although the volume of wood destroyed by the rot is
small, the total volume lost through wind-throw is quite large. Mature,
suppressed, and weakened off-site trees are commonly attacked by the
fungus. It is assumed that the fungus may also spread from infected to
healthy trees through root contacts and grafts. The only outward signs
of decay are the annual sporophores, which develop in late summer and
fall during moist weather. Conks formed at the base of infected trees
are bracket shaped, while those arising from decayed roots appear
circular, sunken in the center, and supported by a short stalk. When
fresh, the upper surface is velvety, concentrically zoned, and
reddish-brown in color with a light yellow margin. The underside is dark
olive or green with large irregular pores.
In forest stands, no method of controlling the disease is known. Losses
may be prevented to some extent by reducing the amount of root damage
and wounding from heavy logging equipment. The prevention of basal fire
scars in conifer stands will also reduce the incidence of this disease.
Trees which show signs of advanced root and butt rot should be removed
from around recreation areas, parking lots, power lines, and buildings
to avoid damage from wind-throw and wind breakage.