_Polyporus hispidus_ is the cause of trunk cankers and localized decay
of hardwoods throughout eastern, central, and southern United States.
The fungus is also known to attack hardwoods in Oregon and California.
Reported hosts are: hickory, ash, mulberry, willow, walnut, and oak. In
the south, _P. hispidus_ is common on oaks, including willow, water,
black, white, Nuttall, and cherrybark.
[Illustration: Fruiting body and canker of Hispidus on oak.]
The fungus usually enters the tree through dead branch stubs, from which
it grows into the heartwood. After becoming established, the fungus
begins penetrating the sapwood and attacks the living cambium. Callus
folds are formed by the host around the dead cambial area, forming an
elongated swollen canker, commonly referred to as a “hispidus canker”.
The cankered area of the stem is bark-covered and sunken, usually
containing a remnant of a branch stub or branch scar. During late
summer, fall or early winter, the fungus produces conks (sporophores) on
the surface of the cankers. The annual bracket-shaped conks are large,
spongy, and yellowish-brown to rusty-brown on the upper surface. When
fresh, the under surface is a light tan color. After a few months, the
mature conks dry to a rigid black mass and fall from the canker. Old
conks are commonly found at the base of cankered trees during the spring
and early summer. Decay produced by the fungus appears spongy, light
yellow, and is commonly separated from the sound wood by a black zone
line. The rot is of the delignifying white rot type. On southern oaks,
the rate of canker elongation has been estimated at one-half foot per
year, with the internal rot usually extending about one foot above and
below the cankered area.
No effective control in forest stands is known. Removal of diseased
trees provides additional growing space for crop trees. Some degree of
shade tree protection can be obtained by pruning of dead branches flush
with the main stem of the tree.