Hills beetle has killed billions of feet of marketable
timber in the Black Hills of South Dakota. The hickory bark
beetle, the Douglas fir beetle and the larch worm have been very
destructive.

Forest fungi cause most of the forest tree diseases. A tree
disease is any condition that prevents the tree from growing and
developing in a normal, healthy manner. Acid fumes from smelters,
frost, sunscald, dry or extremely wet weather, all limit the
growth of trees. Leaf diseases lessen the food supplies of the
trees. Bark diseases prevent the movement of the food supplies.
Sapwood ailments cut off the water supply that rises from the
roots. Seed and flower diseases prevent the trees from producing
more of their kind.

Most of the tree parasites can gain entrance to the trees only
through knots and wounds. Infection usually occurs through wounds
in the tree trunk or branches caused by lightning, fire, or by
men or animals. The cone-bearing trees give off pitch to cover
such wounds. In this way they protect the injuries against
disease infection. The hardwood trees are unable to protect their
wounds as effectively as the evergreens. Where the wound is
large, the exposed sapwood dies, dries out, and cracks. The fungi
enter these cracks and work their way to the heartwood. Many of
the fungi cannot live unless they reach the heartwood of the
tree. Fires wound the base and trunks of forest trees severely so
that they are exposed to serious destruction by heartrot.

Foresters try to locate and dispose of all the diseased trees in
the State and Government forests. They strive to remove all the
sources of tree disease from the woods. They can grow healthy
trees if all disease germs are kept away from the timberlands.
Some tree diseases have become established so strongly in forest
regions that it is almost impossible to drive them out. For
example, chestnut blight is a fungous disease that is killing
many of our most valuable chestnut trees. The fungi of this
disease worm their way through the holes in the bark of the
trees, and spread around the trunk. Diseased patches or cankers
form on the limbs or trunk of the tree. After the canker forms on
the trunk, the tree soon dies. Chestnut blight has killed most of
the chestnut trees in New York and Pennsylvania. It is now active
in Virginia and West Virginia and is working its way down into
North and South Carolina.

[Illustration: SECTION OF A VIRGIN FOREST]

Diseased trees are