The Black 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.