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.