U.S.A. THE maintenance of sustained yields in high quality timber is one of the prime aims of research in forestry. In the disease category one finds not only the pathogenic agents that destroy foliage and kill trees but also those that cause root, butt, and trunk rot. In the United States, the total growth impact of forest diseases is estimated to be about 45 percent of the losses caused by all destructive agents. Timber resources for America's future . In North America, the invasion of foreign forest tree diseases has caused tremendous losses.
Examples are chestnut blight, white pine blister rust, and Dutch elm disease.
The latter is insect borne. The importance of injurious insects that might come from abroad also is clearly recognized. However, the insects are outside the scope of the present consideration. Epidemic diseases are much more serious for forest trees than for comparable agricultural crops. If a field of wheat should be destroyed, something else could be planted the next year.