Protecting Forest Resources From Disease

Harry R. Powers, Jr., chief research plant pathologist, Forestry Sciences Laboratory, Forest Services, Athens, GA.

Nations that fail to protect their forests risk serious economic and social consequences. That is why conservationists work hard to protect forests from over-exploitation and unwise use.

Recreational use of National Forest Lands not set aside as wilderness is likely to increase over the coming decades; settings that are neither primitive nor highly developed will see more intensive management.

Many people do not know, however, that forest tree diseases can be even more devastating than misuse. Diseases carelessly introduced from Europe and Asia have caused billions of dollars of lost revenue from America's forests, and modern forest-management practices have worsened the impact of some of our native tree diseases. Perhaps the worst examples of introduced diseases are chestnut blight and white pine blister rust, which were brought into the United States around the turn of the century. The former destroyed our most valuable native hardwood species, and the latter decimated white pine stands from New England to the Pacific Northwest. Before the advent of high-yield plantation forestry, fusiform rust of southern pines was not much of a problem. Now it is causing over $128 million a year in damage to southern forests.

Costs Must Be Low

The devastation of forest diseases is easy to recognize; the appropriate corrective action is less apparent. Although a forest may be worth a great deal, each individual tree in it is worth little. Even in the South, where trees grow very rapidly, it takes 25 years for a pine to reach pulpwood size and 35 years or more to reach saw-timber size. In the West, trees in managed forests typically are harvested when they are 80 to 120 years old. These are years of waiting for returns on investment. They are also years during which the trees are at risk from disease and insect attack.

How can our forests be protected from diseases at a reasonable cost? Years ago foresters came to the scientific community with that question. The research has been long and difficult, but answers are emerging. The answers will be somewhat different for each disease, but often they will include some tree breeding for disease resistance. Chemical treatments in forest situations are not economically feasible: the individual trees just aren't worth that kind of investment. Yet, as a group, those planted trees represent much of the Nation's timber supply for the 21st century.

Most of the economically important diseases of forest trees are caused by fungi, and trees often vary in their resistance to infection and damage by these fungi. Breeding of trees for disease resistance has proven practical for some diseases, particularly the rust diseases that produce cankers on stems and branches. As knowledge of the genetics of hosts and pathogens (disease-causing agents) increases, the number of diseases that can be controlled in this manner is likely to increase.

Breeding for resistance to tree diseases is most advanced in loblolly and slash pines, the two most commonly planted tree species in the Southern United States. The purpose has been to reduce the devastation of fusiform rust. In the next century, the experiences with these species and this disease are likely to be repeated for other trees and other diseases, so a review of progress with these southern pines is instructive.