DISEASES AND THE FOREST

L. M. HUTCHINS.

Trees, no less than other large forms of life, are subject to diseases that reduce their growth, destroy their usefulness, or bring death. The threat of disease is ever present, from the time that a tree emerges as a seedling to the end of its useful life.

In the virgin forests this threat was lessened because through centuries of interaction a certain degree of natural balance between the trees and their disease enemies had been achieved. Man, in his need for land, shelter, fuel, and communication, however, upset this balance by cutting, clearing, burning, and planting. He brought in exotic tree species from foreign lands, too, and otherwise so changed the forests from their original, natural state that over most of the country the once-stabilized relations no longer exist and the danger of disease has increased.

With the new tree species or their products from abroad came new diseases, which have found here a more congenial environment than in their native habitats. Thus, chestnut blight was brought in from Asia; white pine blister rust on infected pine seedlings and the Dutch elm disease and its insect carriers on elm burl logs were brought in from Europe. For our native chestnut, the results have been disastrous. Our white pines have been saved only by the development of an effective method of control. Losses in American elm from the Dutch elm disease have been heavy, and the future of the species is still in doubt, despite progress in means of control.

Losses from presumably native diseases that have become epidemic are also assuming serious proportions in several places. A highly destructive virus disease, known as phloem necrosis, has killed thousands of elms in several midwestern cities. Littleleaf, a disease whose cause we do not yet know, is making heavy inroads into stands of shortleaf pine in the southern Piedmont. Another disease of undetermined cause, provisionally named pole blight, is spreading in second-growth western white pine stands in Idaho, Montana, and Washington. Altogether, since the turn of the century, more than 25 new forest-tree diseases, introduced or apparently native, have been discovered in this country. Not all have proved equally important, but the aggregate loss of trees from them has been tremendous.

Most of the losses, however, are not from diseases of the spectacular epidemic type, but rather from the many relatively inconspicuous diseases at work always in our forests in and on leaves, bark, wood, roots, seedlings, saplings, old trees. Best estimates place the annual saw-timber loss from heart rots in the forests of the United States at 1 1/2 billion board feet. It is these everyday insidious losses, as well as those from the spectacular epidemic diseases, that must be guarded against if our forests are to continue to supply the wood we need.

Everybody knows how necessary it is to protect farm and orchard crops cotton, tobacco, vegetables, grains, and fruits against disease. Even more important is the protection of forest trees, which occupy the land many years before they are harvested.

TREE DISEASES are of two main types, parasitic and nonparasitic. The parasitic or infectious diseases are frequently highly contagious. They are caused mainly by low forms of life, such as bacteria and fungi, by viruses, by microscopic eel worms or the nematodes, and by seed plants such as mistletoes and dodders.

Among the nonparasitic diseases are such disorders as the sunscald, winter injury, drought injury, root drowning or suffocation, nutritional excesses and deficiencies, and injury from gases, smoke, and fumes.

FUNGI cause most of the major losses from disease in forest trees and are the chief destroyers or deteriorating agents of forest products. They produce leaf spots and defoliation, wilts, blights, cankers, galls, heart rots, and root diseases. Trees that are weakened by fungi often are more susceptible to wind-throw and to attack by insects. In forest products, other fungi cause stains, molds, and decays that are responsible for much deterioration and loss in lumber, posts, poles, buildings, containers, and in wood used for other purposes.

Not all of the fungi in the forests arc harmful : Many fungi contribute to the health and growth of trees by converting the fallen leaves, twigs, and other forest debris into humus, an important constituent of forest soils and a source of nutrient elements for tree growth. Others combine intimately with the tiny feeding roots on some trees to form special absorbing bodies, called mycorrhizae, which are believed to enable the trees to take up nutrients from the soil more effectively than they would otherwise be able to do. Through the production of humus, fungi also tend to create a soil reaction that is unfavorable to the disease fungi that attack the roots of seedlings and young trees. A few fungi attack disease-producing fungi directly.

Diseases and fungi causing deterioration are commonly spread by wind, water, insects, and bird life. Soil fungi causing root rots and wilts may be carried on the wheels of vehicles or the feet of men or animals. The fungus causing canker stain of the planetree is carried on pruning tools and equipment. Virus diseases, such as the phloem necrosis of elm, are almost invariably spread by insects, as also are some stain and decay fungi.

THE EFFECTIVE CONTROL Of forest diseases must be based on a sound knowledge of them and of the forest environments under which they occur. Both direct and indirect methods are employed. Direct methods include the use of sprays, dusts, and soil treatments, the removal and destruction of affected trees or parts, the prescribed use of fire, and the removal of alternate hosts. Sprays and soil treatments are used in the nurseries to protect the seedling trees against diseases, and sprays and dusts to destroy the insect carriers of diseases of shade trees, such as the Dutch elm disease and the elm phloem necrosis. Eradication is particularly important where a dangerous disease has been accidentally introduced into a locality and is known to be of limited distribution. This was the case when the European larch canker was discovered in a limited area in Massachusetts. Thorough eradication was undertaken immediately, and the disease apparently has been eliminated.

FIRE IN THE FOREST ordinarily does more harm than good, but against the brown spot disease of longleaf pine in the Southern States it has a sanitary effect when properly timed.

The white pine blister rust offers an example of a disease that can be controlled through the removal of the alternate hosts, currants and gooseberries. The rust cannot spread directly from pine to pine, but the spores from the rust on pine are carried by the wind and are able to infect currants and gooseberries. Spores from the rust developed on these are, in turn, capable of infecting white pines. The removal of the currant and gooseberry bushes to a safe distance from white pines effectively protects the pines from the rust.

If a disease has become widespread and well-established, eradication is usually impracticable, and we may have to learn to live with it and to reduce losses through indirect methods of control. This applies to most of our native diseases.

The red rot of the ponderosa pine in the Western States is an example. The causal fungus enters the trunk through naturally occurring lower dead branches and it results in an average loss of about one-fourth of the total timber volume. It rarely enters through branches less than an inch in diameter, however. Control of the disease is possible either by pruning off the lower branches before they die or by growing the trees so closely together that the lower branches are shaded out before they become large enough to support the fungus.

Fire wounds are important places of entry for decay fungi, and the prevention of fire in the woods therefore is an effective indirect means of reducing losses from timber decays.

Other indirect methods involve the proper timing of cutting, the control of stand composition to give mixtures of tree species instead of pure stands, and the development and use of disease-resistant varieties.

VARIETIES RESISTANT to disease have been successful in field and fruit crops; there is every reason to expect that they should prove equally valuable in our future forest- and shade-tree plantings. Although work along this line has scarcely more than begun, an American elm resistant to the Dutch elm disease and others resistant to phloem necrosis, strains of mimosa resistant to the mimosa wilt, and white pine resistant to blister rust have been selected and tested. These resistant trees are now being propagated and soon will be available. Although the use of resistant varieties will not save the present susceptible stands of trees, it does offer a promise of future safety in their replacement.

The prevention of deterioration, such as from stain and decay, in forest products is an effective way of extending our national timber supply. It requires different methods from those that can be used on living trees. The young-growth timber now coming into use is less resistant to decay than the wood from the older stands. Modern chemical treatments and more careful drying and storage practices make it possible to avoid damage to lumber, logs, pulpwood, and similar products, however, and are lengthening the useful life of posts, poles, railway ties, and other wood used in contact with the ground. The prompt salvage of timber that is killed by fire, insects, and disease is saving for use much timber that was formerly left to rot.

IN BUILDINGS AND OTHER CONSTRUCTION, the chances of decay is reduced by drainage of sites, use of seasoned lumber, elimination of direct contacts of wood with soil, care to keep rain from entering joints, ventilation or soil coverage under basementless houses, the judicious placing of vapor seals, and the use of preservatives.

With the ever-quickening disappearance of accessible stands of old-growth timber and with world-wide timber shortages brought on by war, the importance of disease in its effect on the future timber crop is rapidly increasing. Losses that formerly passed almost unnoticed can no longer be tolerated from the standpoint of solvency of the timber owner no less than from the public interest. We must be able to grow good wood and grow it profitably. That can be done only if disease losses are held to a reasonable minimum.

The field to be covered by the specialist in forest diseases is immense. More than 100 tree species of commercial importance occur in the forests of the United States; each presents an individual disease problem. When account is taken of the fact that our forest industry ranks fourth in importance in the Nation, the investigative effort devoted to diseases affecting this resource up to the present does not seem proportionate to the values at stake.

L. M. HUTCHINS is head pathologist in charge of the Division of Forest Pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering. He is known for his extensive investigations of virus diseases of trees.