Littleleaf in Pines in the Southeast

W. A. Campbell, Otis L. Copeland, Jr., George H. Hepting.

Littleleaf, a disease of shortleaf (Pinus echinata) and loblolly (P. taeda) pines, was first observed in Alabama in 1934. The early symptoms are a yellowing of the foliage and a reduction in growth. Shoot growth and needle length become greatly reduced as the disease progresses, and often the affected trees produce an abundant crop of small cones. Trees with advanced symptoms are conspicuous because of their short, yellow needles, which are confined to the ends of the branches. The disease rarely affects trees younger than 20 years of age and is most prevalent in stands that are 30 to 50 years old.

Littleleaf occurs in varying amounts in Mississippi, Alabama, Georgia, South Carolina, North Carolina, Virginia, and Tennessee. The littleleaf belt, from Virginia to Mississippi, embraces approximately 30millionacres, of which over half is in forests. Areas of abundant littleleaf cover about 5 million acres of this forest land. But even in areas of greatest abundance, little-leaf is not uniformly distributed throughout stands of shortleaf pine. Age of stand, soil conditions, and degree of erosion are important factors in the distribution of the disease. Stands with severe littleleaf may be found close to healthy stands. The spotty occurrence of littleleaf in many areas and often the relative lack of the disease in nearby stands are generally determined by soil relationships.

A survey made in 1947 in South Carolina disclosed that 118 million board feet of shortleaf pine were recently dead or in the advanced stages of the disease. That means that about 39 million board feet would die each year, because trees in the advanced stages have a life expectancy of less than 3 years. It has been estimated for the South as a whole that littleleaf kills nearly 200 million board feet of shortleaf pine and 20 million board feet of loblolly pine each year. In addition, more than 250,000 cords of pine in the 6-8-inch age classes are annually killed by the disease.

Not all expected mortality can be considered as loss, because much salvage cutting of diseased trees before death is done in areas affected by littleleaf. Diseased trees may live 10 or more years from the onset of symptoms, but they do not increase materially in size. Another type of indirect loss arises from the accelerated conversion of pine stands to hardwoods, which takes place as trees are killed by littleleaf.

The symptoms of littleleaf develop largely from the gradual killing of the fine roots by the parasitic fungus Phytophthora cinnamomi and probably to a lesser extent from other causes. New root ends and very young roots are most susceptible to invasion by the fungus. The gradual loss of the fine roots interferes especially with the absorption of nitrogen, and leads to a slow cessation of growth, yellowing of the foliage, and premature death.

The fungus, widely distributed in the soils of the Southeast, occurs in soils in the Coastal Plain and elsewhere where littleleaf is absent. Its ability to attack roots severely depends to a great extent on soil type and condition. Experimental work has shown that root infection is limited in sandy soils, more common in looms, and severe in silts and clays. As the fungus requires abundant moisture to produce the motile spores that cause infection, root damage increases as the subsoils become more poorly drained. The relationship between internal drainage of the soil and root infection helps to explain why the disease is confined to the heavier soils of the Piedmont and related areas.

A person needs to understand the dependence of the root parasite on certain soil conditions in order to develop plans for the management of littleleaf areas. The disease does not normally spread directly from diseased trees to healthy trees. The fungus is already present in most southeastern pine soils and the development of littleleaf is conditioned almost entirely by soil factors that cannot be changed materially except possibly over long periods of time.

Early studies suggested that severe littleleaf was associated with frequent burning, poor pine sites, soil erosion, and steep slopes. Then in 1948, on the Calhoun Experimental Forest near Union, S. C., on various soil associations, the severity of littleleaf was found to increase as internal drainage of the soil became poorer. This discovery stimulated a more extensive study of the relationship of soils to the disease throughout the littleleaf region of the southern Piedmont in 1949. Forest vegetation, soil characteristics, and severity of disease were analyzed in the search for factors associated with the disease. The analysis disclosed that although disease severity may vary from site to site on any one particular kind of soil, the average severity follows a fairly regular relationship to the internal drainage condition of a particular soil series. Internal drainage is defined as the quality of a soil that permits the downward flow of excess water through it. Soil texture, structure, porosity, height of water table, and other factors influence it. Soils with poor internal drainage thus are usually more nearly saturated with water and less well aerated for long periods. In such soils pines develop poor root systems and infections by root fungi are favored.

Although more than 30 factors of soil, vegetation, or fungus have been studied in relationship to littleleaf, only internal drainage and the degree of soil erosion are significantly correlated with the disease severity.

In Piedmont soil groups a striking relationship exists between the internal drainage of soils and the occurrence of littleleaf. In Durham, Alamance, Cecil, Georgeville, Davidson, Lockhart, Lloyd, and Nason soils with excellent to good internal drainage the percentage of diseased shortleaf pine was 3.9. On Louisa, Madison, Appling, and Helena soils with good to poor internal drainage it was 12.1 percent. On Cataula, Mecklenburg, Grange, Vance, Tatum, Herndon, Manteo, and Wilkes soils with generally poor internal drainage 25.5 percent of the trees had the disease.

Although pine stands as young as 20 years of age may be affected, most of those that become diseased first show symptoms when they are 30 to 50 years old. In general, the younger the age of an attacked stand, the more severe the effect will be obviously, greater loss will result in the stands that show disease at an early age, because most of the diseased trees that should be salvaged will be too small to market except as pulpwood. Stands contracting the disease when they are older often produce a sizable volume of saw timber.

The first problem in dealing with littleleaf stands is that of properly spacing cuttings to minimize the loss of Merchantable timber. Suggested intervals for harvesting are based on the amount of disease in the stand and the rate of deterioration of affected trees.

On the basis that littleleaf trees live for an average of 7 years following the Onset of symptoms and trees in advanced stages less than 3 years, the following cutting rules are advised for stands that are readily accessible: Where only an occasional shortleaf pine shows littleleaf, cuts can be light and spaced at least 10 years apart. When 10 to 25 percent of the trees show unmistakable littleleaf, cut on at least a 7-year cutting cycle, cutting all diseased or suspected trees at each cut. Where more than 25 percent of the trees show littleleaf, cut all shortleaf Pine as soon as merchantable.

With the harvesting of the present littleleaf stands, the question arises as to the future prospects for shortleaf pine on the same sites. Because the disease is associated with soil conditions that cannot be greatly changed except over long periods, succeeding crops of shortleaf pine may be expected to fail at the same or an even earlier age. As many of those sites provide satisfactory yields of pulpwood however, they may be managed on short rotations without undue interference from the disease.

Soil improvement has been suggested as one way to combat littleleaf. That can be accomplished by encouraging hardwoods, especially such species as dogwood, yellow-poplar, and hickory, which are known to improve soils. Hardwoods are more effective than pine in building humus. Practically all hardwoods return more nitrogen, calcium, and other elements to the soil than pine; the eventual change to a greater proportion of broadleaved species in littleleaf areas may serve to reduce the incidence of disease by lowering the number of trees that are susceptible to attack and also by providing more favorable growth conditions for the existing pine. Any effect on pine would be on future stands, however, and the older individuals now occupying the site could scarcely be expected to benefit immediately by conversion to hardwood.

Loblolly pine on severe littleleaf sites is approximately one-third as susceptible as shortleaf and should, therefore, be favored wherever possible. Other pines, such as Virginia, pitch, or longleaf, should also be encouraged where feasible in order to build up the percentage of resistant or immune species.

W. A. CAMPBELL is a senior pathologist, OTIS L. COPELAND, JR., a soil scientist, and GEORGE H. HEPTING, a principal pathologist, in the division of forest pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering. All have had years of experience with the littleleaf disease. Their studies of the disease have been made in cooperation with the Forest Service and several State agencies.