Paul A. Mistretta1
1Supervisory Plant Pathologist, U.S. Department of Agriculture, Forest Service, Southern Region, State and Private Forestry, Forest Pest Management, Pineville, La.
Littleleaf disease is the most serious disease of shortleaf pine in the Southern United States. Affected trees have reduced growth rates and usually die within 6 years.
The disease is caused by a complex of factors including the fungus Phytophthora cinnamomi Rands, low soil nitrogen, and poor internal soil drainage. Often, microscopic roundworms called nematodes and species of the fungal genus Pythium are associated with the disease.
Affected stands are found in the Piedmont area from Virginia to Mississippi, with additional scattered pockets of the disease in eastern Tennessee and southeastern Kentucky (fig. 1). The disease has its greatest impact in Alabama, Georgia, and South Carolina.
In the most recent general survey, littleleaf disease was found over 35 percent of the commercial range of shortleaf pine and was severe enough to be a factor in timber management on about 1.4 million forested acres. Losses attributed to littleleaf exceed $15 million per year.
Shortleaf pine is the most seriously damaged host, with loblolly pine damaged to a lesser extent. Littleleaf disease has also been reported on Virginia, pitch, slash, and longleaf pines.
The first symptoms of littleleaf disease are those of nutrient deficiency: a slight yellowing and shortening of the needles and reduction of shoot growth.
In the later stages of the disease, the symptoms become progressively more distinctive. The crown of an infected tree appears thin and tufted: New needles are discolored and shorter than normal, and the tree loses all but the new needles near the tips of the branches (fig. 2). Branches begin dying, starting in the lower crown and progressing upward through the crown (fig. 3). During this time, the tree's diameter growth is markedly reduced.
About 3 years before death, diseased trees commonly produce abundant crops of small cones. Most of the seeds in these cones are sterile. Littleleaf killed trees can often be recognized by these undersized cones, which remain attached to the branches. (See cover.)
Littleleaf disease rarely occurs in younger trees less than 20 years old and becomes increasingly severe in older stands. Some diseased trees die within the year after the first noticeable symptoms appear, while others live for 12 or more years. The average tree, however, dies about 6 years after the first symptoms appear.
Phytophthora cinnamomi, which causes the disease, infects the rootlets of susceptible trees. Where the soil is characterized by low nitrogen and poor internal drainage, the disease results in rootlet mortality. The effects of the disease are often made worse by the activity of nematodes and species of the fungal genus, Pythium, which also attack the rootlets.
Phytophthora cinnamomi is common in soils in the Southeast. It has been isolated from dead roots of outwardly healthy shortleaf pines, so vigorously growing trees can apparently overcome the root damage it causes. However, when soil conditions are bad enough to cause low vigor in trees, their root systems deteriorate faster than new rootlets form, and the trees slowly starve, causing the symptoms of nutrient deficiency that are the first indication of littleleaf disease.
Spores of Phytophthora cinnamomi are mobile, but their mobility is influenced by the soil's internal drainage. On sites with normally high soil moisture, the disease is favored. On sites with well-drained soils, the movement of spores is restricted.
To harard rate forested areas, simply observe the condition of the pine. If littleleaf disease is present, the area is a high-hazard site for the next tree crop.
Copeland and Campbell (1954) present a very reliable method of hazard rating sites that are not currently supporting shortleaf or loblolly pines (table 1). This method is useful in areas where type conversion or old-field reclamation is planned. It is based on measurements that are easily made in the field. The soil characteristics evaluated in this system are degree of erosion and internal drainage (three subevaluations: subsoil consistency (when moist), depth to zone of greatly reduced permeability, and subsoil mottling).
Table 1. Numerical system for field rating sites for littleleaf disease hazard based on soil characteristics (Campbell and Copeland, 19541).
These four characteristics are rated numerically and the scores totaled. High-hazard sites score 0 to 50 points; moderate-hazard soils score 51 to 74 points; and low-hazard soils score 75 to 100 points.
On sites where hazard is low (no littleleaf present in an established stand or hazard rating of soil shows low hazard for littleleaf disease), land managers need not consider the disease; and normal management of pine will be possible.
Where the disease is present or the hazard is high for littleleaf, preventive or control measures should be taken.
A set of effective management recommendations has been developed for use in high-hazard areas - areas where littleleaf has been or is expected to be a problem. These recommendations have been incorporated into a decision key to aid in selecting appropriate management strategies (fig. 4).
Prevention is accomplished through species manipulation or site rehabilitation.
Planting resistant species, such as hardwoods or nonsusceptible pines, will avoid the problem of littleleaf disease. If shortleaf pine is still desired in an area where the current stand shows littleleaf disease symptoms, the best uninfected trees should be left as seed trees to confer some resistance to the next stand.
The main site rehabilitation technique for use with littleleaf is breaking up any of the bricklike, extremely compact layer of clay (hardpan) present in the soil. This technique permits better drainage on the site, thus reducing the spread of the fungus.
Interplanting or seeding a regeneration area with a legume, such as a species in the genus Robinia or Lespedeza, will produce a short-term increase in available soil nitrogen.
Control can be achieved in two ways: by tree removal or by fertilization. The removal of infected trees is undertaken primarily to minimize the volume of timber lost when a tree becomes diseased.
One ton of 5-10-5 fertilizer plus one-half ton of ammonium sulfate per acre can be used for high-value trees or ornamentals. Occasionally, fertilization is used to boost a forest stand into the next higher value class, for example, from pulpwood to pole. Fertilization will delay the development of symptoms for about 4 years. Trees can appear to recover since the needles produced during this period will be very close to normal in color and size.
Bryan, W.C. Height growth of shortleaf pine progenies from trees selected for resistance to littleleaf disease. Res. Note SE-185. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station; 1973. 7 p.
Campbell, W.A.; Copeland, Otis L., Jr. Littleleaf disease of shortleaf and loblolly pines. Circ. 940. Washington, DC: U.S. Department of Agriculture; 1954. 41 p.
Hepting, George H. Managing pine in littleleaf areas. Forest Farmer 8: 7, 10; 1949.
Roth, Elmer R. Spread and intensification of the littleleaf disease of pine. Journal of Forestry 52: 592-596; 1954.
Roth, Elmer R.; Toole, Richard E.; Hepting, George H. Nutritional aspects of the littleleaf disease of pine. Journal of Forestry 46: 578-587; 1948.
Zak, B. Inheritance of resistance to littleleaf in shortleaf pine. Res. Note SE-88. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station; 1955. 2 p.
Revised October 1984
Formatted for the Internet April 1998