Forest Pests: Insects, Diseases & Other Damage Agents

Anderson, R. L.; Mistretta, P. A. Management strategies for reducing losses caused by fusiform rust, annosus root rot, and littleleaf disease. Agric. Handb. 597. Washington, DC: U.S. Department of Agriculture, Forest Service, 1982. 30 p.

Littleleaf Disease

Significance

Littleleaf disease is the most serious disease of shortleaf pine in the Southern United States. It rarely affects trees younger than 20 years of age and becomes increasingly severe in older stands. The typical infected trees dies within 6 years after initial symptoms become evident; however, some may survive for up to 15 years. It is common to find southern pine beetle attacks on littleleaf-infected trees. About 15 million acres of commercial shortleaf pine are affected, with losses exceeding $15 million per year.

Symptoms

Littleleaf disease is caused by a complex of factors including the fungus Phytophthora cinnamomi Rands; poor soil fertility; poor internal soil drainage; and often, nematodes and species of the fungal genus Phythium. Early symptoms of littleleaf disease, which are only discoloration (yellowing) and a slight shortening of the needles (fig. 14), are not specific and closely mimic the symptoms of nutrient deficiency.

Figure 14. - Healthy (left) and littleleaf-diseased (right) shortleaf pines. F-703150.

Figure 15. - Littleleaf-infected trees showing typical thin, off-color crowns with a tufted appearance. F-703151.

Figure 16. - Littleleaf-killed shortleaf pine with persistent, abnormally small cones. F-703152.

Trees in later stages of the disease have more distinctive symptoms (fig. 15). The crown is thin and ragged, resulting from the shortening and discoloration of the new needles. Moreover, only the new needles near the branch tips are retained, causing the branches to have a tufted appearance. The death of branches tends to thin the crown further. During this time, tree growth is reduced. Finally, about 3 years before death, trees commonly bear an unusually large crop of abnormally small cones ("stress crop") (fig. 16).

Shortleaf pine stands with these symptoms can be considered as littleleaf stands for management purposes. However, to be certain that the stand is suffering from littleleaf disease, you must isolate P. cinnamomi from the soil around affected trees. Also, the soil must have a nitrogen deficiency and poor internal drainage. Graying and mottling of a clay soil are the most obvious indicators of poor internal drainage. Isolation of a Pythium sp. or plant-parasitic nematodes will further strengthen the diagnosis.

Hazard Rating

The most efficient way to evaluate a site for littleleaf hazard is to consider its history. A site with no previous history of littleleaf is a low-hazard site. Sites with a history of littleleaf are high hazard. Figure 17 presents a map showing the known distribution of littleleaf disease. A more detailed site-hazard-rating method, based on soil characteristics, is presented by Campbell and Copeland (1954). This method (table 2) is designed for field use and is helpful when older shortleaf pine is not present on the site. The soil characteristics evaluated in this system are degree of erosion and internal drainage (as three subevaluations –subsoil consistency (when moist), subsoil mottling, and depth to zone of greatly reduced permeability).

Figure 17. - Map showing the known distribution of littleleaf disease

These soil characteristics are rated numerically and the scores totaled. Those sites totaling 75 or more are expected to be free of littleleaf; scores of 51 to 74 indicate light to moderate hazard; a score of 50 or lower indicates high littleleaf hazard.

As a general rule, soil that is severely eroded, has a firm texture, has a shallow permeable layer, or has strong mottling of the subsoil should be further hazard rated. These are potential high-hazard sites.

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