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5.1 Native Diseases of Conifers

5.1.1 Fusiform Rust

Fusiform rust, caused by the fungus Cronartium fusiforme f. sp. fusiforme, occurs primarily on slash and loblolly pines. It is considered the most destructive disease of southern pines, causing cigar-shaped galls on the main stem that are generally fatal (Anderson and others 1980, Czabator 1971).

Extensive planting of susceptible slash and loblolly pines since the 1930s has resulted in an epidemic of fusiform rust. Infected trees can be found throughout the southern pine region (Figure 4), but losses are most serious on Coastal Plain sites from Louisiana to southeastern South Carolina. Several variables including weather, amount of inoculum, abundance of oaks (the alternate host), and susceptibility of the pine species govern incidence of the disease.

Nonindustrial private and industrial forest landowners own a majority of the pine host type in the South. Over 13.4 million acres southwide have at least 10 percent of the slash and/or loblolly pines infected (Starkey and others 1997).

Control strategies designed to minimize the impacts of fusiform rust are documented in several publications. They include genetic selection, silvicultural manipulation, and chemical treatment (Anderson and others 1980, Belanger and others 1991, Dinus and Schmidt 1977, Matthews and Anderson 1979, Schmidt 1998, USDA Forest Service 1971).

More intensively managed areas generally are at higher risk from fusiform rust. The more rapidly a tree grows, the greater its risk of becoming infected. Most practices that improve pine growth, therefore, favor rust development (Dinus and Schmidt 1977, Schmidt 1998)

The incidence and impact of fusiform rust is projected to remain stable or increase slightly in the future. A study by Starkey and others (1997) showed that there was a slight regional trend towards higher infection rates in slash pine and a slightly reduced rate for loblolly pine. In the long term, fusiform rust could be reduced by planting disease-resistant seedlings.

5.1.2 Annosus Root Disease

Annosus root disease (ARD), caused by the fungus Heterobasidion annosum, produces significant losses of conifers across the South. On sandy, well-drained sites, this disease causes growth loss or kills trees. It is most often associated with thinning of loblolly, longleaf, shortleaf, slash, and white pine plantations. Slash and loblolly pines are the most commonly planted species in the South and are both very susceptible to ARD (Robbins 1984, Stambaugh 1989).

A survey of ARD in the South documented 2-3 percent mortality and a 44-60 percent rate of disease occurrence in planted pine. Documented rates of radial and height growth are significantly less for diseased than for healthy pines (Applegate 1971, Froelich and others 1977, Morris 1970).

The fungus enters a stand by infecting freshly cut pine stumps. It progresses into roots, and, thereafter, it grows from tree-to-tree via root contacts and grafts. First entry into a stand can be prevented by treating susceptible new stumps with borax.

The primary risk factors associated with ARD are the amount of host type available, the timing and degree of management activity, and the soil and site conditions. Risk of damage caused by ARD decreases as clay content in the surface layer of soil increases, giving us an effective risk mapping tool (Figure 2). In the Southeast, risk of ARD is high or moderately high on an estimated 163.5 million acres, not all currently forested (Hoffard and others 1995). Silvicultural and chemical controls can be used to minimize the impact of ARD on high-risk sites. A biological control that appears to be effective does not have EPA registration and is currently unavailable to managers.

Private industry generally favors intensive plantation management of loblolly and slash pine on short rotations of 30 to 35 years. Severity of ARD in this type of management is directly related to the number of thinnings in the stand and the proportion of sand in the soil. Industrial owners are more likely to use a full range of management options. Short rotations and intensive management generally result in low ARD caused mortality on industry lands.

On managed public land, the current trend is to restrict the amount of intensive plantation management in favor of longer rotations for watershed protection and recreation. Restoration of longleaf pine is being promoted. Of the southern pines, longleaf is considered the least susceptible to root disease and its restoration on sites currently occupied with other pines will lessen the impact of ARD.

When pine stands managed on longer rotations have few intermediate cuts, the risk of ARD development is generally reduced. However, strategies that promote uneven-age management with frequent cuts will likely increase incidence and severity of ARD. Management for red-cockaded woodpecker habitat, which requires frequent mid-rotation thinnings, may also increase ARD on high-risk sites (Cram 1994).

On public reserved land, where management activities are minimal, ARD will have little impact.

Private nonindustrial land, which includes 69 percent of the South's forest lands, is managed in a variety of ways, creating a range of risk for ARD. The Conservation Reserve Program (CRP), which has assisted private landowners to reforest thousands of acres of erodable cropland, has resulted in increased risk for ARD in the plantations it supports by favoring early thinning. Approximately 400,000 of the 2 million acres enrolled are on high-risk soils for ARD development (Anderson and Mistretta 1982).

5.1.3 Brown Spot Needle Disease of Longleaf Pine

Brown spot needle disease, caused by the fungus Scirrhia acicola, is considered the most serious disease of longleaf pine. It causes seedlings to remain in the grass stage (an early growth stage of longleaf in which the seedling looks like a clump of grass) for an abnormally long time, delaying initiation of height growth and causing loss of potential wood production. Severely infected trees often die. Young longleaf trees become more resistant to this disease once they grow out of the grass stage.

This disease occurs from Virginia to Texas, primarily on the Atlantic and Gulf Coastal Plains. It is more severe in certain geographic areas (Figure 3). It has been estimated to reduce total annual growth of southern pine timber by 16 million cubic feet (0.453 million cubic meters).

At present, longleaf pine occupies only about 5 million acres of its former 60 million acre range. Difficulties in storing and handling longleaf pine seedlings have discouraged managers from planting this species.

Recent work has led to the production of healthier seedlings for planting; planting success has improved on sites where, historically, longleaf was the dominant species (Cordell and others 1989, Kais 1989).

Several possible treatments are available for managers to limit the impact of this disease on their grass-stage longleaf pine seedlings. They include silvicultural, fire, and fungicidal options.

Chemical treatment of seedlings and prescribed burning are most likely to be used by managers of private industry land and managed public land. State forestry agencies are having success assisting private nonindustrial landowners in controlling brown spot, but there are a huge number of landowners to contact and this effort is very slow.

It is expected that disease incidence will increase as attempts are made to return longleaf to its native range.