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The Health of Southern Forests, USDA Forest Service, Forest Health Protection, Southern Region

Stressors of Pine Forests

Annosum Root Disease

Annosum root disease contributes significantly to pine mortality, reduced growth, and increased vulnerability to bark beetle attacks in the South. Fruiting bodies of the disease-causing fungi appear at the root collar and are often obscured by dead needles and soil detritus (Figure 12).

Figure 12. Fruiting bodies of annosum root disease appear at the root collar.
Figure 12. Fruiting bodies of annosum root disease appear at the root collar.

The first comprehensive survey of annosum was conducted in 1960 by the Southeastern and Southern Forest Experiment Stations. Results of this survey showed 2.8 percent mortality in planted loblolly stands and 2.2 percent mortality in slash plantations. Damage was more severe on sandy soils and former croplands, and damage increased as the number of thinnings increased.

Since the 1960 survey, other studies have shown a significant growth difference between healthy and diseased loblolly trees, with a reported 4 percent per year loss in radial growth and a cumulative 5-year loss of 19 percent. Similar losses have been reported in slash pine plantations affected by annosum root disease. Volume losses within high-risk pine plantations have ranged from 0.1 cord per acre per year to 0.5 cord per acre per year over the first 5 to 9 years after initial thinning.

Although the disease is most often associated with thinned plantations on sandy, drained sites, it can be found on a variety of sites, soils, and forest conditions. The primary pine hosts include loblolly, longleaf, shortleaf, slash, and white pines. Slash and loblolly are the most commonly planted species in the South, and both are very susceptible to annosum root disease.

The primary risk factors associated with annosum root disease include the amount of host type available, timing and degree of management activity, and the soil-site conditions. Annosum spores colonize freshly cut stumps. The fungus grows through the root system and into adjacent healthy trees through root contacts and grafts. Within the Southern Region, there are an estimated 163.5 million acres of high and moderate risk sites.

Sites classified as high risk are well-drained soils containing sand within the upper surface layers of 10 inches and greater. Increased annosum risk also has been attributed to reduced organic matter and high pH range in the soil. Soil variables, including organic matter, pH, and texture, are the factors considered most likely to influence host resistance, as well as the root-infecting fungi or soil-inhabiting microorganisms that may compete with annosum. These risk factors are used to predict the rate of spread if a stand becomes infected with annosum root disease.

Variables such as weather patterns, management intensity, species composition, and site productivity all influence host susceptibility to root disease.

Risk of damage caused by annosum decreases on sites with increased clay content in the surface layer. Loamy and silt-loam soils are considered moderate risk for annosum development. Clay sites prone to flooding and those that are predominately wet are classified as low-risk sites.

Evaluation of risk factors prior to implementation of management strategies is an effective tool for reducing annosum root disease. Risk evaluations based on soil maps and examination of soil-site conditions enable forest managers to implement preventive measures.

Annosum prevention includes applying a borax formulation to stumps during first thinnings. A second method, using a spore suspension of a competitive fungus, can be applied to stumps during thinnings on sites where annosum is already present in the stand. Other preventive measures include prescribed burning prior to thinning, increased spacing of seedlings when regenerating pines on sites at risk for annosum, and summer thinning from May through August in areas below 34 degrees north latitude.

Evaluation of annosum risk on a regional scale is beneficial for assessing an ecosystem approach or a holistic landscape viewpoint for resource management. Soil association maps for the Southern States were evaluated for key attributes pertaining to risk factors for annosum root disease. A data base containing polygon coverage of soil types, digitized from State survey maps, was used to prepare the maps.

The regional soil risk map shown in Figure 13 should not be used to evaluate site-specific management areas. County boundaries are shown only to assist in identifying risk areas and do not indicate that all soils and sites are high, moderate, or low risk as identified by the color codes.

Figure 13. Soil risk map for annosum root disease.

Figure 13. Soil risk map for annosum root disease.