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.

Annosus Root Rot

Significance

Annosus root rot (caused by Heterobasidion annosum [Fr.] Bref.) is primarily a problem in thinned pine stands on well-drained, deep, sandy soils. Infected trees begin to die within 2 to 3 years after thinning. Additional mortality may occur for several years. Trees that survive in spite of infection may have reduced mean annual growth increments and are more susceptible to windthrow and bark beetle attack. While damage occurs throughout the South, it is most severe (and hazard is greatest) on deep, sandy soils with good internal drainage. These soils occur on about 20 percent of the land base in the South.

Symptoms

Figure 8. - A single pine showing the off-color, declining crown symptomatic of annosus root rot. F-703144.

Figure 9. - Annosus root rot infection center containing dead and declining trees. F-703145.

The first aboveground symptoms of annosus root rot are crown thinning and foliar discoloration to light yellow green (fig. 8). Infected trees, which generally die, are often adjacent to stumps from previous thinnings or occur in pockets as the fungus spreads from root to root through root contacts and grafts (fig. 9). Commonly, tree mortality occurs over an extended period of time. Annosus-infected stands are often mistaken for those attacked by pine bark beetles. A check at the base of the affected trees may reveal the conks (fruiting bodies) of the fungus that causes annosus root rot. However, the absence of conks does not mean that the tree is not infected.

Conks produced by H. annosum are light gray to dark grayish brown or reddish brown on the upper surface and irregular in shape. They range in size from small buttons to brackets several inches across (fig. 10A and B). The creamy-white underside, which darkens with age, has small pores. The conks are leathery and difficult to tear. They are perennial, but often deteriorate after a short time and thus can appear to be annual. Conks are formed on stumps; slash; dead trees; and at the root collar or on roots of living, infected trees. It is often necessary to remove the duff at the base of the trees to see them (fig. 11).

Figure 10A. - Annosus root rot conks range from small, button-sized sporophores (A) to large, obvious brackets (B). F-703146 and F-703147.	Figure 10B. - Annosus root rot conks range from small, button-sized sporophores (A) to large, obvious brackets (B). F-703146 and F-703147.
Figure 11. - Fruiting bodies of Heterobasidion annosum are generally located on the root collar of an infected tree. Using a small hand rake to remove the duff will expose conks when they are present. F-703148.	Figure 12. - Soft, stringy (rotten) roots, which break readily, leading to windthrow, are symptomatic of annosus root rot. F-703149.

Examination of the decayed roots shows a yellowish, stringy rot. Advanced decay appears as narrow, elongated, whitish pockets that run together and reduce the wood to a spongy or stringy mass. This type of decay is best seen in areas where trees are windthrown. Roots of healthy trees have the sharp, firmly splintered break of sound wood, while diseased roots are shredded and stringy at the break (fig. 12).

Hazard Rating

The best way to determine hazard for annosus root rot is by onsite soil examination. High-hazard sites have sandy-or sandy loam-textured soils at least 12 inches (30 cm) deep. They also have good internal drainage and do not have a high seasonal water table. Sites differing from those just described are considered low-hazard sites. Figure 13 is a distribution map of high-hazard soil types for annosus root rot. However, areas of low hazard occur in the high-hazard zones, and areas of high hazard are found in the low-hazard zones. Therefore, an onsite examination is the best method for stand hazard prediction.

Figure 13. - Generalized map of soils with high hazard for annosus root rot; red is high mortality hazard; blue is high hazard for growth loss.

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