A Guide to Common Forest Pests in Georgia

Terry Price, Forest Health Specialist, Georgia Forestry Commission

Annosus Root Rot

Annosus root rot can be a serious problem of pines in plantations that have been thinned one or more times. Loblolly, slash and white pines are affected most but shortleaf and longleaf are sometimes infected.

The fungus which causes annosus root rot, Heterobasidion annosum, enters pine plantations when its airborne spores land on freshly cut stump surfaces. The spores progress downward into the roots and then spread to adjacent healthy trees through root contacts in the soil. Roots wounded during the plowing of fire breaks or road building may become infected as do freshly cut stumps.

Infected trees begin to show symptoms 1-3 years following inoculation. Crowns will thin, turn light green to yellow and have shortened needles that may be tufted at the branch ends. These symptoms are very noticeable when sick trees are compared to healthy trees (Figure 40). The fungus can rot the roots so badly that some trees may fall over on their own or do so during moderate to heavy winds. Wind-thrown trees in a pine plantation may indicate root rot (Figure 41). The roots of windthrown trees can be examined for resin soaking (an early infection symptom) or for white stringy decay (an advanced infection symptom) (Figure 42). These symptoms along with windthrow are usually a positive sign that root rot is present. Occasionally the fungus will develop fruiting bodies of conks at the base of living and dead trees or stumps. These conks are hard to see because they are frequently formed below the litter layer around the tree or stump base and are most prevalent during the cool, wet winter months (Figure 43).

Annosus root rot is found throughout Georgia on certain sites. There are several soil series that are high hazard for disease (Figure 44). It is best to consult with a soil scientist or forester for a more detailed description of a particular site. Below is a broad classification of sites for possible root rot occurrence:

  • High Hazard - Sandy or sandy loam soils with at least 65% sand in the upper 12 or more inches above a clay layer and with no high seasonal water table.
  • Intermediate Hazard - Silt and silt loam soils 12 or more inches deep.
  • Low Hazard - Poorly drained clay and clay loam soils or those with high water tables.

Managing Established Stands

If plantations are severely infected (50+ %), they should be clearcut and regenerated. Usually by this time infection centers are scattered throughout the stand and future thinnings will necessitate removing too many trees thus leaving the stand understocked. When regenerating high hazard sites a wider spacing should be used. This will delay the first thinning and ultimately reduce the total number of thinnings made during the rotation. Low hazard sites require no special planting treatments.

Uninfected plantations that have never been thinned should have the stumps treated with granular borax at the first thinning (Figure 45). If the stand remains uninfected prior to each subsequent thinning then borax should be used after each thinning. The biocide, Phlebia gigantea, containing the antagonistic fungus, should be used on stump surfaces in stands that are already infected. Borax is of no value once a stand has become infected.

Figure 40 - Pines infected with root rot can be distinguished from healthy trees by their pale green color. (infected tree left)
Photo by USFS Archives

Figure 41 - Root sprung or wind thrown trees indicate root rot.
Photo by USFS Archives

Figure 42 - Roots damaged by root rot
Photo by USFS Archives

Figure 43 - Fruiting conks of root rot
Photo by USFS Archives

Figure 44 - Distribution of root rot in the South. Red indicates high hazard soils. Blue indicates moderately hazard soils.
Photo by USFS Archives

Figure 45 - Treating stumps with borax
Photo by USFS Archives

Summer thinnings appear to lessen the damage from root rot. High stump temperatures over 900F are likely to kill the fungus spores as they land on the surfaces. High temperatures also retard H. annosum spore production. Summer thinnings may create a bark beetle problem in some areas of the state when beetle populations are high. Land managers should contact the Georgia Forestry Commission office nearest them for information on beetle activity before doing summer thinnings. Summer thinnings may be considerably less effective in mountains of North Georgia where maximum summer temperatures are usually much lower than in more southernly portions of the state.

For further information on borax and the biocide write to the Georgia Forestry Commission, P. 0. Box 819, Macon, GA 81298-4599.

Appreciation is extended to Dale A. Starkey, Plant Pathologist, USDA Forest Service, Pineville, LA for some of the information used here.

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