A Guide to Common Forest Pests in Georgia

Terry Price, Forest Health Specialist, Georgia Forestry Commission


Miscellaneous Diseases of Hardwoods

Wilt Diseases

Wilt diseases affect trees' ability to transport water and other materials. This results in moisture stress which leads to wilting. Although water may be plentiful, the vessels that carry it within the tree are clogged up by the action of the disease-causing organisms. Wilt diseases can be introduced into trees by insects or by spores gaining entrance through wounds or root grafs. Once infected, a tree may die gradually or suddenly depending on the nature of the disease and the vigor of the tree before infection. A common wilt affecting elm trees in Georgia is Dutch elm disease.

The fungus that causes Dutch elm disease was introduced into the United States many years ago. It was first identified in Georgia in 1967. This disease, Ceratocystis ulmi, affects all native elm species. The earliest symptoms are wilting, curling and yellowing of leaves on one or more branches, followed by leaf fall and death of the branches (Figure 61). Trees of all ages can be affected. Trees may die within a few weeks of the onset of symptoms or may die a limb at a time over a period of a year or more. The springwood of the last annual ring of an infected tree shows in cross section as a dark-brown ring or as a series of dark dots (Figure 62). The only sure way of identification of the disease is by culturing the fungus in the lab from wood samples taken from infected trees.

Figure 61 - Early sypmptons of Dutch elm disease
Photo by Ed Barnard, Florida Division of Forestry

Figure 62 - Discoloration caused by Dutch elm disease
Photo by Ed Barnard, Florida Division of Forestry

Dutch elm disease is transmitted to healthy trees by two species of bark beetles: the smaller European elm bark beetle and the native elm bark beetle. Both beetles breed in dead or dying elms. Upon emergence from infected trees, the adults will seek out a healthy tree and begin to nibble in the crotches of twigs, thereby, innoculating the tree with the fungus.

Native elm species that are infected usually cannot be saved when more than 5 percent of the branches are dying. Pruning out the dying branches followed by the use of a systemic fungicide (see appendix), is the best method of controlling the disease in trees having 5 percent or less branch wilting. Healthy elms can be protected by removing any dead or dying trees to eliminate potential breeding sites for the beetles. Uninfected trees can be sprayed with an insecticide to prevent the beetles from feeding, but the chemical registered for this purpose is a restricted pesticide and can be applied only by a licensed pest control person. Some of the Asian and European elms are moderately resistant to the dlisease and should be favored over the native species.

Two other wilt diseases that should be mentioned are mimosa wilt and oak wilt. Mimosa wilt is common throughout the south and is a very serious disease on native mimosa trees. Oak wilt has not been positively identified in Georgia, but it is present in South Carolina, North Carolina, Tennessee, Virginia, Kentucky and Texas. Species of the red oak group are the most susceptible.

Hypoxlon Canker

Hypoxylon canker is a common disease of oaks and hickories in Georgia. All Hypoxylon spp. are wood parasites. Most of the species are weak pathogens in that initial infections are in trees that have been injured by drought, flooding, fire, lightning, construction damage or storms. There are many different species of Hypoxylon.

Figure 55 - Brown spot needle blight
Photo by USFS Archives

Older trees which have been weakened by root, branch or trunk injury, or lack of moisture are especially susceptible to infection. The most conspicuous symptoms of this disease are mottled grayish areas where the bark has sloughed off (Figure 63a). These areas generally appear first on the deadbranches and later on the main stem. Usually one to several branches in the upper crown will turn brown and die one to two years prior to tree death. Ambrosia beetles usually attack infected trees causing a buildup of wood borings at the tree base.

There is no control for this disease. Dead trees should be removed as soon as possible to prevent the fungus from spreading to nearby uninfected trees.

Oak Decline

Oak decline is a name often used to describe the decline and death of various oak species for lack of a better descriptive name. Several species of fungi have been isolated from the roots of oaks that are exhibiting decline symptoms. The syndrome appears to be associated with oaks growing on poor soils with symptoms being prevalent following extended droughts. Several thousand acres have been affected in Georgia; the most recent year being 1985. The early symptoms are leaf discoloration and dieback in the tops of trees. Several years may be required for death to occur. Tree death is more sudden during droughts. Hypoxylon spp.are often associated with oak decline.

Since the decline occurs over a period of years, land managers have time to harvest infected areas before tree mortality becomes too serious. Urban trees should be watered during droughts and fertilized periodically to keep them in a vigorously growing condition.

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