Wilt diseases affect trees’ ability to transport water and other materials. This results in moisture stress that 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 grafts. 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 (DED), Ophiostoma ulmi, was introduced into the United States many years ago. It was first identified in Atlanta, Georgia in 1967. This disease 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. 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 127). The only sure way of identification of the disease is by culturing the fungus in the lab from wood samples taken from infected trees.
DED is transmitted to healthy trees by two species of bark beetles: the smaller European elm bark beetle, Scolytus multistriatus, and the native elm bark beetle, Hylurgopinus rufipes (Figure 128). Both beetles breed in dead and dying elms. Upon emergence from infected trees, the adult beetles will seek out a healthy tree and begin to nibble in the crotches of twigs, thereby, inoculating 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 are the best methods of controlling the disease in trees having 5 percent or less branch wilting. Removing dead or dying trees can protect healthy elms. This eliminates potential breeding sites for the beetles that carry the fungus. Trenching can prevent transmission of the disease through common root systems.
Uninfested 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 disease and should be favored over the native species.
Elm yellows (formerly known as elm phloem necrosis) is caused by a micoplasmalike organism (MLO) and is often confused with Dutch elm disease. With elm yellows all of the branches are affected at once. Infected phloem tissues turn a tan or brown color and when placed in a vial for a few minutes will produce a wintergreen odor. The disease is transmitted by the white-banded leafhopper. Some success in suppressing elm yellows has been achieved by the use of tetracycline antibiotics injected into trees and trenching to prevent the spread through root grafts.
Three other wilt diseases that should be mentioned are mimosa wilt, Fusarium oxysporum, persimmon wilt, Acromonium diospyri, and oak wilt, Ceratocystis fagacearum.
Mimosa wilt is common throughout the range of mimosa and is a very serious disease. A few wilt resistant mimosa selections have been propagated to a limited extent.
Persimmon wilt has devastated persimmon trees in ten states from Tennessee and North Carolina, south to the Gulf, and west to Texas. The disease enters healthy trees via wounds. Diseased trees should be removed and destroyed to prevent the disease from spreading. Insects that may spread the disease to healthy trees often attack drought stricken 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. It probably occurs in Georgia but just hasn’t been detected.