Elm Phloem Necrosis
Einar W. Palm
Department of Plant Pathology
Wilfred S. Craig
Department of Entomology
Phloem necrosis of American elm is a disease of unknown origin of many years standing, which is now widespread in the region of the central and lower Ohio River watershed. It extends to northern Mississippi, eastern Oklahoma, Kansas, Nebraska, and is generally distributed throughout Missouri. It damages the tissues that conduct food to all parts of the tree, and it causes wilting and death. Both forest and shade trees are affected, and trees of all ages and all degrees of vigor are susceptible. Trees may be infected for six months to one or more years before symptoms appear. The majority of infected trees usually die from 1 to 1-1/2 years, although some may die in three to four weeks and others may survive for two or more years. Since all infected trees die eventually, this is a serious disease of elms.
Phloem necrosis is caused by a mycoplasma-like organism (formerly believed to be a virus infection), and is usually spread from diseased to healthy elms by an insect, a leafhopper, Scaphoideus leuteolus (Van Duzee). The virus may also spread through root grafts in close plantings or dense natural stands of elm. The disease affects only the American and winged elm; other species of elm are resistant.
When the leaves of American or winged elms suddenly wilt, turn yellow or dry and drop from the trees, phloem necrosis may be responsible, especially if a laboratory assay for Dutch elm disease proved to be negative. A considerable portion of the root system is affected before there is evidence of disease in other parts of the tree. There is a gradual decline of the entire crown, but in large trees the symptoms may first appear in a single branch or in a part of the top. Leaves droop and curl slightly, the foliage appears sparse and the leaves turn yellow and then fall. Death of the tree follows.
These symptoms are not enough to positively identify phloem necrosis, but the disease can be identified by examining the inner bark. Make some small cuts through the bark at several places around the lower truck or buttress roots. Pry the bark from the wood so that the inner bark may be examined. If the inner bark, which lies next to the wood, is yellow or the color of butterscotch, and sometimes flecked with brown or black, the tree probably has phloem necrosis. If the inner bark is white when cut, and turns brown only after exposure to the air, the tree is probably not infected with phloem necrosis.
If you find the yellow or butterscotch color, remove a small piece of bark that is still fresh and not invaded by insects. From this piece of bark, strip the thin discolored inner layer and confine it in a stoppered vial. If a faint odor of wintergreen can be detected after confining the tissue for a few minutes, the disease is probably phloem necrosis. If the discoloration seems typical for phloem necrosis but the wintergreen odor is difficult to detect, secure a new sample from another part of the tree and confine it in the stoppered container for several hours in a warm place. The wintergreen odor will be evident if the disease is present. The discoloration of the inner bark and the wintergreen odor are specific symptoms for this disease and serve to differentiate it from all other elm diseases.
There is no known cure for elm trees that have been infected with phloem necrosis. Because the leafhoppers that carry the phloem necrosis virus inject it into the trees as they feed on elm leaves, spread of the disease can be greatly reduced by spraying with an insecticide to eliminate these insects.
Leafhoppers can be controlled with a spray containing 1 pound Carbaryl (2 pounds 50 percent, or 1-1/2 pounds 80 percent Sevin wettable powder) per 100 gallons of water. In hydraulic sprayers, apply at the rate of 20 to 25 gallons per 50-foot elm.
Apply the spray once in early June. If additional new growth occurs during late July or early August, a second application may be needed to protect this new growth.
Severing root grafts with a soil fumigant material such as Vapam (sodium N-methyl dithio carbamate) is recommended to reduce spread of phloem necrosis from infected to adjacent healthy trees.