When Dutch elm disease was introduced into the United States by wood imported from Europe, the devastating infestation stripped city streets and parks across the nation.
Dutch elm disease has been the most destructive shade-tree disease ever seen in the United States. The infection was first described by Marie Beatrice Schwartz in the Netherlands in 1921 and thus became known as Dutch elm disease; there is no tree called the Dutch elm. In 1930, the disease was brought into the United States in elm logs imported from Europe for making veneer. At least four ports of entry were involved; initially, the infestation was limited to areas near New York City.
The logs were being shipped in partial payment of World War I debts. These particular logs were valued because they had burls—hard, woody growths on the sides of trees that are usually the results of entwined clusters of buds. Burls produce unusual and often attractive grain patterns that are considered valuable in wood used to make furniture. Wooden crates containing dishes from Europe were also found to contain the disease.
The range of the American elm (Ulmus americana) is the largest of any tree in North America. It includes the eastern United States and southern Canada and extends as far west as central Texas, North Dakota, and eastern Saskatchewan. Other elms are found in smaller ranges within the same area. Most elms, including the slippery or red elm (Ulmus rubra) and the rock elm (Ulmus masii), are susceptible to the disease. The Siberian elm (Ulmus pumila) and the Japanese elm (Ulmus davidiana japonica) show resistance to Dutch elm disease.
A major factor in the spread of the disease in the United States was that the American elm had been planted both in large numbers and close together. Urban planners in the early 1900’s lined city streets with them. New neighborhoods were often crowded with houses, and streets were narrow; with the American elm’s graceful shape, the mature trees arched across the streets to form a cathedral-like roof. The result was beautiful and redeeming in the cramped neighborhoods but devastating to the elms; their proximity to one another rendered them more vulnerable to the spread of the disease. The Dutch elm infection, which had previously infested Europe, spread in the United States and killed millions of elm trees, stripping city streets and parks of their shade trees. Every year, local governments and individuals spent many millions of dollars cutting down dead and dying trees.
The disease is caused by the fungus Ceratocystis ulmi. Technically, the fungus is a parasite while the tree is alive and continues as a saprophyte after the tree dies. In both cases, the fungus releases enzymes that digest the cells of the tree. This digestion occurs outside the cells of the fungus, and the degraded material is then absorbed as food by the fungus. The fungus is a member of the sac fungi, so named because they produce a sac (ascus) containing reproductive cells called spores.
The spores allow the fungus to spread from tree to tree. Agents that carry the spores and transmit the infection are called vectors. The principal vectors for Dutch elm disease are the elm bark beetles that tunnel under the bark. When the beetles are under the bark, the spores from the fungus stick to them. When the beetles fly to healthy trees to feed on tender twigs, they carry spores. In 1934, U.S. Department of Agriculture entomologist William Middleton was the first to report that spores are deposited on the feeding wounds as the beetles eat. The spores germinate and grow into the conductive tissues of the tree. Birds, water, and wind can also act as vectors. Bark beetles, however, are the main culprits.
Two species of small bark beetles act as vectors. The European elm bark beetle (Scolytus multistriatus) is another immigrant from Europe. In 1904, this insect was found and identified near Boston. It was not considered a problem until the subsequent arrival of the fungus, for which the beetle became the major vector. A native species (Hylurgopinus rufipes) also helps in spreading the disease. Without the beetles, the fungus would not present much of a threat.
The growing fungus not only kills and digests tree cells but also plugs the vital conductive tissues (especially the xylem) and cuts off areas of living sapwood. The conductive tissues form a ring of vertical tubing just under the bark. The center of a tree does not conduct water; two types of tissue allow water to travel: Xylem takes water up to the leaves, and phloem takes water and the products of photosynthesis down. The first sign of trouble in the conductive tissue is when the upper branches of the elm wilt and curl because they are not getting enough water. Leaves then yellow and fall as the chlorophyll breaks down, and branches die. The conductive tissue may also carry spores internally to start new infections in other parts of the tree. Trees may take several years to die or may die in the same season in which they are infected.
Bark beetles generally produce two broods of young each year in large numbers. As many as twenty-five thousand beetles can emerge from one square foot of bark. The first brood spreads more of the fungus than the second. When they leave a tree with fungus in order to feed on new growth, the beetles are likely to carry spores. After feeding, they fly to dead or dying elms to find breeding places for the next brood, which emerges in August. The closely planted trees helped the beetles to multiply rapidly.
The federal government limited its response to Dutch elm disease to research and did not involve itself in direct efforts to stop the spread of the disease; hence programs to save elms were local and uncoordinated. Some communities concluded that the loss of the elm was inevitable and decided to do nothing. Some control measures were undertaken, but they met with limited success.
The treatment of plant disease is usually focused on the survival of the crop, or the whole species or genus. Only occasionally is an attempt made to save older or especially valued trees.
Control efforts included the cutting and quick burning of infected trees to prevent spread of the fungus. Not only did the beetle take the fungus from tree to tree, but the fungus also was found to spread when roots of an infected tree touched another tree’s roots. In such a case, a root graft is formed, allowing the spores to cross over. For a time, it seemed that the spread of the disease might be stopped by the use of dichloro-diphenyl-trichloroethane (DDT),
Numerous research efforts using chemotherapy (injection of chemicals) were unsuccessful and costly. Individual prized trees were drilled at the base, and fungicides were introduced into the conductive tissue; nevertheless, many such trees were eventually lost. Some success was reported with the use of the fungicide benomyl.
The infestation that was introduced in the 1930’s became an epidemic in the 1950’s and peaked by the 1970’s. Many cities that had planted only elms were left barren.
The Dutch elm infestation stripped the city and park landscapes in thirty-one U.S. states. No tree had a wider range across the nation. Whole cities and neighborhoods that had invested in massive elm plantings were changed. Often, every elm on a street was lost. Nevertheless, none of the affected elm species went extinct as a result of the disease, because the reproductive rate of these species is high. The major impact of the infestation was the realization that careful planning was required to create and protect urban forests.
Scientists warn that the practice of monoculturing, the extensive cultivation of a single crop, is hazardous. What happened to the elm populations in the United States is a lesson in how overplanting one species or related species allows infestations to spread quickly and prosper. Some planners responded only by switching from the use of elms to the use of another species, such as ash, but any species planted in high concentration is more vulnerable to infestation than are more diverse groupings.
Moreover, the epidemic vividly illustrated the dangers involved in the importation of exotic plants, animals, and even fungi. Such introduced species may have no natural enemies. Under new conditions, they may be able to prosper at the expense of native species and displace them. The case of the Dutch elm infestation demonstrates how a previously harmless exotic such as the European bark beetle may become harmful when it is combined with another species.
Another issue that surfaced during the infestation was the tendency to rely solely on modern technology as a solution. Rachel Carson convincingly argued that chemical spraying was not the answer to Dutch elm disease; nevertheless, chemical use continued. The relative amounts of chemicals sprayed on household lawns in the 1990’s to maintain monocultures exceeded that used to produce food. Golf courses tend to use even more chemicals, averaging seven times the amount that farmers use on each acre. Such use requires care and understanding that are often lacking.
Any widespread loss of trees is cause for regret. Trees restore the air by making oxygen. By taking carbon dioxide out of the air, they reduce the greenhouse effect. Trees hold large amounts of water and prevent erosion and flooding. They slow the wind; they shade and cool homes. Trees are required as hosts for much animal life. The beauty of trees restores the spirit and makes the land livable. Nevertheless, they do not need to be monocultured to be beautiful, nor do they need to be planted closely in rows. One of the effects of the loss of the elm monocultures was to call into question the nature of beauty in horticulture.
Hundreds of thousands of elms were lost each year when the infestation was at its peak. Now that many of the elms are gone, the beetles and fungus have also been reduced in number. The trend has been to replant about one tree for every four that were lost in the cities. The Dutch elm infestation showed that such replanting requires careful planning. Researchers continue to study resistant strains and genetically engineered varieties of elm for urban use. Most important to the recovery of affected areas, however, is the recognition that the problems of the past can be avoided through careful planning, the use of native species, and the preservation of diversity.
-
Carson, Rachel. “And No Birds Sing.” In Silent Spring. Boston: Houghton Mifflin, 1962. Chapter in Carson’s classic attack on chemical spraying deals with efforts to save the elms. The individual cases that Carson cites to support her argument are fascinating. -
Lucas, George B., C. Lee Campbell, and Leon T. Lucas. Introduction to Plant Diseases: Identification and Management. 2d ed. New York: Springer, 1998. General reference on plant infestations presents thorough information on identification of diseases and the fundamentals of disease management. Good for gardeners or anyone interested in plants. Includes glossary, list of suggested readings, and index. -
Strobel, Gary A., and Gerald N. Lanier. “Dutch Elm Disease.” Scientific American 245 (August, 1981): 56-66. Presents good illustrations of the infestation and the bark beetles. Includes a map with dates showing the progress of the epidemic. -
Tippo, Oswald, and William Louis Stern. Humanistic Botany. New York: W. W. Norton, 1977. Unique book focuses on the interactions of plants and humankind. The chapter on fungi includes a useful section on plant pathology. -
Wilson, Edward O. The Diversity of Life. Rev. ed. Cambridge, Mass.: Belknap Press, 1999. Outstanding book for the general reader. Does not directly address the problem of Dutch elm disease but presents eloquent arguments regarding the value of each species and explains why the reduction of biodiversity should be avoided. Includes glossary and index.
American Farmers Increase Insecticide Use
Federal Food, Drug, and Cosmetic Act
Müller Discovers the Insecticidal Properties of DDT