Dutch elm disease is caused by a type of fungus that kills elm trees. The disease is spread by elm bark beetles. Originally from Asia, the disease was accidentally brought to America, Europe, and New Zealand. In these areas, it has harmed both native and non-native elm trees that were not resistant to the disease. The name "Dutch elm disease" comes from its discovery in the Netherlands in 1921 by scientists Christine Buisman and Marie Beatrice Schol-Schwarz, who worked with Johanna Westerdijk. The disease affects many species in the Ulmus and Zelkova genera and is spread by several types of beetles, especially those in the Scolytus genus.

Efforts to treat or prevent the disease have included using insecticides to kill the beetles. This method was used in America during the 1940s and 1950s but harmed other animals, such as insects and birds. Fungicides can protect individual trees if injected every two to three years, as long as the roots are not infected. A vaccine called Dutch Trig uses a different fungus, Verticillium albo-atrum, to help trees build immunity. Scientists in the Netherlands, America, and Italy have also tried breeding elm trees that are resistant to the disease.

Overview

Dutch elm disease affects several types of trees in the Ulmus and Zelkova genera. The English elm (Ulmus procera) was severely affected by outbreaks in the 20th century. The smooth-leaved elm (Ulmus minor) has some resistance to the disease. The wych elm (Ulmus glabra) is mostly avoided by the beetles that spread the disease.

Dutch elm disease is caused by a type of tiny fungus called ascomycete microfungi. Three species of this fungus are now known:

• Ophiostoma ulmi, which spread across Europe starting in 1910 and reached North America through imported wood in 1928.
• Ophiostoma himal-ulmi, a fungus found only in the western Himalaya.
• Ophiostoma novo-ulmi, a highly harmful fungus from Japan. It was first discovered in Europe and North America in the 1940s and has caused widespread damage to elms in both regions since the late 1960s.

In Europe, several species of elm bark beetles, including Scolytus scolytus, S. triarmatus, S. multistriatus, and S. laevis, carry the disease. S. scolytus is most often linked to spreading Ophiostoma novo-ulmi. Other beetles that may carry the disease include S. sulcifrons, S. pygmaeus, Pteleobius vittatus, and P. kraatzi. In North America, the native elm bark beetle (Hylurgopinus rufipes) is a vector.

Field resistance refers to the ability of elm trees to survive in areas where Dutch elm disease occurs. The European white elm (Ulmus laevis) has little or no genetic resistance to the disease but is less attractive to beetles, which helps it avoid serious harm. Trees like U. laevis, U. minor, and U. pumila produce certain chemicals, such as alnulin, β-amyrin, friedelin, ilexol, lupenone, lupeol, methyl betulinate, and moretonol. These chemicals make their bark less appealing to beetles.

To stop the fungus from spreading, trees produce plugs in their xylem tissue using gum and tyloses, which are balloon-like extensions of the xylem cell walls. These plugs block the flow of water and nutrients up the tree’s trunk, eventually causing the tree to die.

The first sign of infection usually appears in the upper branches. During summer, the leaves in these areas begin to turn yellow and wither, months before the normal autumn leaf drop. This damage spreads throughout the tree, causing more branches to die back. Eventually, the roots die due to a lack of nutrients from the leaves. In some species, such as the English elm, the roots may produce new shoots called suckers. These shoots can grow for about 15 years before dying.

• Asexual structures of the fungus Ophiostoma ulmi can survive in bark and wood during winter and be spread by beetles.
• Scolytus scolytus, the larger European elm bark beetle, is a major carrier of the disease.
• Beetle feeding tunnels on the trunk of a wych elm.
• Branch death, or flagging, occurs in multiple areas of the crown of a diseased elm.

Disease range

Dutch elm disease was first noticed in continental Europe in 1910. It spread slowly over time and eventually reached all countries except Greece and Finland. Barendina Gerarda Spierenburg recorded information about trees showing symptoms in the Netherlands from 1900 to 1905. She shared this information in a publication in 1921, which helped start research to stop the disease. In the same year, Bea Schwarz, a Dutch plant disease expert, isolated the fungus that causes the disease in the Netherlands. This discovery led to the disease being named Dutch elm disease.

In the 1920s and 1930s, Christine Buisman, working in the Netherlands and the United States, identified the sexual stage of the fungus and developed methods to infect elm seedlings in experiments. These methods helped scientists find elm trees that could resist the disease. In Britain, the disease was first found in 1927 by T.R. Peace on an English elm in Hertfordshire.

In about 1967, a more dangerous strain of the disease arrived in Britain. It likely came through east coast ports on logs of rock elm from Canada. This strain was confirmed in 1973 after another shipment was examined in Southampton. This new strain spread quickly and killed over 25 million elms in the United Kingdom. France lost 97% of its elms. The disease moved northward and reached Scotland within 10 years.

By 1990, few large elms remained in Britain or much of Europe. The English elm, which is especially favored by Scolytus beetles, was very vulnerable. Many of these trees died, but some survived in hedgerows because the roots grew new shoots. These shoots often grew to 10 meters tall before dying from another infection. However, hedges that were regularly trimmed remained healthy after the disease began.

Today, the largest groups of mature elms in Europe are in Amsterdam and The Hague. In 2005, Amsterdam was named the "Elm City of Europe" because its streets and canals have at least 75,000 elms, including trees developed through research. In The Hague, about 30,000 of its 100,000 mature trees are elms, planted because they tolerate salty winds. Since the 1990s, programs that inject antifungal treatments into the most important elms and remove infected trees have reduced elm losses in The Hague to less than 1% a year. New disease-resistant trees are planted to replace lost ones.

In England, the largest group of surviving mature elms is in Brighton and Hove, East Sussex. In 1983, there were 30,000 elms there, and by 2005, 15,000 remained, including some over 400 years old. Their survival is due to the area’s isolation between the English Channel and the South Downs, as well as local efforts to quickly remove infected parts of trees. Laws passed in 1988 allow local authorities to order the removal of infected trees, which has helped reduce the number of elm bark beetles.

In the Isle of Man, about 250,000 elms remain because the climate limits beetle activity. Beetles need temperatures of at least 20 degrees Celsius and wind speeds under five meters per second to fly, which is not common there.

As of 2016, Edinburgh city council reported that 15,000 trees survived the disease. A policy of removing infected trees kept losses to about 1,000 a year. In Paris, elm trees were common from the 17th century, with about 30,000 in the 1970s. Today, only 1,000 remain, mostly in large avenues and parks. Disease-resistant trees like the Dutch-French research elm 'Nanguen' are now planted to replace lost trees.

Dutch elm disease was first reported in the United States in 1928. Beetles likely arrived on logs from the Netherlands used for furniture in Ohio. Quarantine rules kept the disease near New York City until 1941. The disease spread westward and southward, destroying most elms in New Haven, Connecticut, and reaching cities like Detroit, Chicago, and Minneapolis by the 1970s. Of about 77 million elms in North America in 1930, over 75% were lost by 1989.

In Washington, D.C., the disease first appeared on American elms planted along the National Mall in the 1950s. It peaked in the 1970s. The National Park Service used methods like pruning, injecting trees with fungicide, and replanting with disease-resistant trees to control the disease. They also trapped and sprayed insecticides to reduce the number of beetles. These efforts helped preserve the elms on the Mall and surrounding areas for over 80 years.

Dutch elm disease reached eastern Canada during World War II and spread to Ontario in 1967, Manitoba in 1975, and Saskatchewan in 1981. In Toronto, 80% of elms were lost to the disease. Many cities like Ottawa and Montreal also saw large losses in the 1970s and 1980s. Quebec City has about 21,000 elms remaining due to a prevention program started in 1981. Alberta and British Columbia are the only provinces in Canada without the disease, though one tree in Alberta was found infected in 1998 and destroyed immediately.

In Alberta, Manitoba, and Saskatchewan, it is illegal to prune elm trees between April and September, which is when beetles are most active. It is also illegal to use, store, sell, or transport elm firewood in these regions.

The largest surviving group of elm trees in North America is in Winnipeg, where about 200,000 elms remain. The city inspects trees, controls beetles, and removes or replaces infected trees but does not use injections to treat the disease.

Dutch elm disease has also reached New Zealand. It was found in Napier and eradicated, and later in the Auckland region in 1989. A national management program was funded but canceled to focus on other pests. The disease has not been seen again in New Zealand since.

Treatment

Early efforts to stop Dutch elm disease involved cutting away and burning sick parts of trees. In 1963, The New York Times reported that this method, along with spraying trees with DDT, did not work.

In the United States, as the disease spread from the Atlantic coast, efforts focused on controlling the bark beetle by using insecticides like DDT and dieldrin. These chemicals were sprayed on elm trees twice a year, once in the spring and again in the summer with a smaller amount. Early on, people believed that pesticides slowed the disease’s spread, but by 1947, concerns grew because many birds died after eating poisoned insects. In the 1950s, people in sprayed areas noticed deaths among birds like the American woodcock, American robin, white-breasted nuthatch, brown creeper, and chickadees. Biologist Rachel Carson later suggested better cleaning practices and stopping elm spraying, arguing that these methods had worked better in areas with more experience fighting the disease. By 1962, spraying to control beetles dropped quickly, partly because of the use of fungicides.

The fungicide carbendazim phosphate is sold as a treatment for Dutch elm disease under names like Eertavas. Another treatment, Arbotect (thiabendazole hypophosphite), is effective but must be injected every two to three years to keep the disease from spreading. Once a tree is infected, the disease cannot be completely removed. Arbotect does not work on infections from root grafts between nearby elm trees. Another treatment, Alamo (propiconazole), is also effective. Neither Alamo nor Arbotect works on trees with infected root systems.

Female elm bark beetles produce a chemical called multistriatin, which can be made in a lab. In 1983, scientists thought this chemical might help trap male beetles, which carry the fungus that causes the disease.

Because of rules banning chemical use on street and park trees in the Netherlands, the University of Amsterdam created a biological vaccine by the late 1980s. Dutch Trig is a nontoxic treatment made from spores of a weakened strain of the fungus Verticillium albo-atrum, mixed in distilled water and injected into elm trees in spring. This strain is believed to still have enough ability to cause disease to help elms build immunity. This process is called induced resistance. Tests on American elms in Denver, Colorado, showed that after one year, Dutch elm disease losses dropped from 7% to between 0.4% and 0.6%, a faster reduction than seen in other tree care programs.

Breeding resistant trees

Research to find resistant types of elm trees began in the Netherlands in 1928. In 1937, scientists in the United States started studying U. americana. However, U. americana does not mix well with European elms, making it difficult to create hybrids that look like American elms and resist disease. In 2005, the U.S. National Elm Trial tested 19 types of elm trees across the country over a 10-year period.

In Italy, the Istituto per la Protezione delle Piante in Florence worked to develop disease-resistant trees suited for the warmer Mediterranean climate. They used Asiatic elm species crossed with the early Dutch hybrid 'Plantyn' to protect against future changes in the disease.

In 2007, the Elm Recovery Project at the University of Guelph Arboretum in Ontario, Canada, reported that cuttings from healthy old elms found across Ontario were grown into a collection of resistant trees. These trees were used for selective breeding to create highly resistant varieties.

In 2007, A.E. Newhouse and F. Schrodt from the State University of New York College of Environmental Science and Forestry found that young transgenic American elm trees showed fewer disease symptoms and normal root development.

Among European elm species, the European white elm (U. laevis) has little natural resistance to Dutch elm disease. However, the beetles that spread the disease avoid this tree because of organic compounds like triterpenes and sterols in its bark, which make the bark unattractive to the beetles.

Possible earlier occurrences

Scientists studied pollen in peat samples and found that elms were very common before 4000 BC but almost disappeared from northwestern Europe during the mid-Holocene period. This event, called the "Elm Decline," also affected elms to a smaller degree around 1000 BC. In the mid-20th century, experts thought the decline was caused by Neolithic farmers clearing forests and cutting elms for animal food. However, the number of people at that time was too small to explain the widespread loss. Later, the damage caused by Dutch elm disease in the 20th century offered another explanation. Evidence from old elm wood showing signs of the disease and the presence of elm bark beetles from that time support this idea. Experts now believe both farming practices and the disease likely contributed to the Elm Decline.

A less severe form of the disease, caused by a different fungus, may have existed in northwestern Europe for many years. Dr. Oliver Rackham from Cambridge University found evidence of an elm disease outbreak in northwestern Europe between 1819 and 1867. He noted that tree rings from infected elms showed dark stains, confirming Dutch elm disease was present by 1867. He also quoted descriptions of sick elms from Richard Jefferies' 1883 book, Nature near London. Earlier, Rackham mentioned that the elm bark beetle, named Scolytus destructor, was linked to elm damage near Oxford as early as 1780.

In Belgium, elm trees showed signs of dying in 1836, 1896 in Brussels, and 1885–1886 in Ghent. Later outbreaks were linked to the elm bark beetle. In 1658, Sir Thomas Browne wrote in The Garden of Cyrus about an elm disease spreading through English hedgerows, describing symptoms similar to Dutch elm disease.