Dutch elm disease
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What is Dutch elm disease?
Dutch elm disease is a destructive fungal illness affecting elms and certain other trees, first identified in the Netherlands. It is caused by three species of Ophiostoma fungi, primarily spread by bark beetles. The disease has significantly reduced elm populations across Europe and North America.
Symptoms include wilting, yellowing, and browning of leaves, which may then curl and drop. Infected trees can die within months or take longer, showing brown or black discoloration in the sapwood. Control measures focus on preventing beetle spread through practices like burning infected wood and using insecticides. Some elm species, such as Chinese and Siberian elms, show good resistance.
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Which trees are affected by Dutch elm disease?
Dutch elm disease primarily affects elms (Ulmus species). While the American elm is particularly susceptible, other elm species, as well as related trees like Zelkova and Planera, can also be affected to varying degrees. Species such as the Chinese elm (U. parvifolia), Siberian elm (U. pumila), and smooth-leaf elm (U. carpinifolia) have shown good resistance to the disease. The disease is caused by fungi in the genus Ophiostoma and is spread by bark beetles.
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What causes Dutch elm disease?
Dutch elm disease is caused by three species of ascomycete fungi in the genus Ophiostoma, primarily O. ulmi and O. novo-ulmi. These fungi are spread by bark beetles, such as the smaller European elm bark beetle (Scolytus multistriatus). When female beetles excavate egg-laying galleries in dead or weakened elm wood, they can pick up fungal spores. These spores are then carried to healthy elms when the beetles feed in leaf axils and twig crotches. The fungus enters the tree's water-conducting vessels, where it reproduces rapidly, leading to wilting, yellowing, and eventual death of the tree. The disease can also spread through natural root grafts between trees.
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How does Dutch elm disease spread between trees?
Dutch elm disease spreads primarily through the actions of bark beetles. Female beetles excavate egg-laying galleries in dead or weakened elm wood, where fungal spores from the Ophiostoma genus are produced in large numbers. When young adult beetles emerge, they carry these spores on their bodies.
Healthy elms become infected when these beetles feed on them, dislodging spores that enter the trees' water-conducting vessels (xylem). The fungus then reproduces rapidly, weakening the tree and attracting more beetles, thus continuing the cycle. Additionally, the fungus can spread between trees via natural root grafts, reaching up to 15 meters (50 feet). The disease is characterized by wilting, yellowing, and browning of leaves, ultimately leading to the tree's death.
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What are the signs that a tree has Dutch elm disease?
The primary sign of Dutch elm disease is the sudden wilting of leaves on one or more branches of an elm tree. These leaves will turn a dull green, yellow, or brown, then curl and may fall off prematurely. In younger trees, this can lead to death within a couple of months, while older trees might take longer to succumb. A brown to black discoloration under the bark of wilting branches is also indicative of the disease. Positive diagnosis, however, requires laboratory culturing, as these symptoms can resemble other tree ailments.
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How can people try to prevent or control Dutch elm disease?
Preventing and controlling Dutch elm disease primarily involves managing the bark beetles that spread the fungal pathogen. This includes removing and destroying dead or weakened elm wood before spring, as this is where beetles lay their eggs. Applying insecticides to the bark in the dormant season can kill beetles before they transmit the fungus. While some fungicides can be injected into trees, these are generally more protective than curative. Planting elm species that show resistance, such as Chinese or Siberian elms, is also a key strategy.
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Dutch elm disease, widespread fungoid killer of elms (Ulmus species) and certain other trees, first described in the Netherlands. Spread by bark beetles, the disease has decimated elm populations throughout much of Europe and North America.
Dutch elm disease is caused by three species of ascomycete fungi in the genus Ophiostoma. One of these, O. ulmi (also known as Ceratocystis ulmi), was probably introduced into Europe from Asia during World War I. The disease was first identified in the United States in 1930. A federal eradication campaign in the late 1930s and early ’40s sharply reduced the numbers of infected elms but could not stop the disease’s spread into regions wherever the very susceptible American elm (Ulmus americana) grows. In the late 1940s, another virulent species, O. novo-ulmi, was described in Europe and the United States, and heavy elm losses continued. This species was introduced to Auckland, New Zealand, in 1989, where it was nearly eradicated with aggressive control measures; the country suffered a major outbreak in 2013 due to a decline in funding for these efforts. The third species, O. himal-ulmi, was discovered in 1993 and is endemic to the Himalayas.
Spread of the fungus normally occurs by the smaller European elm bark beetle (Scolytus multistriatus), less commonly by the American elm bark beetle (Hylurgopinus rufipes). Female beetles seek out dead or weakened elm wood to excavate an egg-laying gallery between the bark and the wood. If the fungus is present, tremendous numbers of fungal spores (conidia) are produced in the galleries. When young adult beetles emerge through the bark, many carry the spores on and in their bodies. The infection of healthy elms occurs when beetles feed in the leaf axils and young twig crotches of healthy trees. Some spores are dislodged and get into these trees’ water-conducting vessels (xylem), in which they reproduce rapidly by yeastlike budding. The weakened elm is quickly colonized by hordes of beetles, and the cycle is repeated. The fungus can also spread up to 15 metres (50 feet) from diseased to healthy trees by natural root grafts.
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The leaves on one or more branches of a stricken tree suddenly wilt, turn dull green to yellow or brown, curl, and may drop early. Young, rapidly growing elms may die in one to two months; older or less vigorous trees sometimes take two years or more to succumb. A brown to black discoloration occurs in the white sapwood of wilting branches just under the bark. Because symptoms are easily confused with other diseases, especially elm phloem necrosis and diebacks, positive diagnosis is only possible through laboratory culturing.
The control of Dutch elm disease largely involves the exclusion of beetles. All dead, weak, or dying elm wood with tight bark should be burned, debarked, or buried before elms leaf out in early spring. A single, annual dormant spray that coats all bark surfaces with long-lasting insecticide (e.g., methoxychlor) can kill many beetles before they deposit fungus spores. Claims of fungal control have been made for certain fungicides that are injected into the sapwood. Such measures appear to be more protective than curative. Although other species of elms, as well as species of the related Zelkova and Planera, are susceptible in varying degrees, the smooth leaf (Ulmus carpinifolia), Chinese (U. parvifolia), and Siberian (U. pumila) elms have shown good resistance, and experiments with hybrids of American and Asiatic elms have met with much success.