Dutch elm disease (DED) is a vascular wilt of elms (Ulmus spp.) caused by fungi and transmitted by bark beetles and root grafts. It is considered one of the most devastating diseases in the world.
It is believed that the fungi causing elm disease originated from Asia. The name “Dutch” refers to the identification of this disease and causal agent in the Netherlands by a Dutch scientist.
Since the early 1900s, the disease has killed millions of elms in Europe and the U.S. There were two massive outbreaks of the disease, first in the 1930s and then again in the 1970s.
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Causal organism of Dutch elm disease
The fungus, Ophiostoma ulmi, a relatively weak pathogen of elms, was responsible for the first outbreak of DED in Europe in the early 1900s. The pathogen was introduced into the United States in the 1930s and spread throughout the country. The fungus Ophiostoma novo-ulmi is the most aggressive pathogen involved and was described in 1991. Both of the species are thought to have originated separately in Asia, with limited hybridization occurring between the two species.
There are two subspecies of O. novo-ulmi, referred to as the Eurasian – O.novo-ulmi and the North American – O. novo-ulmi.
Vector
The native elm bark beetle (Hylurgopinus rufipes), smaller European elm bark beetle (Scolytus multistriatus), and potentially the banded elm bark beetle (S. schevyrewi) are the vectors responsible for the transmission of disease.
Host- The disease affects the trees of Ulmaceae (elm family).
Morphology of pathogen (Ophiostoma spp.)
O. ulmi and O. novo-ulmi is an ascomycete fungus consisting of white-greyish hyaline hyphae and a perithecium that encloses the asci.
The asexual stage consists of two types of conidia- micro and macroconidia. The macroconidia are generally larger, cylindrical, or club-shaped with rounded ends whereas the microconidia are more elliptical and produced on slimy droplets on the conidiophores.
The sexual stage consists of dark-brown flask-shaped fruiting bodies called perithecia which consist of sac-like ascus containing ascospores. These ascospores are bicellular, crescent-shaped formed as a result of sexual reproduction.
Symptoms of Dutch elm disease
Symptoms vary between the trees infected through root grafts and those infected via beetles.
Direct Spread through Roots: Adjacent, closely spaced elms often have roots joined together. When DED fungus infects one elm, it rapidly spreads directly into adjacent trees through these roots. It causes rapid, localized death, often killing an entire block or row of elms.
The impact spreads rapidly from tree to tree, bypassing the activity of the beetle and creating sudden death in surrounding trees.
Beetles as Vectors: Elm bark beetles, belonging to the genera Scolytus and Hylurgopinus, are the main vectors through which DED spreads over long distances. Adult beetles of elm bark beetles inoculate spores of the fungus into healthy trees by feeding wounds on the bark. The beetles acquire spores from diseased or dead trees, where the fungus grows in breeding galleries.
The spread of the disease is through beetles that extend the infection over large areas to create new outbreaks in otherwise healthy tree populations. The fungus infection within the xylem of the tree causes disconnections of water flows, and this causes wilting, and dieback of branches, and ultimately leads to the death of the tree.
Localized, root graft transmission leads to rapid spread to neighboring trees, while beetle transmission spreads the disease long distances.
Symptoms seen in different plant parts are as follows-
Leaves– In the early phases, leaves begin to wilt which is commonly referred to as “flagging” with a dull green or yellowish color, especially in one or two branches.
During the late phase, with the full browning of leaves, drying, and early defoliation which will strip parts of the tree bark.
Branches and Twigs- Early symptoms include localized wilting in one or more branches, often at the top of the tree. In late stages, multiple branches die back, leaving the canopy sparse, and irregular. Dead branches can hide beetle galleries under the bark.
Trunk and Vascular System- In the early stages, the fungal infection disrupts water transport in the tree’s xylem, causing obvious wilting and water stress. Dark streaks appear in the sapwood, while the xylem fully blocks in late stages, leading to wide canopy dieback.
Root System- Though early symptoms in the roots are not observable, infected trees can transfer the fungus through root grafts. In late stages, root systems die off, weakening the tree and allowing the spread of the fungus to neighboring trees through root connections.
Disease cycle of Dutch elm disease
1. Initial Infection by Beetles
The disease cycle of Dutch elm disease (DED) starts when elm-bark beetles Scolytus and Hylurgopinus species) act as vectors for the DED fungus Ophiostoma ulmi or Ophiostoma novo-ulmi. The adult beetles come out of infested or dead trees and they bring the fungal spores with them on their bodies. They eat healthy elms and in eating them they make little feeding wounds in the bark, especially in the upper branches. The fungal spores enter the tree’s vascular system through these wounds and that is when the infection begins. This usually happens in the spring and early summer when the beetles are out.
2. Fungal Growth and Vascular Blockage
After entering the tree, the fungal spores rapidly germinate and grow throughout the xylem (which is the water-transporting tissue). This fungus reproduces and fills the tree’s xylem with its mycelium. The tree in turn responds by creating tyloses (essentially growths within the xylem) in an attempt to block the fungus, which of course further hinders the flow of water. This in turn causes the tree to wilt leaves turning yellow or brown, and then the branches die back. If the infection isn’t checked then the tree’s water supply is completely cut off and the tree will die within a growing season or two.
3. Beetle Reproduction in Infected Trees
The tree withers and dies and it serves as a host for the beetles. The adult beetles burrow underneath the bark of infected trees to lay their eggs and create galleries to reproduce. The DED fungus grows in these galleries and releases sticky fungal spores that cover the maturing beetles. Then when the new adult beetles come out they take the spores on their body and fly to other healthy trees and the cycle of infection continues. That way the fungus finds its way to new trees each year through the process of beetle reproduction.
4. Root Graft Transmission
Besides being transmitted by beetles, DED moves from tree to tree via root grafts. Elm trees growing close to one another can have intertwined root systems, allowing the fungus to move directly from the roots of an infected tree into the roots of a healthy tree. This root graft transmission frequently leads to explosive, localized epidemics of the disease where entire rows or groups of trees become infected without the agency of the beetle. It tends to occur in urban or planted areas where the trees are densely populated, which can cause large mortality.
5. New Infection Cycle
After a tree is initially infected, either through beetle activity or root grafts, it then serves as a reservoir for the spread of the disease. The infected tree will die in a few seasons, acting as a new host for beetles that will transport the fungal spores to other trees, thus a new cycle begins.
Management of Dutch elm disease
Disease management for Dutch elm disease (DED) involves a multifaceted approach that incorporates preventive strategies, early detection, and control measures. These are necessary to keep the disease from wiping out elm populations and to keep these beautiful trees around for a long time.
1. Tree Selection and Planting
One of the best ways to combat DED is to plant resistant tree species. Disease-resistant varieties like “Ulmus americana” ‘Valley Forge’ or “Ulmus parvifolia” would go a long way in preventing infection.
These trees have been specifically bred for their tolerance to DED, allowing for healthier populations even in areas where the disease is prevalent. Also, in urban and sub-urban areas a variety of tree species should be planted to avoid the dangers of monoculture, which is more likely to be destroyed by disease.
2. Monitoring and Early Detection
Monitoring DED regularly and early detection are a must to properly manage the disease. They may be checked often, especially in the late spring and summer, for early signs of infection, like wilting leaves, yellow foliage, and dark streaking of the bark. The earlier the disease is found then the more controllable it will be.
3. Vector Control
One of the aspects of disease management is the control of the elm bark beetle population, the major carriers of DED.
IPM (Integrated pest management) techniques can be used to minimize the beetles and the damage they cause. Insecticide applications can be done in early spring when beetles are at their peak and this can prevent healthy trees from being infested. And timing and methods of application must be done carefully to avoid killing non-target species and damaging the environment. Also, pheromone traps can be used to track and trap beetles, and the information gained about the population and movement of these beetles can greatly decrease the risk of disease spread.
4. Pruning and Removal
Good DED management also includes the removal and pruning of infected trees. Periodically checking trees for infection and immediately cutting off any infected branches will greatly reduce the rate at which the disease spreads through a tree. It is very important that when one prune, they dispose of the materials properly so that the fungus does not spread to other trees. When a tree is highly infected or already dead, immediate removal is imperative.
Dead and dying trees are susceptible to beetle infestations and sometimes these beetles will move to neighboring healthy trees. In turn, by taking down these infested trees, communities will be able to save their urban forests and limit the number of beetles.
5. Root Graft Management
Another thing that must be done to control DED is to eliminate root graft transmission. When elm trees are planted near each other, their root systems will eventually join, making it easier for the fungus to jump from tree to tree. One way to stop this type of spread is to cut root grafts between infected trees and healthy trees. This may include actual digging or some other way of severing the roots. This procedure if used will greatly decrease the spread of the disease and the infection of other healthy trees.
6. Chemical Treatments
There is no known cure for DED but chemical treatments may provide a certain amount of protection for healthy trees, especially those of great historical or aesthetic significance.
Preventative systemic fungicides may be used to treat healthy elms, especially in DED-infected regions. These therapies are most successful when administered before infection or upon the onset of symptoms of disease. However, the use of fungicides should be approached with caution and conducted under the guidance of professionals to minimize any potential environmental impact.
7. Research and Education
Continued studies on DED (Dutch elm disease) and its management may lead to new methods or refinements to current practices. Keeping up to date on the current research and methodologies allows communities to change their management practices. Participating in workshops, training sessions, and community forums focused on urban forestry and disease management can enhance knowledge and skills among tree care professionals and community members alike. Collaborating with local extension services, arborists, and forestry experts can provide valuable insights and assistance in managing DED effectively.
8. Community Involvement
DED management strategies must include community participation. Engaging residents in monitoring and managing elm populations can foster a sense of responsibility and pride in community greenspaces. Tree care programs and volunteer days for planting and caring for trees would foster stewardship and educate people about DED. This along with the promotion of local government ordinances that require a certain amount of biodiversity in tree planting, and the discouragement of elm planting in highly infested areas will also help to minimize the effect of the disease.
By implementing these comprehensive management practices, communities can effectively mitigate the impact of Dutch elm disease, protect existing elm trees, and preserve the overall health of urban and rural forests. This preemptive action not only protects important forest trees but also strengthens a community’s ability to resist future disease infestations.
References
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- Bernier, L. (2022). Dutch elm disease. In Elsevier eBooks (pp. 291–309). https://doi.org/10.1016/b978-0-323-85042-1.00010-0
- Dutch Elm Disease and its Control – Oklahoma State University. (2017, February 1). https://extension.okstate.edu/fact-sheets/dutch-elm-disease-and-its-control.html
- Gibbs, J.N. 1978. Intercontinental epidemiology of Dutch elm disease. Annual Review of Phytopathology. 16:287-307.
- Dutch Elm Disease. (n.d.). https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/cankers/dutch-elm-disease
- Dutch Elm Disease – MSU Extension | Montana State University. (n.d.). https://wetlands.msuextension.org/extension/Full_HTML_Pubs/a-guide-to-pests-problems-and-identification-of-ornamental-shrubs-and-trees-in-montana/diseases/dutch-elm-disease.html
- Martín, J. A., Domínguez, J., Solla, A., Brasier, C. M., Webber, J. F., Santini, A., Martínez-Arias, C., Bernier, L., & Gil, L. (2021). Complexities underlying the breeding and deployment of Dutch elm disease-resistant elms. New Forests, 54(4), 661–696. https://doi.org/10.1007/s11056-021-09865-y
- Dutch elm disease / RHS. (n.d.). Royal Horticultural Society. https://www.rhs.org.uk/disease/dutch-elm-disease
- The Editors of Encyclopaedia Britannica. (1998, July 20). Dutch elm disease | Description, History, Symptoms, & Control. Encyclopedia Britannica. https://www.britannica.com/science/Dutch-elm-disease
- Dutch elm disease. (2015, October 28). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/dutch-elmdisease/54488994
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