Pathogens & Vectors
Dutch elm disease (DED) is the single most important disease of American elm (Ulmus americana) and has killed millions of elms throughout North America (Sinclair and Lyon 2005). DED is a lethal vascular wilt disease caused by the non-native fungal pathogens Ophiostoma ulmi and O. novo-ulmi (Melin and Nannfeldt 1934; Brasier 1991). DED was introduced into North America on elm logs from Europe. The first detection occurred in 1930 from Ohio, while a second and much larger outbreak was discovered in 1933 from New York and New Jersey (May 1930; 1934). The first wave of DED was caused by O. ulmi (Holmes 1981), but this species was subsequently displaced by O. novo-ulmi, which caused a more severe second wave of disease (Brasier 1991). The genetic profile of O. novo-ulmi is complicated, as two subspecies are recognized (ssp. americana and ssp. novo-ulmi) and have been found to readily hybridize (Brasier and Kirk 2001; Konrad et al. 2002; Brasier and Kirk 2010). Furthermore, subspecies americana occurs throughout North America (Brasier and Kirk 2000) and is composed of two genetically distinct lineages (Hessenauer et al. 2020). A third Ophiostoma species (O. himal-ulmi) also infects elm but is not known to occur in North America (Brasier and Mehrotra 1995; Bernier 2022).
Overland spread of DED from diseased to healthy elms is primarily facilitated by the smaller European elm bark beetle (Scolytus multistriatus), the banded elm bark beetle (S. schevyrewi) from Asia, and the native elm bark beetle (Hylurgopinus rufipes) (Johnson and Lyon 1991; Negrón et al. 2005; Lee et al. 2009; Santini and Faccoli 2013). The disease also spreads from diseased to healthy elms through root grafts when trees are near one another (Sinclair and Lyon 2005).
Hosts
All North American elms are susceptible to DED to some degree. American elm, the most abundant elm species in North America, is highly susceptible while slippery elm (U. rubra), rock elm (U. thomasii), winged elm (U. alata), cedar elm (U. crassifolia), and September elm (U. serotina) vary from susceptible to somewhat resistant (Sinclair and Lyon 2005).
Several DED-resistant cultivars of American elm have been developed, with U. americana 'Princeton' and U. americana 'Valley Forge' the most widely available from tree nurseries (Townsend et al. 2005; Haugen and Bentz 2017). These elms resist DED, in part, by more rapidly detecting Ophiostoma after infection and triggering defense-related genes (Sherif et al. 2017). While these cultivars offer good to excellent resistance to DED, they both suffer from poor canopy structure and are prone to stem and branch breakage under high winds and snow (Copeland et al. 2023). There are several other DED-resistant American elm cultivars that are not as widely available, but offer more desirable canopy structure and form, include U. americana 'Jefferson' and U. americana 'New Harmony' (Haugen and Bentz 2017; Copeland et al. 2023).
Many European elms are also susceptible, such as Scotch elm (U. glabra), European white elm (U. laevis), field elm (U. minor), and Dutch elm (U. × hollandica) (Sinclair and Lyon 2005). Japanese elm (U. davidiana var. japonica), Siberian elm (U. pumila), and Chinese elm (U. parvifolia) are highly resistant and rarely succumb to the disease in the landscape (Sincliar and Lyon 2005).
Symptoms & Disease Cycle
Symptoms of DED, when transmitted by elm bark beetles, typically first appear on upper canopy branches as wilting and yellowing leaves, a symptom referred to as “flagging” (Sinclair and Lyon 2005). In some cases, the leaves turn from green to brown and are rapidly shed from the canopy. For many American elms, once a branch becomes symptomatic, adjacent branches also quickly show symptoms, followed by major canopy dieback (Sinclair and Lyon 2005). Across southern New England, DED symptoms often first appear from late June to early July but can continue to develop later in the summer. Drought or other abiotic stresses (e.g. construction injury) may intensify symptom development and weaken natural defenses, making trees more susceptible (Elgersma 1981).
When the bark is peeled or cut on infected branches, longitudinal, brown-colored bands or streaks in the outer rings of the sapwood are almost always visible. This symptom is referred to as vascular staining and it’s the most distinctive symptom of DED that can be observed in the field (Sinclair and Lyon 2005). Care must be taken to search for vascular staining immediately after the bark is cut or peeled, as sugars in the outer sapwood can oxidize when exposed, turning healthy tissues brown. Flagging symptoms alone may not be sufficient to diagnose DED, as elm anthracnose, fungal branch cankering, drought, among other stresses, can also lead to flagging symptoms in the upper canopy. Therefore, it is important to have DED confirmed prior to intervention.
American elms infected by DED suffer xylem disfunction that results in water starvation and death (Sinclair and Lyon 2005). This occurs when Ophiostoma spreads and parasitizes the xylem vessels, resulting in occlusion (blockage) and cavitation (breakage of the water column) (Bernier 2022). Additionally, antimicrobial compounds (e.g. tyloses, gels, and phenolics) produced through the tree’s natural defense response can block the vessels to compartmentalize the damage (Sinclair and Lyon 2005; Bernier 2022). Physical alterations to the vessels may also occur to create barrier zones to impede pathogen spread (Bernier 2022).
The overland spread of DED depends on the activity of its insect vectors, the European, banded, and native elm bark beetles. Elm bark beetles breed under the bark of dying or dead elms (Johnson and Lyon 1991). When their eggs hatch, the larvae feed on inner bark and sapwood, forming a network of galleries in the wood. Ophiostoma grows and produces fruiting structures topped with sticky spore masses (synnemata) in the beetle galleries of infected elms. The spores then coat the bodies of adult beetles as they emerge from diseased trees or cut logs (Santini and Faccoli 2013). Adult elm bark beetles chew into twig crotches in the upper canopy of healthy elms and chew through the bark of branches and the main trunk on stressed elms to feed on the inner bark or to create galleries for overwintering and egg laying (Johnson and Lyon 1991; Lee et al. 2009). In doing so, the beetles carry the spores of Ophiostoma into or near severed xylem vessels as they feed, where the spores germinate and infect the water-conducting tissues.
The second mode of infection from an infected tree to an adjacent healthy tree is via root grafts (Copeland et al. 2023). In general, elms must be within 30’ of one another for graft transmission (Sinclair and Lyon 2005). Water conducted through the connected root systems carries Ophiostoma spores along with it. Once Ophiostoma is introduced into the roots of the healthy elm, it slowly moves within the vascular system to the upper canopy. DED may progress rapidly, killing infected elm during the same season after infection or it may gradually cause branch dieback over several years in elms that exhibit some level of resistance.
Management
Regularly scout elms for flagging branches in the upper canopy, especially from mid-June onward during the growing season. When flagging is observed, immediately prune symptomatic branches and peel the bark, to both confirm vascular staining is present and determine the point at which it ends. To ensure Ophiostoma has been successfully excised, prune the flagging branch at least 10 feet beyond the border or interface of vascular stain and clear sapwood (if possible) (Campana and Stipes 1981). Pruning should commence as soon as flagging symptoms are observed if there is hope of successfully eradicating the pathogen. To confirm Ophiostoma is the causal agent, submit symptomatic branch material to a diagnostic laboratory.
Elm bark beetles can transmit the fungus throughout the entire growing season, approximately May through September (Santini and Faccoli 2013). Protect American elms from infection with preventative fungicide injections at 1 to 2-year intervals. Fungicides labeled for use against DED include Arbotect 20S (thiabendazole hypophosphite), Alamo (propiconazole), and Tebuject 16 (tebuconazole). Alamo and Tebuject 16 injections can occur early in the growing season (e.g. May) while Arbotect 20S injections are typically conducted once the foliage is fully hardened off in June to avoid the risk of foliar phytotoxicity. While current research is lacking, previous studies have shown that thiabendazole can cause serious injury to wood tissue surrounding the injection site and foliage in the canopy (Andrews et al. 1982; Lanier 1987; Stennes 2000). Arbotect 20S residues have been shown to persist in the canopy longer than Alamo residues, translocating into wood tissues formed after injection, and Arbotect 20S is believed to provide longer term protection (Stennes 2000; Stipes 2000). Another preventative treatment is the biocontrol Dutch Trig (Verticillium albo-atrum strain WCS850), which is widely used in Europe with high success rates (Postma and Goossen-van de Geijn 2016).
When DED infections are detected, therapeutic injections with Alamo or Arbotect 20S can be performed in conjunction with immediate pruning of diseased branches (Stennes 2000; Stipes 2000). Alamo is generally preferred for therapeutic treatment because it rapidly translocates into the canopy, but when more than 10–20% of the canopy is symptomatic, treatment is often too late to be effective (Stipes 2000). Even if elms have been preventatively treated, therapeutic injections can occur during the same growing season. Ensure that injections are being made on the root flares and that repeated injections do not result in injury to the tree.
Elms infected with DED that have no hope of recovery should be removed immediately. Ensure that all debris that is more than 1 inch in diameter is chipped and that logs are not stored for use as firewood. Sanitation activities reduce local inoculum and remove elm bark beetle brood sites, which slows the spread of DED. If dying elms are left standing, they release chemicals that are highly attractive to elm bark beetles, become major breeding sites, and increase the likelihood that nearby elms will become infected (Bernier 2022).
Protect susceptible elms against insect pests (e.g. spongy moth, Japanese beetle, and elm leaf beetle) and opportunistic pathogens to maintain high vigor. Establish a large mulch ring around mature elms to avoid mower and string trimmer injury to lateral roots and the base of the trunk. Provide young American elms with supplemental water during extended dry periods to avoid the development of drought stress. While management of DED comes with a financial cost, there is a price to be paid when untreated elms are infected and killed. Specifically, the loss of elms to DED reduces ecosystem services and benefits such as stormwater and airborne pollutant capture, heat island amelioration, and reduced property values, among others (Hauer et al. 2020). Additionally, dead elms represent a risk and must be removed at a cost to municipalities.
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