Elms are tough trees, but they have two well-known killers: Dutch elm disease and elm yellows. Both can kill a tree within a year or two, and both spread to nearby elms. That makes it important to recognize the warning signs early. Not every brown leaf means the end, though. Drought, leaf spots and wetwood can look alarming while doing little lasting harm. Here is how to tell the difference.

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The short answer: an elm may be dying if leaves on one or more upper branches suddenly wilt, curl, turn gray-green or yellow and then brown in early summer (called flagging), especially with brown streaks in the wood under the bark. That pattern points to Dutch elm disease. Yellowing and wilting across the whole crown at once suggests elm yellows. Both need fast action: get a lab diagnosis and call a certified arborist. Thin leaves, slow growth and scattered dead twigs more often point to drought or root stress.

Sign 1: flagging branches in early summer

The Morton Arboretum says Dutch elm disease (DED) is most easily detected during early summer, when leaves on an upper branch curl and turn gray-green or yellow and finally brown. This is called flagging. Penn State Extension describes the same pattern: leaves on one or more branches wilt, yellow and fall prematurely, and progressively more branches show symptoms.

A flag alone does not prove DED. The Morton Arboretum notes brown streaks in the wood beneath the bark of affected branches are further evidence, and only laboratory testing can confirm the disease.

Sign 2: brown streaks under the bark

Cut a wilting twig and peel back the bark. Penn State Extension notes the outer layers of the sapwood of DED-affected branches have brown streaks. Healthy sapwood is pale and clean. Streaking is a strong reason to send a sample to a diagnostic lab.

Sign 3: the whole crown yellows at once

Elm yellows, also called phloem necrosis, looks different. Penn State Extension describes leaves yellowing and wilting over the entire crown and falling prematurely in summer, with trees possibly dying within a year. The inner bark turns yellow or butterscotch colored and has a distinct wintergreen smell right after cutting.

Other signs that are less serious

Scorched leaf edges in mid to late summer. Penn State Extension lists bacterial leaf scorch, which causes marginal leaf burn with a wavy yellow border, on a few branches at first. It worsens over years rather than weeks.
Foul-smelling ooze on the trunk. This is wetwood. Penn State Extension says it does no apparent damage unless the tree comes under some other severe stress. Do not drill holes or insert pipes.
Thin crown, small leaves, dead twigs. These often follow drought or root damage. The Morton Arboretum recommends deep watering during dry spells.
Shelf-like fungi at the base. Penn State Extension notes fruiting bodies of root rot fungi mean serious damage, and a tree with them near people or property should be assessed for removal.

Which elm do you have?

Susceptibility depends on the species. The Morton Arboretum says American elm is extremely susceptible to DED, and all native and European elms are susceptible, while Asiatic elms such as lacebark elm and Siberian elm are highly resistant. If you are not sure which you have, see how to identify a slippery elm.

How DED spreads

According to The Morton Arboretum, two bark beetles carry the DED fungus. They breed in infected trees and carry spores to healthy elms when they feed on twigs. The disease also spreads through root grafts between neighboring elms, which are common where elms grow close together along streets.

Can a dying elm be saved?

It depends how early you act:

Get a diagnosis fast. The Morton Arboretum advises sending samples to a diagnostic lab as soon as DED is suspected.
Prune out early infections. The Morton Arboretum says an infected tree may be saved by pruning out the diseased branch promptly after the first flag, and pruning works best when no more than 5% of the tree shows symptoms. Penn State Extension advises finding the lowest point of streaking and cutting at least 12 feet below it. Hire a certified arborist for this.
Ask about fungicide injection. Penn State Extension lists fungicide injection as a management option for DED. It is done by trained arborists.
Remove trees that are too far gone. The Morton Arboretum says a tree with many flags, or that has wilted completely, should be removed quickly so beetles and root grafts do not spread the disease. Root grafts to nearby elms should be severed before removal where possible.
Handle the wood carefully. Do not keep elm firewood with the bark on or move it to new areas. The Morton Arboretum recommends chipping or debarking infected wood.

Inspect valuable elms regularly

The Morton Arboretum suggests inspecting valuable elms weekly from early May through July and monthly through September. Early detection is the difference between losing a branch and losing the tree.

Replanting

If you lose an elm, many DED-resistant cultivars are available. Penn State Extension lists more than twenty, including Accolade, Princeton, Valley Forge and the Morton series.

Sources

Dutch Elm Disease, The Morton Arboretum
Elm Diseases, Penn State Extension
American Elm, The Morton Arboretum
Siberian Elm, The Morton Arboretum
Watering Trees and Shrubs, The Morton Arboretum

Top photo: AHA, CC BY-SA 4.0, via Wikimedia Commons

How to Tell If an Elm Tree Is Dying