Dutch Elm Disease in Illinois: Signs & What to Do

Most of the big American elms that once arched over North Shore streets are gone. Dutch elm disease arrived in the Chicago area in the 1950s and worked through the canopy for forty years. What is left is a smaller population of survivors, replacement cultivars, and volunteer elms in back yards - and the disease never actually went away. It is still here, still killing elms, just with far fewer elms left to kill.

If you have a mature elm, it is worth knowing what the first symptom looks like, because the window where anything can be done is short.

What the disease is

Dutch elm disease is a fungus, Ophiostoma novo-ulmi, that clogs the elm's water-conducting tissue. The tree tries to wall off the infection, plugs its own vessels in the process, and the crown dies of thirst. Functionally it is the same mechanism as oak wilt, which is why the two diseases get confused.

It moves two ways:

Elm bark beetles. The beetles breed in dying and dead elm wood, emerge carrying fungal spores, and feed on healthy elm twigs. That feeding wound is the infection point. Beetle flight runs roughly April through July, with the heaviest flight in spring.

Root grafts. Elms planted close together, which is exactly how they were planted along parkways, fuse roots underground. The fungus walks from tree to tree through those connections, and it does not care about any beetle spray program.

The first symptom: flagging

The signature is flagging - one branch, usually high in the crown and on one side of the tree, whose leaves wilt, turn yellow, then brown and curl while the rest of the tree looks perfectly healthy. The dead leaves often hang on instead of dropping.

Timing matters for diagnosis:

• Flagging in May or June is the bad version. It usually means a fresh beetle-borne infection in the crown, and it spreads downward fast.
• Flagging in late summer is somewhat more often a root-graft infection moving up from below, and it can be slower.

Peel the bark off a wilted branch and you will often see brown or olive streaking in the wood just under the bark, sometimes as a ring of dots in cross-section. That streaking is the most useful field sign. It is not proof - a lab culture is proof - but it is strong.

What it is not: a whole crown that thinned gradually over five years, scattered dead twigs in the interior, or an elm dropping leaves in a drought. Elm yellows, verticillium, and plain drought stress all mimic parts of this. The specific pattern worth acting on is one healthy-looking elm, one suddenly dead branch, in the growing season.

What can actually be done

Be clear-eyed about the odds. Response depends almost entirely on how much of the crown is affected when you catch it.

Under about 5 percent of the crown, caught early: therapeutic pruning plus fungicide injection has a real chance. The infected limb is removed well below the visible symptom and below the streaking - typically 8 to 10 feet of clean wood past the last streak - and a systemic fungicide is injected at the root flare. This is not a DIY job and it is not cheap. It is also the only version of this story with a good ending.

More than roughly 10 percent of the crown, or symptoms in the main stem: the tree is lost. Injection at that point buys appearance, not survival.

Preventive injection for a high-value, healthy elm is a legitimate program. It runs on a two- to three-year cycle, and it is a commitment, not a one-time purchase. For a specimen American elm shading a house, the math often works. For a volunteer elm in a fence line, it does not.

Root-graft severing between a confirmed infected elm and a valuable neighbor is a real option where the trees are close, using a vibratory plow or trencher to cut the underground connection before the fungus uses it.

An honest arborist consultation is where this starts, and the honest version sometimes ends with "this one is not worth saving." Ask for the crown percentage estimate out loud.

The timing rule for pruning elms

Do not prune elms April through July. Fresh cuts during beetle flight advertise the tree. Keep elm work in the dormant season, which is the better pruning window for almost everything anyway. The exception is the same as for oaks: a hazardous limb after a storm comes off when it is hazardous, season be damned.

This is the same rule covered in oak and elm care for North Shore homes, and it is the single cheapest thing a homeowner can do for a mature elm.

The wood is the problem

Elm bark beetles breed in dead and dying elm bark. A stack of elm logs sitting in your yard through the spring is a beetle nursery pointed at every elm within a few hundred feet, including your own.

If an elm comes down for confirmed Dutch elm disease:

• Chip it or remove it from the site. Do not stockpile the rounds.
• Do not keep it as firewood with the bark on. Debarked elm is fine. Barked elm stacked by the garage is not.
• Do not move elm wood to another property. This is how the disease covers distances it could never travel on its own.

More on disposal generally in what happens to the wood and chips.

If it has to come down

A dead or dying elm is a brittle-wood removal with a wide, heavy, often asymmetric crown, frequently over a driveway or a roof because that is where the old elms were planted. That combination - weight plus compromised wood plus no good drop zone - is the profile where crews rig in sections or bring in equipment rather than felling. Large tree removal cost covers the pricing, and crane-assisted removal covers the cases where the crane is cheaper than the alternative.

Do not wait on a dead elm the way you cannot wait on a dead ash. The wood degrades, the limbs get unpredictable, and the price of the job goes up as the risk does.

The short version

1. One dead branch on an otherwise healthy elm in May or June is a five-alarm symptom. Get it looked at that week, not that season.
2. Never prune an elm April through July.
3. Get the crown percentage before you spend on injection. Under 5 percent, spend. Over 10 percent, plan the removal instead.
4. Get the wood off the property.

Surviving mature elms are scattered through Evanston, Wilmette, Winnetka, Highland Park, and Lake Forest, and they are worth the attention. If an elm on your property is flagging, request a free on-site estimate for a diagnosis, or use the online estimate tool for a cost range if you already know it is coming down.