Dutch elm disease first shows up as a cluster of yellowing, wilting leaves on one or a few branches, usually high in the canopy. These leaves then turn brown, curl inward, and die but stay attached to the branch instead of dropping. This “flagging,” where a section of otherwise green canopy suddenly looks scorched, is the most recognizable sign of infection. Symptoms typically appear in late spring or early summer, though they can show up anytime during the growing season.
Flagging in the Upper Canopy
The earliest visible sign is a patch of leaves that wilt and yellow while the rest of the tree looks healthy. This usually starts in the upper canopy, affecting one branch or a small group of branches. Starting around mid-June, this is what arborists and homeowners look for when scouting elms. The contrast is striking: a single limb turns dull yellow-green or brown against a backdrop of normal foliage.
Within days to weeks, those yellowed leaves turn brown, curl tightly, and shrivel. They cling to the branch rather than falling, which helps distinguish this from normal autumn leaf drop or wind damage. The dead, curled leaves hanging on an otherwise living tree are one of the clearest visual red flags.
How Symptoms Spread Depends on How Infection Started
The disease enters an elm in two very different ways, and each produces a distinct pattern of decline.
When bark beetles carry the fungus into a tree, they typically feed on small twigs in the upper crown. Yellowing starts in one section, usually on a single branch. From there, the disease spreads to adjacent branches and eventually into the trunk over one or more years. This is the more gradual version, giving you a window to spot the problem early.
When the fungus spreads through root grafts (underground connections between neighboring elms), the progression is much faster and more dramatic. Trees infected this way often wilt rapidly in spring, shortly after leafing out, and symptoms move upward from the lower branches. A tree that looked fine one week can appear half-dead the next. This pattern, starting low and moving up, is the opposite of beetle-transmitted infections and often kills the tree within a single season.
Twig Deformities
Infected twigs sometimes curl downward at their tips into a shape called a “shepherd’s crook,” a distinctive hook that forms as the growing tip dies while the rest of the twig keeps elongating briefly. Affected shoots die back from the tip, leaving bare, curved ends visible against the sky. This sign is easier to spot on younger trees or lower branches where you can see individual twigs up close.
Brown Streaking Under the Bark
The most definitive visual sign you can check yourself is hidden just beneath the bark. If you peel back the bark on a symptomatic branch, you’ll see brown streaks running through the otherwise tan outer wood. This discoloration is caused by the fungus growing inside the tree’s water-conducting vessels, blocking the flow of water and nutrients to the leaves.
To check for this, look at branches that are wilting but still alive. Cut a section and examine the cross-section or peel the bark lengthwise. Healthy sapwood is pale tan. Infected sapwood shows distinct brown or dark streaks, sometimes forming a partial or complete ring in the cross-section. This vascular staining is present in symptomatic branches and is one of the key ways to distinguish Dutch elm disease from other problems like drought stress or elm yellows, which typically doesn’t produce this branch-level discoloration.
Bark Beetle Galleries
On dead or dying branches and trunks, you may notice another telltale sign: intricate tunnel patterns carved into the wood just beneath the bark. These are created by the elm bark beetles that spread the disease. A female beetle burrows a single vertical channel through the inner bark and lays eggs along both sides. When the larvae hatch, they eat their way outward, each creating its own tunnel that widens as the larva grows. The result is a sunburst-like pattern radiating from a central groove.
These galleries are most visible on branches where the bark has loosened or fallen away. You can also find them by peeling bark off dead limbs. While the galleries themselves don’t cause the disease directly, their presence on an elm confirms that bark beetles have been active in the tree, and those beetles are almost certainly carrying fungal spores.
What Causes the Damage
The fungus responsible for modern outbreaks is far more aggressive than the one that caused the first wave of the disease a century ago. The original species was largely displaced within a few decades by a more virulent relative that now threatens elm populations worldwide. Two subspecies of this more aggressive fungus exist globally, with one particularly widespread in North America, Japan, and New Zealand.
Once inside the tree, the fungus colonizes the vessels that transport water from roots to leaves. The tree’s own immune response compounds the problem: it produces gummy substances trying to wall off the infection, which further clogs those vessels. The result is that leaves starve for water even when the soil is moist, producing the wilting and browning visible from the ground.
What Dutch Elm Disease Doesn’t Look Like
Several other conditions can mimic early symptoms. Drought stress causes wilting and browning, but it affects the entire tree more or less evenly rather than starting in one section. Elm leaf beetle damage creates skeletonized leaves with holes, not the curling and clinging pattern of Dutch elm disease. Elm yellows, a different disease, causes leaves to yellow and drop (rather than curl and hang on), and it often produces a faint wintergreen smell in the inner bark.
The combination of one-sided canopy flagging, dead curled leaves that stay attached, and brown streaking in the sapwood is what sets Dutch elm disease apart. If you see flagging in an elm during summer, checking for that brown streaking under the bark is the fastest way to move from suspicion to a working diagnosis before sending samples to a lab for confirmation.