Dutch elm disease

Cause

Two fungi, Ophiostoma novo-ulmi and O. ulmi

Hosts

American and European elms

Spread

By native and European elm bark beetles, and through root grafts between trees

First sign

Flagging: upper-canopy leaves curl, wilt and turn gray-green or yellow

Confirmation

Brown streaks under bark of a wilted branch; a lab test through Extension

Dutch elm disease is caused by the fungi Ophiostoma novo-ulmi and Ophiostoma ulmi. It is a serious disease of American and European elms, but wilting elms have other causes too, including drought and the elm leaf beetle. This page explains the signs, how it is confirmed and what pruning or removal looks like. Battle Born Tree Service does not spray, inject or apply products to trees. We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved.

What are the signs of Dutch elm disease?

Montana State University Extension describes the first symptoms in summer, on branches in the upper canopy. The leaves curl, wilt and turn grayish-green or yellow, and the branch looks like a flag against the rest of the green crown. Nearby branches follow, and a tree can lose a large share of its canopy in a season. Drought can make the symptoms look worse.

The test that arborists use is the wood under the bark. When an arborist cuts into a flagging branch and peels the bark back, a diseased branch shows distinct brown streaks in the outer wood. Oklahoma State University Extension adds that samples must come from branches that are actively wilting and that show this discoloration, and that confirmation is by a laboratory through your county Extension office.

How does it spread?

The fungi overwinter in infected elms and are carried to new trees by the native elm bark beetle and the European elm bark beetle. Beetles breed in dead and dying elms, so a dead elm left standing or a pile of elm wood with bark on it is a nursery for the carrier. The disease also moves through the roots: elms growing close together often have grafted roots, and the fungus passes through the graft from tree to tree. See the Nevada rules on moving firewood.

Which elms are involved?

The Montana State guide lists American and European elms as hosts. The elm on most old Northern Nevada lots is the Siberian elm, and the more common reason for a Siberian elm to look scorched in July is the elm leaf beetle or drought, not this disease. See elm leaf beetle and Siberian elm. A diagnosis needs the wood and a lab, not the leaves alone. If you are unsure, take photos of the flagged branch and call 775.232.8243.

What does an arborist check?

An arborist checks whether the wilting is on one branch or spreading, looks for brown streaking under the bark, and rules out other causes such as drought scorch or root damage. They also check whether nearby elms share roots and whether dead wood could harbor beetles or fall.

• Whether the wilting is confined to one branch or spreading, and its position in the crown
• Brown streaking in the wood beneath the bark of a symptomatic branch
• Signs of other causes: skeletonized leaves, drought scorch, root damage, a cankered trunk
• Whether other elms are close enough to share roots
• Dead or dying wood that could be a beetle breeding site and a falling hazard

When is pruning or removal the answer?

Montana State's advice is to prune symptomatic branches promptly, and arborists disinfect their tools between cuts. A University of Idaho guide says there is no recognized cure for the disease and that pruning only helps when less than 10 percent of the tree is infected. So a tree with a single flagging branch caught early may be pruned and watched, but a tree with a large share of its crown wilted is unlikely to be saved.

Once an elm is dead, or is more than lightly infected, removal is the usual answer, and sooner is better because dead elm wood is where the beetles breed. A large dead elm is brittle and should come down in a controlled way; see tree removal and large tree removal. Do not store the wood with the bark on near other elms. For permits, a city right-of-way tree needs the city first; see tree rules by jurisdiction.