By Joe Zeleznik, Forester

NDSU Extension

It’s tough on trees in North Dakota. Bitter winter cold. Scorching summer heat. Floods. Droughts. Wind. And that’s just an average year!

Okay, maybe I’m exaggerating a bit. But there’s some truth in there. We do experience many of these environmental extremes regularly. Our native trees are mostly adapted to them.

But situations change. In addition to the ever-changing weather, add in some non-native weeds for increased competition, plus some exotic pests, and things get especially difficult.

As an example, let’s look at American elm, North Dakota’s state tree.

American elm trees grow quickly and are highly adaptable in those flood-prone areas along rivers, and they can also handle highly modified soils in urban areas. Their iconic vase-shaped crowns would form archways on either side of city streets, providing a beautifully framed view.

And then an exotic pest – Dutch elm disease – came to North Dakota.

DED was first found in Mandan in 1969. In four years, it had spread to the eastern part of the state.

The biology of the disease is rather complex. DED is caused by a fungus, but the fungus is spread by an insect. Actually, there are two different fungi that can cause DED, and at least three different bark beetle species can spread it.

As I said, it’s complicated.

Once a beetle introduces the disease into a tree, it grows downward through the stem’s large pipes that move water and nutrients up from the soil. Those pipes get blocked, and the upper branches start dying from drought. The classic symptoms are wilting, curling and yellowing of leaves (flagging) on one or more branches in the upper crown. Then those symptoms move down the crown over the next one to three years.

Oh yeah, there’s another way that the fungus can enter a tree: through root grafts with a different elm tree that’s already infected. If a tree becomes infected through the root system, the entire crown will wilt all at once, more or less.

Once a tree is infected, it’s very difficult to treat it effectively. Pruning out infected branches quickly is difficult, and therapeutic (postinfection) fungicide treatments are not nearly as effective as preventative treatments.

So how do we manage Dutch elm disease?

DED is best managed at a population level. That means that when a tree is identified as infected, it is quickly removed within two to four weeks. This removes the beetles’ preferred habitat – dead and dying elm trees – and prevents them from spreading further.

This technique – called sanitation – is highly effective. In communities with a sanitation program, they lose only 1%-2% of their American elms each year. Without sanitation, 10%-20% of the elms will die each year, and all the elms will be gone within 12-15 years.

It’s a tough situation because sanitation means prompt removal of all infected trees, even those on private property.

Some communities have a tree ordinance that allows city officials to require such removals. Other communities don’t have tree ordinances. Some cities that do have an ordinance don’t necessarily enforce it. Not many people want to be the “tree cop.”

Preventative fungicide treatments are proactive, highly effective measures that can work, but they’re also a bit pricey. More information about managing Dutch elm disease is available at ndsu.ag/Dutch-elm-26.

Dutch elm disease is manageable but difficult, and it takes community support.

It’s complicated.

NDSU Agriculture Communication – July 21, 2026

Source: Joe Zeleznik, 701-231-8143, joseph.zeleznik@ndsu.edu

Editor: Dominic Erickson, 701-231-5546, dominic.erickson@ndsu.edu