Dutch Elm Disease

Dutch Elm Disease

Part of Ep. 2004 Whiffs, Wasps and Wonders

The streets of most American cities used to be lined with gorgeous elm trees providing a lot of beautiful shade.  Thanks to Dutch elm disease, that’s a thing of the past.  Unfortunately the disease is still very much around.  Plant Pathologist Brian Hudelson teaches us how to identify Dutch elm disease.

Premiere date: May 12, 2012

TRANSCRIPT+
Wisc Gardener Transcript: 

SEGMENT #2: DUTCH ELM DISEASE

 

Shelley:

The streets of most American cities used to be lined with gorgeous elm trees providing a lot of beautiful shade. Well, that's a thing of the past. I'm with UW Extension plant pathologist Brian Hudelson to talk about why that's a thing of the past.

 

Brian Hudelson:

Yeah, again like you said, we used to have a lot of elm trees along the streets of America, and unfortunately there was a fungal disease that came in and basically has killed most of the elms certainly in the eastern United States and certainly here in the Madison area and Wisconsin.

 

Shelley:

Decimated is a good word.

 

Brian Hudelson:

Decimated is a great word to use for that. It's what's called a vascular wilt disease which are some of the toughest ones to deal with from a control standpoint. The fungus actually enters in typically into a tree through bark beetles that bring in the fungus that colonizes the water conducting tissue. That's called the vascular tissue of the tree and it blocks it off so any point above that point of the tree will not get water from the root system.

 

Shelley:

So, vascular, you have an example then of the water being blocked off.

 

Brian Hudelson:

Right, the water conducting tissue is right underneath the bark. When this fungus infects the trees, it will cause a browning of that particular tissue and that's what I use as a visual cue when I'm doing a diagnostic in my clinic.

 

Shelley:

Well, where does Dutch elm come from? I mean, I think Dutch is the key word right there.

 

Brian Hudelson:

Right, and that's more from the fact that it arrived in Europe before it arrived in the United States. There was a lot of initial work that was done by the Dutch on the disease. But the fungus actually originates, people think in Asia. If you look at where there is resistance to the disease, it tends to be in Asiatic species of elms.

 

Shelley:

This hit our American elms about when?

 

Brian Hudelson:

Around the 1920s here in the United States, a little bit later here in Wisconsin, because it took some time to travel from the east coast into the middle part of the country.

 

Shelley:

Well, we are walking along one of the many bike paths in Madison because there's some really good examples of this disease here.

 

Brian Hudelson:

That's right. People don't think there a lot of elms left, but there are actually a lot of volunteers that have grown up and there's certainly a lot along this bike path. Some of the symptoms I've been seeing in the tress around here, the elm trees, is flagging, which is an initial symptom where you tend to see individual branches start to die back.

 

Shelley:

So out at the outer edges?

 

Brian Hudelson:

Yeah, usually from the tip back, down towards the trunk. Oftentimes that's where the bark beetles will enter into the tissue. They'll do some feeding, drop off the fungus there, and then it colonizes back into the main trunk. It'll eventually move down into the root system, colonize the entire tree, and then it'll do what it's done here, which is kill the entire tree.

 

Shelley:

So when we see a tree that has flagging, is there anything we can do to stop it at that point?

 

Brian Hudelson:

Usually at that point it's a lost cause.

 

Shelley:

Really? Even that early.

 

Brian Hudelson:

Right, when we're seeing those visual cues, that usually means that the fungus is not only in the branch that you're looking at, but also way down in the trunk, which means there's no real way to treat the tree. So we usually recommend removal in that particular case.

 

Shelley:

So there's no cure basically.

 

Brian Hudelson:

No cure. There are some preventative treatments. If you have for example, a really nice showcase elm in your front yard that you want to try to protect, there are fungicide treatments that you would inject into the base of the tree.

 

Shelley:

As a preventative.

 

Brian Hudelson:

That's a preventative. It helps prevent the tree from becoming infected. These are not easy treatments to do, so you really need to get a certified arborist to come and do that. Once you have a symptomatic tree, it's an issue of removal. If you have a cluster of trees, you've got to be very careful to break what are called root grafts where the roots of various trees have fused together underground. Because the fungus can move from tree to tree that way.

 

Shelley:

So that tree's a hazard.

 

Brian Hudelson:

That is a hazard, both from potentially moving to other elms in the area underground, and also the insects that can move the fungus around can revisit that tree, pick up the fungus, and then move on to other elms.

 

Shelley:

Okay, so, I mean, it's an inoculant right now basically. It can spread it even though it's deader than a doornail.

 

Brian Hudelson:

That's right.

 

Shelley:

I live along a bike path as well, and on the bike path property are three trees with no bark whatsoever and they're elms. Are they a hazard?

 

Brian Hudelson:

I wouldn't worry about those, because once the tree is debarked, loses their bark, then I would not expect the Dutch elm disease fungus to be active at that point.

 

Shelley:

So they're just dead and ugly but they're not something I'm going to have to panic about?

 

Brian Hudelson:

No, and actually I would check around the base of those for morels because oftentimes you have morels growing around the base of dead elm trees.

 

Shelley:

Oh, okay! So they might be worth keeping around for a little while.

 

Brian Hudelson:

Yes, for a little while.

 

Shelley:

What about planting elm trees that are resistant?

 

Brian Hudelson:

Definitely that's an option. There are some very resistant varieties that are hybrids between American elms and Asiatic species of elms. They usually don't have the real nice vase sort of form.

 

Shelley:

That beautiful shape.

 

Brian Hudelson:

They're more like a typical tree. But there are also true American elms that have been bred for resistance. There's an Independence elm, an American Liberty elm, a Valley Forge elm, Princeton elm. There are several varieties that are available. I wouldn't grow a lot of those because even those elms although they are resistant, they are not immune, which means they can get the disease.

 

Shelley:

So take good care of them. Maybe use the preventative treatment, and make sure they're not stressed because that just helps their resistance.

 

Brian Hudelson:

That's right, and just make sure you're only planting one here or there. Don't do a monoculture like we used to along our streets where it was all the same tree.

 

Shelley:

Which is why Elm Street no longer has any elms.

 

Brian Hudelson:

That's true.

 

Shelley:

Okay, thanks, Brian.

 

Brian Hudelson:

You're welcome.

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