Learn About Dutch Elm Disease | Gardener’s Path

Ophiostoma ulmi, O. novo-ulmi, O. himal-ulmi

If you want to strike fear into the heart of any elm tree lover, mention the words “Dutch elm disease.”

This fungal disease is a worldwide problem that devastates elm trees from Asia and New Zealand to Europe and across the pond to the Americas.

It has wiped out entire populations of wild and cultivated trees and has scared many gardeners away from planting elms. Don’t let it scare you off, though.

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While I won’t minimize how awful this disease can be, there are ways to avoid infection or mitigate infection. If it’s too late and your plant is already sick, this guide can help with that, too.

In our guide to growing elms, we discuss how to cultivate these trees in your landscape. Coming up, we’ll talk about Dutch elm disease, and how to identify and manage an infection.

Here’s what I’ll cover:

What Is Dutch Elm Disease?

Dutch elm disease, often abbreviated to DED, is a fungal disease that impacts elms, as the name suggests. In spite of the name, this disease isn’t Dutch in origin.

It likely originated in Asia, but it was first named in the Netherlands in 1921 when phytopathologists Bea Schwarz and Christine Buisman, under professor Johanna Westerdijk, identified fungi in the Ophiostoma genus as the infectious agents that cause the disease.

By 1930, it had been identified in trees in Ohio and has since spread around the US from there.

In North America, both Ophiostoma novo-ulmi and O. ulmi cause the disease, while O. himal-ulmi is only found in the Himalayan region. O. novo-ulmi is more destructive and common than the other species.

All species in the Ulmus and Zelkova genera are susceptible to this disease in varying degrees, but North American native species like American (U. americana), red or slippery (U. rubra), or rock (U. thomasii) are the most susceptible to infection.

Those indigenous to Asia, such as Chinese (U. parvifolia), Japanese (U. davidiana var. japonica), and Siberian (U. pumila) are generally quite resistant.

The fungus that causes Dutch elm disease is spread by species of elm bark beetles in the Hylurgopinus and Scolytus genera as they move around and feed on the trees.

Worldwide, a variety of different native and invasive beetle species can spread the disease as well. For instance, the larger European elm bark beetle (S. scolytus) spreads the disease in Europe.

All are about an eighth of an inch long, and black or brown with a shiny carapace as adults.

The adult beetles and larvae overwinter in the tunnels they make in the wood, or in nearby wood piles or leaf litter. Once the warm spring weather arrives, they climb out of their hidey-holes and start feeding.

Depending on the local climate, there can be up to four generations of beetles per year.

The fungal spores overwinter under the bark and in tunnels created by the elm bark beetles. These spores attach to any adults or beetle larvae within the tunnels and are also ingested as they feed.

As the beetles move on to other trees, the spores are introduced to previously healthy specimens and the disease spreads, continuing the cycle.

The pathogen can also be spread between roots of trees growing close together.

Known as “root grafts,” the roots of neighboring trees of the same species may join together, effectively sharing water and nutrients – and pathogens.

When the fungus is transmitted by this method, it often causes the most rapid death because the pathogen moves so quickly through the newly-infected tree.

When the fungus finds a host tree, it invades the vascular system and reproduces in the xylem, which transports the nutrients and water from the roots throughout the rest of the plant.

The fungus causes blockages in the xylem, which results in death starting at the parts of the canopy furthest away from the roots.

As it moves further throughout the plant, it causes the entire plant to die.

Depending on the condition of the tree and how aggressive the pathogen is, the tree could die in a few weeks, or it could take years.

Young or fast-growing specimens are more susceptible to rapid death. Older trees or those growing in drier conditions tend to have slower-moving infections.

Symptoms

If a tree is infected, you’ll start to see leaves on the outer edges of the canopy wilt or “flag” and then turn yellow.

As the disease progresses, the wilted foliage will turn brown and eventually fall from the tree. Next, the leaves closer to the interior of the canopy start turning yellow and then brown.

The symptoms generally impact one branch at a time, though heavily infected specimens can have multiple symptomatic branches.

Underneath the tree, it will start to look like fall arrived early, with brown, fallen leaves scattered about the place.

If you peel away some of the bark of the symptomatic branches, you will often see brown, gray, blue, or tan splotches or streaking underneath in the sapwood. If you take a cross-section of an infected branch, you’ll see a ring of discoloration.

You can also sometimes see the white fungal spores as well as evidence of beetle feeding tunnels and frass under the bark.

This all starts happening in spring or early summer, but it’s not unheard of for the symptoms to appear at any time of year.

Symptoms are generally noticeable in spring because as the tree puts out new growth, it’s actively moving nutrients and water – and the fungus – throughout its vascular system.

In the summer, wood growth slows, and so the infectious agent moves more slowly.

To be certain the symptoms you are seeing are as a result of DED, you can take a sample of a symptomatic branch to your local extension office, the experts there will be able to tell you for sure.

Organic Control Methods

Addressing Dutch elm disease requires a multi-pronged approach and it’s not easy.

In many cases, the tree is destined for the chipper as there is little to do in the case of a severe infection.

That said, catch it early and you have lots of options for treatment. You’ll also need to take steps to control the beetles.

Grow Resistant Varieties

There are no two ways about it: this disease is bad news. The best way to avoid it is to plant resistant cultivars. The important word here is “resistant” as there are no elms that are completely immune.

The Asiatic cultivars and hybrids Accolade™,‘Cathedral,’ Commendation™, Danada Charm™, ‘Discovery,’ ‘Sapporo Autumn Gold,’ ‘Triumph,’ and ‘Urban,’ are highly resistant.

Accolade™

Nature Hills Nursery carries the lovely Accolade™ (U. davidiana var. japonica ‘Morton’). It’s fast-growing, tolerates drought and pollution, and rarely contracts DED.

If you prefer American hybrids and cultivars, which tend to be larger and have more distinctive leaves, look for ‘American Liberty,’ ‘Homestead,’ ‘Independence,’ ‘New Harmony,’ ‘Princeton,’ Prairie Expedition®, ‘St. Croix,’ and ‘Valley Forge.’ These are more resistant than other American types.

U. americana ‘Lewis & Clark,’ commonly called Prairie Expedition®, has a lovely umbrella-like shape and doesn’t flinch at pollution or DED.

Prairie Expedition®

Fast Growing Trees carries this adaptable plant in four- to five- or five- to six-foot-tall sizes.

Be aware that you will need to carefully and consistently prune American types for the first decade or so after planting to keep them healthy and promote resistance to DED.

Remove any dead, diseased, or dying branches and prune any codominant branches – those that are growing close together in parallel.

Weeping elms are typically more resistant because the beetles that spread the disease aren’t good acrobats.

They don’t like being upside down, so they generally avoid trees with drooping branches.

Finally, you could consider picking up and moving to the southwestern states where there is currently no signs of the disease!

Pruning

Assuming you catch the problem before the symptoms extend beyond a few branches, you can potentially save the tree through some careful pruning.

If more than 10 percent of the tree is symptomatic, this method won’t work.

For each symptomatic branch, cut it off at least six feet below the diseased area. If you make the cut and you see discolored wood, cut another six feet down.

If more than half of your tree is symptomatic, you’ll need to get rid of it. I know it hurts, but the infected specimen is acting as a host to beetles that will spread the disease to other specimens. It won’t recover and you should cut it down. Burn and bury or chip the wood.

If you have trees that are growing very close together, dig a trench between them to prevent the roots from one coming in contact with those of the other. This way, if one becomes infected, it won’t automatically spread the disease to the tree nearby.

Dig a trench at least 40 inches deep and a foot wide around the tree just beyond the drip line. Think of it as a moat to thwart marauding pathogens.

Once you dig the trench, place metal lining or fill it with cement before covering with soil.

Use sticky bands to catch the beetles as they move up the trunks of susceptible trees.

Catchmaster Sticky Bands

You can make your own or buy Catchmaster sticky bands in rolls of 30 feet, available via Amazon.

You should also remove any loose mulch or wood piles in the vicinity to deny the beetles a place to overwinter.

Finally, pray, cast a spell, do a dance, or whatever it is you do to try and influence the weather because a late freeze will often destroy the beetles and prevent an outbreak.

Biological

Once upon a time before the fungus arrived in North America and invasive bark beetle species arrived on its shores, these pests were no big deal.

They lived in harmony with the trees and did little to no damage. That’s not the case anymore.

Now, we need to limit these pests to prevent the spread of the disease.

One way to do this is to encourage woodpeckers to hang out in your yard. These birds can’t get enough of bark beetles. They’re like me with a cheese platter.

Other birds like these pests, too. So, set up that suet feeder, hang a nice bird feeder, and plant wild bird favorites like sunflowers, coneflowers, elderberries, and grasses.

Parasitic wasps (Oomyzus gallerucae) and flies (Erynniopsis antennata) will kill the pupae.

You can sometimes purchase these locally, or you can encourage them to visit your garden by planting lots of native flowers. Some municipalities have a program in place to regularly release these parasitoids to protect local elms.

You should also control ants in your garden, since some ants eat the pupae and eggs of these beneficial insects. Try to avoid using broad-spectrum pesticides in your garden because it can kill the good guys as well as the bad.

Organic Pesticides

Technically, you can try using pesticides to control the beetles, but they’re generally not effective in the long term – and their use won’t save your tree if the disease is present in the area.

But you can use this in addition to other methods discussed here to try to save a tree or prevent one from being infected.

Keep in mind that using insecticides can harm beneficial insects, so only spray on calm, wind-less days and only as often as absolutely necessary.

Pesticides should be applied in the fall to kill the last generation of beetles and prevent the females from laying the eggs that will hatch in spring.

Then, spray again in the spring when the adults start to emerge.

Monterey Take Down Garden Spray

Use something that contains pyrethrin like Monterey’s Take Down Garden Spray.

Arbico Organics carries 32-ounce ready-to-use, or pint or gallon concentrates.

Better yet, apply a product containing the beneficial fungus Beauveria bassiana in the spring, summer, and fall to kill the beetles before they can transform into adults.

These products target the pupal stage and shouldn’t be used to try and kill adults. That’s why this is more of a preventative option than a treatment.

BotaniGard 22WP

BotaniGard 22WP is a good option, and you can find it available at Arbico Organics.

Chemical Control

There are fungicides available that will kill the pathogens, but only if you apply them before more than five to 10 percent of the tree is infected.

And when I say “you,” I mean a trained professional who can inject the tree with the appropriate fungicide at the appropriate time.

You shouldn’t try to inject your tree unless you’ve been trained to do so, and the use of the appropriate chemicals might be restricted in your area.

An expert will inject the root flares with the fungicide but at the same time, you’ll need to prune off all the symptomatic branches. Fungicide treatment is not a substitute for pruning.

Also, keep in mind that treatment is pretty expensive, so you’d better really love that tree if you decide to go this route.

In terms of pesticides to control the beetles, use a systemic product that contains imidacloprid. This is an effective grub and beetle control that lasts for a long time in the garden.

Mineiro 2F is a reliable option.

You can purchase a gallon from the Atticus Store via Amazon, then apply following the manufacturer’s directions as soon as the soil can be worked in spring and again in late May or early June.

A Devastating Disease

It’s never fun dealing with any type of plant disease, but Dutch elm disease is a particular challenge.

It’s best to start by growing resistant cultivars, but not all of us have that luxury, whether we inherited an existing tree or simply can’t find them in our area.

Are you seeing symptoms on your elm tree? What’s going on? Let us know what you’re experiencing in the comments section below and we’ll do what we can to help.

If you need a little help with some other tree diseases, and you found this guide useful, here are a few worth checking out next: