You asked, and Holden Forest & Garden’s beech tree expert, David Burke, answered.

By David Burke & Anna Funk

If you live anywhere near northeastern Ohio, you may have noticed your beech trees are looking a bit ragged lately. Unfortunately, there are now not just one but two major pests — beech bark disease and beech leaf disease — that are bothering our region’s beeches. 

Near Holden, beech leaf disease is the more prevalent concern. Luckily for us, our Vice President for Science and Conservation, David Burke, is an expert. We asked him some of our most pressing questions about beech leaf disease and how we best take care of our trees.

This conversation has been edited for length and clarity.

What causes beech leaf disease?

Beech leaf disease is caused by an invasive nematode that’s native to the Pacific Rim. It’s newly recognized, meaning scientists only just discovered and named the species. Right now, no one’s entirely sure why the leaves are affected the way they are. We think the nematodes enter into the tree’s buds and feed on the tissues inside, which is what causes the damage.

Why aren’t we sure? Well, we know that the nematode is required for beech leaf disease — that is, no nematode, no disease. But that doesn’t mean that the nematode is directly causing those leaf effects. There might be, for example, a bacteria or fungi involved, either assisting the nematode in feeding, or causing opportunistic infections, or something else. It’s still a debate as to whether the nematode is acting alone.

What’s a nematode? 

A nematode is a microscopic animal that kind of looks like a worm. You can’t see it unless you look at it through a microscope. (Nematodes aren’t actually worms, but they’re long and wriggly.)

How did you discover the new nematode?

The new nematode species was first identified in Japan in 2018. It was found on a Japanese beech tree. Then in 2018, after we’d found beech leaf disease in the U.S. and started to study it, David McCann with the Ohio Department of Agriculture noticed there were nematodes in some plant material sent to him from a nursery. As we began looking into it, we found nematodes on beech trees — and were able to identify them as this newly recognized species from Japan. 

How do I know if my beech tree has beech leaf disease and not something else?

There are a number of different diseases that affect beech. Sometimes you’ll see beech with yellow spots, or red spots — that’s not beech leaf disease. The most characteristic symptom of beech leaf disease is what’s called interveinal banding. Essentially, the leaves look striped: It’s extra dark green in-between the veins. You’ll sometimes also see the leaves getting really thick and leathery, and in the most extreme cases, the leaves will even look small and deformed, like they didn’t develop properly. That’s in extreme infestations. 

Can the nematodes that live on beech trees spread to other types of trees? What about me or my family — or my dog? Can I touch beech trees?

This particular nematode is definitely only interested in plants. There’s no reason to believe it would cause people or other animals to become ill. We also don’t see any evidence yet that it could affect other trees. We have detected DNA from the nematode on other trees, like maple trees, but those trees haven’t shown any sign of disease. 

The nematode does affect other species of beech beyond American beech (Fagus grandifolia), though. You might see beech leaf disease on European beech, Oriental Beech, or Chinese beech. At least, that’s what we’ve seen within our collection at Holden. European beech (Fagus sylvatica) is a popular tree planted as an ornamental — the leaves are a little different, a little less pointy and a bit more scalloped. 

What kind of beeches can I plant in my yard? What about a cultivar like a copper beech?

Unfortunately, any beech is probably going to be affected. Unfortunately, for now, I recommend not planting beech at all. Select something else.

I’m pretty sure I have a beech with beech leaf disease in my yard. How do I save it?

If you have a beech that’s affected by beech leaf disease, honestly, we’re not yet sure what you can do. Beech leaf disease is so new — we’ve only known about it since 2012 — we haven’t found any treatments that we’re confident enough to recommend. We’ve been working with some local tree care companies to see if certain pesticides will work, including chemicals that have been found to work for other pests like emerald ash borer and some spruce borers. But so far, none have worked (including emamectin benzoate, found in the product TREE-äge).

Our best suggestion is to prune your trees, especially before late August and September. 

Our best suggestion is to prune your trees, especially before late August and September. If you have a tree that’s really thick and full, pruning will open it up a little bit, to get more light and air circulation to avoid excess moisture from lingering on the leaves. Anything you can do to dry out the leaves can interrupt a key stage of the nematode’s life cycle: It uses moisture collected on the outside of leaves to crawl out of the leaves and essentially “swim” into the buds for overwintering. If we could prevent this step, the nematodes would get stuck in the leaves and fall off in autumn, reducing the tree’s symptoms in the next year significantly. [Read more about why pruning might help beech leaf disease.]

If you have a lot of trees in one area, especially if it gets really shady, I’d guess that some of the trees are looking really bad. Only in that case would I consider taking a couple of them out — just to open the canopy up a little bit and get more light and air circulation in there to help the remaining trees.

Should I just cut all my beech trees down? 

No, please don’t cut down all your beech trees, and especially not proactively (that is, before you even see beech leaf disease symptoms)! Some of your beech trees may be resistant to the diseases. For example, with beech bark disease, we’ve learned that about five percent of trees are naturally resistant. We use these resistant trees for breeding selection programs with the U.S. Forest Service. 

My beech is doing fine, actually…

There are a lot of reasons some beeches might not be affected. Maybe your tree has really good nutrient conditions in its spot, and that’s helping it resist the disease. But maybe it’s genetically resistant.

If you have an American beech on your property that looks great, especially if a lot of the beech around it are dying, let us know! We might want to come sample your tree.  (But note that if you’re in a developed area, the beech in your backyard is likely an ornamental European beech. We’re thrilled for you, just not as interested in it for our research.)

What are you doing with potentially resistant trees?

When we identify a potentially resistant tree, we’ll typically take a cutting and bring it back to the greenhouse at Holden. We graft the branch onto a healthy rootstock growing in a pot, which allows us to run tests on it. We’ll test these grafted trees — genetic clones of the trees out in nature — with nematodes to see if they really are resistant. If they are, we’ll breed them in hopes of using them for forest restoration all across the affected region. 

[Read more about the Great Lakes Basin Forest Health Collaborative.]

I’d like to have someone come look at my beeches. Who should I call?

You could call any local tree care expert and have them come out and look at your beech. But realize that this is an emerging disease — even the world’s experts are still trying to figure out the biology of how it works. Which means we’re also still trying to figure out how to control it. Like I said, although the nematode is certainly required for beech leaf disease, there could be another component that we haven’t discovered yet. 

If you do call someone, be aware that products like emamectin benzoate (found in TREE-äge) don’t seem to work, and other products are largely untested. So unless a company provides some really good data to suggest what they’re offering works, you may want to be skeptical about it. Especially if they’re charging a lot of money. Someone called me and said a tree care company wanted to charge $8,000 to treat their tree — with something that we have no evidence if it works!

If you’re a homeowner, it’s up to you whether you want to try to treat the beech tree in your yard. But what if you’re a landowner with forest property? We’re not going to be able to go and spray forests, even if we did find a pesticide that worked against the nematodes. That’s why we’re shifting our efforts to try and find resistant trees for restoration and replanting.

Where is beech leaf disease? 

Currently beech leaf disease is radiating outward from northeastern Ohio into Ontario, Pennsylvania, New York, and eastward. It was just recently discovered for the first time in Michigan. 

Who can I contact if I still have questions about beech trees and beech leaf disease?

If we haven’t answered your questions, please feel free to reach out to the Holden Arboretum! We are here to be a resource to you. Email questions to Rachel Kappler at [email protected].