By John Schwartzer

Coleguita2022, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

In our cities across the United States, the 2nd most common street named for a tree is Elm Street. Over 4000 cities have one, but when walking down Elm Street, it’s unlikely anyone will find a single elm tree. American elms (Ulmus americana), a deciduous tree in the Elm family, were historically a popular, disease-free street tree and city planners planted them often.

There are roughly 35 species of elm in the northern hemisphere across the North American, Eurasian, Northern African, and European continents. Eight native species of elms inhabit North America. The three most common elms found in Pennsylvania are the American elm (Ulmus americana), slippery elm (Ulmus rubra), and the invasive Siberian elm (Ulmus pumilla). Elms are tall, gracefully forking trees that produce vase-shaped crowns. Their unassuming flowers are wind pollinated in the early spring.

Due to an invasive fungal pathogen that swept the United States starting in the 1930s, North America has lost an estimated 77 million elms. The fungus to blame is the Dutch elm disease. Dutch elm disease isn’t from the Netherlands as the name suggests, but was named for the folks that isolated it first in 1921. Dutch elm disease is spread to new trees in the late spring and early summer by native and exotic elm eating beetles. This Asian fungus is an ascomycete that triggers an immune response in the tree’s xylem cells. The water moving cells develop tyloses (or “plugs”), starving the tree of life-giving water.

Immediate pruning can stop an infection if caught early. Some American elms are variable in resistance to the fungus and have been used in breeding new resistant cultivars: ‘Princeton’, ‘Lewis and Clark’, ‘St. Croix’, ‘Jefferson’, ‘Valley Forge’, and ‘New Harmony’.

Elm yellows, a phytoplasma, or a bacterium, is another threat to elms. This systemic disease is transmitted through leafhoppers and spittlebugs. Phytoplasmas attack the phloem cells of the tree’s vascular system. A sign an elm is infected is yellowing leaves and butterscotch-colored streaking under the bark that smells like wintergreen. There is no known cure for elm yellows.

If the disease issues weren’t enough, in 2020 a new invasive pest was detected in North America: the elm zigzag sawfly (Aproceros leucopoda). This Asian wasp’s larvae feeds exclusively on elms and has rapidly spread across the East Coast.

Ed Joice. How to Find Buckets of Morels. 4 April 2021. https://www.mnforager.com/post/how-to-find-buckets-of-morels-how-to-identify-elm-trees

Elm trees have a symbiotic relationship with the morel mushroom (Morchella spp.) in Pennsylvania. The root area of a recently dead elm tree is a great place to look for morel mushrooms in the springtime. When the tree dies, the morels growing in the root zone typically send out flushes of fruiting bodies. The edible mushrooms are a spore lifeboat for the fungus to spread to a new tree.

Elms have edible samaras, or winged fruits. So, if you struck out on morels this spring, you could still bag a tasty snack. The immature pea-flavored samaras can be eaten right off the tree or as a soup or salad addition. As the fruits mature, the inner nutlet can be shelled and used like a pinenut or sunflower seed. Siberian elm fruits are less fuzzy than American elms and are invasive. Pick responsibly.

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