In the early 20th century, American elm (Ulmus americana) was the dominant street tree across much of the Midwest, admired for its graceful form and hardiness. When Dutch elm disease (DED) arrived, it decimated entire neighborhoods. Though the most severe outbreaks occurred mid-century, and are no longer making headlines, DED is still active today and continues to impact our urban forest.
Here’s what you need to know to spot it early, manage it effectively, and help protect the remaining elms in our neighborhoods.
What Is Dutch Elm Disease?
Dutch elm disease is caused by two closely related fungi: Ophiostoma ulmi and Ophiostoma novo-ulmi.
These pathogens are spread in two main ways:
- Elm bark beetles, which carry fungal spores from infected trees to healthy trees, typically entering through pruning wounds or natural injuries.
- Root grafts, where neighboring elm roots, of the same species, naturally fuse allowing the fungus to move underground
Once inside the tree, the fungus quickly impacts the vascular tissue, blocking the flow of water and nutrients. Elm bark beetles carry fungal spores with them when they travel to healthy trees to feed on twigs and upper branches. From the feeding sites, the spores travel into the tree’s vascular system. Chemicals produced by the tree itself, during its attempt to fight the disease, contribute to the plugging of the vascular system, causing the tree to wilt. An infected elm tree can die in just one season.
Symptoms to Look For (early detection is key):
- Flagging: sudden wilting and yellowing of leaves in one branch (earliest symptom typically observed early/mid-summer.)
- Leaf browning and curling, followed by premature drop
- Progressive dieback from the top down
- Brown streaks in the sapwood when bark is peeled back (this can be seen upon closer inspection on branches showing symptoms.)
Prevention & Management
- Avoid Pruning During Beetle Season
Elm bark beetles are most active from April through October. Avoid pruning during this window, best to be done during the dormant season (November – March), as fresh cuts attract beetles carrying the disease. If you must prune, seal wounds immediately with industry approved wound/pruning sealant.
- Remove Infected Trees Promptly
Infected trees serve as beetle habitat and breeding grounds for beetles. Removal should be done quickly and properly. Never store infected elm wood as firewood.
- Break the Root Connection
If trees are growing close together, a certified arborist may recommend root trenching to sever root grafts and stop underground spreading.
- Consider Preventive Treatments
High-value elms trees can benefit from fungicide injections. These treatments are most effective when used preventatively, not after symptoms appear. Treatment must be administered by professionals.
Plant for the Future
- Replanting with DED-resistant American elm cultivars such as ‘Princeton,’ ‘Valley Forge,’ and ‘New Harmony’, to name a few. These varieties retain the classic vase-shaped canopy of American elms but offer improved resistance to disease. These cultivars and other available hybrids are resistant to the disease but are not completely immune.
- Remember, diversity is key. A mix of species makes your landscape more resilient against future pests and diseases.
Dutch elm disease may feel like a problem of the past, but it’s still quietly affecting trees across Illinois. The good news? With proper care, early detection, and smart planting, we can continue to protect our remaining elms.