Name: Ophiostoma novo-ulmi

Photo Credit: Penn State

Photo Credit: Penn State

Introduction: Dutch elm disease is caused by the fungus Ophiostoma novo-ulmi, which is spread by the native elm bark beetle and introduced European elm bark beetle. The pathogen was accidentally introduced in the United States in the 1930s, and has since spread throughout the country and into Canada .

Why are they harmful?: The spread of Dutch elm disease was absolutely devastating to elm trees in the United States. Most large American elm trees succumbed to the disease and were lost as it spread throughout the country. In the 1970s, researchers sampled 100,000 American elm trees to see if any were resistant to the disease. None were. 

The disease is spread when bark beetles feed on healthy trees, or through root grafts. The disease impacts the flow of water within the tree, causing the tree’s leaves to yellow and wilt, then ultimately turn brown and fall off. The disease spreads from branch to brach until the tree dies.  

Dutch elm disease is widely recognized as the largest threat to elm trees in the United States. 

Methods of control: Removing infected trees quickly and thoroughly can help stem the spread of the disease to healthy trees. It’s incredibly important to educate local communities about transporting elm wood—the disease can live in the sap long after the tree has died. Moving elm wood products or firewoods to uninfected areas could facilitate the spread of the disease. 

Chemicals are also available that protect healthy trees for about three years, a method which is costly but effective. 

Although no elm trees have been found to be resistant to the disease, some are more tolerant than others. Of the 100,000 trees sampled for resistance, six of those showed significant levels of tolerance. The USDA Forest Service embarked on a breeding program to produce more tolerant trees to repopulate areas heavily impacted by the spread of the disease. 

Sources: Forest Health Protection—Rocky Mtn Region, Ontario’s Invading Species Awareness Program

Photo credit (cover photo): NatureServe / Flickr