Fungal facts: Dutch elm disease

A stately American elm  (Ulmus americana),  pre-Dutch elm disease

A stately American elm (Ulmus americana), pre-Dutch elm disease

What do fungi, invasive species, beetles, birds, and transatlantic shipments have in common? They’re all characters in the story of Dutch elm disease.

As Andrew Tomes wrote in his post “Fungi: A Tree’s Best Friend,” fungus is critically important to trees. Myccorhizal fungi allow trees to obtain nutrients from the soil that they wouldn’t otherwise be able to access. Fungi are very diverse; this group of organism includes mycorrhizae, molds, yeasts, mushrooms, and pathogens. While some fungi help trees, like mycorrhizae, others do not. 


Dutch Elm Disease

Organisms catch rides across oceans on cargo all the time, and once in a while, this has disastrous effects (see other posts about invasive species). It is presumed that in the 1920’s, a fungal disease that affects elms, now known as Dutch elm disease, travelled from Europe, where it was not a spectacularly devastating disease, to North America. In North America, the trees had not evolved in tandem with the fungal disease and so they had no immunity to it. The rest is history: thousands of elms succumbed to the disease across the continent. In many cities and towns, you might have noticed an Elm Street downtown that doesn’t have any elms. While elms still persist today in smaller numbers, they usually die from the disease before they grow very large. Some areas were able to prevent the disease from taking hold with the use of fungicides and other measures, so elms can still be found in some cities, like Winnipeg. 

A postcard of an elm-lined street in Salem Massachusetts.

A postcard of an elm-lined street in Salem Massachusetts.

In 1967, Dutch elm disease found itself a stowaway on yet another trip across the Atlantic, back to Europe. This time, the disease wreaked havoc as it spread from England to continental Europe, due to the fact that it had mutated over time into a more destructive form.

This fungal disease spreads from tree to tree via elm bark beetles. When Dutch Elm disease was in the process of destroying elms across the US, DDT was used to try to kill elm bark beetles and stop the spread of the disease. Unfortunately, birds like to eat bark beetles. As you may know, DDT was commonly used to control insects, which worked quite well, but it was very harmful to birds and led to Rachel Carson’s book “Silent Spring.” Fungicides used to kill the fungus directly have also been used, hopefully with less disastrous side effects.

Bark beetle damage on an elm.  Photo courtesy of Stefan Czapski.

Bark beetle damage on an elm. Photo courtesy of Stefan Czapski.

One species of elm bark beetle,  Scolytus scolytus.  There are at least three species. 

One species of elm bark beetle, Scolytus scolytus. There are at least three species. 

So what can we learn from this? For one thing, accidentally transporting diseases and other organisms to new environments can have unfavorable consequences. Besides that, we can look at the bright side: diversity provides stability in forests (and all ecosystems!). Losing elms has changed the composition of forests and city streets, but diversity prevents diseases like Dutch Elm disease from having even more dire consequences. Variation amongst individuals means that some trees are more resilient to the disease than others and maybe could someday (a long long time from now) lead to healthy elms that could repopulate the forests. Diversity amongst species means that it is quite unlikely that one disease could destroy a forest. Other species of trees and plants can fill in the gaps left by elms.

In the face of threats such as exotic pathogens, climate change, and development, biodiversity is important to maintain, so that we may have robust, resilient ecosystems.