Dutch elm disease was introduced to North America in the 1930s and began killing millions of native elm trees. It has been identified in every county in Iowa and has claimed more than 95 percent of Iowa's urban elm trees.
Unfortunately, Dutch elm disease became famous after devastating native elm populations. The fungus is native to Asia and was introduced to Europe shortly after World War I. From Europe, it traveled to North America in crates made from infected elm logs. The disease quickly infected elms across the United States since native elms did not have natural resistance to the introduced pathogen.
"It's during this time of year that we are reminded that the disease is still out there; as numerous elms are currently dying in the landscape," said Tivon Feeley, with the DNR's forest heath program. This year, Dutch elm disease has been prevalent in urban landscapes and in woodlands. Wilted, bright yellow leaves draw attention to elm trees that are infected and begin to die.
Typically, the topmost leaves start to yellow, eventually turn brown and fall off the tree. Branches will then begin to die until the entire tree is killed. This process can take a few weeks or can stretch out over several months.
The fungus, Ophiostoma novo-ulmi, which causes Dutch elm disease, finds its way into elm trees in two ways. One way is elm bark beetles inadvertently carry the fungus on their backs and infect healthy trees when they feed and breed just under the bark. These beetles can move the fungus from diseases to healthy trees over a distance of several miles.
The other way is through the root system. The roots of elms located within 50 feet each other can root graft together allowing the fungus to travel through the roots systems. Trees that are infected this way usually die quickly.
Once inside a tree, the fungus does its damage by growing inside the water-conducting vessels, blocking the flow of water to the top of the tree and causing the typical wilting pattern. Although chemical treatments to prevent Dutch elm disease work, they have been reserved for the rare specimen tree due to the high cost of semi-annual treatments.
There may be hope for those who want native elms as part of their landscape. Researchers have been selecting and developing elms that are tolerant of the disease. Some of these elms are hybrids with Asian varieties, and some are true native American elms that have shown resistance. However, the elms that sprout up in yards and woodlands are extremely unlikely to be resistant and should either be managed or removed before they grow into larger shade trees that are expensive to cut down.