Dutch elm disease was introduced to North America in the 1930s and began killing millions of native elm trees. Dutch elm disease has been identified in all Iowa counties, and it's estimated that more than 95 percent of the urban elm trees have succumb to this disease.
Unfortunately, Dutch elm disease became famous after devastating our 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 our native elms did not have natural resistance to the introduce pathogen.
It is 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. This year, Dutch elm disease has been very prevalent in the urban landscapes and woodlands. Wilted, bright yellow leaves draw attention to elm trees that are infected and begin to die.
Typically, the topmost leaves start to yellow and eventually turn brown and fall of the tree. Over the next several days, the branches begin to die until the entire tree is killed. This process can take a few weeks or can stretch out over a period of several months.
The fungus Ophiostoma novo-ulmi, which causes Dutch elm disease, finds its way into elm trees two different ways. 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.
Another way the fungus can infect a healthy tree 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. This blocks the flow of water to the top of the tree and results in 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 still be a glimmer of hope for those that want to have 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 elm that have shown resistance. However, the elms that sprout up in yards and woodlands are extremely unlikely to be resistant and should be managed or removed before they grow into larger shade trees that are expensive to cut down.