CORVALLIS - Dutch elm disease came to the United States in the early 1930s on a freighter full of green logs destined for a veneer factory. By 1960 it swept through the Midwest, killing about 42 million trees. About 25 years later it was first sighted in eastern Oregon.
Now, Dutch elm disease is regularly showing up and is spreading in some western Oregon cities, parks and home landscapes.
The disease is generally fatal to trees unless you can catch it when only one or two percent of the tree is infected, says Melodie Putnam, chief diagnostician at the Oregon State University Plant Disease Clinic. Even then, trees need annual injections of expensive fungicides to save them.
"Most Dutch elm disease prevention strategies involve complete removal and burning of the infected tree to reduce the chance of spread to other elm trees," she said.
What are the signs of Dutch elm disease?
"Look for yellowing, wilting or flagging (leaves hanging limply) near the top of the tree," Putnam explained. "There are other problems that can cause yellowing, but they generally don't cause wilting."
Since both removal and treatment are expensive options, homeowners will probably want to get a branch sample analyzed to confirm the presence of Dutch elm disease before they take any action. It is difficult to detect in the fall or winter, so look this year before the leaves start to turn or wait till next summer, Putnam suggested.
Putnam says you need to get a sample that is about as thick as your thumb and at least 12 inches long, with some leaves still intact. Since the disease first appears at the top of the tree, you likely will need the help of a professional tree pruner or at least some extended handle pruning tools.
"Once you have a diseased limb, a positive check for Dutch elm diseases is to cut away the bark down into the wood and look for brown streaking in the wood," she said. "This would positively identify Dutch elm disease."
According to Gene Milbrath, Oregon Department of Agriculture plant pathologist, workers in most cities and parks are on the lookout for Dutch elm disease, but it takes the cooperation of homeowners to stop the spread.
It's difficult to weed out diseased trees because not only are you asking people to get rid of a large, beautiful shade tree, but it may cost $700 or more to have the tree removed. Milbrath suggests talking to your local city government to see if they have any provisions to help fund the removal of diseased trees.
Putnam says homeowners can contact local OSU County Extension offices for instructions on how to collect diseased plant or tree samples for analysis.