The Dutch elm disease first appeared in this state in 1977 in Walla Walla. It was first noticed in Spokane in 1990, and detected in Tacoma and Bellevue in 1994.
A fungus that grows in the xylem tissue causes DED. Xylem is basically the tree's plumbing system that moves water from the roots up through the trunk and out to the leaves. The disease restricts water movement through the tree resulting in wilted and yellowing leaves, twig and branch dieback, premature defoliation, and often the death of the tree. Look for brown streaks if you cut into the sapwood of infected branches. The disease may progress slowly or kill the tree within a week after you notice symptoms.
The only positive way to determine if your tree has DED is to isolate and culture the fungus. Send samples from suspected trees to a diagnostic laboratory. Select twigs about 6 inches long from recently killed branches that show brown streaking of the wood just below the bark.
The DED spreads primarily by the feeding and breeding activity of elm bark beetles. It also can spread when the roots of adjacent elm trees grow together. This often happens when trees are within 40 feet of each other.
The best way to control the spread of Dutch elm disease is to monitor all elm trees regularly for symptoms. Remove infected trees as soon as you diagnose the disease. Remove and burn all dead wood from diseased trees before elm bark beetles emerge from the wood in the spring. Sever root grafts with a mechanical trencher to prevent the spread of the disease between roots of adjacent trees. Trunk injections of systemic fungicides may provide some protection if applied annually, but they are only recommended for extremely high-value trees.
Perhaps the best hope for the future is to plant elm species that are resistant to Dutch elm disease. Most North American and European species of elms, including American, slippery, Scotch, and English elms, are susceptible. The Chinese elm (Ulmus parvifolia) and Siberian elm (Ulmus pumila) are quite resistant, although nurseries seldom recommend the latter species due to other problems. Plant breeders have developed a number of disease-resistant cultivars by crossing European and Asian species. The zelkova tree, a close relative of the elm, is also resistant to Dutch elm disease and makes a nice substitute.
The impact of DED on our landscape will be less severe than in the East and Midwest because fewer elms were planted here. But where elms are planted, watch closely for symptoms of DED.
For further information contact your local WSU Extension Office.
From The Gardener, Vol 6 No. 3, Autumn 1995.