While the City of Seattle has taken affective measures to control the spread of Dutch Elm disease, the Department of Transportation is warning residents that many new infected trees have been discovered across the city.

Dutch Elm disease was first discovered in Washington in 1974 but was not seen across the mountains in Seattle until 2001. That year three infected trees were identified followed by five more in 2002.

Between 2002 and 2007 no more than 2 percent of Seattle's elm trees were infected, but during this summer that number has increased. Ross Playfield is just one location where infected trees have been found.

City of Seattle arborist Nolan Rundquist said the diseased elm trees will likely be replaced with a hybrid species of elm that is less likely to contract Dutch Elm disease.

Dutch Elm disease originates as a fungus, Ophiostoma ulmi, which invades the vascular system of elm trees and prevents water flow throughout the tree.

The disease is primarily spread through the elm bark beetle, an insect that lives in elm wood and transfers fungal spores of Ophiostoma ulmi.

Diseased elm trees can also spread the disease through natural root grafts, where they share nutrients and water.

Once infected, the leaves of elm trees will rapidly wilt and turn brown. Branches can also be checked for brown streaks in the wood beneath their bark. Healthy elm wood is white with hardly any markings. Any discoloration could be a symptom of Dutch Elm disease.

Unfortunately, once elm trees are infected, they are difficult to save. Because the disease affects a tree at the tips of its branches first, later spreading to the roots, it is possible to save a tree by pruning away wilted braches.

Still, the process is difficult. Branches must be cut back 10 feet further than the infected area.

"It takes a pretty good effort to excise the disease from the tree," Rundquist said. "It can often get quite ugly."

Typically, after two years diseased elm trees are fully engulfed with the fungus and must be removed to prevent the disease from spreading to other elms. The longer the infected tree stands the more likely it is to infect other trees.

Following Seattle Mayor Greg Nickel's policy, the city has replaced each removed elm tree with two new trees.

There is currently no cure for Dutch Elm disease, however prevention measures have been successful. A fungicide called Arbotect is used by the city every three years on trees on public property. So far, no trees treated with the fungicide have contracted Dutch Elm disease.

The city is also treating elm trees with Dutch Trig, an experimental treatment developed in the Netherlands.

Currently, the Seattle Department of Transportation is monitoring all elm trees on city streets and city parks, although Rundquist said that the vast majority of diseased elm trees have been found on private properties.

The department is working to inform residents of the disease so that they might observe their own elm trees for symptoms.

"They need to be aware of what kind of trees they have in their yards," said Barbara DeCaro, integrated pest management coordinator.

Certain human activities have been known to spread Dutch Elm disease. Rundquist said firewood from elm trees should not be stored unless the bark has been removed. The elm bark beetle can live in firewood and, if infected firewood is transported, spread the disease to healthy elms.

Rundquist added that it is also best to refrain from pruning elm trees between April and October, when the elm bark beetle is actively flying.

Residents who wish to treat elm trees on their private property with Arbotect or Dutch Trig should contact a local arborist. If residents are concerned that an elm tree on their property is infected they may contact the Seattle Department of Transportation to have it tested.

"We want to help folks out as best as we can," Rundquist said. "If someone wants us to come out and take a look at their tree we'd be happy to do so."

Rose Egge can be reached at 932-0300 or rosee@robinsonnews.com.