Dutch elm disease, or DED, is one of the most destructive plant
pathogens in the United States and Europe, having killed
millions of elms and persisting despite efforts to control it.
With approximately 3,500 susceptible elms throughout the city,
DED would have a catastrophic impact on Portland’s urban forest
if allowed to prevail unchecked. PP&R Urban Forestry works
diligently to manage the impact of DED in Portland. DED is
caused by a fungus, Ophiostoma spp., which invades the vascular
tissue of elms and prohibits water movement in the tree. There
are three ways the DED fungus spreads: bark beetles, root
grafts, and human activity. Elm bark beetles breed, feed, and
overwinter in elm wood. In infected trees, spores of the DED
fungus stick to the backs of bark beetles and are transported to
healthy elms when beetles emerge to feed. The fungus spreads
most rapidly through root grafts, which form between trees
growing in close proximity. Human activity, such as transporting
infected elm wood, also spreads the d