Dutch Elm Disease of Trees

Key points

• Dutch elm disease is a serious lethal disease that infects a large number of elm species including American, winged, slippery, rock, and September elms.
• Symptoms typically appear in May as wilted branches that show yellow, then brown, leaves.
• Dried leaves may remain attached to the branches or may fall, leaving bare twigs.
• As symptoms progress, entire limbs may show wilt symptoms and once the infection reaches the trunk the entire tree may wilt.
• In older trees, wilt symptoms may progress throughout the canopy for several years before death occurs.
• Infected twigs and branches typically show vascular streaking under the bark.

• This fungal disease enters through feeding wounds made by the native elm bark beetle and the more prevalent, smaller, European elm bark beetle (see photo).
• Beetles breed beneath the bark of dying or recently dead elms.
• Infected or stressed trees actually emit an odor that attracts beetles to them for breeding and egg laying.
• In addition, female beetles also emit attractants that lure additional beetles to declining trees.
• There are up to three generations a year with the first emergence starting in April from overwintering larvae.
• Beetles that emerge from infected trees typically are covered with fungal spores growing in the pupal chambers.
• Beetles flying to other trees spread the spores to other trees.

Management

• The chief strategy for control of Dutch elm disease is through early removal of newly infected branches to interrupt the spread by elm bark beetles.
• Ideally, removal should occur within two to three weeks after symptoms appear during the growing season.
• Diseased wood should be chipped and removed from the site.
• Pruning cuts should be made at a branch fork at least 10 feet below visible streaking in the sapwood.
• Pruning will be most effective if less than five percent of the crown is affected.
• High-value trees may be appropriately injected with systemic fungicides, administered by a licensed arborist. This is expensive and may need to be repeated in one to three seasons. If injections will be used, prune after the injections are complete.