This disease is not of Dutch origin, but because early work on the disease was done by Dutch pathologists in the 1920s, the disease has been called Dutch elm disease (DED). In all probability, the disease is of Asiatic origin.
Dutch elm disease is caused by a fungus called Ophiostoma ulmi (formerly Ceratocystis ulmi) that was introduced to the U.S. in the early 1930s. The American elm, Ulmus americana, is extremely susceptible and the disease has killed hundreds of thousands of elms across the U.S. All native elms are susceptible, as are European elms, but the Asiatic elms, U. parvifolia (Lace bark elm) and U. pumila (Siberian elm) are highly resistant to the disease. The disease is still a threat today, but fortunately, several resistant American elm and hybrid elm selections are available or being developed.
Two insect vectors are responsible for transmitting DED: the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark beetle (Scolytus multistriatus). These beetles lay their eggs in infected trees. When the adult beetles emerge, they carry the fungus with them when they travel to healthy trees to feed on twigs and upper branches. From the feeding sites, the spores travel to the tree’s water-conducting cells or xylem. Chemicals produced by the tree during its attempt to fight the disease contribute to the plugging of the xylem, causing the tree to wilt.
The beetles typically have two generations per year in the Midwest. DED can also be transmitted through root grafts. Root grafts between trees are especially prevalent in cramped urban and suburban parkways. The disease usually does not spread in this manner beneath roads because the road foundation prevents root grafts between trees on opposite sides. Driveways and sidewalks are usually not effective barriers to root growth.
The disease is most easily detected during early summer when the leaves on an upper branch curl and turn gray-green or yellow and finally brown. This condition is known as “flagging,” but a flag alone is not absolute assurance that the tree has DED. Brown streaks in the wood beneath the bark of affected branches is further evidence, but only laboratory isolation and identification can confirm positively that the tree has DED.
Samples should be sent to a diagnostic laboratory as soon as DED is suspected. Secure branch samples at least 1/2 inch in diameter and at least 8 inches long from a branch that shows active wilting (but is not completely dead). Wrap the sample in plastic wrap or place in a plastic bag to prevent the sample from drying out (do not add water or damp paper towels to provide moisture).
The University of Illinois Plant Clinic will confirm DED for a nominal charge. The address: Plant Clinic, University of Illinois, 1102 South Goodwin, Urbana, IL 61801. Phone: 217-333-0519.
Both the beetles and the fungus need to be considered for control of DED. Control is possible through prevention, early detection of the disease, and replanting with resistant elms. Valuable trees should be inspected frequently, e.g. weekly, from early May through July, and monthly through September. An infected tree may be saved by pruning out the diseased branch promptly after seeing the first “flag.” A final pruning cut 7-10 feet below the lowest evidence of discolored (streaked) wood is necessary, but the saw blade should be wiped (sterilized) with 10% bleach (1 part bleach: 9 parts water) or denatured alcohol before the final cut is made. Injecting trees with systemic fungicides (see below) may be done at this time. If a tree shows many flags or completely wilts and dies, it must be removed quickly so that beetles and root grafts do not transmit the disease further. Root grafts should be severed before removal of a diseased tree whenever possible.
Sanitation
The bark beetles breed in standing dead or dying elm trees and piles of elm wood with the bark attached. Therefore, trees that completely wilt and die are suitable for beetle reproduction and should be felled. Destroy the infected wood and bark by chipping and composting (chips must attain temperatures of at least 120 degrees F), or at a minimum, remove the bark from cut logs and let the logs dry out. Cut logs from diseased trees should not be kept for firewood unless all of the bark has been removed and there is no evidence of bark beetles. Transporting diseased elm firewood may spread DED to otherwise disease- free areas. Covering and sealing cut logs and chips in clear plastic during the summer will allow the sun to heat up the wood and is another way to kill the beetles and fungus. Prolonged sunny weather and high temperatures are necessary, however, for this method of sanitation, called “solarization,” to be effective.
Chemical Protection and Therapy
At present, treatments of affected trees with injected fungicides show promise and should only be applied by licensed, certified arborists. If properly applied, American elms may be protected for 3 years. Be aware that repeated injections with a systemic fungicide may damage the bark and water-conducting tissues.