Although the number of Dutch elm disease (DED) cases in Illinois has dropped since the epidemic of the 1950s, we can’t expect Dutch elm disease to disappear completely from Illinois. The University of Illinois Plant Clinic confirms Dutch elm disease every year from infected Illinois trees. We confirmed 13 cases in 2002, 9 in 2003, and 21 in 2004. This season started May 2, and already the Plant Clinic has confirmed 3 cases in Cook County.
DED symptoms generally begin in early summer. Although it is unlikely that you will be able to save an infected tree, you can help nearby healthy elms.
American elms are very susceptible to the DED fungus. Although Chinese elm and Siberian elm are known to be more resistant, infection of these species can occur as well. Breeding programs have produced the more resistant Sapporo Autumn Gold, American Liberty, and Urban elms. Alden Townsend of the National Arboretum released the first two American elm cultivars (‘Valley Forge’ and ‘New Harmony’) tolerant to DED to nurserymen and the public. For more information on his work, visit this site, http://www.usna.usda.gov/Research/~amt.html. Ask about DED resistance when purchasing elms.
Dutch elm disease is caused by the fungus Ophiostoma ulmi (Ceratocystis ulmi). Watch for yellowing of the elm leaves, followed by wilting and browning. A single branch usually shows symptoms first (called flagging), with rather rapid spread to adjacent branches and the entire tree. Look for vascular discoloration to help with diagnosis of this disease. Peel back the bark of a symptomatic branch to reveal the brown streaks in the otherwise tan outer sapwood. Positive identification would require laboratory culturing of the fungus, but send sections you have not peeled. The fresh wood sections can be sent in plastic or foil to the Plant Clinic for testing. Chilling the wood is necessary for oak wilt suspects but should not be necessary with Dutch elm suspect samples. Expect about 7 days of lab time for the fungus to grow to the point where it can be positively identified. There is a $12.50 fee for this service.
Confirmation of Dutch elm disease involves isolating the fungus from infected wood. First, wood samples are surface disinfected. The bark on one end of the branch is removed, and chips of wood are taken from the exposed wood. These chips are placed in a potato dextrose agar and incubated for 5 to 7 days before the fungus can be positively identified. Keep in mind that the sampled wood represents the entire tree, so the sample must be taken from areas of the tree showing symptoms. Laboratory isolations are most successful when samples contain live wood that exhibits vascular discoloration. Samples need to be about thumb thickness and 6 to 8 inches long. If you have questions about sampling, refer to Report on Plant Disease, no. 647, “Dutch Elm Disease and Its Control.” This report is available in Extension offices or on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm.
There are no chemicals available to homeowners for control of DED. Some products available to commercial applicators are used as preventive or therapeutic treatments when the disease is caught early. Consult the 2005 Commercial Landscape and Turfgrass Pest Management Handbook for details. Use of Alamo or Arbotect is discussed, as are sanitation measures and prevention.