Many homeowners who have mature elms in their landscapes do not realize that Dutch elm disease is still prevalent in Illinois and in the Midwest. The fact that a tree was not killed by the epidemic of this disease in the 1950s does not mean it is resistant. It is far more likely that the tree escaped infection. We have many (100+) confirmed cases of Dutch elm disease at the Plant Clinic each year. The Morton Arboretum in the Chicago area has confirmed positive cases. Private tree care companies in Chicago have also confirmed positive cases of Dutch elm disease.
Dutch elm disease (DED) is caused by a fungal pathogen, Ceratocystis ulmi. The fungus works much like the other vascular pathogens (see oak wilt in issue No. 6 and Verticillium wilt in issue No. 8 of this newsletter) causing plugging of the vascular tissues and resultant wilting and death of foliage. American elms are very susceptible to this pathogen. Although Chinese elm and Siberian elm are known to be more resistant, infection of these species can occur as well. Work is still under way to develop resistant elms. So far, work has produced the more resistant Sapporo Autumn Gold, American Liberty, and Urban elms.
Watch for yellowing of the leaves in the elm, fol-lowed by wilting and browning. Often this happens so quickly that the problem is first noticed when branches with brown leaves appear in the canopy “overnight.” A single branch will usually show 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. As with oak wilt, DED will cause a streaking of the sapwood. Peel back the bark of a symptomatic branch to reveal the brown streaks in the otherwise tan outer sapwood. We generally select branches of about thumb thickness with wilted leaves. Verticillium wilt and Dothiorella wilt can also cause this streaking in elms. Positive identification would require laboratory culturing of the fungus. Cut several 6- to 8-inch long sections from wilting but living branches that show definite streaking in the sapwood. The fresh-wood sections should be thumb thickness and can be sent in plastic or foil to the Plant Clinic for testing. Chilling the wood 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. Remember, payment must accompany the sample, or it will not be processed.
For more information on DED, including control procedures, consult RPD No. 647, available on the VISTA Extension Web site. It is generally too late to save a tree once it is infected, but an accurate diagnosis of the problem may help save nearby elm trees. A similar disease caused by a phytoplasma is discussed in RPD No. 660, Elm Yellows or Phloem Necrosis and Its Control.