Dutch Elm Disease (DED) is caused by a fungal pathogen, Ceratocystis ulmi. The disease works much like Verticillium wilt and oak wilt, plugging vascular tissues and causing the foliage to wilt and die. Elms are the hosts of DED, and American elms are very susceptible. Although Chinese and Siberian elm are known to be more resistant, infection of these species can occur as well. Breeders have developed more resistant elms, available as Sapporo Autumn Gold, American Liberty, and Urban elms.
Watch for yellowing foliage, followed by wilting and browning. A single branch usually shows symptoms first, with rather rapid spread to adjacent branches and the entire tree. To help with diagnosis of this disease, look for vascular discoloration. As with oak wilt and Verticillium wilt, DED causes streaking of the sapwood. Peel back the bark of a symptomatic branch to reveal brown streaks in the otherwise tan outer sapwood. We generally select branches of about thumb thickness that have wilted leaves. Sometimes the streaking is more evident when a branch is viewed in cross-section: a ring of discoloration can be seen just under the bark. Since Verticillium wilt and Doth-iorella wilt can also cause this streaking in elm, positive identification requires laboratory culturing of the fungus. Cut several six- to eight-inch sections from wilted but living branches that show definite streaking in the sapwood. These fresh wood sections should be 1/2 to one inch in diameter and can be sent in plastic or foil to the Plant Clinic for testing. Expect about seven days of lab time for the fungus to grow to the point where it can be positively identified. There is a $10 fee for this service.
Many people have the mistaken impression that this disease is a thing of the past, since there are now so few elms around. Not so! We receive many elm specimens at the Plant Clinic each year to be tested for the Dutch Elm Disease fungus. Unfortunately, we are still able to make many positive isolations.
For more information on DED, including control procedures, consult Report on Plant Diseases No. 647. A similar disease caused by a phytoplasma is discussed in RPD No. 660, Elm Yellows or Phloem Necrosis and its Control. Also see Elm Yellows in issue no. 4.