Symptoms of Dutch elm disease (DED) generally begin in early summer. The Plant Clinic has received several positive cases in the last few weeks; and the Morton Arboretum in northern Illinois also reports many. Monitor elms for development of DED. It is not likely that you will be able to save the infected tree, but 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. Breeding programs have produced the more resistant Sapporo Autumn Gold, American Liberty, and Urban elms. Ask about DED resistance when purchasing elms.
Dutch Elm Disease (DED) is caused by the fungus, Ophiostoma ulmi (Ceratocystis ulmi). The fungus works much as the other vascular pathogens, causing plugging of the vascular tissues and resultant wilting and death of foliage. 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. As with oak wilt (discussed in issue no. 11 of this newsletter), DED causes a streaking of the sapwood. Peel the bark of a symptomatic branch to reveal the brown streaks in the otherwise tan outer sapwood. Verticillium wilt and Dothiorella wilt can also cause this streaking in elm. Positive identification requires laboratory culturing of the fungus; this service is offered by the University of Illinois Plant Clinic. Cut several 6- to-8-inch-long sections from wilting, but living, branches that show definite streaking in the sapwood, but send sections you have not peeled. 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 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.
There are no chemicals available to homeowners for control of DED. Some products are available to commercial applicators. These products are used as preventive or therapeutic treatments when the disease is caught early. Consult the Commercial Landscape and Turfgrass Pest Management Handbook for details. For more information on DED, including control procedures, consult Report on Plant Disease (RPD), no. 647. A similar disease caused by a phyto-plasma is discussed in RPD, no. 660, “Elm Yellows or Phloem Necrosis and Its Control.” These reports are available in Extension offices or on the Extension Vista Web site.