We have had a few confirmed cases of Dutch elm disease (DED) at the Plant Clinic this year, and The Morton Arboretum in the Chicago area has seen a few as well. Many believe this disease is a thing of the past because there are so few elms around now. Not so! Each year, at the Plant Clinic, we receive many requests for testing for this fungus. Unfortunately, we are still able to make many positive isolations.
Dutch elm disease (DED) is caused by a fungal pathogen, Ceratocystis ulmi. The fungus works much like the other vascular pathogens, 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, this work has produced the more resistant Sapporo Autumn Gold, American Liberty, and Urban elms.
Watch the elm for yellowing of the leaves, followed by wilting and browning. A single branch will usually show symptoms first (called flagging) with 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 last week), DED causes 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 that are about as thick as a thumb with wilted leaves. Verticillium wilt and Dothiorella wilt can also cause this streaking in elm. Positive identification requires 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 suspect samples. Expect about seven 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, and payment must accompany the sample, or it will not be processed.
For more information on DED, including control procedures, consult Report on Plant Disease No. 647. A similar disease caused by a phytoplasma is discussed in RPD No. 660, “Elm Yellows or Phloem Necrosis and Its Control.”