There are still many elms in Illinois, so be aware of this lethal disease. Some growers assume that a mature, healthy American elm must be resistant, as it did not succumb to infection when Dutch elm disease was rampant in the 50s. It is more likely that the tree is an escape, luckily avoiding infection via beetles. |
Dutch elm disease (DED) is caused by a fungal pathogen, Ceratocystis ulmi, which causes plugging of vascular tissues and resultant wilting and death of foliage, much as we see with oak wilt or Verticillium wilt. American elms are very susceptible to the DED pathogen. Although Chinese elm and Siberian elm are known to be more resistant, infection of these species may occur as well. Work is still under way to develop resistant elms. So far, research has produced the more resistant Sapporo Autumn Gold, American Liberty, and Urban elms. Some of this research is being conducted at the U.S. National Arboretum near Washington, D.C. Other work is being done at the Morton Arboretum in Illinois, with some selections available through the Chicagoland Grows, Inc., program. Refer to issue no. 2 of this newsletter for information on disease resistance.
Watch for yellowing of the leaves in the elm, followed by wilting and browning. Often this happens so quickly the problem is first noticed when branches with brown leaves appear in the canopy “overnight.” 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, 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 of about thumb thickness 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 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. The fee for this service is $12.50. Remember, payment must accompany the sample, or it will not be processed.
For more information on DED, including control procedures, consult Report on Plant Disease (RPD), no. 647, available on the Extension’s VISTA Web site. It is generally too late to save a tree once it is infected, but an accurate diagnosis 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.”