Many of you have likely experienced the following scenario: “Several large elms in my neighborhood have been killed by Dutch elm disease (DED) over the last year or so. What can I do to protect my elm? My neighborhood is aware of this disease, and we are doing our best to watch for early signs of the disease and get rid of the infected wood. However, I want to ensure that my tree never gets diseased.”
Never is a very long time. How do you respond to this person? You might begin by telling him or her to keep up the good work, because sanitation is a vital part of communitywide DED management. However, be honest and say that although sanitation is the best first line of defense, it is not a guarantee against DED infection. In fact, there is no documented method or product that guarantees that the tree will not become infected. DED management is analogous to protecting yourself against a car accident; to protect yourself, you might
Learn how to drive well./Learn how to identify DED early.
Be a defensive driver./Prune and practice sanitation when you see DED.Avoid distractions./Fighting DED for one season won’t cut it.
Wear a seatbelt./ Sever root graft to prevent DED from moving from tree to tree.
Install a roll cage./Inject the tree with a fungicide.
Wait a minute, who installs a roll cage in their car? Race-car drivers do! Why? Because despite other precautions, they are at high risk for an accident. Like the race-car driver, if your community has recently lost a number of elms to DED, your tree is at higher risk of becoming diseased compared to a community essentially free of DED casualties.
Researchers and practitioners continue to investigate the use of systemic fungicides in an attempt to provide safe, long-lasting protection against DED. Although there are a number of fungicides in, or entering, the DED-management market (Abasol, Alamo, Arbotect 20-S, Eertavas, Elm Fungicide, Fungisol, Imisol, Phyton 27, and Tebuject), a recent literature review by Stennes and Haugen (Plant Disease Quarterly 1999 20:29-38) points to Arbotect 20-S and Alamo as being the most effective and well documented products for use against DED. Arbotect 20-S and Alamo are labeled for use as preventive and therapeutic injections. Certainly, preventive injections are more effective and reliable than therapeutic injections. However, keep in mind that fungicides move primarily upward upon injection and therefore are not effective against infections that come from roots grafted to nearby infected trees.
Compared to Alamo, Arbotect 20-S carries a somewhat higher risk of causing phytotoxicity (tissue damage) to the injection site and crown. However, this risk may be offset by the fact that Arbotect 20-S is known to last longer in elm than Alamo, providing protection for up to three growing seasons in northern climates. While Alamo can be applied using the newer “micro-injection capsules,” most practitioners prefer the traditional “macro injection” (or root-flare) technique. The drawback to using any of the current fungicides is cost (typically $300 or more per tree) and the need for retreatment in 1 to 3 years. In addition to the cost of treatment, there is still the issue of risking tree health due to many years of injections. Although there are many trees that have been safely injected for 15 to 20 years or more, there are cases where trees have been essentially girdled because of too many injections. Thus, fungicides are suggested only when high-value trees are in danger and only when used in conjunction with a good community wide sanitation and root-graft control program. Tree injections should only be made by trained arborists or others trained in DED diagnosis and injection techniques. Keep in mind that roll cages don’t always fully protect race-car drivers, and fungicide injections don’t always fully protect plants, but each can be valuable in high-risk situations.
Are there any biological-control products out there? Several different biological-based products (for example, “Dutch Trig” and “Elm Vaccine”) are currently being studied for their ability to protect elms against DED. Simply stated, these products are preventatively injected into elms, where they trigger the elm tree’s own natural resistance. Although the sponsoring companies claim encouraging results, we await peer-reviewed research reports and marketing of the commercial products in the United States. For more information about DED, refer to the June 21 issue of Home, Yard and Garden Pest Newsletter (http://www.ag.uiuc.edu/ cespubs/hyg/html/200009e.html).