Forty-one years ago, Swingle instituted this communication on a half sheet of paper providing personalized information on American elms and Dutch elm disease. With the onset of Dutch elm disease, our attention focused on treatments to save healthy trees while quickly removing the diseased elms. Ash, honeylocust, linden, maple, and oak were sourced as replacement street trees. Elm planting became taboo soon disappearing from tree nurseries.
Today, as it has in recent years, this newsletter focuses on the care and re-population of elms. Insects have become a serious nuisance and just as top of mind as Dutch elm disease. Geneticists, botanists, arboretums and nurserymen have successfully bread and brought new elms to market. These elms are worthy of your consideration and inclusion in your landscape.
Last year (2012) no trees on our Elm Preservation Program were lost to Dutch elm disease. Again in 2012 disease incidence was again low in eastern Colorado. In other parts of the United States Dutch elm disease continues to be a formidable foe. The spread of the Dutch elm disease fungus is depended upon elm bark beetles. We surmise, along with Colorado State University researchers, this may be due to the apparent displacement of the smaller European elm bark beetle by the banded elm bark beetle. The banded beetle may be less effective in spreading Dutch elm disease.
Dripping, honeydew, and ghost rain are phenomena synonymous with American elm. The culprit is the ever ubiquitous European elm scale of which Swingle has records dating 60 years. In that time, we have applied no less than eighteen different insecticides on this insect. Several of these insecticides have since been regulated out of existence while others, once effective, seemed to have lost their punch. The last nearly twenty years, the tree care industry has relied heavily upon systemic insecticides, those that have are introduced into the tree through the soil or tree trunk and taken up into the canopy and foliage. While these insecticides and processes still work, the result is a reduction in scale – not complete control. The insect is prolific; each female can give rise to 400 young scale crawlers. Insecticides alone cannot control this insect. A disease, predator, or change in climate will be needed.
Of course at Swingle we aren’t satisfied with saying – well that’s just the way it is and leave it at that. We are continuing to try different materials in our quest to control scales and verify our results. This novel chemical, when sprayed on the foliage, prevents scale crawlers from maturing into adults. We will set out trials this summer, count insects this fall and report to you in 2014.
Usually research on trees occurs in areas that have a lot of trees – not Colorado. A surprise and well kept secret are the National Elm Trials. As the name implies there are sixteen evaluation sites in fifteen states throughout the United States. However, there is a plot in Fort Collins, Colorado and Colorado State University is the central reporting station for the sites. The elms have been under evaluation since 2005 and now good information is available.
The “new” elms are developed or parented in two ways. American elms that show heightened resistance to disease have been bred. Elms have also been developed from seed selected from northern China. The chart at the right shows a few of the elms in the Fort Collins plot that show promise and are available in local nurseries. While Valley Forge performs well, it picks up European elm scale and this may be objectionable. We know Accolade and Prospector have performed well several years after planting.
In Closing
Swingle’s elm preservation program is the most extensive and researched program in the state. Each year we inspect all of the American elms providing information and recommendations. Please do not become complacent; it was diligence that got us to where we are. Do continue to water your elm trees especially after treatment to help move the insecticide up into the tree canopy. Dutch elm disease is a tree disease in Denver and is still a threat. We recommend continuing with your disease injections.
