MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The loss of many landscape trees in Michigan caused by the emerald ash borer, drought and urban decline problems has emphasized our need for more choices for replanting. The loss of the monocultures of American elms which once graced our streets, followed by loss of our Ash species begs the question; what next, locusts, maples? One can only guess. Certainly, the solution to preventing devastating loss of street trees in the future is to prevent monoculture plantings.

To this end, cities in Michigan are actively planning for a greater diversity of tree species to replace our lost trees and avoid the repeated mistake of monoculture plantings. To meet the new goals, some recommendations suggest that no more than 10 percent of street trees be of one species and no more than 20 percent be of one genus. But a challenge remains; where can we find enough tree species to adequately diversify our landscapes?

Selecting trees from nature and testing them for tolerance to urban conditions is relatively easy and this research is ongoing on MSU’s campus where sweet gums, Corylus  species, and many other trees are being evaluated. Breeding trees known to have good urban tolerance to improve their resistance to problematic diseases and insect pests can be much more difficult. Have you ever wondered what scientists have been doing to bring us new disease and insect resistant American elms, White pines, or green ash? In this article, we will have a look at what has been happening with incorporating resistance to Dutch elm disease (DED) into the elms.

Following the spread of the devastating DED worldwide, early breeding efforts began in the Netherlands. Unfortunately, the fruits of these labors were not sufficiently winter hardy for most of North America, and the cultivars had weak growth habit. They came short of the mighty American elm in stature. In the United States, we had several active tree breeders that worked a life time on improving elms, including Drs. Eugene Smalley from the University of Wisconsin, Denny Townsend from the National Arboretum, and George Ware of the Morton Arboretum. The breeding was complex because it was necessary in most instances to hybridized DED-resistant elms of Asian origin with American or European species. Such hybrids produce mostly severely stunted and unthrifty progeny. However, small percentages of the progeny exhibit the strong growth and suitable horticultural characteristics desired. From this small pool of the resultant hybrid elms that exhibited resistance to DED, further breeding and selection aimed to increase resistance to many common insect pests of elm. Some of these hybrids are crosses between two, three and even four species of Ulmus. Their parentage from North America can include three species Ulmus americana, Ulmus rubra, and Ulmus thomasii. Their parentage from Europe can include Ulmus glabra and Ulmus carpinifolia; and from Asia Ulmus parvifolia, Ulmus pumila, Ulmus japonica, and Ulmus wilsoniana. Elm cultivars that possess Asian or European parentage have resistance to the disease Elm yellows or Elm phloem necrosis caused by a leafhopper transmitted phytoplasma. Phytoplasmas are especially small and fragile bacteria that have lost their cell walls and many genes, and can survive only in plant phloem and in their insect vectors. Elm cultivars that possess parentage of U. wilsoniana or U. japonica have resistance to elm leaf beetle herbivory.

Ultimately, several new elms were created which matured into spreading, vase-shaped trees, reminiscent of the grand American elms like those evident on the MSU campus. These hybrids include: ‘AccoladeTM,’ ‘Homestead,’ ‘Pioneer,’ and ‘VanguardTM.’ The latter is particularly drought tolerant. At MSU, these hybrids are included among the twenty DED-tolerant cultivars that have been planted with five replications for a total of 100 trees. The DED-tolerant cultivars are being evaluated every year to determine how they perform in the Michigan climate. Their horticultural form, growth rate, fall color and other characteristics are recorded. Some cultivars may possess resistance or tolerance to other plant pathogens such as those that cause branch and truck cankers and leaf spots. Additionally, they may possess tolerance to insects that feed in twigs and trunks, and on foliage. Just as important will be whether they may possess resistance breakage during high winds, heavy snows or ice storms.

The MSU elm trial includes a number of special selections of the American elm, U. americana, that have proven resistant or tolerant to DED in earlier trials. These cultivars include 'Princeton' discovered around 1920 in New Jersey which may also have some tolerance to feeding by the elm leaf beetle. Dr. Denny Townsend developed the cultivars known as ‘Valley Forge’ and ‘New Harmony’ which we have planted. 'Valley Forge' is considered to have greater tolerance to DED than 'New Harmony,' but the latter has the more attractive vase-shaped habit. A triploid U. americana known as ‘Jefferson' has also been planted. It was discovered by researchers from the National Park Service. ‘Jefferson’ has tested repeatedly as DED-tolerant in other climates.

The planting and maintenance of the MSU elm trial was sponsored by the Michigan Department of Agriculture.The planting at MSU is a part of the National Elm Trials, and similar research and teaching plots are planted throughout the United States where willing participants have agreed to maintain them. While the trees at MSU are still rather small, they will remain available for study for many years to come and ultimately many will be planted out into the general campus landscape. We at MSU’s Department of Plant Pathology hope that you will have several opportunities to examine these trees in guided tours during local nursery, landscape, urban forestry, master gardener and other plant industry meetings and workshops.

Active breeding of improved elms for disease and insect resistance has vanished from the nation’s universities since the 1980s. University administrators failed to support the maintenance of the continuation of the breeding programs and also the existing elms from the life’s work of the former generation of University professors. Therefore, the elms atMSU and in the National Elm Trial are part of an attempt to rescue the best products of the older work and get information about these selections out into the nursery trade. Only commercial interest in these elms will ensure their continued propagation and availability.

A listing of the cultivars, their trade names, parentage, and the nurseries that donated the plants is available on the web site for the National Elm Trial established at Colorado State University by Dr. William Jacobi. The table included in this article is reproduced in relation to the MSU trial. Measurements and data recorded yearly at MSU are sent to the Colorado organizers and often posted on the website:

http://treehealth.agsci.colostate.edu/research/nationalelmtrial/NationalElmTrial.htm

We gratefully acknowledge the nurseries that donated these plants for the National Elm Trials.

Spring 2008 at MSU Plant Pathology Research Farm