In 1987, a plant pathologist in Montana ended an incomplete experiment by cutting down fourteen young American elm trees (Ulmus americana). At the time, Dutch elm disease (Ophiostoma ulmi, DED) was taking hold in parts of Montana. The only management practices then available in Montana were tree removal or pesticide sprays to stop the movement of the vectors, elm bark beetles. The pathologist, Gary Strobel, had been hoping to develop an unconventional method of disease management—vaccinate the tree with a genetically engineered bacterium (Pseudomonas syringae) to fight the fungal disease.
In lab trials, Strobel and his colleague Donald Myers had demonstrated that Pseudomonas syringae produced natural antibiotics that suppressed the spread of DED through vascular tissue. Still, Strobel needed permission from the Environmental Protection Agency to proceed with a field experiment that involved a genetically engineered organism. The bureaucratic machinations promised to delay the field experiments for another year, so Strobel moved forward with his experiments and injected fourteen trees before receiving formal approval. This moment captured national headlines focusing attention on the debate over genetically engineered organisms and drawing unwelcome attention to Montana State University, where Strobel was a researcher. Rather than put his colleagues at risk of losing federal funding due to his decisions, Strobel volunteered to destroy his field experiment.
As an unanticipated result, however, the news also generated a newfound interest in tree breeding work occurring in a Chicago suburb. At the Morton Arboretum in Lisle, Illinois, George Ware had been busy developing DED-resistant elms the conventional way, through targeted breeding efforts using disease-resistant germplasm. In a 1987 New York Times article titled “Fighting Elm Disease the Natural Way,” Ware is quoted as saying, “Dr. Strobel was trying to help one kind of elm quickly. We’re looking more toward diversity over the long run.” This quote highlights the nature of Ware’s Elm Improvement Program and the efforts that he went through in developing the next DED-resistant elms.
George Ware arrived at the Morton Arboretum in 1968, a mere two years after Marion Trufant Hall was hired as director and charged to lead the arboretum in an initiative to expand our research capacity. Ware was enlisted as the research director and the in-house ecologist and dendrologist. In 1972, four years into his tenure, Ware began noticing a peculiar tree in the arboretum collections: a stately elm growing outside the former study of Joy Morton, the arboretum’s namesake and founder.
At the time, much of the landscape in and around Chicago had been devastated by Dutch elm disease—as was the case across the United States. The graceful American elm had been widely planted in the Chicago area, along streets and in parks. The prevalence of this species, which was also a ubiquitous forest tree, would ultimately be its undoing, enabling the rapid spread of both the vector and the disease by air and by root-to-root transmission.
The first detection of Dutch elm disease in the United States was recorded in 1929 by Curtis May, a plant pathologist for the United States Department of Agriculture (USDA). May received samples collected in Ohio by plant pathologist Paul Tilford. The trees in Ohio were dying and a cause for concern. Later, in 1933, a USDA inspection would discover the source of the introduced disease: shipments of imported burl logs harboring the European elm bark beetle (Scolytus scolytus). The larvae of elm bark beetles, including our native species (Hylurgopinus rufipes), feed on the vascular tissue of infected trees, picking up spores with their bodies. When they mature and emerge from the tree, they can move to uninfected trees, introducing fungal spores.
Newspapers across the country began raising the alarm about the rapid loss of trees as the disease continued to spread in the East and Midwest. By 1970, the Chicago region was reported to have lost more than fifty thousand trees and was projected to lose another fifty thousand within two years. It was amidst this devastation, in 1972, that Ware noticed the tree growing outside of Joy Morton’s study window at the Thornhill Estate. It was an elm (Morton accession 2352-24*1) with gracefully arching branches, healthy and green foliage, and no symptoms of the disease.
The original elm, fondly referred to by Morton Arboretum staff as the Thornhill Elm, was accessioned into the collections in 1924, shortly after Joy Morton founded the arboretum on his estate in 1922. With guidance from Charles Sprague Sargent, the director of the Arnold Arboretum, Morton established a 735-acre arboretum that included an herbarium, library, and nurseries, along with staff to manage it all by the time of his death. The most integral component of the arboretum—the living collections—included many accessions of plants initially sourced from the Arnold. In the initial establishment of the Morton collections, Sargent provided access to seed, clonal propagation material, and plants.
One such packet of seed was labeled Ulmus crassifolia (the cedar elm), and records indicated that the seed had been wild-collected in Brownwood, Texas, by botanist Ernest Jesse Palmer. It was accessioned into the arboretum collections, and seeds were germinated and grown in the nursery. Eventually, a sapling was planted outside the bay window of Morton’s study. As the years passed, the tree witnessed Morton’s family and guests enjoying summer afternoons by the pool. The sloping vista beneath the elm was crowned by hawthorns for which the estate was named. There were staff picnics for Morton Salt Company and the Morton Arboretum in the coming decades. The tree overlooks the Morton family cemetery and bore witness to family funerals, but it also provided shade to guests at weddings and garden soirees. Eventually, when the crumbling mansion was demolished long after Morton had died, the tree stood guard over Morton’s study, which was preserved as part of a new facility for educating the public about plants and the rest of the natural world.
In 1972, Ware looked at this tree and recognized that it was, in fact, not Ulmus crassifolia. The leaves were too large, the bark not quite right, and the form much too refined. As a dendrologist who had been a faculty member at Northwestern State College in Louisiana, Ware was familiar with U. crassifolia, which is native to that region. In fact, one of the first deposits Ware made into the arboretum collections in 1968 was a packet of cedar elm seed (Morton accession 385-68) that he had collected from the wild in Seguin, Texas. After further investigation (and even a visit to Arnold), Ware confirmed that the Thornhill Elm was U. davidiana, a species native to eastern Asia. Noting the native origin of the species and the lack of symptoms in the tree, Ware saw the possibility that the Dutch elm disease pathogen had Asiatic origins itself. Perhaps the presence of Ophiostoma ulmi in the natural habitat of U. davidiana had led to coevolution of the species such that the David elm had adapted a natural biochemical defense mechanism to combat the disease. In this tree, Ware saw great potential.
|Species||Geographic distribution||Ware Description||Selections available in US nursery trade?|
|Ulmus davidiana (syn. include U. japonica, U. wilsoniana, U. propinqua)||China, Japan, Korea, Mongolia, Siberia||Variation in habit; tolerant of hostile conditions||Yes; many introductions made in the past couple of decades|
|U. glaucescens||Gansu Province (China), northern China||Small tree; small leaves, fine texture; yellow to orange fall color; tolerant of urban conditions based on its distribution||No|
|U. laciniata||Humid areas of northern China, Korea, Siberia, and Japan||Small to medium tree; potential drought hardiness; lobed leaves; Zelkova-like branching||No|
|U. macrocarpa||China, Mongolia, Korea, and Siberia||Strong wood; shrub to medium-sized tree; adapted to humid and arid regions; tolerant of “hostile” conditions||No|
|U. parvifolia||China, Korea, Japan||Tolerant of drought, pollution, poor soils; attractive lace bark; glossy leaves||Yes; many introductions made in the past couple of decades|
The Thornhill Elm inspired the development of the first breeding program at the Morton Arboretum, the Elm Improvement Program. As a trained ecologist and dendrologist, Ware understood the necessity for genetic diversity within a population. He was soon on the search for more parent material to include in his germplasm collection. By 1980, Ware had clones of the Thornhill Elm propagated and under evaluation. That same year, he published two articles in the Journal of Arboriculture focusing on the qualities necessary for trees to survive in human-built landscapes and the attributes of Asian elm species that made them ideal candidates for such an environment. These publications were an effort to raise awareness within a community of tree experts about the possibilities that were held within the genetic resources of Asian elms. While American elms were being felled across the eastern United States, Ware was proposing a new solution to a decades-old problem: Let’s plant Asian elms, he suggested, given that these species are adapted to both the constructed environment and the devastating disease. Clones of the Thornhill Elm are now widely available in the commercial nursery trade under the name Ulmus davidiana ‘Morton’ Accolade™.
When developing any plant breeding program, a breeder must first start with objectives and further refine them by identifying specific desirable traits. Ware’s primary objective was to develop elm trees with Dutch elm disease resistance. Second to that, he aimed to develop trees that were not preferred by the elm bark beetles or elm flea weevils (Orchestes alni). Beyond pest and disease resistance, Ware would focus on species adapted to environments of the extremes: temperature, drought, flood, high winds, blizzard, and “hostile” soils. He defined hostile soils as those with high pH, poor aeration, and minimal organic matter. He noted that these are all common conditions of the Midwest and Great Plains, and coincidentally, these are the same conditions faced by trees in developed landscapes regardless of the region. Ware went on to list and describe Asian elm species that should be considered for breeding programs.
Meanwhile, Ware began the process of hybridizing elms that were available within the Morton collections. He collected branches covered in rounded floral buds and brought them into his lab. He placed the cut stems in vessels containing water and positioned them upon white sheets of paper spaced out along lab benches. As the forced stems began to flower, yellow piles of pollen would accumulate on the paper, signaling the pollen was ready to be collected and stored. Ware then used a ladder to take this pollen into the canopy of a female parent tree, where he secured a bag over a flowering stem. Making an opening in the bag, he dispersed pollen inside and mimicked the movement of the wind to ensure the pollen made contact with the receptive stigma. Once the bag was securely shut, he climbed down from his ladder and waited. This process led to the development of several new hybrid elms, including Ulmus ‘Morton Glossy’ Triumph™. This selection is one of the most popular of Ware’s elm introductions due to its low maintenance requirements in both commercial nursery production and municipal tree management.
While a breeder can develop the best possible plant selection, the plants would not get very far out of the breeding program without help from the nursery industry. Ware was acutely aware of this. While his initial collaborations were with arborists, foresters, and botanists, he would go on to develop strong working relationships with the nursery industry, specifically Keith Warren, the former manager of new plant development for J. Frank Schmidt & Son, based in Boring, Oregon. The two first discussed the possibility of evaluating Ware’s elm selections after a Metropolitan Tree Improvement Alliance conference, hosted at Thornhill in June of 1990. This meeting would lead to a collaboration between the Morton and J. Frank Schmidt that continues today, enabling hybrid elm selections to be propagated on greater scales and evaluated in field research.
The first grafting of Ware’s elm hybrids at J. Frank Schmidt occurred in 1994—just twenty-two years after Ware recognized the tree’s potential and seventy years after being received as seed labeled Ulmus crassifolia. The Oregon Department of Agriculture helped the collaborators set up a screened isolation and quarantine area at the commercial nursery, ensuring that DED would not be introduced into the Oregon landscape due to the nursery trade. By 1995, additional propagation material was distributed for in vitro propagation evaluation by Microplant Nurseries, a tissue culture lab based in Gervais, Oregon, managed by Gayle Suttle. At that time, there were not yet any cultivars of U. davidiana available through the commercial industry.
Ware also recognized that for elm breeding efforts to be effective, additional genetic material needed to be collected from the wild. When he began his research, he found that few elms of wild provenance were available in the collections of North American public gardens, potentially creating a genetic bottleneck for any North American elm breeding program. The total number of elm species is somewhere within the range of twenty to forty, depending on taxonomic classification, and the center for this diversity is unmistakably in eastern Asia. The Flora of China indicates that more than half of all elm species are native to the region. Ware and his colleagues ultimately visited China five times and the Soviet Union three times, developing relationships with forestry researchers willing to collect seeds in the wild and ship them to Ware. Today, the Morton’s elm collection contains 329 accessioned individuals representing thirty-three species and thirty-four cultivars, a dramatic increase from 1968, when Ware arrived. At that time, the elm collection included fifty-one trees, which represented nine species and ten cultivars. Of the newer individuals accessioned into the Morton collections, eighty-one came directly as plants from Ware’s breeding and research program.
Ware also actively distributed seed and plants throughout the United States. He coordinated a seedling distribution program through which he distributed one thousand seedlings to Midwestern nurseries, aiming to popularize the Asian elm species. Municipal foresters and park managers regularly arrived at the Morton Arboretum’s service gate searching for elm seedlings he had promised. As the current manager of the program that Ware initiated, I still receive notes from recipients of such gifts who recount fond memories of Ware and his generosity. Today, the seedling trees that he distributed can be found from Oregon to New York and Illinois to Louisiana. Several of Ware’s elms were even planted in the late 1980s on the course of the Winged Foot Golf Club, the prestigious host of multiple US Opens in Mamaroneck, New York. This planting was a direct result of a 1987 New York Times interview of Ware following the Strobel controversy.
By 1990, Ware had several elm selections in the pipeline and a greatly expanded collection of germplasm. He then began the process of developing a new breeding population. Working with large, wind-pollinated, late-winter-flowering trees presents unique challenges to a breeder. The flowers are insignificant and often located more than six feet above the ground. (I can attest to the complications of these factors as a breeder working with elms today.) To simplify the hybridization process, Ware developed an isolation block of sorts in a local cemetery. He knew the cemetery would not be paved and that the trees would be left alone until they declined from old age. While Ware retired in 1995, he continued to develop his vision of trees for the future as a research associate of Morton Arboretum until 2009.
|Cultivar and trade name||Parentage/origin||Traits||USDA Hardiness Zones||Dimensions (feet)|
|Ulmus ‘Morton’ Accolade™||Chance seedling |
U. japonica × U. wilsoniana
|Vase-shaped habit and vigorous grower; foliage fine-textured, dark green, and glossy with yellow fall color; DED and elm yellows resistance; resistant to elm leaf beetle||5 – 8||20 year|
30’ H, 15’ W
50 – 60’ H
30 – 40’ W
|Ulmus ‘Morton Plainsman’ Vanguard™||Chance seedling|
U. japonica × U. pumila
|Relatively upright branching and rounded habit in youth; requires corrective pruning to avoid included bark; dark green foliage with yellow fall color; DED and elm yellows resistant; susceptible to elm leaf beetle, Japanese beetle, and leafminer||5 – 7||Mature|
45 – 50’ H
40 – 50’ W
|Ulmus ‘Morton Glossy’ Triumph™||Controlled cross|
U. Accolade™ × U. Vanguard™
|Grower favorite due to ease of training; lustrous dark green foliage with yellow fall color; upright oval form that ages to vase shape; strong branching; excellent DED resistance; moderate pest resistance||4 – 9||Mature|
50 – 60’ H
40 – 50’ W
|Ulmus ‘Morton Stalwart’ Commendation™||Controlled cross|
U. Accolade™ × (U. pumila × U. carpinifolia)
|Symmetrical arching branches, upright oval habit; large, dark green leaves with yellow fall color; rapid growth and broad adaptability; excellent DED resistance; moderate susceptibility to elm leaf beetle, Japanese beetle, and gypsy moth||(4)5 – 9||Mature|
50 – 60’ H
40 – 50’ W
|Ulmus ‘Morton Red Tip’ Danada Charm™||Chance seedling|
|Rounded habit in youth maturing to large and elegant vase-shape; fast grower; glossy green foliage with red-pigmented new growth; yellow fall color; excellent resistance to DED and elm yellows; moderate susceptibility to Japanese beetle and elm leaf beetle||(4)5 – 9||Mature|
60 – 70’ H
50 – 60’ W
The selection criteria that Ware developed for this population include tolerance to DED and elm yellows (a phytoplasma disease, Candidatus Phytoplasma ulmi, which causes leaves to suddenly wilt in late summer), pest resistance, cold hardiness, vigor, and red fall color. Red foliage is not commonly seen in elms. Typically, the fall color is a muddy yellow. Ware, however, had noticed an intriguing trait in a group of Asian elm seedlings: red pigmentation in emerging leaves. He understood that if the seedlings could produce red pigmentation (anthocyanins) in leaves during the spring, they should be able to use the same biochemical pathway to produce anthocyanins in the fall. This unexpected discovery led to red fall color becoming a new breeding objective.
I was hired as the tree and shrub breeder for the Morton Arboretum in 2016. When I arrived here, I was certainly not an elm expert. I had spent my graduate school years working primarily with shrubs and herbaceous perennials. It took some time to unearth the details of the Elm Improvement Program, but today, I can say that we are continuing to make progress with Ware’s legacy project. The program is now part of the Daniel P. Haerther New Plant Development Program, named in honor of a generous benefactor of the arboretum who was one of many that Ware inspired to appreciate the development of trees for the urban landscape. Ware consulted about elms on Haerther’s estate, and in the process, the two would develop a relationship centered on a love of trees.
Currently, we have sixty-one seedling selections from the breeding population that Ware left behind for the next generation. These were all selected for fall colors ranging from oranges to reds and purples. The breeding population includes the Ulmus davidiana complex, a variable group that was historically treated as three separate species (U. japonica, U. wilsoniana, and U. propinqua). The population also includes three other Asian species (U. macrocarpa, U. parvifolia, U. pumila), an Asian hybrid (U. ‘Sapporo Autumn Gold’), and the European field elm (U. minor). Our primary focus has been on the U. davidiana complex. We have selected a tree that will serve as the seed parent. It has an attractive form and relatively petite stature, along with somewhat glossy and predation-free foliage in the summer.
We also continue to expand the program, particularly with work on the lacebark elm (Ulmus parvifolia). In 1996, Ware published two short articles in Landscape Plant News regarding this Asian species. He had participated in a USDA-sponsored research exchange trip to China led by Eugene Smalley from the University of Wisconsin–Madison. Ware and four American colleagues joined Smalley in the field to collect seed and determine the natural range of U. parvifolia. They were also very much focused on building relationships with Chinese researchers. Ware considered this elm species to be an especially promising selection for built environments of the South due to its broad adaptability to heat, flooding and drought, hostile soils, and both humid and arid conditions. However, he also noted that it would not perform well in northern states due to limited cold hardiness.
Today, lacebark elms have demonstrated some hardiness with proper site selection. At the Morton Arboretum, seventeen individuals of this species (along with three cultivars and three unnamed hybrids) have survived multiple polar vortexes. I have also witnessed the lacebark elm growing and thriving from North Carolina to New York City and Las Vegas to New Orleans. This widespread adaptability, however, is accompanied by legitimate concerns about weediness. Even though the lacebark elm has not been widely planted in the Midwest, it is already listed as a weed of concern in Wisconsin. Colleagues at public gardens in other regions have expressed similar apprehensions about the species. This concern has led us to develop a new elm improvement project at the Morton Arboretum focused on developing selections with reduced fertility.
Breeders have long used methods of mutation breeding to develop seedless plants. The most commonly known examples include the seedless watermelon and banana. These were developed through a traditional breeding method referred to as interploidy hybridization. Ploidy is the number of complete sets of chromosomes found in the cells of an organism. Humans typically carry two sets of chromosomes (diploid)—one set inherited from our mother, the other from our father. A plant, however, can carry many more sets of chromosomes within its cells. Having three sets of chromosomes (triploid) often causes issues in reproduction due to the odd number of chromosomes that cannot segregate evenly during meiosis. To develop a triploid, a breeder must hybridize a diploid and a tetraploid (four sets of chromosomes). Tetraploids can be developed through the application of chemical mutagens known as mitotic spindle fiber inhibitors. (One such chemical is colchicine, a toxic compound found in the autumn crocus, Colchicum autumnale.) We currently have several tetraploid lacebark elms, but now we must wait for them to mature. Once these trees have reached maturity, we will hybridize them with diploids in our collections that are reasonably cold hardy.
Meanwhile, from the Ulmus crassifolia seed that Ware deposited in the Morton Arboretum collections in 1968, we have selected a tree with a remarkably symmetrical and pyramidal form that has survived severe winters and flooding events unscathed. We are building numbers of rooted cuttings to grow in evaluation blocks, inoculate with DED, and distribute to partners for evaluation around the country. Additionally, both this species and the lacebark elm are fall-flowering species. Considering they are wind-pollinated and not self-compatible, we have begun collecting open-pollinated seed from our cedar elm selection and an adjacent lacebark elm, and we plan to evaluate the resulting seedlings. According to a paper published by USDA researcher Frank Santamour in 1973, not only are the two species compatible, but the lacebark elm confers increased DED resistance to its hybrid progeny.
As Ware noted in his 1987 New York Times interview, the Morton Arboretum’s effort to develop new trees for the American landscape has been focused on traditional breeding efforts. These slow and steady methods have required several decades, spanning multiple careers. Yet, the value in Ware’s approach to breeding and outreach is evident in today’s nursery catalogs and landscape. Once there were monocultures of American elm planted across the country in the built landscapes of cities and suburbs, but today the monocultures have been replaced with DED-resistant Asian elms. This diversity includes more than thirteen cultivars of elms from the Ulmus davidiana complex, in addition to at least thirteen cultivars of U. parvifolia. Many more selections of various species are still in the pipeline from academic and commercial breeding programs around the country.
Through tenacity and vision, George Ware managed to inspire the nursery industry to adopt a new crop and introduce an unfamiliar Asian elm species to the North American landscape. The work has resulted in further diversification of our tree palette. It all began with the original Thornhill Elm, distributed to the Morton Arboretum by the Arnold Arboretum almost a century ago. Now, this very selection graces the landscape of the city of Boston, having come full circle in its journey from seed to cultivated tree.
Chicagoland Grows. 2017. The Morton Arboretum Elms. Plant Release Bulletin (no. 44). http://www.chicagolandgrows.org/downloads/MortonArboretumElms.pdf
King, S. S. 1971, September 7. Dutch elm disease spreads westward. New York Times, 45.
Malcolm, A. H. 1987, November 1. Fighting elm disease the natural way. New York Times, 9.
Myers, D. F. and G. A. Strobel. 1983. Pseudomonas syringae as a microbial antagonist of Ceratocystis ulmi in the apoplast of American elm. Transactions of the British Mycological Society, 80(3): 389–394. https://doi.org/10.1016/S0007-1536(83)80034-5
Santamour, F. S., Jr. 1973. Resistance to Dutch elm disease in Chinese elm hybrids. Plant Disease Report 57(12): 997–999.
Schneider, K. 1987, September 4. Tearful scientist halts gene test. New York Times, A1, A11.
Ware, G. H. 1980a. Little-known Asian elms: Urban tree possibilities. Journal of Arboriculture, 6: 197–199.
Ware, G. H. 1980b. In search of new kinds of elms. Journal of Arboriculture, 6: 233–237.
Ware, G. H. 1996a. Notes on elms observed on a trip to China. Landscape Plant News, 7(10): 4–6.
Ware, G. H. 1996b. New elms for urban landscapes. Landscape Plant News, 7(10): 6–8.
Kim Shearer is the tree and shrub breeder at the Morton Arboretum, where she is also manager of the Daniel P. Haerther Charitable Trust New Plant Development Program and woody plant liaison for Chicagoland Grows.
Citation: Shearer, K. 2021. George Ware and the Thornhill Elm: A vision of trees for the future. Arnoldia, 78(4): 38–47.