Skip to main content
ARS Home » Northeast Area » Washington, D.C. » National Arboretum » Floral and Nursery Plants Research » Research » Research Project #425048

Research Project: Evaluation and Genetic Improvement of Woody Ornamental Landscape Plants for Disease and Pest Tolerance, Non-Invasiveness, and Ornamental Traits

Location: Floral and Nursery Plants Research

2014 Annual Report

The overall goal of this project is to breed, evaluate, select, and release improved landscape trees and shrubs that are tolerant of biotic and abiotic stress, are not invasive, and have superior ornamental value. This goal is achieved through the following objectives: Objective 1. Characterize, evaluate, breed, select, and release improved woody landscape plant germplasm. Subobjective 1.a. Characterize germplasm and develop hybrids or breeding lines in genera currently under investigation, including Catalpa, Cercis, Corylopsis, Lagerstroemia, Nyssa, Prunus, Tsuga, and Ulmus; Subobjective 1.b. Propagate, evaluate (in-house and via cooperators), and release advanced selections developed in previous project cycles; and Subobjective 1.c. Initiate a breeding program in Buxus by collecting and characterizing germplasm for resistance to boxwood blight and making crosses to determine breeding parameters, and combining traits of interest. Objective 2. Develop non-invasive cultivars via wide hybridization and inter-ploid crosses to limit naturalization and gene-introgression into natural populations. Subobjective 2.a. Determine ploidy or genome size in genera that will be used for future interploid breeding work, including Acer, Buxus, Corylopsis, and Prunus, and develop methods for ploidy manipulation; and Subobjective 2.b. Conduct interploid and wide crosses in order to combine traits of interest and create sterile hybrids. Objective 3. Develop and apply molecular techniques to accelerate characterization, identification, selection, or evaluation of priority plant materials for key traits. This research will contribute to basic knowledge of breeding, ploidy, and genetics of several important woody genera, as well as new cultivars that are valuable to the nursery and landscape industries and that will enhance the diversity and sustainability of the urban and suburban landscape.

The approach to achieving the objectives of this Project will build upon previous work, but will also establish populations, knowledge, and germplasm that will be used for the next 10-20 years. Over the next five years, the long-term objective will be accomplished through the following general approach. Parental germplasm will be collected from native habitats, botanical repositories, and commercial sources, and will be evaluated for traits of interest. Controlled intra- and interspecific hybridizations will be carried out in the field or greenhouse to produce hybrid progeny, to determine compatibility among parents, and to study breeding systems and inheritance of traits of interest. As needed, molecular markers will be used in conjunction with classical taxonomy to determine genetic relationships among taxa and verify parentage of hybrids. Resultant progeny will be evaluated, and promising selections will be transplanted to the field. Selections developed during previous project cycles that have performed well will be propagated. These include elite clones of Catalpa, Deutzia, Ilex, Lagerstroemia, Magnolia, Platanus, Prunus, Syringa, and Tsuga, among others. These plants will be evaluated in replicated field trials and released if warranted. To initiate a new boxwood breeding program, germplasm will be collected through existing national and international collaborators. Accessions will be screened for ploidy, resistance to boxwood blight, and horticultural traits, and selected taxa will be used in hybridizations to create novel genotypes that combine disease resistance with quality ornamental and production traits. Accessions that will be used for ploidy manipulation will be screened to determine genome sizes and relative ploidy levels using flow cytometry. Chromosomes of select taxa will be doubled as needed, and plants will then be used to conduct appropriate inter-ploid crosses for taxa-specific breeding goals. Treated material will be tested for ploidy and then stabilized and propagated.

Progress Report
Our breeding programs continue to advance through controlled pollinations within our priority genera. First generation hybrids were intercrossed in Catalpa to produce segregating populations for novel flower and foliage color, powdery mildew tolerance, compactness, and improved habit. New test crosses were performed to determine breeding barriers between Catalpa species and Chilopsis and develop novel xChitalpa hybrids. More than 350 Catalpa hybrid seedlings from the prior year were planted in field nurseries for long term evaluation. Additional provenances of Ulmus americana were obtained from the National Arboretum taxonomy program for inclusion in our long term evaluation trials of American elm. We have selected our first triploid xChitalpa for propagation and distribution to Cooperators for evaluation. Novel Chinese fringetree (Chionanthus retusus) and Chinese pistache (Pistacia chinensis) were distributed for propagation and buildup to a commercial nursery for future release. Hemlock woolly adelgid-resistant hemlock (Tsuga) hybrid selections have been made, propagated, and distributed to Cooperators in 2013 and 2014. Two novel Viburnum selections, identified from the National Arboretum germplasm program, have been propagated for distribution to Cooperators in 2015. We have established a boxwood breeding program to develop boxwood blight resistance boxwood (Buxus) for the American nursery industry. Crosses were made to determine intra- and interspecific crossability and degree of selfing. A comprehensive ploidy screen was conducted on the National Boxwood Collection to determine ploidy levels of available germplasm. SSR markers were developed and screening of ~250 taxa was initiated in order to determine genetic relationships and answer questions about identity of some taxa. In vitro screening assays using detached leaves or stems were developed in order to efficiently screen taxa and hybrids for resistance to blight. With a cooperator at Ohio University, SSR markers were developed to evaluate Corylopsis species and cultivars for genetic diversity and identity. This information will be combined with previously determined ploidy data to develop a complete picture of the genetics, identity, and diversity of Corylopsis taxa.

1. Release of a new pink-flowering Deutzia hybrid ‘Nikko Blush’. The introduction of new woody ornamental plants is one of the driving forces behind the sustained growth of the ornamental horticulture industry. Researchers in Washington, DC released a new cultivar of pink-flowering Deutzia named ‘Nikko Blush’. This cultivar was selected for its abundant pink-blushed flowers and flower buds that appear in late spring contrasting with medium-green foliage on a low-growing mounded shrub. It is one of the few pink-flowering Deutzias on the market, and is well-suited as a flowering groundcover, massed on a wall or terrace, in the front of a shrub border, as a low-growing hedge, or as a container plant. It transplants readily and withstands pruning to rejuvenate or shape as needed, so will be valuable to growers, landscapers, and the gardening public.

2. Identification of historic and novel gene pools within Chinese fringe trees. Chinese fringe tree (Chionanthus retusus) is an increasingly popular flowering tree valued for its wide adaptability across North American landscapes; however, little was known about the diversity and origin of material in the nursery industry. Molecular markers developed by ARS researchers at Poplarville and Stoneville, Mississippi and Washington, D.C. were used to determine relationships among cultivated Chinese fringe trees in the United States. Commercially available material originated from three sources, the majority from northeastern China, followed by Taiwan, and Japan. Additional novel material from China and Korea are available in botanical collections, with the U.S. National Arboretum housing the most diverse and complete collection. For the first time, morphology and performance of cultivated Chinese fringe trees are linked with their geographical origin which allows researchers to more effectively collect and distribute adaptable material to the nursery industry.

Review Publications
Pooler, M.R. 2013. 'Helen Taft' and 'Abigail Adams' flowering cherries. HortScience. 48(9):1195-1196.