Location: Floral and Nursery Plants Research2016 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 was made on all three objectives and their subobjectives, although progress was significantly affected by a critical scientist vacancy that has existed since before May 2015. Under Objective 1, we performed interspecific hybridizations in Buxus, Catalpa, Chilopsis, Nyssa, Prunus, Cercis, Corylopis, and Syringa; propagated Buxus, Catalpa, Chilopsis, Nyssa, Ulmus, Cercis, Prunus, Syringa, Halesia, and Viburnum (seed, cutting, or grafting); and planted out advanced selections for evaluation. We also continued work on breeding and evaluating boxwood, including developing parameters for in-vitro screening assays for determining blight resistance. In collaborative work with ARS scientists in Kearnyesville, West Virginia, we established explants of diverse woody ornamental genera in tissue culture in order to examine the possibility of using gene manipulation to alter plant architecture (dwarf, pillar, weeping). Under Objective 2, we performed interploid crosses in Prunus and Corylopsis, and completed evaluation of ploidy of boxwood and Catalpa. We also evaluated and confirmed tetraploid plants of Catalpa species created in vitro. Under Objective 3, we completed our study of genetic diversity of Asian Cercis taxa using SSR markers, and determined the diversity of populations of native Magnolia ashei using SSR markers to guide conservation efforts.
1. Effective bioassays for assessing resistance to boxwood blight. Each year, more than 13 million boxwood plants are sold in the U.S., making boxwood one of the most popular shrubs in the landscape. A new blight disease, caused by the fungus Calonectria pseudonaviculata, threatens growers and landscapes throughout the U.S. Developing disease resistant cultivars of boxwood is the only long-term sustainable solution to this problem. ARS scientists at the U.S. National Arboretum in Washington, District of Columbia developed a bioassay using detached leaves that can be used to quickly and effectively screen boxwood taxa for resistance to boxwood blight. The assay uses fungal mycelium instead of spores, so can be readily scaled up for multiple assays. It is an efficient tool for breeders and pathologists to rapidly evaluate large numbers of boxwood genotypes for blight resistance.