Location: Floral and Nursery Plants Research2015 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, all of which fall under NP 301. Under Objective 1, we initiated field trials of advanced selections of flowering cherry, hemlock, and chitalpa, and propagated selections to be sent for evaluation by nursery cooperators. We continued intergeneric crosses in Chilopsis and Catalpa, as well as crosses within Nyssa sylvatica. We continued the breeding program in boxwood (Buxus) by amassing germplasm and characterizing the entire collection using SSR markers, as well as making crosses with select taxa and growing out seedlings from previous years. In addition, we finalized parameters of an in-vitro screening assay using detached leaves or stems to efficiently screen taxa and hybrids for resistance to boxwood blight. Under Objective 2, we continued ploidy studies on Buxus and Prunus, and established in vitro cultures of these genera to facilitate ploidy manipulation. We also examined ploidy in in-vitro cultures of Catalpa. Under Objective 3, we tested AFLP markers in Hedera, Fatsia, and their hybrids in order to determine genetic identity of several accessions. We also developed SSR markers in Magnolia ashei to determine diversity in native populations to guide conservation efforts. We used previously developed SSR markers in Cercis to characterize the relatedness of the Asian taxa at the U.S. National Arboretum, including accessions used for breeding.
1. Molecular characterization of boxwood cultivars. Each year, more than 13 million boxwood plants are sold in the United States (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. ARS scientists at the U.S. National Arboretum, Washington, District of Columbia, developed molecular markers and used them to assess the genetic diversity of boxwood taxa to facilitate breeding aimed at creating disease resistant boxwood plants. The molecular makers and the information derived from them will be useful in cultivar identification and genetic conservation, as well as guiding breeding strategies.
Guo, Y., Olsen, R.T., Kramer, M.H., Pooler, M.R. 2015. Effective bioassays for evaluating boxwood blight (Calonectria pseudonaviculata) susceptibility using detached stem inoculations. HortScience. 50(2):268-271.
Thammina, C., Olsen, R.T., Wight-Malapi, M., Crouch, J., Pooler, M.R. 2014. Development of polymorphic genic-SSR markers by cDNA library sequencing in boxwood, Buxus spp. (Buxaceae). Applications in Plant Sciences. 9(12):1400095.