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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

2018 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
This is the final report for Project 8020-21000-061-00D, which ended on 2/13/2018 and was replaced by 8020-21000-071-00D. Significant progress was made on all three objectives, which fall under NP301. Field planting and evaluation of advanced selections of hybrids created in previous years continued. This included new plantings of Catalpa and Lagerstroemia, and evaluation of Nyssa, Prunus, Tsuga, and Corylopsis. Hundreds of new crosses were made in Buxus, focusing on germplasm identified as more disease tolerant. Hybrids were also attempted in Catalpa, Chilopsis, and xChitalpa, with limited success. Buxus seed and hybrids from previous years’ crosses were stratified, germinated, tested for disease tolerance (using previously developed lab assays), or prepared for field planting. Accessions of several genera are being propagated for in-house testing, distribution, or multiplication, including Catalpa, Tsuga, Buxus, Ulmus, Lagerstroemia, and Prunus. Hemlock advanced hybrids were sent to cooperators for testing under a Plant Evaluation Agreement. Our evaluation of the genetic relationships of the USNA boxwood collection was completed in 2017; we are evaluating at least 50 additional selections and species that we have obtained since then, and will update the phenogram to include all accessions. We have improved our lab-based disease resistance screening assay and are working on a collaborative project with industry and university scientists to screen and rank disease resistance among the most important cultivars in the industry. Leaf samples of a diverse collection of Corylopsis species and hybrids were collected and evaluated for ploidy and for genetic relationships using SSR primers developed in our lab. We are also using SSR markers to determine the identity of flowering cherry selections that were mixed up in a stakeholder’s field.

1. Resistance to cherry leaf spot in ornamental flowering cherries. Ornamental flowering cherry trees are popular plants for street, commercial, and residential landscapes, with an annual value of over $33 million in the U.S. However, they are prone to a fungal disease called cherry leaf spot that causes susceptible trees to lose their leaves in late summer, leading to weakening or even death of the tree. In order to identify resistant plants for use in landscape plantings and in our breeding program, ARS scientists in Beltsville, Maryland, developed a laboratory assay to screen diverse ornamental flowering cherry trees for resistance to cherry leaf spot. We found clear differences in susceptibility among the accessions, with seven accessions developing virtually no symptoms at all. We developed leaf assay which is an effective method for screening large numbers of plants for relative resistance to cherry leaf spot.

Review Publications
Guo, Y., Kramer, M.H., Pooler, M.R. 2018. Screening ornamental cherry (Prunus) taxa for resistance to infection by Blumeriella jaapii. HortScience. 53:200-203.
Kidwell Slak, D.L., Pooler, M.R. 2018. A checklist of Cercis (Redbud) cultivars. HortScience. 53:148-152.