"From Arboretums to Your Garden"

For almost 75 years, the U.S. National Arboretum's Floral and Nursery Plants Research Unit (FNPRU) has been a leader in the development of new and improved floral and nursery products. The 446-acre arboretum's advances in genetic improvement and disease control of landscape plants and major cut flowers have, in no small part, contributed to the rapid growth of the floriculture crop sector of American agriculture—which had a $3.5 billion wholesale value in 1998. (See "Floral Gems," Agricultural Research, September 1997, pp. 8–13.)
FNPRU's research activities have helped spur the burgeoning floral and nursery crops industries in myriad ways. From their germplasm improvement, taxonomy studies, and development of virus- or pest-resistant plants to the creation of plant pathogen detection methods and genetic transformation technologies, the research unit's scientists continue to engage some of the horticulture industry's most enduring challenges.
"A 1996 horticulture research initiative spearheaded by ARS Administrator Floyd P. Horn has been tremendously beneficial to the industry," says Mary Ashby Pamplin. She previously directed horticultural research for the American Nursery and Landscape Association.
"Through labs like the FNPRU, ARS does something that we as an industry cannot; that is, conduct the long-term, high-risk, and costly basic research that forms the building blocks for the industry's own research programs," Pamplin says. "ARS is in a perfect position to coordinate research across disciplines at a very high level and to effectively communicate its findings."
This cooperation with industry informs much of what ARS scientists accomplish at FNPRU and at the more than 100 ARS laboratories across the country and abroad and provides researchers with an incentive to find solutions to difficult challenges.

Following are some research programs that highlight the depth and breadth of FNPRU's activities:
Ornamental Virology Over the last 10 years, the FNPRU has been engaged in detecting, identifying, and characterizing several viruses affecting ornamental crops. Among the tools scientists use are antibodies, electron microscopy, and nucleic acid hybridization. These approaches include developing control procedures to prevent or minimize virus transmission, finding reagents to quickly detect and screen viruses, and quarantine interception and epidemiology.
Using Genetics To Plum the Depths of Plant Diseases The plum pox virus (PPV) research program, led by plant pathologist John Hammond, has yielded sensitive detection techniques that not only confirm the presence of PPV—a disease that affects stone fruits like plums, peaches, and their ornamental relatives—but also distinguish between severe and less virulent strains of the disease and help determine their origins and modes of transmission. (See related story on p. 9.) To the $1.3 billion annual U.S. stone fruit industry, this kind of research is critical. Hammond uses a technique called the polymerase chain reaction (PCR), which multiplies the number of target molecules in a sample's nucleic acid. Having enough copies helps researchers detect any particular strain of PPV. "We've been able to develop PCR primers that initiate this process," says Hammond. "The process allows us to more readily detect all PPV strains." If scientists can detect different strains, it may help them find a correlation between a particular strain and its ability to infect a specific type of host plant.

New Redbuds and Lilacs Grace America's Gardens FNPRU's Margaret R. Pooler has released a new Chinese redbud cultivar named Don Egolf, a variety of Cercis chinensis whose profusion of rosy-purple flowers, compact structure, ease of propagation, seedlessness, and apparent high tolerance to Botryosphaeria dothidia canker have made it a welcome newcomer to nurseries across the country. Since 1994, cooperating nurseries throughout the eastern, southern, and midwestern United States have evaluated Don Egolf with high acclaim. Its ease of propagation by rooted cuttings is an especially valuable trait, because redbud cultivars are notoriously difficult to propagate. Because the cultivar is seed-sterile, it produces no fruit—enhancing the shrub's winter appearance.
In addition, Pooler's release of a new Syringa cultivar named Betsy Ross has provided the industry with a new lilac acclaimed for its fragrant white flowers, lush green foliage, compact growth habit, disease tolerance, and adaptation to warmer climates. One significant advance has been the new lilac's tolerance to powdery mildew—the biggest disease problem for lilacs in the Washington, D.C., area. The new shrub thrives under full sun and can be used as a background planting in a shrub border or as a specimen plant or hedge. It can be planted throughout USDA hardiness zones 5 to 7.

A Rose Is a Rose Is a Rose Researchers at FNPRU have also developed a new technique to enable transformation of genetically diverse varieties of roses—the number-one cut flower in the United States. One of the principal obstacles to the genetic engineering of roses has been an inability to develop a whole plant from genetically engineered cells. "This technique has already been applied to three rose varieties with great success," says Kathryn K. Kamo, a plant physiologist. The new method can potentially be used to genetically introduce traits such as resistance to black spot, along with quality traits, like scent, color, and heat tolerance.

Nightmare on Elm Street Is Over The tree-breeding program at FNPRU has successfully bred red maples with both good fall color and leafhopper resistance; elms with tolerance to Dutch elm disease and elm yellows; superior alder and hackberry; and hemlock with wooly adelgid resistance. New hemlock hybrids are being verified with the use of molecular markers.
Over the past 20 years, plant geneticist Denny Townsend has worked to develop the first commercially available elm varieties that are tolerant to Dutch elm disease. After their long-awaited arrival in wholesale nurseries in 1997 and retail nurseries in 1999, the American elm is well on its way to gracing our boulevards and backyards once again.
Novel Approaches to Plant Breeding Plant geneticist Robert J. Griesbach, with FNPRU, has been developing five new Ornithogalum hybrids. The best-known species of this bulbous plant is the Star-of-Bethlehem. These new hybrids—for which patent applications have been filed—introduce new colors and growth habits through interspecific breeding and embryo rescue techniques. The introduction of disease resistance is projected in the future through genetic engineering. According to Griesbach, he and colleagues have been able to develop whole plants from rescued embryos that have been germinated from immature seeds. "Under normal circumstances these embryos would die," he says. "The resulting plants have larger, fuller flowers and stronger, longer stems."