US National Arboretum


Floral and Nursery Plants Research Unit

Dr. Denny Townsend
Research Geneticist
{Glenn Dale, MD}

CRIS Project Title:   Genetic improvement of landscape trees for superior pest resistance.

INTRODUCTION:  The majority of trees planted in urban landscapes have not been selected for resistance to insect or disease pests or tolerance to environmental stresses. As a result, these trees suffer from poor survival and adaptability, engender high maintenance costs, and require the application of potentially toxic pesticides for optimum development. There is a great need for the development of insect- and disease-resistant and stress-tolerant cultivars of trees for use in American landscapes.

The overall objective of this project is to select, breed, evaluate, and release genetically-improved landscape trees which are disease- and pest-resistant, tolerant of environmental stresses, horticulturally superior, and not invasive. This objective will involve identifying and evaluating genetic variation in resistance to biotic and abiotic stresses in seedling populations and selections of elms, hackberries, hemlocks, maples, alders, and tupelos; making controlled crosses in order to develop new hybrid progenies with disease- and insect-resistance; using genetic markers to verify hybridity and assess genetic diversity; and evaluating clonal selections for horticultural desirability and geographic and climatic adaptability. Exceptional clones with superior ornamental qualities combined with disease- and pest-resistance will be named and released to nurserymen.

PROGRESS:   Released five new insect-tolerant red maple (Acer rubrum) cultivars, 'Brandywine', 'Somerset', 'Sun Valley', 'New World', and 'Red Rocket'. Also released the first two American elm (Ulmus americana) cultivars tolerant to Dutch elm disease, 'Valley Forge' and 'New Harmony'. Three clones of European alder (Alnus glutinosa) were selected for cold hardiness, leafminer tolerance, and rapid growth rate; propagated; and shipped to nursery cooperators nationwide for evaluation. Controlled pollinations were made between five hemlock species (Tsuga) species from eastern North America and Asia. Resulting seeds were collected and planted, and over 5700 seedlings germinated. A selected number of putative hybrids from each cross were selected for authenticity by DNA fingerprinting. Although attempts to hybridize eastern hemlock (T. canadensis) with 3 Asiatic species were unsuccessful, 59 authentic hybrids from crosses between Carolina hemlock (T. caroliniana) and Chinese hemlock (T. chinensis) were identified. Crosses between the Asiatic species also were successful. Selfing was demonstrated in all species. Fundamental information of this type may enable plant breeders to successfully hybridize woolly adelgid-tolerant species with the susceptible native hemlock species to create new trees that are both tolerant to the adelgid and horticulturally desirable. With elms, we recently inoculated 20 clones (each replicated 28 times) of American elm (Ulmus americana) with the fungi (Ophiostoma ulmi and Ophiostoma novo-ulmi) which cause Dutch elm disease. Data on foliar symptoms 4 weeks after inoculation were recorded.

PLANS:    Record crown die-back data in large American elm test, those clones inoculated with Ophiostoma in 2002. Complete outplanting of verified hemlock and hackberry hybrids. Begin hemlock study to determine the nature of the crossability barrier that prevents eastern hemlock from crossing with Chinese hemlock. Collect data on tupelo progenies (F1) outplanted in replicated field tests; and use AFLP techniques to identify advanced generation (F2), interspecific tupelo seedlings. Make controlled crosses between superior, non-American elm clones. Propagate superior clones of alder, elm, hornbeam, maple, and zelkova for future distribution to nursery cooperators.


1994. Townsend, A.M., and L. W. Douglass. Variation among Alnus progenies grown in Ohio. J. Arboric. 20: 165-169.

1995. Townsend, A. M., R. W. Hall, and W. O. Masters. 'Patriot' elm. J. Environ. Hort. 13: 82-85.

1995. Wilkins, L. C., W. R. Graves, and A. M. Townsend. Development of plants from single-node cuttings differs among cultivars of red maple and Freeman maple. HortScience 30: 360-362.

1995. Wilkins, L. C., W. R. Graves, and A. M. Townsend. Responses to high root-zone temperature among cultivars of red maple and Freeman maple. J. Environ. Hort. 13: 82-85.

1995. Townsend, A. M., S. E. Bentz, and G. R. Johnson. Variation in response of selected American elm clones to Ophiostoma ulmi. J. Environ. Hort. 13: 126-128.

1996.  Townsend, A. M., and L. W. Douglass.  Variation in growth and response to Ophiostoma ulmi among advanced-generation progenies and clones of elms. J. Environ.  Hort. 14: 150-154.

1997.  Zhang, H., W. R. Graves, and A. M. Townsend.  Water loss and survival of stem cuttings of  two maple cultivars held in subirrigated rooting medium at 24 to 33 C.  HortScience 32: 129-131.

1997.  Bentz, J. A., and A. M. Townsend.  Variation in adult populations of the potato leafhopper (Homoptera: Cicadellidae) and feeding injury among clones of red maple.  Environ. Entomology 26: 1091-1095.  1997.

1998.  Zwack, J. A., W. R. Graves, and A. M. Townsend.  Leaf water relations and plant development of three Freeman maple cultivars subjected to drought.  J. Amer. Soc. Hort. 123: 371-375.

1998.  Townsend, A. M., and L. W. Douglass.   Evaluation of various traits of 40 selections and cultivars of red maple and Freeman maple growing in Maryland. J. Environ. Hort. 16: 189-194.  1998.
1999.  Bentz, J. A., and A. M. Townsend.  Feeding injury, oviposition, and nymphal survivorship of the potato leafhopper on red maple and Freeman maple clones.  Environmental Entomol. 28: 456-460.  1999.

1999.  Zwack, J. A., W. R. Graves, and A. M. Townsend.  Variation among red and Freeman maples in response to drought and flooding.  HortScience 34: 664-668.

2000.  Townsend, A. M.  USDA Genetic Research on Elms.  Pages 271-278 in (C. L. Dunn, Ed.): The Elms: Breeding, Conservation, and Disease Management. Kluwer Academic Publishers, Norwell, MA.

2000. Sinclair, W. A., A. M. Townsend, H. M. Griffiths, and T. H. Whitlow. 2000. Responses of six Eurasian Ulmus cultivars to a North American elm yellows phytoplasma. Plant Dis. 84: 1266-1270.

2001. Bentz, J. A., and A. M. Townsend. 2001. Leaf element content and utilization of maple and elm as hosts by the potato leafhopper (Homoptera: Cicadellidae). 2001. Environ. Entomol. 30: 533-539.

2001. Sinclair, W. A., A. M. Townsend, and J. L. Sherald. 2001. Elm yellows phytoplasma is lethal to Dutch elm disease-resistant Ulmus americana cultivars. Plant Dis. 85: 560.

2001. Townsend, A. M., and L. W. Douglass. 2001. Variation among American elm clones in long-term dieback, growth, and survival following Ophiostoma inoculation. J. Environ. Hort. 19: 100-103.

2002. Bentz, S. E., L. G. H. Riedel, M. R. Pooler, and A. M. Townsend. 2002. Hybridization and self-compatibility in controlled pollinations of eastern North American and Asian hemlock (Tsuga) species. J. Arboric. 28: 201-206.

2002. Pooler, M. R., L. G. H. Riedel, S. E. Bentz, and A. M. Townsend. 2002. Molecular markers used to verify interspecific hybridization between hemlock (Tsuga) species. J. Amer. Soc. Hort. Sci. 127: 623-627.

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