Research Horticulturist

Professional Biographical Information:

Ph.D. in Plant Breeding and Genetics (1978), Department of Horticulture, Purdue University

M.S. in Plant Breeding and Genetics (1975), Department of Horticulture, Purdue University

B.A. in Biology (1969), Indiana University

1980-present: Research Horticulturist, USDA-ARS, Appalachian Fruit Research Station, Kearneysville, West Virginia

1978-1980: Associate Horticulturist, Department of Horticulture, University of Illinois, Urbana, Illinois

1974-1978: Graduate Research Assistant, Department of Horticulture, Purdue University, West Lafayette, Indiana

Description of Research Projects:

The long-term objective of the USDA pear improvement program is the development of genetic solutions to the major problems affecting pear production and consumer acceptance.  The program seeks to develop new and improved pear cultivars which combine excellent fruit quality, high productivity, precocious fruit bearing, and long postharvest storage life, and which have improved levels of resistance to the major diseases and insect pests.  Current research and breeding focuses on resistance to fire blight, resistance to pear psylla, and development of productive dwarf pear trees.  Projected impacts include (1) enhancing the economic returns to commercial orchardists by providing a high quality product and reducing production costs due to pesticide usage, (2) providing home orchardists with high quality pear cultivars which can be grown with fewer or no pesticides, and (3) providing consumers with high quality fruit with improved quality and ripening characteristics.  The breeding program seeks to utilize the broad genetic resources of pear available worldwide.  The research program characterizes these genetic resources for important traits and determines their genetic control in order to more effectively develop new cultivars.  Traditional breeding, molecular genetic mapping, as well as genetic transformation is utilized for appropriate traits.

Current research and breeding goals include:

Breeding new fire blight resistant pear cultivars

Goal:   Develop new cultivars and germplasm of pear with resistance to fire blight. 

Background:    The major varieties of pear grown in the U.S. are highly susceptible to fire blight, caused by the bacterium Erwinia amylovora.  Fire blight is the most severe disease affecting pear worldwide.  Blossoms, actively growing shoots, and immature fruits are most readily infected, but the trunks and roots may become infected as well, resulting loss of fruit, branches or entire trees.  The disease is a constant threat, even in the climatically favorable production areas of the Pacific Northwest.  It is difficult to control even when all recommended practices are followed.   Breeding for the west European pear (Pyrus communis L.) in the USDA and other programs has been based primarily on   resistance derived from ‘Seckel’, ‘Old Home’, ‘Maxine’, and a few other P. communis cultivars.  Limited use has been made of germplasm from the Asian species, P. pyrifolia and P. ussuriensis.   Prior cultivar releases include ‘Magness’, ‘Moonglow’, and ‘Dawn’ (all released in 1960), ‘Potomac’ (1992), ‘Blake’s Pride’ (1998), and ‘Shenandoah’ (2003).  

Approach: Hybridizations utilizing new and diverse germplasm will be incorporated into the program. European-type buttery flesh pears for the fresh market are the primary focus of the breeding program.   A broad range in seasons of maturity and flavor types are being sought, but with added emphasis on dual-purpose cultivars with ‘Bartlett’-type flavor and processing characteristics, and “winter” pears with improved post-harvest storage life and consistent ripening.  In addition, crisp-fleshed European (P. communis) x Asian (P. pyrifolia, P. ussuriensis, and P. xbretschneideri) hybrids, which can be eaten without softening are being evaluated. These are particularly interesting because of the possibility of multiple disease and insect resistance, including resistance to pear scab (Venturia pirina), Fabraea leaf spot (Fabraea maculata), and the pear psylla (Cacopsylla pyricola).  Crosses are made according to diallel or test cross designs to facilitate genetic studies for resistance and fruit traits.  The long term breeding strategy is recurrent mass selection in a population derived from parents containing all necessary traits.  Advanced selections are tested by cooperators located in all major pear growing regions.  

Collaborators:  Jay Freer, Cornell University; Rachel Elkins, University of California Extension-Lake County; Ed Fackler, Rocky Meadow Nursery; Stephen Castagnoli and David Sugar, Oregon State University; James Nugent and William Shane, Michigan State University; Greg Reighard, Clemson University; Bob Gix, Blue Star Growers; Diane Miller, Ohio State University.

USDA-ARS Pear Cultivar Fact Sheet

Breeding for host resistance to pear psylla

Goal:  Develop new cultivars and germplasm of pear with resistance to pear psylla.

Background:  The pear psylla (Cacopsylla spp.) is a major pest of European pear (Pyrus communis) in orchards in most temperate regions where the crop is grown.  In North America, the only species present is Cacopsylla pyricola.  While some progress in biological control and integrated pest management, including the development of more selective pesticides, has been made, the level of control necessary still requires the use of substantial amounts of insecticide, at considerable cost to producers.  The pest has rapidly developed resistance to many classes of pesticides. Thus, development of cultivars with resistance has become a priority in breeding programs in North America and Europe.

Host plant resistance has been characterized in the East Asian pear species, P. ussuriensis and P. xbretschneideri. and in P. ussuriensis x P. communis hybrids.  Within P. communis, we have previously identified 15 old European cultivars with high levels of resistance.  Ovipositional antixenosis, nymphal antibiosis, and feeding antixenosis have been identified as modes of host plant. 

Approach: The two breeding populations currently utilized consist of (1) second and third generation backcross hybrids between P. communis and the Chinese species P. ussuriensis  (primarily Illinois 65 and Illinois 76), as well as (2) first generation hybrids between east and west European P. communis germplasm.  Additional third and fourth generation modified Asian x European backcrosses (to non-recurrent P. communis parents) and intercrosses have been made.  Resistance to pear psylla in seedling selections will be verified using bioassays for nymphal feeding antixenosis, ovipositional antixenosis, and antibiosis.   The general breeding strategy is based upon a combination of modified “backcrosses” to unrelated parents with high fruit quality and intercrosses to increase recombination.

Genetics of Host Resistance to Pear Psylla

Goal:  Determine the mode of inheritance of host resistance to pear psylla in pear seedling populations

Background:  Resistance in P. ussuriensis x P. communis populations has been characterized as quantitative, polygenic, and moderately heritable, and positively correlated with large fruit size.  Preliminary data on nymphal feeding from our program with P. ussuriensis-derived populations confirms the polygenic nature.  The mode of inheritance of resistance derived from East European pear cultivars (P. communis) has not been characterized.   Knowledge of whether the inheritance is due largely to one (monogenic) or a few major genes (oligogenic), or to many genes (polygenic), and the nature of gene action is important in designing an appropriate breeding strategy.  

Approach: Psylla-resistant east European pear cultivars have been hybridized in most combinations with susceptible parents.  Nymphal feeding antixenosis of seedlings from each cross will be assayed, and the data analyzed to determine heritability and general and specific combining ability of the parents.  In addition, association of molecular genetic markers to genes determining resistance will be investigated.  Such markers could then be used to estimate the number and relative importance of the genes, and used as indirect selection criterion for resistance, that is, in marker-assisted selection.

Genetic transformation for fire blight resistance

Goal: Determine their efficacy of selected lytic peptide genes in enhancing resistance to fire blight in major pear cultivars.

Background: Lytic peptide genes exhibit antimicrobial activity by disrupting the cell membranes of bacteria and fungi, causing cell lysis and death.  Various lytic peptides occur in plants, insects, and higher animals, and synthetic peptides have also been designed.  Naturally occurring peptides such as attacin and cecropin, as well as synthetic cecropins have been shown to confer moderate enhancement of resistance to fire blight in apple and resistance of other crops to some of their most serious diseases.   

Approach:  Transgenic clones of the fire blight-susceptible ‘Bartlett’ pear cultivar have been produced and selected in our laboratory.  One set of clones was transformed with the coding sequence of a synthetic lytic peptide gene with the pea vacillin signal peptide is driven by ubi7 promoter which contains an intron and the first ubiquitin‑coding region. Three transgenic clones containing the same gene and promoter, but without the signal peptide, have also been produced.  Plants of all clones will be rooted, acclimated to the greenhouse, and tested for their reaction to artificial inoculation with a virulent isolate of Erwinia amylovora . 

Development of Transgenic Dwarfing of Pear 

Goal:  Determine the potential of growth regulating genes to produce dwarfed, but precocious and productive, pear trees, either as the scion or as the rootstock.

Background:    Efficient production of pears is limited by large tree size and the long period of time necessary for newly planted trees to produce economically rewarding yield.  Genetically dwarfed scion cultivars have not been commercially introduced.  Pear rootstocks that would result in the degree of tree size control and precocious, high fruit yields in a manner similar to that achieved in apples have not been widely introduced from breeding programs.  Various genes have been found to have an effect on plant growth habit.  Genes from Agrobacterium rhizogenes, rolA, and rolC, have each shown effects on growth habit in woody and herbaceous plants, primarily reduced internode length.  We have previously transformed ‘Beurre Bosc’ pear with the rolC gene and produced transgenic plants which display an extreme dwarf phenotype.  Research in Europe has demonstrated that the rolA gene resulted in reduced height and shortened internodes when transformed into an apple rootstock, and one rolA clone induced shorter internodes.   Other growth regulating genes may have the potential to produce productive, dwarfed trees when transformed into major pear scion cultivars or rootstocks.

Approach:   Pear cultivars expressing the rolC gene and other growth regulating genes will be produced in order to evaluate more fully the effect of this gene on pear tree growth.  Transgenic pears will be evaluated on their own roots, as scions budded onto clonal pear rootstock, and as rootstocks with ‘Beurre Bosc’ as the scion.  Growth measurements in the greenhouse will include shoot length, lateral branch production, internode length, leaf size, leaf number, stem diameter and timing of active growth periods.