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Update on the Landscape Plant Development Center Pyrus Breeding Project: Promising Selections

Rita L. Hummel, Washington State University Puyallup Research and Extension Center

Abstract

A planting of 503 interspecific hybrids of the genus Pyrus is presently growing in the field at the Washington State University research center at Puyallup, WA. Pyrus was chosen because of the number of species representing diverse plant forms, foliage characteristics, and climatic adaptations with potential for recombination through controlled breeding. This project was initiated with the objective of developing varieties of small-statured pear trees with superior landscape characteristics that are tolerant of environmental and biological stresses.

Introduction

The genus Pyrus, a member of the subfamily Pomoideae in the Rosaceae, is generally considered to contain 22 primary species and at least 6 naturally occurring interspecific hybrids. Believed to have arisen in the mountainous regions of western China during the Tertiary period, the genus Pyrus is today indigenous to Europe, temperate Asia, and the mountainous regions of North Africa (Bell et al. 1996). Early humans recognized the value of Pyrus as a source of food and selection for improved fruit characters has been practiced for at least 3000 years. Today pears are second only to apples as the most important deciduous tree fruit crop in the world. The major species of commerce are P. communis L., the common European pear, and P. pyrifolia (Burm.)Nak., the Asian or Oriental pear.

In addition to its importance as a deciduous tree fruit crop, a number of valuable trees for landscape use have come from the genus Pyrus. Most notable of these are the cultivars of P. calleryana Decne. (Dirr 1990). ‘Bradford’ was introduced by the U.S. Department of Agriculture in the early 1960’s and has become the most widely planted of the P. calleryana cultivars. ‘Bradford’ has many fine landscape qualities including early spring flowering, outstanding fall color, and fireblight resistance. Unfortunately, as ‘Bradford’ aged, problems with severe splitting of the tight, upright branch crotches were encountered and newer cultivars have been introduced.

There is considerable genetic diversity in the genus Pyrus for climatic and edaphic adaptation, landscape traits, and disease and pest resistance. Although the gene pool available to fruit breeders is well documented (Bell et al. 1996), the potential of Pyrus for landscape use remains largely unexplored. Current evidence indicates that all species of Pyrus are diploid (2n=34, x=17) and no major interspecific crossing barriers appear to exist in the genus (Zielinski and Thompson 1967; Westwood and Bjornstad 1971; Bell and Hough 1986; Bell at al. 1996). Self-fertility is rare because Pyrus has a gametophytic incompatibility system that insures outcrossing (Crane and Lawrence 1952). Sterility of the interspecific hybrids is generally not a problem for Pyrus breeders (Zielenski and Thompson 1966, 1967; Bell at al. 1996).

Procedures

In 1990, a Landscape Plant Development Center (LPDC) technician collected open pollinated (OP) seeds from Melvin Westwood’s interspecific Pyrus hybrids growing at the National Clonal Germplasm Repository in Corvallis, Oregon. The first controlled crosses of selected Pyrus accessions in the Repository’s collection were made in 1991 (Pellett and Hunt 1992). Additional crosses were made in 1992. The Pyrus species involved in the crosses and their indigenous geographical distribution are as follows: P. amygdaliformis Vill. and P. elaeagrifolia Pall. have a circum-Mediterranean distribution; P. regellii Rehd. and P. salicifolia Pall. are mid-Asian species; P. betulaefolia Bunge, P. calleryana Decne., P. dimorphophylla Mak., P. fauriei Schneider, P. pyrifolia (Burm.) Nakai, and P. ussuriensis Maxim. are East Asian species; while P. nivalis Jacq. is of European origin (Bell et al. 1996). These species were chosen because they represent a diversity of tree forms, foliage characteristics and climatic adaptations with the potential for recombination through breeding.

OP seeds and seeds from Pyrus crosses made in 1991 and 1992 were sent to the Washington State University Research Center in Puyallup, WA where they were germinated and grown to transplantable size (Hummel et al. 1992). In November of 1993 and 1994 trees from these crosses were selected and transplanted to the field. A planting of 503 interspecific Pyrus hybrids is presently growing in the field at WSU Puyallup.

Data on growth, flower, fruit and foliage characteristics were first collected in 1995. Observable variation in the hybrids is extreme. Fall color has been excellent with leaf colors ranging from clear yellow to red and deep burgundy (Fig. 1). Colors of the new foliage in spring also vary with some trees having reddish or silvery new growth. Plant forms vary from narrow upright to broad spreading. Many trees have weeping or dwarf forms. About 40 promising individual trees have been selected and propagated by budding onto pear rootstock.

fall color in field of Pyrus hybrids

Figure 1. Fall color of interspecific Pyrus hybrids growing in the field at Puyallup, WA.
The photograph was taken from a bucket truck on October 27, 1999.

Promising Selections

Three of the 8 siblings from a four-species Pyrus cross with the interspecific F1 hybrid P. ussuriensis x P. calleryana as female and the interspecific P. betulifolia x P. pyrifolia as male were selected for additional testing. The progeny exhibit very diverse growth habits, one is an extreme dwarf, only about 1 meter tall after 5 ½ years in the field, while another is over 7 meters tall and seems destined to be a large tree. Two of the siblings selected for propagation are intermediate in size with a compact, globose form, medium fruit and attractive midsummer foliage.

All ten F1 hybrids from a cross of P. amygdaliformis x P. calleryana ‘Chanticleer’ are quite uniform. Two siblings selected for propagation have shiny, dark green leaves and small fruit. One is nearly columnar while the other has more of an oval form and attractive pink flower buds in the spring. Fall color ratings made the middle of October and the first of November in 1998 and 1999 indicated these trees had predominantly yellow foliage with a reddish tint. They did not develop color until the later rating time.

The cross P. elaeagrifolia x P. amygdaliformis has produced a dense, upright tree with attractive silvery-green leaves. New growth is covered with dense, white pubescence. Individual trees from the crosses P. elaeagrifolia x P. ussuriensis and P. elaeagrifolia x (P. ussuriensis x P. calleryana) also have good potential as small to medium sized trees. The species P. elaeagrifolia and P. amygdaliformis, although of little merit for the landscape due to coarse, spiny growth and poor plant form, are of interest because they are native to Asia Minor and Mediterranean Europe and may produce hybrids with good heat and drought tolerance

A selection from the cross P. calleryana ‘Chanticleer’ x P. elaeagrifolia is very promising (Fig. 2). This tree has attractive silver-green foliage, is upright with a moderately columnar form, has small fruit and, in our field planting, has shown no symptoms of leaf spot. Juvenile growth of this tree was very spiny but spine production has decreased with time and spines are not present on the current year’s growth. Fall color occurs later in the season and is red to reddish yellow. The quality of the fall color display has been extremely variable from year to year. This tree does not have showy flowers.

1

Figure 2. Two siblings of Pyrus calleryana 'Chanticleer' x P. elaeagrifolia were photographed in August of 2000. The height of the pole in the tree on the right is 4.6 meters. Both trees were planted from 3.8 liter nursery containers in November of 1994.

Insect and disease infestations have been allowed to develop naturally in the Pyrus planting. Outbreaks of pear leaf blister mite and pear slug have been observed. Pseudomonas blossom blast and dieback and Entomosporium leaf spot disease have also been detected on some of the trees. One disease that has not developed in Puyallup is fire blight, Erwinia amylovora. To develop, it requires moisture and a temperature of 65° F or higher when the plants are blooming. Puyallup’s climate is usually too cold at bloom time. The pathogen is in the Pacific Northwest because fire blight will occasionally occur. Because of the severity of the disease in other climates, the advanced Pyrus selections from the Puyallup planting will require screening for fire blight. Some of the selections have already been screened by Andrew C. Bell and Dr. Thomas G. Ranney at North Carolina State University.

Evaluation and selection of the Pyrus hybrids for superior landscape characteristics and environmental and biological stress tolerances is an ongoing project. In the future, we will continue to produce and distribute seed to LPDC cooperators, and characterize and select trees for environmental and biological tolerances and for landscape traits. Ultimately superior Pyrus trees for landscapes will be introduced.

Literature Cited

Bell, R.L. and L.F. Hough. 1986. Interspecific and intergeneric hybridization of Pyrus. HortSci. 21:62-64.

Bell, R.L., H.A. Quamme, R.E.C. Layne and R. M. Skirvin. 1996. Pears. p. 441-514. In: J. Janick and J.N. Moore (eds.). Fruit Breeding Vol. I Tree and tropical fruits. John Wiley & Sons, New York.

Crane, M.B. and W.J. Lawrence. 1952. The genetics of garden plants 4th ed. Macmillan, New York.

Dirr, M.A. 1998. Manual of woody landscape plants: Their identification, ornamental characteristics, culture, propagation and uses. Fifth Ed. Stipes Publishing Co. Champaign, IL.

Hummel, R. L., D.W. Privett, Harold Pellett and Lisa Hunt. 1992. Pyrus seed germination: Results from 1991 crosses. Landscape Plant News. 3:13-14.

Pellett, H. and L. Hunt. 1992. Success of 1991 Pyrus crosses. Landscape Plant News. 3:7-8.

Westwood, M.N. and H.O. Bjornstad. 1971. Some fruit characteristics of interspecific hybrids and extent of self-fertility in Pyrus. Bul. Torrey Bot. Club. 98:22-24.

Zielinski, Q.B. and M.M. Thompson. 1967. Speciation in Pyrus: Chromosome number and mitotic behavior. Bot. Gaz. 128:109-112.

Zielinski, Q.B. and M.M. Thompson. 1966. Pollen germination in Pyrus species and species hybrids. Euphytica. 15:195-198.


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