Pears

Abstract

The most important commercial pear species grown are Pyrus communisand P. pyrifolia, although there are significant acreages of several other species. Climatic adaptation is a concern of all pear breeders, as well as fruit quality, season extension, compatibility with major pollenizer cultivars and disease resistance. Increasing fire blight resistance is an important goal in the eastern and southern parts of North America, and many regions of Europe. In the breeding of pear rootstocks, the common goal is to develop rootstocks that induce size control and precocity in the scion cultivar. Resistance to fire blight is quantitatively inherited in an additive fashion, with a few major genes playing an important role. Genes have also been identified for semidwarf or compact cultivars, short-internode dwarfs and short internode compact pears. Varying amounts are known about the genetics of fruit development and quality. Molecular studies have been conducted on the expression patterns of genes during fruit ripening and storage. Several genetic maps have been developed of pear and DNA markers have been linked to a number of resistance genes including black spot disease, scab and fire blight resistance. Transformation strategies have been employed to generate herbicide and disease resistant plants.

Full text

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Chapter 10
Pears
J. F. Hancock 1, G. A. Lobos 2
1 Department of Horticulture, 342C Plant and Soil Sciences Building, Michigan State
University, East Lansing, Michigan 48824, U.S.A. 2 Laboratorio de Ecofisiologia Vegetal,
Escuela de Agronomia, Universidad de Talca, Chile.
E-mail: hancock@msu.edu
Abstract
The most important commercial pear species grown are Pyrus communis and P. pyrifolia, although
there are significant acreages of several other species. Climatic adaptation is a concern of all pear
breeders, as well as fruit quality, season extension, compatibility with major pollenizer cultivars and
disease resistance. Increasing fire blight resistance is an important goal in the eastern and southern
parts of North America, and many regions of Europe. In the breeding of pear rootstocks, the common
goal is to develop rootstocks that induce size control and precocity in the scion cultivar. Resistance to
fire blight is quantitatively inherited in an additive fashion, with a few major genes playing an important
role. Genes have also been identified for semidwarf or compact cultivars, short-internode dwarfs and
short internode compact pears. Varying amounts are known about the genetics of fruit development
and quality. Molecular studies have been conducted on the expression patterns of genes during fruit
ripening and storage. Several genetic maps have been developed of pear and DNA markers have
been linked to a number of resistance genes including black spot disease, scab and fire blight
resistance. Transformation strategies have been employed to generate herbicide and disease
resistant plants.
10.1 Introduction
Pears are grown in all temperate regions of the world. Culivars of the European pear, Pyrus
communis, predominate in Europe, North America, South America, Africa and Australia, and
the sand or Japanese pear, P. pyrifolia, is the main cultivated species in southern and central
China, Japan and Southeast Asia. Other pears grown widely in Asia include P. ussuriensis
(Ussuri pear) and hybrids of P. pyrifolia and P. ussuriensis. Interest in Asian pears, primarily
Japanese cultivars, continues to increase in Western Europe, North America, New Zealand
and Australia, but the European pear has made little impact in Asia, except in Northern Japan,
where most Asian pears do not have sufficient winter hardiness. Pears are eaten fresh,
cooked, dried or made into a fermented cider-like beverage called “perry”. They are also
processed as canned halves, diced pieces for fruit cocktail and as puree for baby food.
The world production of pears is second only to apples among the deciduous tree fruits.
About 19.2 million metric tons of pears were produced in 2005 (FAOSTAT, 2007), up over 2
million tons from 2004. Asia produced the most pears (13.5 million t), followed by Europe
(3.2 million t), South America (0.8 million t), North America (0.8 million t), Africa (0.7
million t), and Oceania (0.2 million t). In Asia, China was the largest producer with 60% of
the world volume, followed by Japan (2.1%) and Republic of Korea (2.0%). The major
producers in Europe were Italy (4.9%), Spain (3.4%) and France (1.2%). In South America,
the highest producers were Argentina (2.7%) and Chile (1.1%). In North America, the

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leading producer by far was the U.S.A. (3.9%), where the main state is Washington with 50
percent of the countries volume (California and Oregon complete the other half). In Africa,
the major producer was the Republic of South Africa (1.8%). Production in Oceania was
highest in Australia (0.8%).
Pyrus is genetically quite diverse, with considerable variability in morphology and
physiological adaptations (Figure 10.1) (Knight 1963, Westwood 1982, Lombard and
Westwood 1987, Bell et al. 1996). European and Asian pear breeders have utilized this
variability to develop high quality cultivars with large size and attractive appearance that are
well adapted to local conditions. The European pears are distinguished by their juiciness,
delicate flavor and aroma, while the Oriental (Asian or nashi) pears are known for their
crispness and sweet flavor. North American breeders have had to focus more on disease
resistance and cold hardiness than the Europeans, although the spread of the bacterial disease
fire blight [Erwinia amylovora (Burrill) Winslow et al.] throughout Europe is forcing the
breeders there to concentrate more on disease resistance (Bell et al. 1996).
Figure 10.1. Diversity in fruit of Pyrus species (Picture by Joseph Postman, USDA-ARS
National Clonal Repository in Corvallis, Oregon.
10.2 Evolutionary biology and germplasm resources
The genus Pyrus is in the subfamily Pomoideae of the Rosaceae. All species of Pyrus are
diploid and interfertile (Westwood and Bjornstad 1971, Bell and Hough 1986). The genus
contains 24 primary species (Table 10.1), up to six natural interspecific hybrids and at least
three artificial hybrids (Bell et al. 1996). The species of Pyrus are located in Europe,
temperate Asia and the mountains of North Africa. The species boundaries are blurred in
many instances, resulting in different species designations by some authorities.
Pomoideae are unique in the Rosaceae by having a basic chromosome number of 17
compared to 7 to 9 for the other subfamilies. The origin of the subfamily may have occurred
when two primitive forms of Rosaceae successfully hybridized, one having a basic
chromosome number of 8 and the other 9 (Sax 1931, Zielinski and Thompson 1967). These
could have been members of the Prunoideae and Spiraeoideae. All the species of Pyrus have
a chromosome number of 2x = 34, except for a few higher polyploid cultivars of P.

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communis. The genus Pyrus probably arose during the Tertiary period in the mountains of
western China and the early taxa likely dispersed east and west through the mountain chains.
Speciation may have been associated with geographical isolation of populations in the
mountain ranges (Rubtsov 1944, Zeven and Zhukovsky 1975).
Table 10.1 Pyrus species of the world (adapted from Bell et al. 1996).
Scientific name Synonyms Distribution
European
P. communis L. P. asiae-mediae Popov
P. balansae Decne.
P. boissieriana Buhse
P. caucasica Fed
P. elata Rubtzov
P. medvedevii Rubtzov
West to southeast Europe,
Turkey
P. korshinskyi Litv. P. pyraster (L.) Burgsd. South central Asia, Afghanistan
P. nivalis Jacq. West, Central and Southern
Europe
P.cordata Desv. S.W. England, W. France,
Spain, Portugal
P. ×salviifolia DC Europe, Crimea
Circum-Mediterranean
P. amygdaliformis Vill P. sinaica Dum. Cours. Mediterranean Europe, Asia
minor
P. complexa Rubtzov Caucasus
P. elaeagrifolia Pall. P. kotschyana Boiss. ex Decne. S.E. Europe, Russia, Turkey
P. syriaca Boiss. Tunisia
P. longipes Coss. & Dur. Algeria
P. gharbiana Trab. Morocco, W. Algeria
P. mamorensis Trab. Morocco
Mid-Asian
P. glabra Boiss. Iran
P. salicifolia Pall. N.W. Iran, N.E. Turkey, South
Russia
P. regelii Rehd. P. heterophylla Regal & Schmalh. South Central Asia
P. pashia Buch.-Ham. ex D. Don. P. kumaoni Decne.
P. variolosa Wall. ex G. Don.
P. wilhelmii C.K. Schneid.
Pakistan, India, Nepal
East Asian
P. ×bretschneideri Rehd. Northern China
P. ×phaeocarpa Rehd. Northern China
P. pyrifolia (Burm.)Nak P. serotina Rehd. China, Japan, Korea
P. ×serrulata Rehd. Central China
P. pseudopashia T.T. Yu P. kansuensis Batalin N.W. China
P. ussuriensis Maxim P. lindleyi Rehd.
P. ovoidea Rehd.
P. sinensis Lindley
Siberia, Manchuria, N. China,
Korea
P. calleryana Decne. Central & S. China, Vietnam
P. betulifolia Bunge Central & N. China, S.
Manchuria
P. fauriei C.K. Scheid. Korea
P. hondoensis Kik. & Nak Japan
P. dimorphophylla Mak. Japan
P. kawakamii Hayata P. koehnei C.K. Schneid. Taiwan, S.E. China

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A number of pear species are grown commercially (Bell et al. 1996). As mentioned
previously, Pyrus communis (common pear) is the most important pear in Europe, North
America, South America, Africa, and Australia. Pyrus nivalis, the snow pear, is grown
locally in Europe to make perry and hybrids of P. communis and P. pyrifolia are grown in
North America for processing. The sand pear, P. pyrifolia, is the primary pear species
cultivated in southern and central China and in Japan, with hectarage of the Ussuri pear, P.
ussuriensis, hybrids of P. pyrifolia and P. ussuriensis, and the Chinese white pear (P. ×
bretschneideri) also being found in northern China and Japan. Pyrus pashia (Pashia pear) is
grown in southern China and northern India. Several species are used for rootstocks including
P.betulifolia, P. calleryana, P. pyrifolia, P. ussuriensis, and P. communis in Europe, North
America and eastern Asia, and P. pyraster, P. amygdaliformis and P. elaeagrifolia in Asia
Minor and central Asia. The small fruited species, P. calleryana, P. fauriei, P. betulifolia, P.
salicifolia, and P. kawakamii are grown as ornamentals.
The fruit of the Asian pears are notable in that they can be eaten right after harvest, unlike the
European pears. They are sweet and juicy and tend to have crisp-textured flesh, but do not
have the smooth, buttery texture of European pears, tend to have less aroma, and some have
abundant stone cells (Bell et al. 1996). The Chinese or Japanese sand pear (P. pyrifolia) is
well adapted to warm climates and is very widespread with hundreds of cultivars grown. The
fruit vary in size from very small to very large and they are sweet, juicy and gritty, with a
russeted skin. Pyrus pashia is well adapted to very hot, humid climates, but has only
mediocre fruit quality compared to the other species.
The Ussurian pear, P. ussuriensis, is the most cold hardy Asian pear and is grown in Northern
Japan where P. pyrifolia is susceptible to winter injury. It has small, globose medium-sized
fruit with persistent calyxes. They are bland flavored but not too gritty and some have
melting flesh (Shen 1980). The Chinese white pear, P. ×bretschneideri is the second most
hardy Asiatic type. Their fruits are of medium size, range in shape from pyriform to obovate
and have the most pleasing texture and flavor of the Oriental pears.
It is likely that interspecific hybridization played an important role in pear domestication. A
number of species have been implicated as being in the background P. communis including
wild populations of P. communis var. pyraster, P. caucasica and P. nivalis (Challice and
Westwood 1973). Rubtsov (1944) felt that modern cultivars of P. communis had
characteristics derived from at least three species, P. elaeagrifolia, P. salicifolia and P.
syriaca. Cultivars grown in northern China may belong to a hybrid complex involving P.
ussuriensis and P. pyrifolia (Bell et al. 1996). The origin of P. ×bretschneideri likely
involved the hybridization of P. betulnefolia and P. ussuriensis or P. pyrifolia (Kikuchi
1946). The most recent molecular data suggests that P. ×bretschneideri is closely related to
P. pyrifolia and P. ussuriensis, and is likely a variety or subspecies of P. pyrifolia
(Yamamoto et al. 2002a, Bao et al. 2007).
Vavilov (1951) identified three centers of diversity for cultivated pears: 1) A Chinese center,
where P. pyrifolia and P. ussuriensis are found. The primitive species, P. calleryana, is
located in this center (Challice and Westwood 1973, Pu et al. 1986). 2) A central Asiatic
center (northwest India, Afghanistan, Tadjikistan and Uzbekistan), where P. communis and
hybrids of P. communis and P. ×bretschneideri are found (Yu and Zhang 1979). 3) A Near
Eastern center (Caucasus Mountains and Asia Minor), where P. communis is also grown. It is
in the Near Eastern center that P. communis may have been first domesticated.

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The USDA-ARS National Plant Germplasm System (NPGS) clonal repository at Corvallis,
maintains an extensive and diverse pear germplasm collection that includes historic varieties,
landrace varieties, and wild species (more than 2,200 different individuals in total). This
collection includes genotypes with desirable traits important for crop improvement, such as
resistance to fire blight, pear psylla, Fabraea leaf spot, and pear scab.
10.3 History of improvement
Originating in the Caucasus region (Southeastern Europe between Black and Caspian seas),
pears have been cultivated for at least 3,000 years in Asia (Kikuchi 1946). Indo-European
tribes spread the pear as they migrated into European and Northern India. The first mention
of pears in the written record was made by Homer in about 1000 B.C., when he referred to
them as one of the “gifts of the Gods” (Hedrick et al. 1921). According to Theophrastus (371-
286 B.C.), pear culture was common in ancient Greece, where cultivars were propagated by
grafting and cuttings. The Roman, Cato (235-150 B.C.), described pear cultural methods that
are very similar to the techniques practiced today. By the time of Pliny the Elder (23-79 BC),
at least 35 cultivars of pear were being grown in Rome, while only three types of apples were
noted. The range in fruit characters of these ancient cultivars were similar to those grown
today. By the end of the Sung Dynasty (China, AD 1279), over one hundred varieties of pear
existed.
In the Middle Ages, pears were grown widely in central and western Europe (Bell et al.
1996). Cordus (1515-1544) described pear cultivars in Germany that had all the fruit
characters possessed by modern cultivars, with the exception of buttery texture. France was
the leading pear-producing country in the sixteenth and seventeenth centuries, and were most
active in the development of new cultivars. In 1628, the amateur fruit collector Le Lectier had
254 pear cultivars in his garden and by the early 1800s there were over 900 cultivars of pears
growing in France. Most of these were crisp-fleshed.
In the eighteenth century, Belgium became the center of pear culture and improvement,
primarily through the efforts of Nicolas Hardenpont and Jean Baptiste Van Mons (Bell et al.
1996). The Belgian breeders developed the first cultivars that had a melting buttery flesh, and
some of their cultivars are still important today, including 'Beurre Bosc', 'Beurre d’Anjou' and
'Winter Nelis'.
The pear was not native to England, but commercial culture probably arose there by 1200. It
is not known when the pear was introduced into England, but it is likely to have been before
the Roman conquest (Bell et al. 1996). The English became expert pear breeders in the 1800s,
evidenced by an 1826 catalog of the Royal Society of London listing 622 cultivars. The most
important cultivar now grown in the world, 'Williams Bon Chretien' ('Bartlett'), was identified
around 1796 (Figure 10.2) (Hooker 1818). Another 19th century selection named
'Conference', is also still planted.
Only two species were bred in Europe, Pyrus communis (the common European pear), and P.
nivalis (the perry pear). Most of the improved types were derived from open-pollinated
seedlings of existing cultivars, although Knight in England was using controlled
hybridization around 1800 (Bell et al. 1996). The father of modern genetics, Gregor Mendel,
was also hybridizing pears at Brno in the late nineteenth century to develop late-ripening
cultivars with superior flesh quality (Vavra and Orel 1971).

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Figure 10.2. The 'Bartlett' pear (watercolor published in The Fruits of New York, U.P.
Hedrick, 1921).
Domestication of the Chinese pear species began about 3300 years ago (Kikuchi 1946) and
commercial orchards have existed in China for more than 2000 years (Pieniazek 1966).
However, pear breeding did not begin in China until 1956. Pears were cultivated as early as
the eighth century in Japan (Kajiura 1966), with large plantings not appearing until 1868.
Kikuchi began the first breeding program in Japan in 1915, which is still active (Kanato et al.
1982).
Pears were introduced to North America by the early French and English settlers; the first
reference to pear culture was made in New England in 1629 (Hedrick et al. 1921). Pears were
introduced into the west Coast by the Franciscan monks, and by 1800, pear cultivation
extended from British Columbia to southern California. In the 17th and 18th centuries, only P.
communis was grown in North America and it was not until the early 1800s that P. pyrifolia
arrived at the west coast of the United States via Chinese immigrants. The first hybrid of
these two species, 'Le Conte', appeared in 1846, followed by 'Kieffer' (1873) and 'Garber'
(1880) (Hedrick et al. 1921). The fruit of these hybrids was not of as high quality as the
existing European cultivars, but the trees were more resistant to fire blight. These early
hybrids were likely accidental, but controlled crosses were soon being made in hopes of
combining high fruit quality with blight resistance.
Cold hardy cultivars of P. communis were introduced to North America from Russia in 1879.
These trees had poor fruit quality and were susceptible to fire blight, but they proved to be an
excellent source of cold hardiness (Magness 1937). Additional sources of cold hardiness
came with the introduction of P. ussuriensis into Iowa by Patten around 1867. Chance
hybrids of P. ussuriensis x P. communis proved to be very cold hardy. Reimer (1925)
introduced a number of European and Oriental cultivars to North America in the early 1900s,
and some of the Oriental introductions had superior blight resistance. This stimulated the U.S.

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Department of Agriculture and several State Agricultural experiment stations to begin
actively breeding for fire blight resistance (Bell et al. 1996).
In general, pear breeding is only a recent activity in South America, Australia, New Zealand
and Africa. However, 'Packham’s Triumph', was introduced commercially in Australia
around 1900 and is still important there and in New Zealand (Lombard and Westwood 1987).
10.4 Current breeding efforts
Climatic adaptation is a concern of all pear breeders. Cold hardiness is of paramount
importance in the more temperate and most northern production regions, while in the warmer
regions the emphasis is on drought and heat tolerance. A goal of most pear breeders is to
generate a series of cultivars that produce a continuous supply of quality fruit throughout the
season. Compatibility with major pollenizer cultivars is important everywhere with an
overlapping bloom period.
Disease resistance is also very important to most pear breeders. Increasing fire blight
resistance is a major goal in the eastern and southern parts of North America, and many
regions of Europe. Resistance is also sought in many areas to pear psylla [Cacopsylla
pyricola (Foerster)], powdery mildew [Podosphaera leucotricha (Ellis & Everh.) E.S.
Salmon], leaf spot and scab (Venturia pirina Aderh.). In Japan, resistance to black spot of
Asian pear (Alternaria alternata (Fr.) Keissler) is particularly important, as well as resistance
to the Asian pear scab (Venturia nashicola Tan. & Yan.) and pear rust (Gymnosporangium
asiaticum Miyabe ex Yamada).
All pear breeders seek high fruit quality, although what constitutes good quality varies by
species and location, as was previously mentioned. A soft, buttery flesh is desired in
European pears along with aromatic flavor and low fiber. A crisp, breaking flesh is preferred
in Oriental pears along with a sweet flavor and lack of grittiness. In European pears, the ideal
fruit is considered large (7 cm long and 6 cm wide) with a pyriform shape. The skin can be a
wide range of colors from golden yellow with or without a red blush, green or greenish-
yellow and even bright red. Russet free skin is generally preferred, although russeting that is
uniform and smooth is acceptable. Fruit are sought that are resistant to bruising, white fleshed
and contain limited quantities of stone cells. In Asian pears, an even larger size is desired (up
to 10 cm wide), along with a regular, round shape (Kanato et al. 1982). The skin can be
yellow or a light green, glossy and uniformly covered with a golden, smooth russet. In both
European and Oriental pears, uniformity of ripening, a long postharvest storage life and a low
susceptibility to physiological disorders are all important.
A trend that is of increasing importance is the ability of a cultivar to be harvested
mechanically both for fresh markets and processing. Pear cultivars adapted to this approach
need to produce fruit of uniform size and maturity that separates easily on shaking, and has a
tough skin that resists bruising (Bell et al. 1996). It is likely that trees adapted to mechanical
harvesting will need to be adapted to trellises or other support systems, and have trunks and
limbs that can withstand the vibrations associated with mechanical shaking.
In the breeding of pear rootstocks, the common goal is to develop rootstocks that induce size
control and precocity in the scion cultivar. They should also be compatible, winter hardy, and
disease resistant. Adaptation to the specific climatic conditions of each production region is

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also very important. A reduction in root suckering contributes to decreased risk of infections
and herbicide damage. Because seedlings rootstocks are derived from parent cultivars which
are highly heterozygous and self-incompatible, the rootstocks are genetically not identical,
although they are usually uniform in all important characteristics. Japanese and Chinese
breeding programs have focused on reducing the physiological disorder “black end”. In
Europe, a strong emphasis has been placed on improved cold hardiness in Pyrus and
Cydonia, along with adaptation to iron chlorosis in quince (Chevreau and Bell 2006). Several
disease problems of rootstocks receive primary attention in Europe and North America
including fire blight and the pear decline phytoplasma.
Much effort was devoted in the past on improving quince rootstocks, but more energy is now
being devoted to developing dwarfing Pyrus rootstocks. There is less incompatibility
encountered between intrageneric than intergeneric grafts, and some of the Pyrus species are
more winter hardy, disease resistant, drought tolerant and have better anchorage than the
quince rootstocks (Bell et al. 1996). Only a modest amount of root stock breeding is being
conducted in the United States, but active programs exist in England, France, Italy, Sweden,
the Soviet Union and Romania.
Numerous pear breeding programs are found across the world. Some of the largest programs
in North America are those of the U.S. Department of Agriculture at Kearneysville, West
Virginia and Agriculture and Agri-Food Canada at Harrow, Ontario. At least three private
companies are also producing pear cultivars in the U.S.A. Some of the most important
programs in Europe are at the Institute National de la Recherche Agronomique at Angers and
Dax, France, the Instituto Sperimentale per la Frutticoltura in Rome and Forli, Italy, and the
East Malling Research Station in England. Other pear breeding programs are found in
Norway, Sweden and Romania.
In Asia, major programs exist in China at Xingcheng (Liaoning Province), concentrating on
P. ×bretschneideri, and Zhengzhou (Henan Province) and Hangzhou (Zhejiang Province),
focusing on P. pyrifolia (Wang 1990). Breeding is also being done in Korea at the National
Horticultural Research Institution (Kim and Ko 1991 and 1992) and in Taiwan (Hsu and Lin
1987). In Japan, the largest breeding program is at Kikuchi (Kanato et al. 1982).
In the southern hemisphere, only a few breeding programs exist. A major program was
established in 1983 at HortResearch in New Zealand that focuses on fresh fruit for export to
the Northern Hemisphere. In the 1990s, breeding work was also begun in South Africa
(Human 2005) and Brazil (Barbosa et al. 2007).
10.5 Genetics of economically important traits
10.5.1 Pest and disease resistance
Fire blight is the most important disease of pears in North America and Western Europe. In
fact, commercial pear growing has been largely abandoned in the warm humid regions of the
southeastern, southern, and central United States because of the severity of this problem. The
most widely grown cultivars are generally susceptible, but several new varieties have been
released that have improved resistance (Quamme and Spearman 1983, Bell et al. 2002,
Hunter et al. 2002a and b). 'Seckel' and 'Old Home' were the original sources of resistance.
Resistance is quantitatively inherited in an additive fashion, with a few major genes playing

9
an important role (Decourtyne 1967, Quamme et al. 1990). The highest levels of resistance
are found in the Asian species P. calleryana and P. ussuriensis (Table 10.2).
Table 10.2. Useful horticultural traits carried by native pear species (from Bell 1991).
Scientific name Useful characteristics
European
P. communis L. Resistance to fire blight, white spot, pear scab, pear decline, pear psylla;
cold hardy
P. nivalis Jacq. Resistance to pear psylla; cold hardy
P.cordata Desv. Resistance to codling moth; adaptation to warm winters, high pH,
drought
Circum-Mediterranean
P. amygdaliformis Vill Resistance to codling moth; adaptation to warm winters, high pH,
drought and clay soils
P. elaeagrifolia Pall. Resistance to codling moth; cold hardy
P. syriaca Boiss. Adaptation to drought
P. longipes Coss. & Dur. Cold hardy; adaptation to wet and dry soils
P. gharbiana Trab. Adaptation to drought
P. mamorensis Trab. Adaptation to low pH
Mid-Asian
P. glabra Boiss.
P. salicifolia Pall. Adaptation to drought
P. regelii Rehd. Resistance to pear psylla
P. pashia Buch.-Ham. ex D. Don. Adaptation to warm winters, low pH, clay soils
East Asian
P. pyrifolia (Burn.) Nak Resistance to pear scab
P. pseudopashia T.T. Yu Resistance to codling moth; winter hardy
P. ussuriensis Maxim Resistance to fire blight, pear psylla; winter hardy
P. calleryana Decne. Resistance to fire blight, Fabraea leaf spot, pear psylla, codling moth;
adaptation to warm winters, low pH, wet, dry and clay soils
P. betulifolia Bunge Resistance to pear psylla, codling moth; adaptation to warm winters, low
pH, wet, dry and clay soils
P. fauriei C.K. Scheid. Resistance to fire blight, pear psylla, codling moth; adaptation to warm
and cold winters, low pH, wet and clay soils
P. hondoensis Kik. & Nak Resistance to fire blight
P. dimorphophylla Mak. Resistance to powdery mildew, pear psylla, codling moth; adaptation to
low pH and wet soils
P. kawakamii Hayata Resistance to codling moth; adaptation to warm winters
Identifying resistance to fire blight has been problematic due to a number of factors,
including 1) age, vigor, succulence, and kind of tissue infected; 2) temperature and humidity
relationships during the pre- and post infection period; 3) inoculum purity and concentration;
4) method of inoculation, and 5) virulence of isolates (Bell et al. 1996). A useful rating
system for estimating blight resistance in the field was devised by van der Zwet et al. (1970).
Leaf blight caused by the fungus Fabraea maculata Atk. is a widespread problem in pears,
along with Pseudomonas blight (also called bacterial canker and blossom blast) caused by
Pseudomonas syringae pv. syringae van Hall. Leaf blight resistant genotypes are relatively
common in a number of Pyrus species, while resistance to Pseudomonas blight is limited in
European and Asian pears (Bell et al. 1996). Red-skinned mutants of 'Beurre d’Anjou' have
been reported to be more resistant to Pseudomonas blight than green-skinned ones
(Whitesides and Spotts 1991).

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Table 10.3. Genetics of pest and disease resistance in pear.
Pest or disease Observations and sources
Bacterial
Fire blight
Erwinia amylovora
Resistance in P. communis is rare but exists (van der Zwet and Keil
1979, Quamme and Spearman 1983, Thibault 1987, van der Zwet and
Bell 1990, Hevesi et al. 2004), High levels of resistance are present in P.
calleryana and P. ussuriensis, but not immunity (Hartman 1957),
Resistance is quantitatively inherited, although major genes have been
implicated (Layne et al. 1968, Thompson et al. 1975, Bell et al. 1977),
Additive effects predominate (Quamme et al. 1990)
Fungi
Black spot
Alternaria alternata
Many sources of resistance exist (Hiroe et al. 1958, Kanato et al. 1982),
Susceptibility is controlled by a single dominant gene (Kozaki 1973)
Leaf blight
Pseudomonas syringae
Resistance has been identified in most Pyrus species (Bell et al. 1996)
Pear scab
Venturia pirina
Resistant genotypes have been found in P. communis, P. pyrifolia, P.
ussuriensis, P. nivalis (Kovalev 1963, Westwood 1982, Bell 1991),
Resistance varies regionally due to the existence of multiple fungal
biotypes (Shabi et al. 1973, Vondracek 1982)
Venturia nashicola The major cultivars are not resistant, but many minor cultivars of many
species carry resistance (Kanato et al. 1982)
Pear rust
Gymnosporangium
haraeanum
Resistance has been identified (Kanato et al. 1982)
Powdery mildew
Podosphaera leucotricha
Resistance has been identified (Fisher 1922, Kanato et al. 1982,
Westwood 1982, Serdani et al. 2006)
Pseudomonas blight
Pseudomonas syringae
Most European and Asian pears are susceptible, red-skinned mutants of
'Beurre d’Anjou' are less susceptible than green-skinned ones
(Whitesides and Spotts 1991)
Phytoplasma
Pear decline Pyrus communis is generally tolerant, P. betulifolia is very tolerant,
while P. pyrifolia, P. ussuriensis and P. calleryana are susceptible
(Lombard and Westwood 1987), Resistance is inherited additively
(Westwood 1996)
Insects
Pear psylla
Cacopsylla spp.
Resistance is common in P. betulifolia, P. calleryana, P. fauriei, P.
ussuriensis, P. ×bretschneideri and P. communis x P. ussuriensis hybrids
(Westigard et al. 1970, Quamme 1984, Bell 1991), resistant genotypes
have been identified in P. nivalis and P. communis (Bell and Stuart 1990,
Bell 1992), resistance was positively correlated with large fruit size in
progeny of P. communis x P. ussuriensis (Harris and Lamb 1973)
Pear sawfly
Caliroa cerasi
Variation in resistance exists (Shaw et al. 2004)
Wooly pear aphid
Eriosoma pyricola
Most pear species are resistant or have variable resistance (Westwood
and Westigard 1969), Immunity has been found in two Indian rootstocks
of P. pashia (Khan 1955)
Nematodes
Root-knot
Meloidogyne spp.
Resistant genotypes of P. communis have been reported (Tufts and Day
1934)
Two species of Venturia cause widespread scab diseases in pears: 1) V. pirina Aderh, which
is particularly important in Europe, but is found everywhere P. communis is grown, and 2) V.

11
nashicola Tanaka & Yamamoto which infects Asian pears throughout their range. Resistant
genotypes have been found to V. pirina in P. communis, P. pyrifolia, P. ussuriensis and P.
nivalis (Kovalev 1963, Westwood 1982, Bell 1991) and resistant cultivars have been released
(Fischer and Mildenberger 2000, Bellini et al. 2000). However, resistance can be variable
regionally due to the existence of multiple fungal biotypes (Shabi et al. 1973, Vondracek
1982). Few major Asian pear cultivars are resistant to V. nashicola, although a number of
minor ones carry resistance (Kanato et al. 1982).
Other locally important diseases that effect pears are black spot [Alternaria alternata (Fr.)
Keissler], Monilia fruit rot [Monilinia fructicola (Wint.) Honey], Fabraea leaf spot (Fabraea
maculata Atk.), powdery mildew [Podosphaera leucotricha (Ell. & Ev.) Salm.], Asiatic pear
rust [Gymnosporangium haraeanum Syd.], along with several virus diseases including pear
bud drop, stony pit and ring spot mosaic. Black spot resistant cultivars have been released
(Kanato et al. 1982, Kajiura 1992) and resistance sources have been identified to all of the
other diseases listed above (Bell et al. 1996, Bell and van der Zwet 2005, Serdani et al. 2006).
The phytoplasma induced disease pear decline is a very widespread problem. It is vectored by
the pear psylla, Cacopsylla spp. (Hibino and Schneider 1970), with C. pyricola Förster being
the causative species in North America, and C. pyri L. and C. pyrisuga Förster being the
vectors in Europe. Pear species vary widely in their resistance to pear psylla, with the east
Asiatic species being more resistant than those from Asia Minor and Europe (Bell et al.
1996). Improved resistance to pear psylla has been developed in Romania (Braniste 2002).
A number of insect pests cause problems in pear orchards (Table 10.3). The woolly pear
aphid (Eriosoma pyricola Baker & Davidson) is a common pest of pear trees in the nursery
and young orchards, particularly in Oregon. Resistance to this aphid is widespread in Pyrus,
with at least eight species carrying resistance (Westwood and Westigard 1969). Other insect
pests that periodically cause problems are the pear leaf blister mite (Eriophyes pyri
Pagenstecher), pear sawfly (Caliroa cerasi L.) and the root-knot nematode (Meloidogyne
spp.). There are no reports of resistance to these pests, except pear sawfly (Shaw et al. 2003
and 2004).
10.5.2 Morphological and physiological traits
Improving winter cold hardiness has played a key role in expanding the range of pear
growing into northern regions of North America, Europe, and Asia. The very cold hardy
species P. ussuriensis from northern Russia has proven extremely useful in the development
of cold hardy cultivars for the northern U.S.A. and Canada (Ronald and Temmerson 1982,
Stushnoff and Garley 1982, Luby et al. 1987, Peterson and Waples 1988), as well as Europe
(Ludin 1942, Zavoronkov 1960, Sansavini 1967). These cold hardy hybrid types do not have
as high quality as the cultivars grown in the main pear-growing regions, but they can
withstand winter temperatures as low as – 30 to – 40 °C and have much higher fruit quality
than their P. ussuriensis parents. Inheritance of cold hardiness has not been studied in pear,
but it is likely to be similar to apple where inheritance is quantitative and mostly additive
(Bell et al. 1996).
Resistance of blossoms to spring frost is also an important goal in cold climates. Two
approaches have been taken to solving this problem: 1) breeding for a late blooming periods,
and 2) identifying genotypes with direct resistance of blossoms to frost. Bloom date has been

12
shown to be highly heritable (Anjou 1954) and late flowering does not appear to be strongly
correlated with late ripening (Baldini 1949). Date of leaf break may be used to predict
flowering date (Bell et al. 1996). A large variation in frost tolerance of blossoms has been
identified, but parental and progeny assessment of frost resistance has proven to be difficult
due to yearly microclimate differences, variations in the impact of fruit set reductions
(Perraudin 1955) and cultivar differences in parthenocarpic fruit set (Simovski et al. 1968).
Reducing the chilling requirement of P. communis is a goal of some southern U.S.A.
breeding programs to expand the range of cultivation. A number of interspecific P. communis
x P. pyrifolia hybrids have been released with low chilling requirement by the University of
Florida. Efforts have also been taken to reduce the chilling requirement of Asian pears at
numerous locations in S. E. Asia, China and India. Detailed information about the chilling
hour requirements of pear varieties can be found at Spiegel-Roy and Alston (1979) and
Ghariani and Stebbins (1994). To our knowledge, no studies have been conducted on the
genetics of chilling requirement in pear.
Drought resistance of scions and rootstocks can be critical when pears are grown in arid to
semiarid conditions. Pruss and Eremeev (1969) found considerable variability in the drought
tolerance of 99 pear cultivars of Russian, European, Australian, and American origin. Pears
grafted onto P. salicifolia performed much better on in dry soils than those on P. communis
rootstocks and had higher tolerance to extreme temperature changes and alkaline soils. They
were also more resistant to pear scab and woolly aphid (Kuznetzov 1941).
Dwarf fruit trees are now favored to standard types, as they are easier to manage and yield
more per area due to more efficient light interception (Tukey 1964). However, there are no
completely dwarf pear cultivars, and potential sources of small size are much harder to find in
pears than in apples. Quince rootstocks are sometimes used to control pear plant size, but the
adaptability of these rootstocks is too limited for widespread use (Bell et al. 1996).
There are several possible approaches to reducing pear tree size (Table 10.4). Semidwarf or
compact cultivars of P. communis have been identified (Tuz 1972) and when they are crossed
with standard types, there appears to be dominance for the compact form. Decourtye (1967)
identified a short-internode dwarf, 'La Nain Vert', that is under single dominant gene control.
This gene has been used in U.S.A. and Italian breeding programs, but the dwarf parent has
poor fruit quality and a very slow growth rate. Jingxian et al. (1988) identified short internode
compact pears from seedlings of 'Jin-xiang' that are also under single gene control. Few
genetically dwarfed scion cultivars have been released to date (Bellini et al. 2000), but active
efforts continue in Italy and France to reduce stature (Rivalta et al. 2002, Chevreau and Bell
2006).
Precocity is very important in pears, both to the breeder who is anxious to evaluate the fruit
and the producer who wants a return on his planting investment as soon as possible. The
length of the juvenile period is heritable and under additive genetic control (Zielinski 1963,
Visser 1967 and 1976, Zimmerman 1976, Bell and Zimmerman 1990). Li et al. (1981) found
that the amount of linear growth to the first flower (“juvenile span”) was positively correlated
with the length of the juvenile period and was heritable. Bell and Zimmerman (1990) found
that in a population of inter-specific hybrid origin, the juvenile period was more dependent on
the individual genotype than species pedigree. Seedlings of P. pyrifolia are more precocious
than P. ×bretschneideri and P. communis (Zhejiang Agricultural University 1978).

13
Table 10.4. Genetics of adaptation, productivity, plant habit and fruit quality in pear.
Attribute Observations and sources
Adaptation
Cold hardiness Quantitatively inherited, largely additive; P. ussurienis is extremely hardy (Bell et al.
1996)
Season of flowering Quantitatively inherited (Anjou 1954, Bell et al. 1996); late flowering only weakly
correlated with late ripening (Baldini 1949)
Harvest date Quantitatively inherited with high heritability observed in P. communis (Thibault et
al. 1988) and moderate heritability in P. pyrifolia (Zhejiang Agricultural University
1977), although some studies have shown parents to be poor predictors of progeny
performance (Crane and Lewis 1949)
Productivity
Juvenile phase length Quantitatively inherited under additive control (Zielinski et al. 1963, Visser 1967 and
1976, Zimmerman 1976, Bell and Zimmerman 1990)
Incompatibility Numerous S-alleles exist, but there are compatible combinations (Sanzol and Herrero
2002, Kim et al. 2004, Moriya et al. 2007, Wu et al. 2007)
Plant habit
Dwarfing Several sources of dwarfing exist that are regulated by single genes (Decourtye 1967,
Jingxian et al. 1988)
Fruit quality
Firmness Quantitatively inherited and highly heritable (Machida and Kozaki 1976, Kajima and
Sato 1990)
Flavor Quantitatively inherited but with low heritability in P. communis (Bell and Janick
1990); poor flavor of P. ussuriensis is inherited quantitatively in interspecific crosses
with some dominance (Lantz 1929); Aroma of P. ussuriensis is highly heritable in
interspecific crosses (Pu et al. 1963)
Juiciness Regulated by a limited number of dominant genes (Zielinski et al. 1965)
Keeping quality Quantitatively inherited (Zielinski et al. 1965)
Russeting Quantitatively controlled in P communis with high heritability (Zielinski et al. 1965,
Crane and Lewis 1949, Bell and Janick 1990); Two genes regulate russetting in P.
pyrifolia, R and I (Kikuchi 1930)
Skin color Major genes determine – yellow dominant to green, blushed recessive to non-blushed
(Zielinski et al. 1965); Deep red regulated by dominant gene C (Zielinski et al. 1963,
Brown 1966); White flesh dominant to colored (Zielinski et al. 1965)
Shape Quantitatively inherited with round and obovate dominant to pyriform and turbinate
(Crane and Lewis 1949, Zielinski et al. 1965, Wang and Wei 1987, White and
Alspach 1996); Moderate to high heritability for ratio of length to diameter (Shin et
al. 1983, White et al. 2000)
Size Quantitatively inherited (Crane and Lewis 1949, Zielinski et al. 1965, Shen et al.
1979, Wang and Wei 1987); Low heritability in some P. pyrifolia populations
(Machida and Kozaki 1976, Shin et al. 1983)
Sugar content Heritability of soluble solids in P. pyrifolia is low (Shin et al. 1983) to moderate
(Machida and Kozaki 1976) depending on population
Texture Quantitatively inherited with relatively high heritability in P. communis (Machida
and Kozaki 1976, Kajiura and Sato 1990); Presence of stone cells dominant to
stoneless in most crosses of Pyrus (Golisz et al. 1971, Pu et al. 1963, Zielinski et al.
1965); Grit content regulated by at least four loci, acting independently and
additively (Thompson et al. 1974)
A number of characters may be useful in the early identification of precocity. Bell et al.
(1996) suggested that juvenile seedlings can be identified by the presence of thorns, irregular
leaf margins and wide-angled branches. Seedlings with short juvenile periods lose these
features quickly. Vigor, measured as stem diameter, is not an accurate predictor of the length
of the juvenile period (Zimmerman 1977, Shen et al. 1982).

14
While there is a need for pear cultivars that ripen all across the season, early ones are
especially important for the fresh market. In P. communis, heritability for date of ripening
was calculated to be 0.49 (Thibault et al. 1988), while in P. pyrifolia a value of 0.33 was
estimated (Zhejiang Agricultural University 1977). Bell et al. (1996) suggested that to breed
most effectively for earliness or lateness, both parents should be either early or late.
10.5.3 Fruit quality
High fruit quality is the prime objective of all fruit breeding programs and encompasses a
wide array of attributes including flavor, texture, appearance, juiciness, postharvest storage
life and incidence of physiological disorders. The most important attribute is eating quality,
which is largely dependent on flavor and texture. Most programs focus on fresh fruit quality,
but some are also interested in the quality of canned or pureed fruit. Considerable attention is
commonly given to core breakdown, bitter pit and superficial scald.
High sugar content is likely the most important factor determining a flavorful pear. The level
of acidity is secondary in importance, as high and low acid pears have been characterized as
good as long as they have high sugar (Visser et al. 1968). Extreme bitterness and astringency
are generally not desirable in dessert pears, although they are important contributors to the
bitter-sharp flavor enjoyed in perry (Luckwill and Pollard 1963). The heritability of flavor in
P. communis has been found to be relatively low (h2 = 0.21) (Bell and Janick 1990).
Subjective perceptions of sweetness and acidity, as well as physical measurements of soluble
solids and acidity, are inherited as independent, quantitative traits (Table 10.4). Estimates of
the heritability of soluble solids in P. pyrifolia cultivars have ranged from quite low (Shin et
al. 1983) to moderate (Machida and Kozaki 1976). Zielinski et al. (1965) found that juiciness,
depending on the parent, was regulated by either a single gene or several, with juiciness being
dominant to dryness. The inheritance of flavor in hybrid populations of P. ussuriensis is
quantitative with poor flavor showing some dominance (Lantz 1929).
It is important that pears can be held in cold storage for long periods of time without
developing internal breakdown, so that they can be sold during the winter when prices tend to
be the highest. Few inheritance studies have been conducted on this characteristic, although
segregation patterns suggest that long term keeping quality is quantitatively inherited
(Zielinski et al. 1965).
A large number of aromatic compounds are produced by pears that make important
contributions to fruit flavor. In 'Bartlett', 77 volatile compounds have been identified with
varying impacts on flavor (Jennings and Tressel 1974). The esters of trans-2, cis-4-
decadienoic acid are highly correlated with the intensity of 'Bartlett' aroma (Jennings et al.
1964), although there are cultivars with high levels of these compounds that do not have that
distinctive aroma (Bell et al. 1996). Seventeen volatile compounds, including toluene, ethyl
butryate, hexanal, (E) 2- hexanal, ethyl hexanoate, a-farnesene, were found to be the most
common constituents of five Asian pear cultivars (Horvat et al. 1992). 'Ya Li' of P. ×
bretschneideri and 'Shinko', a putative interspecific hybrid, lacked five compounds found in
three other P. pyrifolia cultivars ('Chojuro', 'Hosui', and 'Kosui'). The genetics of these
compounds are unknown, although Pu et al. (1963) found that some P. ussuriensis cultivars
had an aromatic quality that was dominant to the sweet, bland flavor of P. ×bretschneideri
and P. pyrifolia cultivars.

15
The desired flesh texture of pears varies from region to region, in part because of species
differences. In Western Europe and North America the favored type is the soft, buttery one
typical of P. communis, while in China and Japan, the preferred texture is the crisp, breaking
one of P. pyrifolia, P. ×bretschneideri, and P. ussuriensis. A minimum of grittiness (stone
cells) is appreciated in all types of pears, although more is tolerated in the Asian than
European ones. Bell and Janick (1990) found heritabilities of 0.30 and 0.45 for texture and
grit scores in P. communis. Flesh firmness has been shown to be highly heritable in
populations of P. pyrifolia (Machida and Kozaki 1976, Kajiura and Sato 1990). Stoneless
flesh was found to be dominant to stony in certain interspecific hybrids of wild Pyrus species
(Westwood and Bjornstad 1971); however, the presence of stone cells was dominant in
several studies of cultivars including crosses involving P. communis and P. ussuriensis
(Golisz et al. 1971), P. ussuriensis crossed with P. pyrifolia and P. ×bretschneideri (Pu et al.
1963) and P. communis cultivars (Zielinski et al. 1965). Thompson et al. (1974) suggested
that the control of grit content is complex, regulated by a minimum of four loci that act in an
independent and additive fashion. In most hybrids derived from crosses of Asian and
European pears, it is difficult to recover pure crisp flesh selections with little grittiness (Wang
1990, Bell et al. 1996).
Several genetic studies have been conducted on skin and flesh color in pears. Zielinski et al.
(1965) found that the background color of pears was under the influence of a major gene,
with yellow dominant to green. They also found that the blushed vs. nonblushed character
was controlled by a recessive gene, and white flesh was dominant to colored, with green and
cream likely being regulated by other alleles at the same locus. Deep red colored sports of
'Bartlett' called 'Cardinal Red' and 'Max Red' were found to be regulated by a single dominant
gene, C by Zielinski (1963). Brown (1966) found that red was dominant to white in one
segregating population of 'Sanquinole' (red) x 'Conference' (white). Not all red skin mutations
are transmitted sexually, as the red coloration of 'Starkrimson' only affects the epidermis and
not the germlines (Dayton 1966).
Russeting of the skin is acceptable if it is smooth, uniform and light tan for the fresh market,
but is not acceptable in processed puree. In P. communis, Zielinski et al. (1965) and Crane
and Lewis (1949) found that the control of russeting was under quantitative control. Bell and
Janick (1990) estimated heritability for russeting to be 0.52. Wellington (1913) originally
suggested that russeting in P. pyrifolia was under the control of a single gene; however,
Kikuchi (1930) proposed that two loci were involved, R and I. In his model, RR__ genotypes
are completely russeted and Rrii genotypes are always partially russetted. RrI_ genotypes
have a partial russetting that is environmentally sensitive (under humid conditions they have
more russetting than under dry conditions). Wang and Wei (1987) suggested that russetting in
P. pyrifolia was recessive to non-russetting.
There is considerable variability for fruit size in Pyrus. Many genotypes of P. betulifolia and
P. calleryana are as small as 1 cm in diameter, compared to genotypes of P. communis and P.
pyrifolia which can exceed 12 cm in diameter (Bell et al. 1996). The size of pears is under
polygenic control with variable levels of heritability depending on breeding population
(Zielinski et al. 1965, Shen et al. 1979, Wang and Wei 1987, White and Alspach 1996).
White et al. (2000) found heritability for length and width ratios to be more than 0.5 in
families of European and Asian pear parentage, while Machida and Kozaki (1976) and Shin

16
et al. (1983) found much lower values in their P. pyrifolia breeding populations. Many
environmental factors influence pear size including water availability, fruit set and yield.
The most common shape of European pear cultivars is pyriform and most Asian pears are
round (Bell et al. 1996). The inheritance of fruit shape is under polygenic control (Crane and
Lewis 1949, Zielinski et al. 1965). Round and obovate shapes are most common in
segregating populations of European and Asian types, suggesting that this shape is dominant
to pyriform and turbinate shapes (Zielinski et al. 1965, Wang and Wei 1987). Shin et al.
(1983) found that the ratio of fruit length to diameter had a moderately low heritability of
0.23.
Several molecular studies have been conducted on the expression patterns of genes during
fruit ripening and storage. El-sharkawy et al. (2004) found several 1-Aminocyclopropane-1-
carboxylic acid (ACC) synthase genes in late ripening pears that were differentially expressed
during cold treatment, and they discovered that cold dependent and independent cultivars had
variant allelic assemblages. Fonseca et al. (2005) followed the transcript accumulation of
seven genes encoding cell wall modifying enzymes during fruit growth, ripening and
senescence. They found that induction of the genes for xyloglucan
endotransglucosylase/hydrolase and expansin 2 was likely associated with cell wall
maintenance, while expansin 1, polygalacturonase 1 and 2, ß-galactosidase and ß-xylosidase
most likely played a role in cell wall disassembly and loosening. Itai et al. (2000) isolated 30
cDNA clones of genes corresponding to mRNAs up-regulated during fruit ripening of
Japanese pear. The cDNAs were sequenced and were found to be associated with stress
response, protein catabolism and pathogenesis. Several of the genes were inhibited by 1-
methylcyclopropane (MCP), an inhibitor of ethylene action.
A number of genes have been isolated from pear which are associated with fruit development
and/or ripening. Lelievre et al. (1997) cloned ACC synthase and ACC oxidase from P.
communis. El-Sharkawy et al. (2003) isolated and characterized four ethylene perception
elements and found them to be differentially expressed after cold and ethylene treatment. Itai
et al. (1999a and b) characterized ACC synthase and ß-D-xylosidase from P. pyrifolia, the
latter likely playing a role in senescence. They found an association between the expression
of two specific ACC synthase genes and ethylene production in 35 Asian pear cultivars. Itai
et al. (2003) developed a rapid method for analyzing fruit storage potential by utilizing CAPS
(cleaved-amplified polymorphic sequences) markers of the two ACC synthase genes. One
marker was associated with high ethylene producers, while a second marker was associated
with moderate ethylene producers. Low ethylene producers had neither of these markers.
Sekine et al. (2006) cloned thirteen cDNAs that encode cell-wall hydrolases in the European
pear and followed their expression patterns during cold storage. Hiwasa et al. (2003) found
seven α-expansin genes to be differentially expressed during growth and ripening of pear
fruit. They identified seven genes that might be associated with the melting texture. Yamada
et al. (2006) cloned two isoforms of soluble acid invertase of Japanese pear and followed
their development during ripening; one was particularly active in young fruit. Tateishi et al.
(2001) isolated a cDNA fragment of the fruit softening enzyme, ß-galactosidase. Several
genes encoding membrane bound proteins have been cloned including arabinogalactan
proteins in P. communis (Mau et al. 1995) and H(+)-pyrophosphatase in P. pyrifolia (Suzuki
et al. 1999). Additional genes that have been characterized in pear include alcohol

17
dehydrogenase (Chervin et al. 1999), polyphenol oxidase (Haruta et al. 1999) and α-L-
arabinofuranosidase (Tateishi et al. 2005).
10.5.4 Rootstocks
Historically, little breeding effort has been devoted to improving pear rootstocks, although
Chevreau and Bell (2006) suggest that “deliberate evaluation and selection of parents and
hybridization has become more common”. Pyrus communis is the species most widely
employed as rootstocks in North America and Europe. In Asia, P. pyrifolia, P. betulifolia, P.
calleryana and P. pashia provide the primary rootstocks, while in Asia Minor and the
Mediterranean region, P. elaeagrifolia, P. syriaca, P. amygdaliformis and P. longipes are the
species most commonly utilized.
Pyrus communis seedlings produce vigorous trees that are adapted to a broad range of
climates and soil types, and they are generally resistant to pear decline and Armillaria root rot
[Armillaria mellea (Vahl:Fr.) P. Kumm.], but are often susceptible to fire blight (Bell et al.
1996). Fire blight resistant types have been found in progeny of the P. communis cross, 'Old
Home' x 'Farmingdale' (OH F rootstock series) and individuals from this family provide some
size reduction (Brooks 1984, Lombard and Westwood 1987) and precocity, but sometimes
have problems with suckering (Raese 1994). Other dwarfing P. communis rootstocks include
'Pyrodwarf' and 'BU 2-33' coming from 'Old Home' x 'Bonne Louise d'Avranches' (Jacob
1998 and 2002), the Blossier series (Brossier 1977, Michelesi 1990) and the Rètuziére series
(Michelesi 1990, Simard and Michelesi 2002). Seedlings of 'Barlett' predominate as
rootstocks in older plantings, with seedlings of 'Winter Nelis' probably being the next most
widely used seedling rootstock.
Among the other species utilized as rootstocks, Pyrus calleryana has been used in the
southern U.S.A., China and Australia because of its fire blight, pear decline and wooly aphid
resistance, drought tolerance and it’s control of vigor, although the species is very susceptible
to winter injury (Cole 1966, Batjer et al. 1967, Lombard and Westwood 1987). Pyrus
betulifolia is used in local areas of the United States, central China, northern Italy and Israel
where temperatures are not very cold; it is particularly useful where clay soils and poor
drainage restrict vigor and it is resistant to salinity, black end, pear decline and the wholly
pear aphid (Lombard and Westwood 1987, Matsumoto et al. 2006). Dwarfing root stocks
have been selected from both P. calleryana and P. betulifolia (Robbani et al. 2006). Pyrus
ussuriensis has proven useful where extreme cold hardiness is required in northeast China
and the Great Plains of North America (Morrison 1965). It is resistant to fire blight (i.e.
Reimer selections) and the woolly pear aphid, but is sensitive to excessive moisture, black
end and pear decline. Pyrus pyrifolia is utilized widely in Japan, China and Korea because of
its tolerance to a wide range of soil textures and soil moistures, even though it is prone to the
physiological disorder black end (hard end) and pear decline (Bell et al. 1996). Pyrus pashia
is used in northern India and southern China; it is resistant to black end, although it is
sensitive to lime-induced chlorosis and is susceptible to the woolly pear aphid (Bell et al.
1996). Pyrus amygdaliformis has high tolerance to salinity stress (Matsumoto et al. 2006).
Cydonia oblonga L. has long been used in Europe as a powerful dwarfing rootstock for
milder climate pear production. It reduces size by 30 to 60% compared to standard P.
communis seedling rootstocks, shortens time to fruiting and increases fruit size. However,
quince rootstocks suffer from several problems including susceptibity to fire blight and

18
Armillaria root rot, poor winter hardiness, low tolerance to wet soils, insufficient soil
anchorage and poor graft compatibity with many common pear cultivars (Millikan and
Pieniazek 1967, Lombard and Westwood 1987). Quince selections are being developed with
improved cold hardiness and greater tolerance to high pH (Loreti 1994, Bassi et al. 1996,
Webster 1998).
In addition to Cydonia, selected clones of Amelanchier and Crataegus possess graft-
compatibility with Pyrus and can be used as dwarfing rootstocks (Lombard and Westwood
1987, Lombard 1989).
10.6 Crossing and evaluation techniques
10.6.1 Breeding systems
As mentioned previously, the pear is likely an ancient allopolyploid but it behaves as a
diploid with disomic inheritance (Crane and Lewis 1940). Most Pyrus cultivars are diploid,
although there are polyploid cultivars, especially in P. communis (Bell et al. 1996). A number
of cultivars are triploid (2n = 3x = 51), a few are tetraploid (2n = 4x = 68), and hexaploid
forms have been produced, but not commercialized (2n = 6x =102).
All pear species are self-infertile with the gametophytic self-incompatibility system (Crane
and Lewis 1942, Westwood and Bjornstad 1971). Therefore, straight selfing to increase
homozygosity can not be employed, although crosses where inbreeding coefficients do not
exceed 0.25, rarely result in significant losses of vigor. A significant, albeit small, association
has been observed between inbreeding and improved flavor, grit, and texture, suggesting that
limited inbreeding can aid in selection for homozygous recessive genotypes (Bell et al. 1981).
A considerable amount of work has been done to characterize the genes associated with the
incompatibility locus (S) locus in European (Sanzol et al. 2006) and Asian pears (Sassa and
Hirano 1997, Ushijima et al. 1998, Ishimizu et al. 1999, Kim et al. 2004). PCR-RFLP and
CAPS (cleaved amplified polymorphic sequences) marker systems have been developed to
genotype the S alleles of cultivars (Kim et al. 2002, Takasaki et al. 2006, Moriya et al. 2007).
One mutated gene of S4-RNase has been described that confers self-compatibility in 'Osa-
Nijiisseiki' (Norioka et al. 1996, Wu et al. 2007). Two other genes have been cloned in pear
that are associated with pollination - the gene for uridine diphosphate (UDP)-glucose
pyrophosphorylase that plays a role in pollen tube wall synthesis (Kiyozumi et al. 1999) and a
pollen- and seed-transmitted RNA-dependent RNA polymerase (Osaki et al. 1998).
10.6.2 Pollination and seedling culture
Pollen is generally collected several weeks before the bloom period. Branches 1 to 1.5 m in
length are collected and forced in the laboratory or greenhouse with their cut ends in water.
About 2 weeks are required before the blossoms are ready for collection if the branches are
gathered while the buds are still dormant. If they are collected at the tight cluster stage, only 2
to 3 days are usually necessary.
The anthers are generally harvested just before they dehisce, by rubbing them on a wire mesh
screen over paper. The anthers are allowed to dry on the paper for 24 hours and then the
pollen is poured into glass vials that are placed in a desiccator with anhydrous CaS04 and kept

19
at about 5 °C. Pear pollen remains viable for two to three weeks at room temperature, but can
be stored for over a year with refrigeration. The pollen is generally transported to the field on
ice in larger vials containing desiccant. The vials of pollen are removed from the cooler when
required for pollination and put back immediately after use.
Emasculation is done using a variety of tools including fingernails, scalpels, tweezers, or
scissors specially modified with a notch in the blades and an adjustable screw to control the
amount of closure (Bell et al. 1996). A cut is made below the sepals and the flower is pulled
off leaving the pistil. One to three blossoms per cluster are generally emasculated at the
balloon stage of development and all other flowers are removed. The larger basal flowers
tend to set more fruit than the smaller terminal ones. Pollen is applied to the stigmas using a
variety of objects including glass rods, fingers or camel hair brushes. Bell et al. (1996)
suggests that approximately two flowers must be emasculated and pollinated to produce one
seed. Bees do not visit flowers without corollas (Visser 1951), so special precautions to
prevent pollen contamination are not necessary for routine crosses.
Pear seeds require stratification to overcome their internal dormancy requirement. This is
commonly done by holding them for 60 to 90 days in moist, finely ground peat moss placed
in polyethylene bags at temperatures a little above freezing (Hartman et al. 1990). Fungicides
are often added to the water to prevent damping off. After stratification, the seeds are planted
into flats or small pots in a sterilized mixture of equal parts sand and peat moss. Germination
generally occurs within 9 to 10 days, and the seedlings are transferred to larger pots filled
with commercial soil mix when they are 15 cm tall.
10.6.3 Evaluation techniques
Pear seedlings have a long juvenile period and remain unfruitful for 4 years or longer (Visser
et al. 1976, Bell 1991). The length of this juvenile period appears to be dependent on how
rapidly the seedlings grow (Zimmerman 1972). Therefore, growing conditions are generally
optimized as much as possible. Other factors have been evaluated to hasten flowering such as
grafting onto bearing trees or quince rootstocks, root pruning and sprays with growth
retardants, but these practices have proved generally ineffective (Zimmerman 1972,
Verhaegh et al. 1988).
There are some seedling characters, such as disease and insect resistance, that can be
conducted in the greenhouse to allow for early seedling selection (Dayton et al. 1983). These
preliminary screens can be followed with an additional screening in the nursery at close
spacing, to verify resistance ratings and eliminate escapes. Some juvenile characters are
correlated with economically important adult characteristics. Bell et al. (1996) list some of
these in P. communis such us seedling vigor with precocity (Zimmerman 1972), juvenile
period with precocity of propagated trees (Visser and De Vries 1970), juvenile period with
red leaf coloration (Zielinski 1963), early flowering before leaf break with high yield (Moruju
and Slusanschi 1959), seed size with germinability and seedling vigor (Schander 1955), and
fruit color with foliage color.
When the plants do fruit, selection is undertaken on such characters as precocity and
productivity, fruit quality, harvest dates and ripening uniformity. The storage and processing
qualities of the most promising types may also be evaluated if there is enough fruit. The
selected genotypes are then evaluated as grafted trees at normal orchard spacing in one

20
location, and then the most promising are further evaluated as grafted trees at multiple
locations.
10.7 Biotechnological approaches to genetic improvement
10.7.1 Genetic mapping and QTL analysis
A wide array of molecular markers have been developed for pear including isozymes,
restriction fragment length polymorphisms (RFLPs), randomly amplified polymorphic DNA
(RAPDs), amplified fragment length polymorphisms (AFLPs) and inter simple sequence
repeats (ISSRs) (Chevreau and Bell 2006). This work has primarily focused on fingerprinting
cultivars and measuring diversity patterns within and among Pyrus species (Monte-Corvo et
al. 2000, Yamamoto et al. 2002a and b, Inoue et al. 2007, Katayama and Uematsu 2006). In
general, cultivars have been readily distinguished through these approaches and species have
clustered according to traditional taxonomic classifications. Many SSRs isolated from apple
have been shown to be transferable to pear (Yamamoto et al. 2001 and 2002c, Pierantoni et
al. 2004), and vice versa (Fernández-Fernández et al. 2006).
Physical maps of chloroplast DNA (cpDNA) have also been generated using restriction
analysis. In a comparison of Asian and Occidental pear species, Iketani et al. (1998) found
four cpDNA haplotypes in Asian pears, but only one in occidental pears. They argued that the
two groups have evolved separately, and that considerable hybridization and introgression
has occurred between species. Katayama and Uematsu (2003) developed a cpDNA map of
Pyrus ussuriensis var. hondoensis and then carried out an RFLP analysis on cpDNAs from
representatives of Pyrus pyrifolia, P. ussuriensis, P. calleryana, P. elaeagrifolia and P.
communis. Two mutations, a recognition-site mutation and a length mutation (deletion), were
found in the cpDNA of P. pyrifolia cultivars.
Several genetic linkage maps have been developed of pear. Isozyme loci were used to
identify three linkage groups in pear that shared a high level of synteny with apple (Chevreau
et al. 1997). Iketani et al. (2001) employed RAPD markers to construct linkage groups of 18
and 22 for the two Asian pears, 'Kinchaku' and 'Kosui'. The linkage map for 'Kinchaku'
contained 120 loci in 18 linkage groups across 768 cM, while that for 'Kosui' had 78 loci in
22 linkage groups spanning over 508 cM.
Yamamoto et al. (2002c) constructed a genetic linkage map of an interspecific cross between
European (P. communis cv. 'Bartlett') and Asian (P. pyrifolia cv. 'Housui') pears using
isozymes, AFLPs, SSRs and morphological traits from pear, apple, peach and cherry. In the
map of the female parent, 'Bartlett', 226 loci were identified on 18 linkage groups over a total
length of 926 cM. In the male parent, 'Housui', 154 loci were represented on 17 linkage
groups encompassing a genetic distance of 926 cM. The position of 14 SSRs from apple
could be placed on the pear map, along with a few SSRs from Prunus.
Pierantoni et al. (2004) used 100 apple SSRs to develop linkage maps of two European pear
families. A total of 41 markers were positioned on the cross of 'Passe Crassane' x 'Harrow
Sweet', and 31 were placed on a map of 'Abbè Fetél' x 'Max Red Bartlett'. Considerable
colinearity was observed in the linkage relationships of the apple and pear genomes. Dondini
et al. (2004) expanded the map of 'Passe Crassane' x 'Harrow Sweet' with a wide array of
markers including SSRs, MFLPs (microsatellite-anchored fragment length polymorphisms),

21
AFLPs and RGAs (resistance gene analogs). They placed 155 loci on the 'Passe Crassane'
map consisting of 18 linkage groups with a coverage of 912 cM. On the Harrow Sweet map
they identified 156 loci on 19 linkage groups with a coverage of 930 cM.
Figure 10.3. QTL for fire blight resistance identified on the linkage groups of the fire blight
tolerant cultivar 'Harrow Sweet'. The probability of association of markers is indicated by the
LOD score. The black line indicates a LOD threshold value of 1.3, which is the 95%
probability value (Dondini et al. 2005).
DNA markers have been linked to a number of genes of horticultural importance in pear.
Banno et al. (1999) identified a RAPD marker that was closely linked to the gene A,
conferring susceptibility to black spot disease in Asian pear. Inoue et al. (2006) found a
RAPD marker linked to fruit skin color in Japanese pear. Iketani et al. (2001) found four
RAPD markers that were loosely associated with the pear scab resistance gene, Vnk; they
were able to place the resistance allele for pear scab and the susceptible gene for black spot
on their linkage map. Terakami et al. (2006) also found an SSR marker closely linked to the

22
scab resistance gene, Vnk, and converted the sequence into a sequence tagged site (STS)
marker, along with the four RAPD markers previously identified by Iketani et al. (2001).
Dondini et al. (2004) found four putative QTL for fire blight resistance (Figure 10.3).
10.7.2 Somatic cell genetics and genetic manipulation
Haplo-diploidization through gametic embryogenesis has been employed in pears to obtain
homozygous lines from heterozygous parents (Germaná 2006). Bouvier et al. (1993) found
haploid plants (2n = x = 17) among seedlings of 12 crosses of European pear and were able to
induce in situ parthenogenesis using irradiated pollen. The immature embryos were cultured
in vitro and 1 haploid, two misoploid and several diploids with the maternal phenotype were
recovered. In subsequent work, Bouvier et al. (2002) treated haploids with oryzalin in vitro to
generate doubled haploids. These were confirmed to be diploid and homozygous using
isozyme and microsatellite markers. Kadotat and Niimi (2002 and 2004) have produced
triploid plants of Japanese pear by anther culture, and tetraploids using in vitro colchicine
treatment.
Regeneration techniques have been developed for a wide array of elite pear cultivars and
genotypes, including those of P. communis (Chevreau et al. 1997, Matsuda et al. 2005,
Yancheva et al. 2006), P. pyrifolia (Lane et al. 1998); P. syriaca (Shibli et al. 2000), P.
pyraster (Caboni et al. 1999) and quince (Dolcet-Sanjuan et al. 1991, Baker and Bhatia
1993). Leaves from in vitro grown plants were used as explants in most work, with a brief
amount of callus growth at the wounding site before bud regeneration in about 3-6 weeks.
Explants were generally exposed to dark and light periods of 2 – 4 weeks. The most common
hormones employed were thidiazuron (TDZ) and naphthaleneacetic acid (NAA). Maximum
rates of regeneration varied from < 20% in P. pyrifolia (Lane et al. 1998) to > 80 % in P.
communis (Chevreau and Leblay 1993) and C. oblonga (Baker and Bhatia 1993).
Palombi et al. (2007) used in vitro regeneration of wild pear (P. pyraster) to generate
somaclonal variants for higher adaptability to calcareous soils. Selective treatments involved
Murashige and Skoog (MS) medium with Fe-EDTA replaced by equimolar amount of FeSO4
with KHCO3 or NaHCO3. Eleven putatively tolerant lines were obtained from vegetatative
shoot apices.
The first transformed pears were the European cultivars 'Passe Crassane', 'Conference' and
'Doyenne du Comice' (Mourgues et al. 1996). Since then, several other cultivars of P.
communis have been transformed including 'Vyzhnitsa' (Merkulov et al. 1998), 'Beurre Bosc'
(Bell et al. 1999), 'Barlett' (Bommineni et al. 2001), wild P. pyraster (Caboni et al. 1998), P.
betulifolia seedlings (Kaneyoshi et al. 2001) and the rootstocks GP217 (Lebedev and Dolgov
2000) and BP100030 (Zhu and Welander 2000). A number of different disarmed strains of
Agrobacterium tumifaciens were utilized in these studies, with kanamycin or hygromycin
being used as the selectable markers. In most cases, leaves were used as explants, with the
only exception being cotyledons of P. betulifolia.
Transformation strategies have been employed in a number of instances to improve disease
resistance. Researchers at INRA in Angers, France, have incorporated transgenes encoding
lytic peptides from insects (attacin and cecropin) (Reynoird et al. 1999), lysozymes from T4
bacteriophage (Mourgues et al. 1998), the lactoferrin gene from bovin (Malnoy et al. 2003a)
and a viral EPS-depolymerase gene (Malnoy et al. 2002) to develop resistance to fire blight.

23
The insertion of the lytic peptide gene (D5C1) was shown to be partially effective against
pear psylla (Puterka et al. 2002). Lededev et al. (2002) introduced plant defensin genes into
pear to enhance pathogen resistance. Two genes have been cloned that may play a role in
defense against pathogens, polygalacturonase inhibitor protein (PGIP) (Stotz et al. 1993) and
thaumatin/PR5-like protein (Sassa and Hirano 1998). Malnoy et al. (2003b) searched for
pathogen-inducible promoters from tobacco that would work in pear and found two, str246C
and sgd24, which were responsive to inoculation by Erwinia amylovora.
In other transformation work, the phophinotricin acetyl transferase (PAT) gene was
incorporated into pear rootstocks to produce herbicide resistance (Lebedev et al. 2002b), and
the super sweet gene thaumatin II was used to modify fruit taste (Lebedev et al. 2002c). The
rolC gene from Agrobacterium rhizogenes was introduced into pear to cause dwarfing (Bell
et al. 1999) and enhance rooting (Zhu and Welander 2000). The gene encoding S-
adenosylmethionine hydrolase (sam-k) was incorporated into 'Bartlett' to improve postharvest
quality and self-life by modifying ethylene synthesis (Bommineni et al. 2000).
10.7.3 Mutation breeding
Irradiation (X-rays) has been used to increase the frequency of mutations in fruit trees
(Ahloowalia et al. 2004). Several kinds of mutations have been identified after irradiation in
P. communis including bloom time, blossom color, ripening time, fruit color (Decourtye
1970, Roby 1972a and b, Predieri and Zimmerman 2001) and growth habit (Visser et al.
1971, Lacey 1975, Predieri and Zimmerman 1997). In P. pyrifolia, mutations have been
induced that effected disease resistance (Matuda et al. 1997) and self-compatibility (Hirata
1989). At least five European and four Japanese pears have been developed through mutation
breeding. One of them, 'Gold Nijisseiki', has had a substantial impact on the Asian pear
industry, and two new self-compatible, black spot resistant varieties show high promise
(Ahloowalia et al. 2004).
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