Pyrus pyrifolia (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Pyrus pyrifolia (N.L. Burman) Nakai

Protologue: Bot. Mag. Tokyo 40: 564 (1926).

Family: Rosaceae

Chromosome number: 2n= 34

Synonyms

• Pyrus serotina Rehder (1915),
• Pyrus sinensis auct. mult. (non Poiret 1816, nec Lindley 1826).

Vernacular names

• Oriental pear, nashi, sand pear (En)
• Poire orientale, nashi (Fr)
• Indonesia: apel jepang
• Malaysia: lai
• Philippines: peras
• Thailand: sali
• Vietnam: lê.

All names, except nashi, refer to other Asian Pyrus species as well. In this text "nashi" is used for the modern high quality pears grown under intensive cultivation; "sand pear" for the hardy traditional types with gritty flesh; and "oriental pear" is used more generally and denotes pears from East Asia.

Origin and geographic distribution

P. pyrifolia is one of a number of Pyrus species originating in north-east and east Asia; wild groves occur in the Szechuan region of southern China. Interspecific hybridization, selection and clonal propagation have given rise to the cultivated oriental pears. P. pyrifolia is the principal progenitor for cultivars in warmer areas where winters are mild; these cultivars range from the hardy sand pears with their gritty fruit to the nashi, the best of which produce fruit of supreme quality. P. pyrifolia was first cultivated in China and Japan, which are still the centres of production. Commercial production has spread to Korea, Taiwan, and more recently to the United States, Australia and New Zealand. Scattered trees and small orchards of sand pears are found in most tropical highlands in South-East Asia, mainly in north Thailand and the drier parts of Indonesia and the Philippines.

Uses

The fruit is eaten fresh, in fruit salads, or more rarely, canned. Fruit of indifferent cultivars is sometimes dried or candied. The nashi is the most important fresh fruit in Japan after citrus and apples. The timber is used for furniture.

Production and international trade

No statistics are available on oriental pears as a group, but world production of the nashi was 1 600 000 t in 1988, of which 1 000 000 t was produced in China. Japan produced 500 000 t on 18 700 ha. About 4% of the production of Japan is exported to the United States, Europe and Singapore. The market demands excellent quality fruit, for which very high prices are paid. Retail prices reached US$ 2 a piece in Japan in 1985 and over US$ 5 per kg in Indonesia in 1989.

Properties

The ideal oriental pear has firm, crisp, juicy flesh, without stone cells. The taste should be very sweet with a delicate flavour reminiscent of cucumber and rosewater, the skin golden yellow or silvery green and with no russeting or blemishes. Fruit weight ranges from 150-500(-1500) g, large fruit generally being preferred.

Few analyses distinguish between the European and the oriental pear. The edible portion of the fruit is about 92% of fruit weight and contains per 100 g: water 85 g, protein 0.4 g, fat 0.2 g, carbohydrates 10-12 g, fibre 1.5 g, and ash 0.4 g. Malic acid in ripe fruit ranges from 0.3-2.5%.

Description

• Erect small deciduous tree, sometimes spiny, up to 15 m tall; young branches glabrous or more often woolly-pubescent.
• Leaves ovate-oblong, rarely ovate, broadly ovate or ovate-orbicular in cultivars, 7-15 cm × 4-6 cm, rounded at base, acuminate at apex, sharply setose-serrate with usually slightly appressed teeth, glabrous or slightly floccose when young; petioles 3-4.5 cm long.
• Inflorescences in terminal umbellate corymbs, mainly on spurs, glabrous or floccose at first, flowering with 6-9 flowers before or with unfolding of leaves, pedicels 3-5 cm long; flowers 3-3.5 cm in diameter; calyx tubular, lobes long-acuminate, almost twice as long as tube; petals ovate, white; stamens 20, anthers usually red; ovary (4-)5-locular, styles (4-)5, glabrous.
• Fruit a pyriform to subglobose pome, 2-3 cm in diameter but up to 15 cm in cultivars, brown or yellow, often reddish-brown; flesh rather hard, in wild and primitive types with many stone cells; walls of locules cartilaginous.
• Seeds blackish.

Growth and development

Oriental pears generally have an upright growth habit. Shoots grow in flushes, the number of leaves being limited in spurs and indeterminate in long shoots. The annual cycle of growth is similar to that described for the apple. Little chilling is required and in most monsoon climates in tropical highlands the annual cycle persists more or less clearly. The trees flower early in the dry season, following quiescence and gradual loss of leaves in the preceding months; the leafless period tends to be short in the tropics. Self-incompatibility is common, but parthenocarpic fruit set occurs, especially if the trees flower in warm sunny weather in the tropics. Fruit ripens after 4-5 months.

Other botanical information

Cultivars of pears can be divided into three main groups originating from 3 species: Pyrus pyrifolia , P. ussuriensis Maxim. and P. communis L. Many interspecific hybrids exist, also with the other species listed below:

Pyrus ussuriensis is a species from north-east China, eastern Soviet Union, Korea and Japan and is also cultivated in northern Indo-China, Canada and the United States. It is the most cold-resistant pear species with small sweet-sour, attractive and aromatic fruits.

Pyrus pashia D. Don occurs from Afghanistan, India and northern Burma to south-west China. It is the species most adapted to a hot humid climate. It is cultivated on a very limited scale in Indo-China and northern India. Its fruits are small and rather insipid. They need to be stored for several weeks to become edible. The flesh of some cultivars is nearly black.

Pyrus bretschneideri Rehder originates in western and central China; it has sweet, juicy, white-fleshy fruits and is also cultivated in Japan and the United States.

Pyrus lindleyi Rehder originates in China and is also cultivated in Japan and northern Indo-China.

Cultivars of P. pyrifolia are sometimes classified as a botanical variety: var. culta (Makino) Nakai, with large fruits and leaves when compared with the wild species. The first high quality, grit-free nashi cultivars "Nijiseiki" and "Chojuro" were chance selections made in Japan at the end of the 19th Century. They are still standard cultivars, but "Shinsui", "Kosui", "Hosui", "Shinseiki" and others have added to the choice.

Cultivars of P. communis (the European pear) have melting flesh and a more aromatic taste. Oriental pears introduced into the United States in the 1800s cross-pollinated with P. communis which had been introduced earlier. The resulting hybrids gave rise to several cultivars, e.g. "Baldwin", "Kieffer", "Hood" and "Carnes", the fruit of which resembles that of the oriental pear. "Kieffer" is often encountered in tropical highlands. The resistance to fire blight and the low chilling requirements of these cultivars attracted breeders, who tried to obtain P. communis hybrids resistant to fire blight and better quality low-chilling pears; the latter include "Flordahome" (from Florida) and "Seleta" (Brazil).

Ecology

P. pyrifolia is a tree of warm temperate and subtropical regions. It requires a growing season with much sunshine and without extremes of temperature and moisture supply. Frost during flowering and fruit set causes considerable crop losses. A cold season is required to break bud dormancy. Lack of chilling leads to delayed foliation and poor, uneven fruit set and weakens the tree. Cultivars differ greatly in their chilling requirements. In Taiwan where superior cultivars are grown above 1700 m elevation, "Hungshan" succeeds below 800 m. "Flordahome" and "Seleta" require only about 250 h below 7°C. In the tropics overcast skies during the rainy season and drought reduce chilling requirements, probably because these factors enhance leaf fall. Protection against wind is important, particularly for high-quality cultivars, because the fruit is easily damaged by rubbing against branches, and even leaves.

Oriental pears are reasonably tolerant of drought and excess soil moisture during the rest season and can be grown on a wider range of soils than most other fruits.

Propagation and planting

In Asia cultivars are mostly grafted onto seedlings of P. pyrifolia. P. betulaefolia Bunge may be used as a rootstock on heavy wet soils and on light calcareous soils. P. calleryana Decaisne is generally used as a stock in Florida and has been recommended for hot climates. Most nashi cultivars appear to be incompatible with the European pears and quinces (Cydonia oblonga Miller). Dwarfing rootstocks are not available. Rootstocks are grafted or budded during the rest period, preferably at the beginning. Nashi seeds should be planted immediately after they have been extracted from the fruit. Once dried, they pass into a dormant state, which can only be broken by stratification.

In the tropics hardwood cuttings are used as propagation material, perhaps because of a shortage of viable seeds. This implies that only cultivars that root easily from cuttings have become popular.

Spacing ranges from 7 m × 7 m in the tropics to 6 m × 4 m elsewhere. The hardy trees may be planted along the contours of sloping land or on the edges of terraces to help control erosion.

Husbandry

The extreme differences between the common sand pears and the best nashi cultivars are reflected in growing techniques. The upright habit of the sand pears facilitates intercropping during the early years. The weight of the first substantial crops - after 6-7 years, when an adequate number of spurs has been formed - bends the branches, transforming tree habit from upright to spreading. Hence, little formative pruning is required. Spurs have an economic life of some 7 years; after that, the old wood is gradually pruned to stimulate new growth. Although no pollinators are planted, good crops are produced in the tropics, at least by some cultivars (e.g. "Kieffer"). Crop care is limited to weeding, manuring and harvesting; little or no crop protection is practised. Depending on the climate, dinitro-orthocresol (DNOC) or cyanamides may have to be sprayed to obtain earlier and better budbreak.

The top nashi cultivars in Asia, on the other hand, are grown in orchards which receive intensive care. The trees are trained on a horizontal wire trellis about 1.80 m above the ground. Irrigation is considered very desirable. The annual labour requirements per ha in Japan and Taiwan indicate the intensity of the production system: training and pruning: 1270 h, soil management and fertilizer applications: 250 h, crop protection (15 treatments): 60 h, artificial pollination: 230 h, fruit thinning: 680 h, fruit bagging: 1680 h, harvesting: 950 h! Cultivars are combined for cross-pollination, but if the bees are lured away by other flowering plants, the trees must be pollinated by hand. Thinning aims at leaving 1 fruit per 20-30 leaves to obtain fruit of the preferred size.

Few horticultural techniques match the sophistication of a grafting technique employed in Taiwan to advance the nashi harvest. Spurs with flower buds are collected from good cultivars in the highlands and grafted on "Hungshan" trees in the lowlands; 100-200 spurs per tree. The operation is timed so that the grafts flower at the same time as the "Hungshan" trees; cross-pollination ensures good fruit set for both. Before being grafted, the spurs are kept in cold storage to supplement chilling; they produce their crop - 1 to 4 fruits per spur - well ahead of the trees in the highlands.

Experiments show that if the trees are defoliated after harvest (before the buds have entered deep dormancy) it is possible to force the spurs to flower once more to obtain an off-season crop! Weather conditions in early spring are not always congenial and many refinements in the grafting and pollination methods have evolved to ensure success even in inclement weather. In the tropics where weather is good enough to allow parthenocarpic fruit set, the methods could probably be greatly simplified.

Diseases and pests

Whereas the common sand pear suffers little from diseases and pests, the improved cultivars require fairly intensive crop protection. "Nijiseiki" and several other older, clear-skinned cultivars are very susceptible to black spot disease caused by Alternaria kikuchiana ; fruits require bagging and about 10 applications of organic phosphorus for protection. Other important diseases are scab (Venturia nashicola) and rust (Gymnosporangium haraeanum). Nashi is less susceptible to fire blight (Erwinia amylivora) than the European pear and some cultivars have considerable resistance. Trunk canker (Phomopsis sp.), nectria canker (Nectria cinnabarina) and powdery mildew (Phyllactinia corylea) may cause problems in some cultivars.

A very large number of pests attack the nashi, especially the oriental fruit moth (Grapholita molesta), oriental pear moth (Monema flavescens) and peach fruit moth (Carposina niponensis). Birds of several species are another serious problem.

Physiological disorders such as hardened fruit, fleshy spot disease and water core occur in some cultivars. The latter may be prevented by early harvesting.

Harvesting

The base colour of the nashi turns yellow when mature, and a colour chart is a useful guide to establish optimum maturity for harvesting. Timing is important, because most of the gain in weight occurs during the last three weeks.

Yield

Under the very intensive management system in Japan, yields of 45 t/ha per year of excellent quality fruits can be obtained. Average commercial production is 25-40 t/ha per year. There is no information on yield levels in the tropics, but growers consider oriental pears to be good and regular croppers.

Handling after harvest

Fruit of the improved nashi cultivars is the most expensive pome fruit on the market, and only immaculate fruit is accepted. As the skin is delicate and easily damaged, the fruit should be picked with great care and be packed immediately in suitable containers. Alveolar trays are used for this purpose and each fruit is covered with a sleeve of polystyrene netting. Most nashi cultivars, except the early ones, store fairly well: about two weeks at ambient temperatures, and up to 4(-6) months under cold storage.

Genetic resources and breeding

Being of very old cultivation, hundreds of cultivars have been developed in China and Japan. Most breeding work in Japan is done at the National Fruit Tree Research Station, Yatabe, Ibakari and is based on the cultivars "Nijiseiki" and "Chojuro". It aims at developing cultivars which combine excellent fruit quality and a wide range of maturity dates with resistance against diseases, especially against black spot disease. Breeding work in Taiwan concentrates on reducing chilling requirements, and selections have been made which do not show delayed foliation even at an altitude of 100 m.

Prospects

The oriental pear appears to be an under-utilized fruit crop in the tropics, particularly in South-East Asia. The common types are hardy and productive, and although fruit quality is inferior to that of the choice nashi cultivars, the fruit is firm, juicy and sweet; properties which generally appeal to consumers in Asia. Moreover, a number of improved low-chilling cultivars have been bred. Some of them might widen the scope for pears in the highlands, where - particularly at higher elevations - the choice of suitable fruit crops is rather limited. The Taiwan example opens a further perspective: there may be scope for intensive systems to produce high-quality pears in the tropics.

Literature

• Buwalda, J.G., Klinac, D.J. & Meekings, J.S., 1989. Effects of time and degree of fruit thinning on fruit size and crop yield at harvest for four nashi cultivars. Scientia Horticulturae 39: 131-141.
• Hsu, H.T. & Lin, S.C., 1987. Oriental pear breeding for high fruit quality and adaptation to subtropical lowlands of Taiwan. In: The breeding of horticultural crops. Proceedings of an International Symposium at National Chung Hsing University, Taiwan, December 1986. FFTC Book Series 35. pp. 95-100.
• Kanato, K. & Kajiura, I., 1982. The ideal Japanese pear. In: van der Zwet, T. & Childers, N.F. (Editors): The pear. Horticultural Publications, Gainesville, Florida, USA. pp. 138-155.
• Lin, H.S., Lee, C.L. & Lin, C.H., 1987. Production of high chilling Asian pear in Taiwan's lowland. Acta Horticulturae 199: 101-108.
• Seike, O., 1973. Japanese pear culture. Farming in Japan 7: 27-30.
• Sherman, W.B. & Crocker, T.E., 1982. Low chilling pears. In: van der Zwet, T. & Childers, N.F. (Editors): The pear. Horticultural Publications, Gainesville, Florida, USA. pp. 130-137.
• Thibault, B., 1985. Peut-on cultiver les nashi et les li en France? L'arboriculture fruitière 376: 30-37.
• Uraki, M., 1982. The culture of Nijiseiki pears in Tottori prefecture, Japan. In: van der Zwet, T. & Childers, N.F. (Editors): The pear. Horticultural Publications, Gainesville, Florida, USA. pp. 77-81.

Authors

L.P.A. Oyen