Prunus (PROSEA Fruits)
- Protologue: Sp. Pl.: 473 (1753); Gen. Pl. ed. 5: 213 (1754).
- Family: Rosaceae
- Chromosome number: x = 8
Major species and synonyms
- Prunus mume Siebold & Zuccarini, Fl. Jap. 1: 29, t. 11 (1826), synonym: Armeniaca mume (Siebold & Zuccarini) Carr. (1885).
- Prunus persica (L.) Batsch, Beitr. Entw. Pragm. Gesch. 1: 30 (1801), synonyms: Amygdalus persica L. (1753), Persica vulgaris Miller (1768).
- Prunus salicina Lindley, Trans. Hort. Soc. London 7: 239 (1830), synonym: Prunus triflora Roxb. (1832).
Information on minor Prunus species in South-East Asia is presented in the chapter on minor edible fruits and nuts.
P. mume :
- Japanese apricot (En)
- Thailand: boir, foung
- Vietnam: phung, mo'.
P. persica :
- peach, nectarine (En)
- Pêcher (Fr)
- Indonesia, Malaysia: persik
- Philippines: peras
- Laos: khai
- Thailand: makmuan (northern), tho (Chinese), hung mon (Chiang Mai)
- Vietnam: dào.
P. salicina :
- Japanese plum (En)
- Indonesia, Malaysia: ijas jepang
- Laos: 'mân 'luang, tsi keu
- Vietnam: mân.
In this text the word "plum" refers both to the European plum as well as to the Japanese plum.
Origin and geographic distribution
The genus Prunus mainly occurs in the temperate regions of the northern hemisphere; some species originate in tropical highlands.
P. mume is native to China, Japan and probably also to the northern parts of Laos and Vietnam. It is cultivated there and in northern Thailand, but not elsewhere in South-East Asia.
P. persica is native to China, where it has been cultivated since ancient times. Cultivation spread from China all over the world, in temperate climates as well as in tropical highlands; in many highland areas - also in South-East Asia - the species is naturalized.
P. salicina originates in China and has been cultivated there and in Japan since ancient times. It is now grown worldwide, mainly in the subtropics and tropical highlands. It is the most important cultivated Prunus species in Indo-China, but in South-East Asia the trees are rare.
All 3 species are grown for their fruit. Japanese apricots are always preserved by drying and salting or pickling, but the green fruit may be eaten as a vegetable. Peach and plum are eaten fresh as well as preserved in the form of jam, fruit halves in syrup, juice, wine, liqueur and other alcoholic drinks. Blossoming trees are very ornamental and flowering branches of Japanese apricot and peach trees in Indo-China are used as New Year decoration.
Production and international trade
Production of Japanese apricots goes unrecorded and is probably negligible compared with that of peaches and nectarines (8 165 000 t in 1988) and plums (6 590 000 t in 1988). Production of these latter crops is expanding. The main peach-producing areas are the United States, the Mediterranean and China; plums are grown mainly in China, Romania, Yugoslavia, Germany and the United States. Preserved Japanese apricots are a very common sight in Thai markets. The area of peach and nectarine in northern Thailand was about 90 ha in 1987. Elsewhere in South-East Asia Prunus fruits are found only in scattered pockets in highland areas where they were introduced during the colonial period.
There is lively international trade, including fruit from tropical highlands: during winter Europe, for instance, imports substantial quantities of plums from Colombia.
- Deciduous or evergreen trees or shrubs; dormant buds with many imbricate scales. Leaves alternate and simple, mostly serrate; stipules small, caducous.
- Inflorescences umbel-like clusters or racemes, flowers sometimes solitary, usually opening in spring and often preceding the foliage.
- Flowers bisexual, white to pink; calyx lobes 5; petals 5; stamens many, inserted with the petals on the hypanthium; pistil 1, 2-ovuled, style elongated, stigma capitate.
- Fruit a drupe, with a fleshy pericarp in which is embedded the hard stone (the endocarp), usually enclosing a single seed.
P. mume :
- Shrub or tree, up to 10 m tall, root system deep, crown wide spreading.
- Leaves ovate to elliptic, 4-10 cm × 3-5 cm, long acuminate, finely and sharply serrate, pubescent or glabrous; petiole 1.5 cm long, glandular.
- Flowers solitary, appearing before the leaves, subsessile, pink or white, fragrant; petals suborbicular, stamens 35-40.
- Fruit globose, 2-3 cm in diameter, yellow or reddish-yellow, soft hairy; flesh dry, clinging to the pitted stone, with very sour taste but pleasant smell.
P. persica :
- Shrub or tree, up to 8 m tall, twigs glabrous.
- Leaves lanceolate, 5-8(-15) cm × 1.5-2 cm, acuminate, serrate, glabrous, shiny above, petiole and blade-base gland-bearing.
- Flowers appearing before the leaves, often a pair of flowers flanking the axillary leaf bud, sessile, pink, 2.5-3.5 cm diameter; stamens 35-40.
- Fruit subglose, 3-8 cm in diameter, fleshy, yellow to red; flesh white, yellow to red, stone deeply pitted and furrowed, very hard.
- Fruit tomentose in peach, glabrous in nectarine.
P. salicina :
- Shrub or tree, up to 10 m tall; twigs glabrous, becoming lustrous red-brown.
- Leaves oblong-obovate to oblong-ovate, 6-10 cm × 2-3 cm, abruptly acuminate, doubly obtuse-serrate, shiny bright-green above; petioles 1-2 cm long, with several glands.
- Flowers usually in clusters of 3, borne on spurs and appearing before or together with the leaves, white, 1.5-2 cm diameter; pedicels 1-1.5 cm long; stamens 25-30.
- Fruit globose-ovoid, 3-7 cm diameter, deeply indented at base, yellow, red or greenish, glabrous, often pointed at apex.
Growth and development
P. mume and P. persica types naturalized in the tropics grow spontaneously from seed, which germinates without a dormant period. Seeds of introduced peaches require 2-3 months stratification in a refrigerator to germinate. Extension growth continues over a long season, particularly in young trees, so that tree size increases rapidly. In peach laterals grow out while the shoot is still extending vigorously (sylleptic shoots or "feathers"); hence branching is not solely dependent on the breaking of dormant buds and there are few problems in shaping the tree. Young plum trees tend to grow several long, virtually unbranched shoots ("whips"). All three species flower laterally on long and short shoots; terminal buds do not flower. Shoots of Japanese apricot range from long (extension) shoots to intermediate and short (lateral) shoots, all of which flower to varying degrees. If tree vigour is adequate, peaches form few short shoots and flowering on long shoots determines the crop. Mature plum trees, on the other hand, produce large numbers of spurs; flowers on the few long shoots contribute little to the crop. Japanese apricots and peaches are self-fertile and excessive fruit set is a common problem in peach and nectarine cultivars in the tropics. In Thailand fruit quality improves up to a leaf : fruit ratio of 40 : 1; beyond 75 : 1 the vegetative condition dominates and fruit retention and flower initiation may be reduced. In plums self-fertility is the exception; even self-fertile cultivars bear heavier crops when cross-pollinated. Many Japanese plum cultivars are also cross-incompatible, so when planting an orchard cultivars must be chosen carefully.
The fruit of these 3 Prunus species ripens 80-120 days after flowering. Normally extension growth stops before harvest, but it may go on much longer, particularly in early cultivars. Eventually the buds go dormant, gradually entering a state in which they cannot be forced to leaf out, however favourable the growing conditions. At high latitudes this dormancy is broken by low temperatures (temperatures below 7°C are believed to be most effective) during winter.
In the tropics shade or overcast skies seem to take over, at least partly, the role of low temperature. In South-East Asia the growing season usually starts after the cool and/or overcast season, perhaps in response to the progressive loss of old leaves. This applies to Japanese apricots and the naturalized (inferior) peach type; improved cultivars of peach and plum require dormancy-breaking treatments, minimally consisting of defoliation. Alternatively, the buds can be forced to flower/leaf out before they have gone dormant, i.e. within about one month after harvest. If the climate is conducive to shoot growth at that time - as in parts of Central America - it is possible to obtain 2 crops per year from a tree, in a similar way as with apple and grape.
Other botanical information
The genus Prunus comprises about 200 species, many of which are of major importance for their edible fruit, and many are prized as ornamentals. In the temperate zones the Prunus fruits are collectively known as stone fruits to distinguish them from the pip fruits (apple, pear, quince, etc.). No agreement exists about the taxonomic delimitation of the genus and various classification systems are used in literature.
Koehne's system, as adapted by Rehder and Kalkman, is followed here.
Prunus is subdivided into 5 subgenera. The more important fruits and nuts, also for the tropics, are found in the subgenera:
- Prunus , including P. armeniaca L. (apricot), P. domestica L. (European plum), P. mume (Japanese apricot), and P. salicina (Japanese plum);
- Amygdalus (L.) Focke, including P. amygdalus Batsch (almond) and P. persica (peach and nectarine);
- Cerasus (Miller) Focke, including P. cerasus L. (sour cherry), P. avium L. (sweet cherry) and P. cerasoides D. Don (an ornamental cherry indigenous to northern Thailand).
Apricot trees are occasionally seen in the tropics, but they fruit irregularly and are not grown commercially. The European plum, almond and cherry are even less adapted to the tropics; some cherries grow well but they hardly flower and fruit.
Cultivars of P. salicina (and hybrids with other species, including P. domestica) often encountered in the tropics are "Methley" (most common, prolific, but small-fruited), "Santa Rosa" (the traditional standard for plum quality in the tropics), "Mariposa", "Kelsey" and "Ogden".
In addition to the local, naturalized Japanese apricot, the cultivars "Jen Tao" and "Ping Ting" - which require only 100-150 hours of chilling and hence are called low-chilling cvs - have been introduced from Taiwan to Thailand and are grown commercially by hill tribes.
Peaches and nectarines for the tropics derive from Honey and Peen-to, two groups of low-chilling peaches in South China, characterized by prominently tipped fruits. For decades breeding programmes in several subtropical countries have turned out improved cultivars which may do well in the tropics. In the 1980s trials with these cultivars were started in South-East Asia. Hill tribes in Thailand grow "Flordasun", "Flordared", "Flordabelle", "Earli Grande" and "Flordaprince" peaches, and the nectarines "Sunred" and "Sundowner". All these cultivars have been bred by the University of Florida.
Peach and nectarine cultivars are grouped according to a few distinctive fruit characteristics: soft or firm flesh, white or yellow flesh, and freestone or clingstone type (the latter indicate whether or not the stone separates easily from the flesh).
The 3 species require a warm growing season and a sufficiently cold rest season to break bud dormancy. Limiting factors at high latitudes are the risk of late frosts killing the flowers or fruitlets, and summer rains leading to diseases, including fruit rots. Late flowering and early ripening are therefore desirable. In the tropics ecological requirements depend on the crop cycle. In the case of an annual cycle, fairly high elevations (1200-2000 m) and a clearcut seasonal weather course (a sunny growing season and a cool or overcast resting season) are preferred. Where the crop cycle is shorter and the trees are cropped continuously, growing conditions should be favourable throughout the year, implying lower elevations (800-1400 m) to ensure year-round warm weather. In both cases the grower will have to force budbreak. There is no sufficient experience with either approach, particularly the latter, to specify ecological conditions.
Stone fruit grows on most soil types. Medium-textured soils are preferred, ideally more clayey in the topsoil and more sandy deeper down, as is found on alluvial river banks. Such soils combine good drainage with adequate water retention. The soil should be slightly acid.
Propagation and planting
Cultivars of all three species are generally propagated by budding; however, air layering is more common for the Japanese apricot in Thailand and Vietnam. Seedlings of the Japanese apricot can be used as rootstock; for peach and plum, seedlings of the naturalized peach are normally used by small growers. For commercial orchards, the cultivar "Okinawa" is the recommended rootstock. Plums grow well on peach stocks, but if plum rootstocks are preferred, these are clonally propagated from root cuttings since seedling populations are not uniform.
Seeds germinate faster and more uniformly if the stone is cracked to extract the seed. In northern Thailand many naturalized peach trees have been topworked with the introduced low-chilling cultivars. This results in substantial crops within 16 months, compared with a period of 3 years for newly planted trees.
Japanese apricots are spaced 7-12 m apart in the field, peaches and nectarines 7 m × 7 m to 6 m × 4 m, and plums from 7 m × 5 m to 5 m × 4 m.
The preferred tree shape is the vase, an open-centre tree with good light distribution even for larger tree sizes. The young tree is headed back to about 50 cm and 2-4 laterals are retained to become the scaffold branches. As the vase widens these branches are allowed to fork. Peach flowers on shoots that were formed in the previous season and pruning is required to stabilize the numbers of shoots in mature trees, to maintain a moderate shoot vigour (25-40 cm) and to make sure that these shoots are not borne ever further away from the scaffold branches.
In the case of plums the first laterals may grow into very long virtually unbranched "whips". In shaping the tree, the whips can be tipped, defoliated and bent down to force many axillary buds to leaf out. This leads to a rapid increase in tree volume through shoots that are more likely to form spurs that will eventually flower.
The main formative pruning is done at the start of the crop cycle; additional pruning may be needed - especially if tree vigour is excessive - while the fruit is on the tree. Additional pruning aims at better light distribution; this is important since partly-shaded shoots tend to flower the next season on the exposed side only. Plums require less pruning than peaches, but to maintain fruit size the fruiting wood should be gradually rejuvenated.
To start a new crop cycle, trees must be defoliated. Since the leaves do not detach easily and trees are fairly large, chemicals are used: sodium chlorate (0.5-1%), copper or zinc sulphate and recently cyanamide (all about 2%).
Orchards are clean-cultivated, preferably with a mulch under the trees. During the rainy season weeds are slashed or a cover crop may be grown. Irrigation is desirable and a must if crop cycles are shortened. Nutrients are applied at the beginning of the growing season, with one or two top dressings during the next 2-3 months. If nutrients are in the form of compound NPK fertilizers, the ratios should be approximately 15:4:12 or 12:4:17, that is, low in phosphorus and potassium content depending on soil fertility. The recommended rate is 200-400 g per tree per year, multiplied by tree age in years, with a maximum of 3 kg/tree/annum.
Fruitlets should be thinned after natural drop and before the seed hardens (about 50 days after flowering), since later thinning has little or no effect on fruit size. Often about half the fruitlets need to be removed. Fruits should be spaced 7.5-15 cm apart, depending on cultivar and leaf size.
Diseases and pests
Diseases of stone fruit in South-East Asia include peach leaf curl (Taphrina deformans), brown rot (Sclerotinia fructicola), scab (Venturia carpophila) and rust (Tranzschelia discolor). These are all fungi which thrive under wet conditions; they are not serious if the growing season is not too wet. The most harmful insect is the oriental fruit fly; the maggots infest peach and nectarine fruits. The main control methods are baiting the flies and destroying infested fruit so that the insect cannot breed. Plums suffer sometimes from persistent attacks by aphids. If biological controls fail, specific insecticides can be used.
In the northern part of Thailand and Vietnam Japanese apricots and early peaches are harvested in March-April, plums follow in May-June, all well ahead of the harvest season elsewhere in the northern hemisphere. In East Java the natural season is September-October following flowering in July. Fruits on a tree do not ripen simultaneously; a cultivar may be picked 2 times a week for 2 weeks. The fruit is picked by hand with the utmost care, using ladders to reach the fruit.
Yield depends very much on what fruit quality is considered marketable. It would seem that 10-25 t/ha per year is possible with the current cultivars and growing techniques.
Handling after harvest
In Thailand peaches are sized in 4 grades, the 2 best grades being sold fresh, while the lower grades are processed into pickles, dried and salted peach, and jam. Single-layer packing and utmost care are required in handling fruit for the fresh market. Shelf life is not long, but where the infrastructure is good it suffices for the fruit to reach markets anywhere.
So many low-chilling cultivars emerge from peach breeding programmes in the United States, Brazil and South Africa, that testing their adaptability to the tropics is a formidable task; criteria are needed to select the most promising new cultivars. With regard to low-chilling plums, the first results of deliberate breeding have only just reached the testing stage.
Temperate-zone fruits have received relatively little attention in South-East Asia, with the exception of the Japanese apricot and strawberry in northern Thailand and the apple in East Java and Timor. Elsewhere in tropical highlands the stone fruits have come to the fore, but in South-East Asia peaches, nectarines and plums are relatively unknown. Whereas most of South-East Asia may be too wet for these fruits, there are drier highland areas where even the production of 2 crops per year may be feasible. However, a sustained effort is required, first in testing potential areas and secondly in experimenting with crop husbandry.
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