2. Major Fruits

2.1 Longan

(Sapindaceae, 2n = 30)

2.1.1 Names:

Scientific: Dimocarpus longan Lour. (Syn. Euphoria longana Lamk., Nephelium longana Cambess, Euphoria morigera Gagnes., and Euphoria scandens Winit & Kerr.)

Vernacular:

English: Longan
French: Longanier, Oeil de Dragon
Indonesia: Lengkeng
Malaysia: Lengkeng
Philippines: -
Thailand: Lamyai
Vietnam: Nhan

2.1.2 General: Longan is a sub-tropical fruit in the family Sapindaceae and a prolific bearer thriving in monsoonal regions (pronounced wet and dry seasons). It is closely related to lychee having similar leaf and flower characteristics but the fruit is quite different, both in color (brown instead of red) and skin texture (smooth instead of bumpy). The major difference between lychee and longan, however, is the taste of fruits. While lychee has both a sweet and sour taste with a pleasing aroma, longan has only sweet with no sour taste and very little aroma. Many people, especially the Chinese, love to eat longan as they believe the fruit provides vitality and energy and possibly has some medicinal properties.

Longan is considered a traditional fruit of Vietnam with the main production areas being in the south: Tien Giang, Ben Tre, Dong Thap, Vinh Long, Can Tho, Baria Vungtau; and in the north: Lao Cai, Yen Bai, Thai Nguyen, Phu Tho, Son La, Hung Yen and Thanh Hoa.

In 1997, total planted areas for longan were 60 000 ha, and reached 75 200 ha in 2002 (of which 49 367 and 14 530 ha were in the Mekong delta and the southern region, respectively). The annual production for 2000, 2001, and 2002 were 535 500, 508 200 and 620 000 tons, respectively (Hong and Chau 2004). About 65-70% were consumed as fresh fruits domestically, while the rest is exported in the form of dry fruits

2.1.3 Origin and distribution: The origin is uncertain; some think the mountainous chain in Myanmar through Southern China while others think Southwest India and Sri Lanka including the lowlands. The crop is mainly grown in southern China, Taiwan and Thailand. Smaller growing areas are found in Vietnam, Cambodia, Laos, Queensland (Australia) Indonesia and Florida (United States). Since ancient times, the Chinese have grown longan in the south, especially in the provinces of Fujian, Guangdong and Guangxi (mentioned in literature in the period of Emperor Cheng Tang in AD 1223). Longan growing extended to India, Sri Lanka, Myanmar then to Australia, the United States (Florida and Hawaii), Cuba, West Indies, and Madagascar in later years.

Longan in Vietnam is classified into subspecies (ssp.) longan consisting of two botanical varieties, var. longan and var. longepetiolulatus. The first botanical variety, var. longan, is a native of southern China and is commercially grown in southern China, Taiwan and northern Thailand, with small acreages in Queensland (Australia) and Florida (United States). It is also commercially grown in northern Vietnam. The second botanical variety, var. longepetiolulatus, probably originated in southern Vietnam and is commercially grown only in this area.

2.1.4 Description: The longan is closely related to the lychee and is similar in growth and fruiting habit. The tree is tougher and less demanding than the lychee that produces fruits in cooler and drier areas. It is a medium to large (10 to 20 m tall) evergreen tree with a dense canopy, brittle wood and corky bark that splits and peels. The tree shape depends greatly on the cultivar variying from erect to spreading. The inflorescence is large (30 to 50 cm long), multi-branched and leafless. The flowers are small and yellow brown. The fruits are similar to those of the lychee, but smaller, smoother and yellowish-tan in color. The translucent flesh or aril is white to off-white or pinkish surrounding a large red-brown, brown, or black seed that separates easily from the flesh, which is sweet and juicy. It is milder in flavor and less acidic than lychee.

Two botanical varieties, longan and longepetiolulatus, of the subspecies longan are grown in Vietnam. The former, var. longan - grown in the north has small petals and requires a cool climate for flowering and fruiting. The latter, var. longepetiolulatus - grown in the south, has large white petals and does not require cool climate for flowering and fruiting. The longan fruit can be eaten fresh, frozen, dried or canned, with canned fruits being considered better than canned lychees.

2.1.5 Ecology: Being a subtropical plant, longan (Dimocarpus longan spp. longan, var. longan) requires a cool climate for its growth, particularly for flowering that is found in the north. However, it can also grow well in tropical areas where there is a prominent change of seasons necessary for satisfactory flowering. A short, cool winter season brings about prolific bloom. Good quality, eating lychee will not flower in these conditions. Many cultivars of var. longepetiolulatus can be grown in southern Vietnam where the winter is not cold.

Longan requires higher soil moisture than lychee; suitable annual precipitation is about 1 500 to 2 000 mm. The tree cannot tolerate drought (as lychee does), but can tolerate shady conditions. Longan prefers soil with a pH of about 6.0 to 6.5 and cannot tolerate acidic soil. Unlike lychee, which can tolerate poor soil, longan prefers sandy soil with high organic matter.

2.1.6 Genetics and improvement: Genetic improvement of longan has not been actively conducted. Selection of the best types among existing cultivars and open-pollinated seedlings has been made in China, Florida (United States), Taiwan, Thailand, Australia and Vietnam. At present, most cultivars in Vietnam are of inferior quality as they have large seeds and very juicy flesh.

2.1.7 Major cultivars in Vietnam: ‘Nhan’ is a local term for longan. It is customary to use it with some longan cultivars but not others for unknown reason. The most popular cultivars in the north are ‘Nhan Lông’ and ‘Nhan Cui’, both of which produce large fruits with small seeds (80 to 100 fruits/kg). In the south, the popular cultivars are ‘Nhan Long’ and ‘Tieu Da Bo’; both have large fruits (80 to 90 fruits/kg). Recently, many cultivars were introduced from other countries and are undergoing cultivar trials. See details in Table 4 below.

Table 4. Longan cultivars in Vietnam with their fruit size and location of cultivation

* Data not available

2.1.8 Propagation: Longan is normally propagated by marcotting (air layering) and the root development is lengthy. Layering can be done at the beginning of the rainy season with the marcots remaining on the mother plant for four to five months. As marcotted plants have a shallow root system, they should not be grown on hillsides. In the north, longans are often propagated by inarching. This procedure involves taking a slice graft from a rootstock grown in a plastic bag and splice grafting on to the branch of the mother tree. The graft is bound with tape until the graft takes; this may take two to four months depending on the weather. Better results are obtained when the scions are taken from healthy mother trees.

2.1.9 Planting: In northern Vietnam, a spacing of 8 x 8 m is generally practiced, but in the south where smaller longan cultivars are grown, a spacing of 5 x 5 m is practiced.

2.1.10 Pests and diseases: Serious pests include the lychee stinkbug (Tessartoma papillosa), which causes greater damage to longan than lychee; fruit borer (Conogethes punctiferalis), mirid leaf miner (Acrovercops cramerella), mealy bug (Pseudococcus sp.), spherical mealy bug (Nipaecoccus sp.), and grasshopper (Oxya sp.). The fruit-eating bat attacks longan fruits when they are ripening. The only prevention is to enclose fruit bunches in a bag or cover the whole tree with a fine plastic or nylon net. Powdery mildew (Oidium sp.) is considered a serious disease for longan

2.1.11 Fruiting season: In the south, production is all year round (with peak period during June-July). In the north, the main season is from July to August. Some newly selected cultivars can be harvested in early September.

2.1.12 Harvesting and yield: Longan fruits are non-climacteric and have to be harvested when ripe. The mature longan fruit has a dark, smooth skin, the inside of which is netted and has sweet taste. Normally, picking is done twice at an interval of 7 to 10 days. Whole panicles are cut with secateurs and bunched together before being packed in a container. In northern Vietnam, longan gives a yield of about 4 to 5 t/ha, while in southern Vietnam, the yield is about 6 to 8 t/ha, although of lower quality. However, a higher figure of 8 to 10 t/ha is provided by NIAPP (1994). Adverse environmental factors, such as rain and cold weather at flowering, drought at fruit set and maturity, play an important role in the yield of longan. Many longan cultivars have a biennial bearing habit unless fruit thinning and good fertilization and water management are practiced. Yield is also determined by its alternate bearing habit.

2.1.13 Post-harvest operations: After harvesting, longan panicles are sorted and bunched together and then placed in a container lined with longan leaves. Sulphur dioxide fumigation may be applied to prolong the shelf life of longan fruits.

2.1.14 Problems: Among the problems of producing longans fruit bats are the most difficult to control since the only way to prevent the fruits from bat damage is by bagging or netting the trees. Prolonging shelflife of the fruits is another problem with longan, although the fruit may be dried to assist preservation.

2.1.15 Prospects: As there is a high domestic demand, longan can be a profitable crop for Vietnam as there seems to be no major climatic problems affecting growth and development in both the north and the south. Exports of fresh, dried and canned longans to southern China and neighboring countries like Singapore and Hong Kong are becoming very significant. However, a high quality standard must be maintained for these export products.

2.2 Lychee

(Sapindaceae, 2n = 28, 30, 32)

2.2.1 Names:

Scientific: Litchi chinenis Sonn.

Vernacular:

English: Lychee, litchi
French: Ce’risier de la Chine, Litchi de Chine
Indonesia: Litsi (In), Klengkeng (Jv), Kalengkeng (Md)
Malaysia: Laici, Kalengkang
Philippines: Letsias
Thailand: Linchi
Vietnam: Vai

2.2.2 General: Lychee has been traditionally cultivated in Thanh Ha district of Hai Duong province. Other production areas are: Bac Giang, Lang Son, Quang Ninh, Thai Nguyen, Cao Bang, in the Northeast, and Hung Yen and Ha Tay.

2.2.3 Origin and distribution: A native plant of south China/northern Vietnam (evidence: wild lychee trees were found by a French scientist in 1942 at the foot of Ba Vi Mountains in northern Vietnam, and in 1970, a famous Vietnamese pomologist, Dr. Vu Cong Hau, found wild lychee trees growing at the foot of Tam Dao and Truong Son Mountains in Binh Province). Wild lychee forests are known in Guangxi province of southwestern China. The climate of northern Vietnam is quite suitable for the growing lychee trees. In Asia, it is cultivated in China, India, Thailand, Taiwan and Vietnam. Elsewhere it is cultivated in Australia, Madagascar, South Africa, and Mauritius.

2.2.4 Description: The lychee tree is round-topped and slow growing with smooth, gray, brittle trunk and limbs. Trees may grow to 12 m in height and have a, dense, rounded, symmetrical canopy extending nearly to the ground. Trees are very attractive with dark green foliage and reddish-colored fruit. The leathery, pinnate leaves are divided into four to eight leaflets. They are reddish or bronze or yellow when young, depending on cultivar and becoming shiny and bright green when mature. The inflorescence is a raceme. Flowers are small, white /greenish, and are borne in terminal clusters that emerge at the ends of branches, with male and female flowers borne separately.

Lychees are eye-catching in spring when huge sprays of flowers cover the tree. There are three flower types: (1) staminate flower, (2) hermaphrodite flower functioning as female flower, and (3) hermaphrodite flower functioning as male flower, all borne on the same inflorescence, but bloom at different times. Male flowers contain a lot of small pistils in the center of the flower, surrounded by 6-8 stamens; the anthers are yellow, releasing pollens when mature. Female-functioned hermaphrodite flowers clearly have visible pistils with two-carpelled ovary, each carpel consists of one ovary; mostly one carpel of the ovary develops into a fruit while the other carpels disintegrates; it has a short filament and the anther does not release any pollen. The male-finctioned hermaphrodite flower is similar to the female-functioned hermaphrodite flower, but the tip of the pistil does not split to allow pollen to adhere, rather the anther breaks open and releases the pollen. Flowering precedes fruit maturity by approximately 140 days. Fruits are borne in terminal loose clusters, which emerge at the end of the branches.

Fruit are borne in loose clusters of 3 to 50 fruits, are round to oval and 25 to 38 mm in diameter. The skin (pericarp) ranges from yellow to pinkish or red and is leathery, with small, short, conical or rounded protuberances. The edible portion (aril) is white, translucent, firm and juicy, with excellent subacid flavor, which is sweet, fragrant and delicious. Fruit contains one shiny, dark brown seed, usually relatively large, but it may be small and shriveled (called chicken-tongue) in some cultivars. Fruit must be ripened on the tree for best flavor.

In some areas lychees tend to be alternate bearers. Fruit splitting is usually caused by fluctuating soil moisture levels.

2.2.5 Ecology: Lychee trees grow well in the tropics and warm subtropics where winters are short, dry and cool but frost free, and summers are long and hot with high rainfall and humidity. Lychees require seasonal temperature variations for best flowering and fruiting. Warm, humid summers are best for flowering and fruit development and a certain amount of winter chilling is necessary for flower bud development. Most varieties need between 50 and 150 hours of standard chilling (0-10^oC). Cool winters with low rainfall are ideal for lychees. The trees become hardier as they age. Mature trees have survived temperatures as low as -4^oC, when fully hardened off. Young trees may be killed by a light frost. Good protection from wind is essential for good harvest.

Being subtropical in origin, lychee requires cool, dry weather especially to induce flowering. For a good harvest, bright sunshine is needed during fruit development period and ripening. If the weather in some years is not cool and dry as the result of rain or even light showers occur during November-December, flowering and/or fruiting do not take place, or do so very sparsely. The habit of erratic bearing of lychee is well known among lychee growers.

Lychee does not like moist climate, especially during flowering. However, lychee requires adequate water for its vegetative growth and fruit development. It is tolerant to hilly acid soils with pH levels of 5.0 to 5.5. However, rich soil with adequate nutrients as found in alluvial or riverside soils with pH levels of 6.0 to 6.5, are preferred.

2.2.6 Genetics and improvement: Genetic improvement has been by selection of existing cultivars, or selection among open-pollinated seedlings. The objectives of selection are: regularity of bearing, high yield, large fruit size, seedlessness or small seed, change in ripening period, flavor and bright-red fruit color.

2.2.7 Major cultivars in Vietnam: ‘Vai’ is a Vietnamese term for lychee and is usually placed in front of cultivar names. The most common cultivar is ‘Vai Thieu’, a native cultivar grown exclusively in northern Vietnam. It has sweet, dry fruit and is regular bearing. Its harvest period is between mid-June to mid-July (late cultivar). Two other cultivars are ‘Vai Lai’ which is derived by somatic mutation from ‘Vai Thieu’ with most characters of ‘Vai Thieu’ except for maturing early by 5 to 7 days; ‘Vai Chua’, a rather sour but otherwise pleasant taste with not as many problems with bearing as ‘Vai Thieu’, matures quite early (20 April to 5 May). Other less common cultivars are listed in Table 5, below:

Table 5. Lychee cultivars in Vietnam with their characters

* Data not available

2.2.8 Propagation: Air layering (marcotting) is the most common method of propagating lychees in Vietnam with the advantage of retaining characteristics of the mother plant. Lychee trees do not come true to seed; seedling trees may take ten or more years to bear fruit and are not reliable producers of quality fruit. Grafting is difficult.

Air layering is quite simple. Mature lychee trees are marcotted twice a year with each mature tree producing 20 cuttings per season. The only disadvantage is that a marcot does not have a strong root system as it lacks the taproot, thus the tree is easily blown by strong wind. The shallow root system means the tree is more affected by long periods of drought than a plant grown from seedling. This has led to an investigation to propagate lychee by grafting, using a rootstock which develops from seedling. Selection of rootstock is important for successful growth of the grafted tree. It must be healthy with a straight stem bearing large leaves and long internodes. The scion must be taken from healthy mother tree that has been properly managed and pruned to produce many new branches to be used as scions. The correct tools and experienced nursery workers are needed to achieve good results. Suitable periods for grafting are between March and April and between September and October. Shade is required for newly grafted materials so that they are not exposed to bright sunlight.

2.2.9 Planting: Normal spacing for a lychee plantation is 8 x 8 m. Depending on soil fertility, the holes of variable sizes (e.g. 60 cm diam. x 40 cm deep) are dug and filled with organic matter such as farm manure and compost. In northern Vietnam, planting is done in January to February when residual moisture is available. The young plant does not require watering since it normally obtains water from the rain during March to May.

2.2.10 Pests and diseases: Major insect pests of lychee are: lychee stem-end borer Conopomorpha sinensis); lychee mid vein borer (C. litchiella); aphid (Aphis gossypii and Toxopleura sp.); lychee stink bug (Tessaratoma papillosa); small green beetle (Platymycterus sieversi) and lychee erinose mite (Eriophyes litchi). Diseases of high economic importance are: lychee sudden death (Pythium sp. and Fusarium solani), anthracnose (Colletotrichum gloeosprioides) and lychee brown blight (Peronophthora litchi).

Good orchard management through the use of biological control, and eradication practices such as the use of pheromone and protein bait against lychee stem end borer and fruit fly, in combination with rational use of fungicide to control brown blight anthracnose are some of the key techniques for their control.

2.2.11 Fruiting season: In northern Vietnam, early-maturing cultivars of lychee flower in February and fruits are harvested in May, while late-maturing cultivars flower in March and fruits are harvested in June. Some early cultivar (e.g. ‘Vai Chua’) can be harvested as early as late April.

2.2.12 Harvesting and yield: Maturity is judged by a particular shape, skin color, and texture, and flavor of each cultivar. The fruit must be allowed to ripen fully on the tree. Overly mature fruit darken in color and lose their luster. The flavor sometimes lacks the richness associated with a certain amount of acidity. Fruit are harvested by cutting the main stem bearing the fruit clusters several inches behind the fruit clusters. Fruit may or may not be detached from the fruit clusters before shipping and storage. Ripe fruit are sweet, plump and of the size and color characteristic of the cultivar. Fruit picked while immature are not sweet, have poor flavor and will not improve with time. In a plantation with full-grown trees, a yield of 7 to 8 t/ha is expected. However, NIAPP (1994) gave a little higher yield of 8 to 9 t/ha.

2.2.13 Post-harvest operations: Lychees are best consumed as soon after harvest as possible. Not only does lychee have a short harvest period, its shelf life is also very short. Fruits are delicate, and with high water and sugar contents, they become spoiled through rotting when exposed to high temperatures. Browning of the peel occurs rapidly at warm temperatures and low relative humidity. Several difficulties are encountered in storage and transportation of the lychee fruits as the color of the skin changes to dark color if kept in a dry place, and the fleshy aril rots in high temperatures. Fruit may be stored for 2 to 3 weeks at cool temperatures. Fruit may be placed in sealed polyethylene bags and stored in the home refrigerator for 7 to 10 days and will retain reasonable color and freshness. However, a new technology of treating the fruits with sulfur dioxide gas, and soaking in a mild acid solution, has solved the problem, resulting in longer shelf life of bright red-colored fruits. Another technique involves the dipping of lychee fruits in the solution of CBZ fungicide. Fruits may be kept for 5 to 6 days at ambient temperature with CBZ treatment while the quality and appearance of fruits remain good and the skin does not turn brown. A combination of refrigeration and fungicidal treatment can keep fruits fresh for a longer period of time up to 3 weeks at 12°C, and 4 weeks at 2°C.

2.2.14 Problems:

2.2.14.1 Confusing cultivar names: In different localities, lychee cultivar names are quite confusing. This can be solved by the use of advanced technologies such as isozyme, RLPD, or PCR (facilities of the latter are now available at RIFAV) to identify cultivar.

2.2.14.2 Lack of elite cultivars: At present, there is only one popular cultivar of lychee. If this cultivar is grown extensively, it is likely to be wiped out by epidemic diseases or pests as the result of its narrow genetic base. This can be solved by selection of good cultivars for propagation. Some high quality cultivars have been introduced into Vietnam at Luc Ngan in the north by a UNDP/FAO project with VEGETEXCO, about 1986. Only grafted materials should be provided to the local farmers for planting.

2.2.14.3 Short harvesting period: Lychee harvesting period is rather short (about one or one and a half months). This makes it difficult for the growers to sell all their harvests as most ripen during a short period. Early and late varieties would expand harvesting period. At the same time, consumers would have more varieties to choose from according to their preference.

2.2.14.4 Lack of technology: Current technologies on lychee cultivation and post-harvest technologies in Vietnam are limited and this is reflected in low yield and low quality. The major technical gaps are: (1) inferior planting materials resulting from poor parent materials and outdated propagation technologies (marcotting), and (2) lack of technical package for orchard management in terms of pruning, fertilization and pest and disease control.

2.2.15 Prospects: The following prospects are envisaged:

2.2.15.1 Suitable climatic condition: North Vietnam has an ideal climatic condition for the growth of lychee. Even with rather poor, hilly soil, lychee is growing well. There is substantial scope for expanding production from existing plantations through better orchard management.

2.2.15.2 High market demand: Lychee is a fruit that is loved by most people. Located close to southern China, Vietnam thus has the advantage of exporting lychee fruits to southern China, which cannot produce enough to meet its demands. It is estimated that as much as 30% of lychees produced in Bac Giang Province alone are exported to China. The arrangement is that about two weeks before the harvest, middlemen active in the export trade, identify those trees in each province that will bear the best fruit, and then contract with the owner to purchase the entire produce of these selected trees. In some cases, Chinese middlemen are introduced to farmers by local Vietnamese liaisons. Lychees exported to China are mainly destined for Guangxi Province. They are transported by trucks to Lang Son, a Vietnamese border town - by the Vietnamese as well as the Chinese middlemen. The lychee fruits are packed in cardboard boxes enclosed by a layer of ice inside a plastic cover. These special boxes are provided to the Vietnamese farmers by the Chinese purchasers to help maintain the freshness of the fruits while they are in transit to China.

2.2.15.3 High potential for processing: Lychee is a fruit with pleasing aroma and taste, both in fresh state and after processing. The latter includes drying of whole fruits, canning of the flesh (seed removed), and juice, all of which are in high global demand as few countries in the world can produce them.

About 40% of lychees produced in Vietnam are processed. The most common form of processing is drying of the whole lychee. Coal is used for firing the drying process and the conversion factor from fresh to dried lychee is about 5:1. The main market for dried lychees is China. A small amount of lychee is also converted into wine by a few private units operating in the province.

2.3 Mandarin

(Rutaceae, 2n = 18)

2.3.1 Names:

Scientific: Citrus reticulata Blanco

Vernacular:

English: Mandarin (En), tangerine (Am)
French: Mandarinier
Indonesia: Jeruk Keprok, Jeruk Jepun, Jeruk Maseh
Malaysia: Limau Langgat, Limau Kupas, Limau Wangkang
Philippines: Sintones
Thailand: Som KhieoWan
Vietnam: Quyt (sometimes spell Quit)

2.3.2 General: Mandarin is one of the popular fruits in Vietnam. However, domestic production is much less than the demand, mainly because of the occurrence of several diseases, notably greening and other virus-like diseases that cause premature death. In order to satisfy domestic demand, mandarin has to be imported from China. Main production areas of mandarin in Vietnam are: Can Tho, Tien Giang, Vinh Long, Ben Tre, Dong Thap, Tra Vinh, Dong Nai, Ha Giang, Tuyen Quang, Yen Bai, Lang Son, Hoa Binh and Nghe An provinces.

2.3.3 Origin and distribution: Mandarin originated in Southeast Asia and the Malaysian Archipelago although some authors specify Indo-China as the area of origin. In many countries, mandarins have been selected and hybridized creating new cultivars, for exmple, the Satsuma mandarins in Japan, King mandarins in Indo-China and the Mediterranean mandarins in Italy. At present mandarins are widely cultivated in all tropical and subtropical regions of the world. Main production areas of mandarins in Vietnam are: Can Tho, Tien Giang, Vinh Long, Ben Tre, Dong Thap, Tra Vinh, Dong Nai, Hoa Binh, Nghe An and Lang Son.

2.3.4 Description: The common mandarin is a small tree with slender twigs. Flowers are borne singly or in small clusters in the axil of the leaves. Fruits are a depressed, globose or subglobose berry with thin, loose peel, easily separating from the segments and bright orange or scarlet-orange when fully ripe. The King mandarin is an upright, small tree; the fruit is large with thick peel and 12 to 14 segments.

2.3.5 Ecology: Mandarin prefers loamy sandy soil with some clay and good drainage. A pH level of 5.5 to 6.5 is preferred. The tree is moderately drought resistant able to survive long dry periods. Air-layered mandarins with a shallow root system are preferred for areas with a high, fluctuating water table where deep-rooting seedling stocks would suffocate in water- saturated soils.

2.3.6 Genetics and improvement: There are few reports on genetic improvement of mandarins. Most existing cultivars are the result of selection of chance seedlings or somatic mutations.

2.3.7 Major cultivars in Vietnam: As mandarin is called ‘Quyt’ in Vietnamese, it is customary to use the word ‘Quyt’ in front of a given cultivar of mandarin, for example. ‘Quyt Chun’. Mandarins grown both in the north and in the south of Vietnam comprise three groups:

2.3.7.1 First group: True mandarin (Citrus reticulata). This group has thin, rather loose skin, and includes such cultivars as ‘Quyt Duong’, ‘Quyt Vo Mong’, ‘Quyt Do’, ‘Quyt Chun’, ‘Quyt Bac Son’, ‘Quyt Hong’, ‘Quyt Vang’, ‘Quyt Chu Sa’, ‘Quyt Dong Khe’, ‘Quyt Ta’, ‘Quyt Tieu’, and ‘Quyt Vo Mong’. The fruit is sweet with little sour taste and is highly valued domestically.

2.3.7.2 Second group: King mandarin once botanically known as C. nobilis, is now known as a natural hybrid of mandarin and orange (C. reticulata x C. sinensis). Vietnamese use the local name ‘Cam’, which means ‘orange’ in front of the cultivars of King mandarin. ‘Cam Sanh’ is a popular cultivar of King mandarin in Vietnam. It has large fruit with thick, loose, yellow-red skin making transportation easy. The pulp is deep yellow, with proportionately sour and sweet taste. Those grown in the south do not develop yellow-red skin and their taste is less sour.

2.3.7.3 Third group: This comprises a natural hybrid of mandarin and pummelo (C. reticulata x C. maxima). Again, the Vietnamese use the local name ‘Cam’ at the beginning of the name; the most popular cultivar of this group is ‘Cam Bo Ha’. See details in Table 6.

Table 6. Mandarin cultivars in Vietnam, their characters and location of cultivation

2.3.8 Propagation: Like orange, mandarin can be propagated by marcotting and grafting. Several related species of citrus, such as Volkerameriana spp., Troyer Citange, Trifoliata etc., can be used as rootstock for mandarin. The suitable periods for grafting are March to April and October to November since the bark is easily removed at that time and there is a good sap flow in the stem.

2.3.9 Planting: Spacing would usually be 2.5 x 2.5m, or 3 x 3m or other arrangements, depending on whether the grower intends to intercrop mandarins with another secondary crop like pineapple. A hole about 60 cm diameter and 40 cm deep is dug. A mixture of natural compost, and fertilizer is mixed with the soil removed from the hole, placed in the bottom of the hole and the tree planted on top.

2.3..10 Pests and diseases: In Vietnam, as well as in various other tropical countries in southeast Asia, mandarins are subject to attack by a wide range of citrus pests and diseases, particularly the phloem-vein virus, Liberibacter (Huanglong bin / greening) and tristeza. Other diseases include phytophthora root rot (Phytopthora palmivora), citrus canker (Xanthomonas campestris p.v. citri), citrus scab (Elsinoe fawcetti), sooty mold (Capnodium citri), melanose (Diaporthe citri), gummosis (Phytophthora nicitianae var. parasitica, P. citropthora, P. palmivora) and brown rot (Phytopthora sp.), to name a few. In certain instances psorosis on mandarin is a serious problem in the south, and post-harvest losses from fruit rots (Phytopthora sp.) can be high. Poorly drained soils and low budding of trees cause serious losses from phytophthora root rot (Phytopthora sp.). Greening or yellow shoot is potentially the most serious disease of mandarins. The most effective control measures for this disease are:

(i) micro-grafting to eliminate the pathogen and produce the free-disease mother plants for propagation

(ii) provide optimum orchard management with emphasis on the vector (psylid - Diaphorina citri) control to protect agaisnt the re-infection for healthy growth.

Major insect pests of citrus, including mandarins are leaf miner (Phyllocnistis citrella), green shoot borer (Adoxophyes privatana), brown stem borer (Coleoptera and Lepidoptera), citrus rust mite (Phyllocoptruta oleivora, Panonychus citri, Polyphagotarsonemus latus), fruit fly (Bactrocera dorsalis), sucking moth, hopper (Hypomeces squamosus), mealy bugs Pseudococcus brevipes) and scale insects (Planococcus citri, Pseudococcus citriculus, Saissetia coffeae, Chrysompalus ficus, Pulvinaris aurantii, Icerya purchasi, etc.), and citrus aphids (Toxoptera aurantii, Toxoptera citricidus). Control of the citrus aphids is often poor in many plantations and thus spreading of tristeza disease would occur easily. Control of citrus psyllid (Diaphorina citri) that transmits greening is highly desirable to reduce the spread of this serious citrus disease. The peak of occurrence of the citrus psyllid population coincides with February-March, June-August and October-November growth flushes. Diflubenzuron gives good control of nymphs at 100 ppm. Phosmet, Dichlorvos, Deltamethrin and Dimethoate also help in controlling these pests.

2.3.11 Fruiting season: In the south: August-February (peak period, December-February); in the north: October-April (peak period, January-March).

2.3.12 Harvesting and yield: Fruits should be harvested when fully mature for the best quality fruits.. Fruit should be cut from the tree to avoid damage; this is especially important with the loose-skin cultivars. The yield is about 5 to 7 t/ha (8 t/ha given by NIAPP, 1994), depending on the healthiness of the orchard. Potential yield would be two to three times higher if trees are healthy.

2.3.13 Post-harvest operations: Mandarins have a thin skin and should be handled with care.. However, the thick-skinned King mandarin can tolerate normal handling quite well. The fruits should be sorted by size and grade (colour and peel quality) before being packed in a box.

2.3.14 Problems: Main problems are the lack of good, disease-free cultivars and facilities to multiply them and the farmers’ inexperience in disease control. Bacterial, virus and virus-like diseases are major problems. Most trees are infected with a virus or mycoplasmamaking them weak and unproductive. Consequently, mandarin growers have to recover their investment in the orchard during the few years after the trees begin bearing and before trees decline.

2.3.15 Prospects: If virus and mycoplasma diseases can be solved by introducing known technologies, such as shoot tip grafting and disease indexing, and culling infected trees, mandarin cultivation in Vietnam has great potential. There are no problems with respect to the weather and soil and demand for fresh and processed fruits are high for both domestic and export markets. For large-scale mandarin growing to be successful in the future, the following measures are suggested:

(i) High quality, good yielding cultivars and rootstocks, propagated from clean planting materials, should be selected for adaption to different localities;

(ii) Effective control of pests and diseases should be implemented, particularly with respects to virus and virus-like disease transmission,which require systematic control measures using new technologies in nurseries and orchards;

(iii) Intensive cultivation techniques should be investigated to improve the yield and quality of fruits;

(iv) Post-harvest treatment should be properly administered in order to improve fruit quality.

2.4 Mango

(Anacardiaceae, 2n = 40)

2.4.1 Names:

Scientific: Mangifera indica Linn.

Vernacular:

English: Mango
French: Manguier
Indonesia: Mangga, Mempelan
Malaysia: Mangga, Mempelan
Philippines: Mangga (Tg), Paho (By), Mango (Il)
Thailand: Mamuang
Vietnam: Xoai

2.4.2 General: Mangoes have traditionally been cultivated in the Central and Southern parts of Vietnam. Mangoes are not commercially grown in the North, except in Mai Son and Yen Chau districts in Son La province, Yen Minh district in Ha Giang, and Lai Chau province where the winter temperature is quite high and humidity is low (which is considered very favorable for pollination and fertilization of mangoes). However, mangoes are often grown as ornamental or shade trees in home gardens and pagodas in many inland provinces in the North. Main production areas in Vietnam are Tien Giang, Dong Thap, Can Tho, Vinh Long and Khanh Hoa. To the east of Cochin China, mangoes are grown in Dong Nai and Baria- Vung Tau province. There has been a proposal to cultivate more mangoes in the North in order to have enough for local consumption without having to rely on transporting from the other regions.

2.4.3 Origin and distribution: Originating in Indo-Burma region, and SE Asia region, mangoes have been cultivated in countries such as India for thousands of years. Mangoes spread throughout southeast Asia about 1500 years ago and are now found in all the tropical and subtropical continent countries of the region..

2.4.4 Description: An evergreen, erect, branched tree usually 10 to 30 m tall, 60 to 120 cm trunk diameter with long taproot and dense, surface mass of feeder roots. Flowers are small, with male and hermaphrodite flowers borne on a long panicle inflorescence. Fruit is a fleshy drupe, very variable in shape, size and color, but usually ovoid-oblong 30 x 10 cm, yellowish-green to reddish. The flesh is variable in thickness and color (yellow to orange), can be fibrous or non-fibrous. Taste can be juicy and sweet to turpentine flavored. Seeds are either monoembryonicor polyembryonic, depending on the cultivar and area of origin. Most Indian cultivars are monoembryonic, while almost all SE Asian origin mangoes are polyembryonic.

2.4.5 Ecology: Mango thrives both in the sub-tropics and tropics. In the sub-tropics, the cold months ensure excellent floral induction but late frost is a major risk that destroys the tender parts of the trees and flowers or young fruits. In the tropics, the mango grows almost everywhere up to 1200 m elevation, but for good commercial fruit production, a prominent dry season lasting more than three months is necessary. Mangoes prefer climates with much sunshine and little rain. A flush of flowers is produced in the dry season. However, flowering is rather erratic in the tropics due to variable temperature and rain falling at the wrong time, and high humidity. Generally, above about 600m in Asia, the climate becomes too cool and often too humid for the commercial cultivars, which have an optimum temperature of around 24 to 27°C.

Weather conditions available in North of Vietnam include rather low November and December temperatures, much sunshine and little rain. These should create favorable conditions for mango flowering, but no commercial production is viable, because cold northwesterly winds, cloudy skies and drizzling rain occur when the mango flowers in January and this causes poor fruit set, seedless small fruits, and a high incidence of diseases that prevent fruit set and development. However, recent research has yielded three cultivars (GL 1, GL 2, and GL 6) that flower quite late in April, and thus allows successful pollination and fertilization to produce fruits with good shape and appearance. These cultivars are suitable for growing in the North. In the South, the conditions are better for mango as rain ceases in November and bright sunshine in December to February, thus induces profuse flowering and fruiting.

Mangoes tolerate a wide range of soils and moisture conditions. They are drought-tolerant and flood-tolerant to a certain extent, especially if grown from polyembryonic seedlings or grafted onto good polyembryonic rootstocks with strong, well-developed taproots. Easy access to water and nutrients tends to stimulate growth at the expense of flowering. Mangoes prefer acid soils with an optimum pH range of 5.5 to 7.0.

2.4.6 Genetics and improvement: Very few reports on genetic improvement of mangoes are available in Vietnam. Existing cultivarswere obtained from selection within populations of spontaneous seedlings. However, a number of cultivars were obtained from breeding programs in other countries, notably India so many commercial cultivars are based on Indian monoembryonic germplasm. Some have been selected as first or second-generation seedlings from a known mother tree, or as chance seedlings of unknown origin.

2.4.7 Major cultivars in Vietnam: ‘Xoai’ is a Vietnamese term for mango. It is customary to use it in front of the hundreds of mango cultivars in Vietnam. The major cultivars in the South are ‘Xoai Cat Hoa Loc’, ‘Xoai Cat Chu’, ‘Xoai Hon’, ‘Xoai Xiem Num’, and ‘Xoai Buoi’ (Table 7). In the North, three new cultivars have recently been released by Research Institute for Fruits and Vegetbles (RIFAV)- GL 1, GL 2 and GL 6 (GL is an abbreviation of Gia Lam, the place where RIFAV is located). These grow well in northern regions of Vietnam, producing flowers quite late (towards the end of April when the weather is warmer, drier and the temperature is higher), which is suitable for pollination and fertilization. ‘Xoai Yen Chau’ is another cultivar that has been grown for a long time in Son La Province where the winter climate is milder than the rest of northern Vietnam.

Table 7. Mango cultivars in Vietnam, their charcters and location of cultivation

2.4.8 Propagation: Polyembryonic cultivars can be propagated by seedling, but vegetative propagation is now more common, as is the case for monoembryonic cultivars. Although cuttings and air layering can also be used, they are insignificant on a commercial scale. Cuttings and marcotting can give a certain success but they are not commercially significant. For budding or grafting, seedlings are raised from polyembryonic cultivars. Seeds are placed on their edge with the dorsal (most prominently curved) edge upwards, to produce a straight stem and taproot. Germination time can be reduced by 7-10 days by removing the endocarp prior to sowing. Grafting can be done at any rootstock age, but the rootstock stem should be sufficiently woody and about the size of a pencil to support cuts for grafting. Various methods of grafting are practised in Vietnam while inarching is preferred in Thailand and India, wedge grafting in the Philippine. Veneer grafting or patch budding are also practiced elsewhere.

2.4.9 Planting: At one to two year-old, grafted rootstocks that have been hardened-off are preferred for field planting. Planting should be done in the rainy season. Spacing varies from 8 to 12 x 8 to 12 m although much closer spacing, such as 2.5 x 2.5, 3 x 3, 4 x 4 m are becoming more popular in Thailand, especially when grown on raised bed in the central provinces where water table is high. Vietnam would be able to adopt a similar system, and in fact in the Mekong delta area, high density planting of mango is used on raised bed converted from paddy fields.

2.4.10 Pests and diseases: Among the most serious diseases are: big eye bug (Idioscopus sp.) that sucks sap from the young shoots and inflorescences; fruit fly (Bactrocera dorsalis) which lay eggs in the fruit and the resulting worms eat the mango flesh; leaf eating caterpillars (Papilio demoleus and P. polytes), stem borers and tip borers (Zeuzera coffeae) which may cause die-back of the whole branches.

Major diseases include anthracnose (Colletrotrichum gloeosporioides) causing circular or sunken black marks on leaf, flower and young fruit, and stalk-end fruit rot (Diplodia spp.).

2.4.11 Fruiting season: In the South fruits are harvested during February to May (peak period in March-April) while in the North fruiting season can last from May to September.

2.4.12 Harvesting and yield: The fruit is handpicked, either by climbing the tree or by using a picking device attached to a bamboo pole. Assessing maturity from the appearance of the fruit is difficult. One way is the length of time after flowering, which is 81 to 85 days in the South, the other is to observe the colour. Fruits begin to turn from green to pale yellow and the fruits become more aromatic. However, experience allows growers to choose the right time for harvest. The average yield of 7 to 8 t/ha is expected, although a higher yield can be obtained with well-maintained orchards and with high-yielding cultivars.

2.4.13 Post-harvest operations: The fruit is perishable and must be marketed as soon as possible. It is graded by size and appearance. The fruit stalk is removed to stop the burn from the corrosive sap and so that it does not puncture other fruit. Since the fruit bruises easily, packing plays an important role in keeping the fruit in good condition enroute to the market. Single-layer trays partitioned into single-fruit compartments, are preferred. Fruits are normally washed and dipped in fungicide or hot water to control anthracnose (Colletotrichum gloeoosporides) and fruit fly (Dacus dorsalis). Cold storage can allow for up to one month of storage for some select cultivars. Most will only keep in cool stroage of around 12^oC for 2-3 weeks.

2.4.14 Problems: Main problems include the lack of good cultivars for specific areas, and the lack of technologies in orchard management, which often results in poor yield and quality. Being a seasonal bearing fruit, mango is available only within a short period of about one to two months. During this period the price is quite low whereas off-season fruit can be sold at a much higher price. Since mango cannot be grown commercially in the North, it has to be transported from the South, adding a high cost to the consumers. A number of cultivars grown in Vietnam are of low quality and the overall yield is low due to poor management practices.

2.4.15 Prospects: Mango is the most sought after seasonal tropical fruit but production in Vietnam remains small. With improved technologies in production and post-harvest operations, including the new techniques of producing off-season fruits, there is a great prospect for large-scale cultivation of mango in Vietnam, both in the South where it has traditionally been grown, and in the North, which has potential to grow selected cultivars in given areas.

2.5 Orange

(Rutaceae, 2n = 18)

2.5.1 Names:

Scientific: Citrus sinensis (L.) Osbeck

Vernacular:

English: Orange, sweet orange
French: Oranger doux
Indonesia: Jeruk Manis, Chula, Choreng
Malaysia: Limau Manis
Philippines: Kahel
Thailand: Som Tra, Som Kliang
Vietnam: Cam (orange), Cam Ngot (sweet orange)

2.5.2 General: Orange is imported from China to satisfy domestic demand since production is hampered by the occurrence of several diseases, particularly greening disease. The main production areas are in the northern provinces of Ha Giang, Yen Bai, Lang Son, Phu Tho, Hanoi, Thanh Hoa, Ninh Binh, Nghe An and Ha Tinh. Oranges are also grown in the South, mainly in Can Tho, Tien Giang, Vinh Long, Ben Tre, Dong Thap, Tra Vinh, Dong Nai.

2.5.3 Origin and distribution: Originating near the border of China and Vietnam, orange has been cultivated almost everywhere in the subtropics and tropics between latitudes 40°

N and 40°S; near the equator they grow at about 2000 m altitude.. Oranges are considered one of the most important fruits in the world.

2.5.4 Description: Orange is a small evergreen tree, 5 to 8 m tall, with a rounded crown. Bisexual flowers are born singly or as few-flowered racemes, which emerge from leaf axils. Fruit is subglobose, 4 to 12 cm in diameter, greenish-yellow to bright orange; peel up to 0.5 cm thick; segments have juicy pulp, yellow to orange-red in color.

2.5.5 Ecology: Being a sub-tropical plant, orange thrives best in the cool climate of northern Vietnam, especially in the highlands. The cold nights allows the fruit to develop a deep orange color, on the tree at maturity and a pleasant aroma and an ideal combination of sweet and sour.

Orange can also be grown in the South of Vietnam where temperatures and humidity and rainfall are much higher than the North.. Such climatic conditions have different effects on oranges. These include:

(i) Growth period: In the South, it takes only 6 to 6.5 months from flowering to fruit maturation while in the North, 8 to 9 months are required.

(ii) Cropping cycle: In the South, two crops of orange can be produced per year, if water is provided, while in the North, only one crop can be obtained.

(iii) Fruit siz: High temperature and humidity in the South result in larger fruit than in the North.

(iv) Juiciness, sugar and acid content: Oranges in the South have a higher sugar content than those grown in the North because of exposure to long sunny period of high temperature. The acid content is lower, because of warmer nights, which makes these oranges less popular with foreigners who prefer an orange with mild acid taste.

(v) Orange color: Due to the absence of a cool night during the maturation period, there is no formation of anthocyanin pigments in the rind and in the vesicle, thus, they appear as green and greenish yellow, respectively. The situation is worse if the fruit is ripening in a period of high rainfall.

(vi) Flavor and aroma: Orange requires a cool climate with bright sunshine with warm days and cool nights for the development of optimum flavor, aroma and orange color. If grown in warmer climates like southern Vietnam, growth is faster and fruit quality is poor.

In northern Vietnam, rainfall is about 1 500 to 1 700 mm a year, and is well distributed. Thus, there is no need for supplemental irrigation since even in the dry season there are occasional rains. However, in certain years there may be a lack of seasonal rainfall, thus moisture must be maintained by watering. In the southern highlands, irrigation is necessary when high output is required. Elsewhere in the world, watering and climatic selection is used to induce flowering and ripening of oranges for specific period of harvesting.

Fertile soils are required for good fruit production. With infertile soils but with favorable climatic conditions, fertilizers, both organic and inorganic, must be applied. This needs to be done in northern Vietnam where the soil is rather poor, as the areas have been planted with staple food crops for many years and fertility has declined.

In the tropical conditions of the Southwhere heavy rainfalls wash away soil nutrients, additional fertilizers are needed, especially Potassium to avoidthe oranges becoming too sour for local tastes.

2.5.6 Genetics and improvement: As in most other citrus species, the improvement of orange depended for many years on the selection of desirable chance seedlings. Breeding programmes in the US, Spain, Italy, Japan and Brazil have yielded a number of improved cultivars. Vietnam has not undertaken any specific breeding programmes to date.

2.5.7 Major cultivars in Vietnam: Vietnamese use the term ‘Cam’ for any citrus, which resembles orange, and ‘Cam Mat’ for sweet orange (Citrus sinensis). For example, ‘Cam’ is used in front of the cultivar names of many citrus hybrids, not necessarily of orange parentage, as seen in mandarin x orange and mandarin x pummelo hybrids. It is also used for orange x pummelo hybrid such as ‘Cam Chanh’, ‘Cam Chap’, as well as sour orange (C. aurantium), as in ‘Cam Chua’. To confuse the issue, the term ‘Cam Mat’ is not used in front of cultivars of sweet orange, as in ‘Xa Doai’, ‘Van Du’ and ‘Song Con’. All three are major cultivars of northern Vietnam and are probably various forms of Valencia, one of the world’s most popular cultivars of orange. In the South, the very sweet ‘Tieu’ and ‘Duong’ (sugar) are popular. Descriptions of major cultivars of oranges are given below.

Xa Doai: named after the commune where the cultivar originated. It has sparse shoots with sharp thorns. Fruits are spherical or elliptical, 180 to 200 g. with good flavour and rather juicy.

Van Du: named after a locality where the cultivar was first introduced in the 1940s. Mature trees have a dense canopy with oblong leaves. Fruits are either spherical or elliptical, with thick skin, very juicy with lots of seeds and fiber. The tree can adapt to soils with low fertility.

Song Son: named after a state farm that was established in the 1960s. It is a local cultivar probably selected from an introduced cultivar. It has vigorous growth and a spherical canopy with dense twigs. Fruits are spherical, 200 g, with good, sweet flavor but many seeds.

2.5.8 Propagation: Although orange can be grown from seed, Vietnamese growers, like most around the world, do not use seedlings as the plants are thorny with a long juvenile period where theydo not produce fruit and are rather tall making harvesting quite difficult. Marcotting and budding are generally practiced with budding being more popular at present. Several rootstocks are used in budding, for example sour pummelo, ‘Chap’ (a natural hybrid of orange and pummelo) is used in the North, while ‘Mat’ orange and Volkamer lime are used in the South. The vegetative propagation ensures uniformity in fruit quality and yield of the trees but can result in the transfer of virus and virus-like diseases should marcots or buds be taken from infected mother trees.

2.5.9 Planting: After land preparation (mostly by hand on sloping areas or by small tractor on flat land) holes 60 cm diameter and 40 cm deep are dug by manual labour at a spacing of 3 x 3 m or more (up to 6 x 6 m).

2.5..10 Pests and diseases: Oranges are attacked by a wide range of insect pests including capricorn beetle (Chelidonium argentalum). The female lays eggs on small branch axils. After hatching, the young worms make holes in the branch, then in the trunk, causing portions above the holes to wither and die. Prevention is quite difficult but can be done by capturing adult insects, spraying with insecticide, removing dead branches and killing the worm inside the hollow branches. Other major insect pests include fruit fly (Bactrocera dorsalis), citru fruit borer (Citripestris sogittifereela), sucking moth (Othreis fullonia), hopper (Hypomeces squamosus) and aphid (Toxoptera aurantii and Toxoptera citricidus).

Orange is affected by a number of virus and virus-like diseases causing a decline in health of the plants. Greening, psorosis, exocortis and tristeza are common diseases in Vietnam. There is no cure if the symptom appears. Prevention is by destroying the infected plants, using virus-free planting material or grafting of young healthy shoots on immune rootstocks.

The use of selected cultivars of scions and rootstocks suitable to different localities is suggested. There should be effective control measures for diseases both in the nurseries and orchards. Other major diseases include citrus canker (Xanthomonas axonopodis pv. citri earlier known as X. campestris pv. citri), gummosis (Phytophthora nicitianae var. parasitica, P. citropthora, P. palmivora or due to viroide: Cachexia xylopose) and brown rot (Phytophthora sp.).

2.5.11 Fruiting season: Year round, with peak season during August to January in the South, and February to March in the North.

2.5.12 Harvesting and yield: Orange matures six to nine months after bloom. Its maturity can be determined by the ratio of total soluble solids (B) and percentage of citric acid (A). In the subtropics, a B/A ratio of 8 is generally used as a minimum, but in the tropics 10 to 16 is the standard. A higher ratio than 20 results in the fruit becoming too sweet for most consumers.

When picking, the fruit must not be pulled straight down, but removed with an upward twisting action or cut from the tree. An average yield of 8 to 10 t/ha is achieved in the North while a much higher yield of 15 to 18 t/ha is normally obtained in the South due to favorable climatic condition of tropical weather, but the quality is poorer. Elsewhere in the subtropics, a much higher yield is obtained; in Florida, for example, a yield of 40 t/ha/yr is considered an average.

2.5.13 Prospects: As the demand for orange is always high in Vietnam (such that it has to import fruit from China), the prospect of large-scale cultivation of orange is good. However, greening and other virus-transmitted diseases must be controlled. A possible approach is to grow oranges in large areas, in complete isolation under strict quarantine, starting with disease-free material obtained from shoot tip grafting technique, and eradication of all diseased trees in the area. Specific prospects in the North and the South are discussed below:

2.5.13 Post-harvest operations: Fruits are normally packed in cartons or boxes or bamboo baskets for transport. Oranges can be stored in a cool place for several weeks, and in cool stores even longer.

2.5.14 Problems: Main problems are the lack of good disease-free cultivars and facilities to multiply them, and the lack of knowledge of farmers in disease control. Because, orange is native to Northern Vietnam, orange has many problems of pests and diseases in its cultivation.

2.5.14.1 Soil fertility: Most areas planted to orange, both in the North and the South, are of low soil fertility resulting in low yield and poor quality of fruits, especially more sour tasting fruit, which indicate a lack of potassium.

2.5.14.2 Diseases: Greening and tristeza, as well as other related diseases caused by virus and mycoplasma result in premature death of most affected orange plants.

2.5.14.3 Pests: Many insect pests considerably accelerate the decline of orange trees. Thus most orchards in Vietnam have a life-span of 10 to 12 years or less as opposed to 20-30 years in other countries where no damage caused by Citrus Huanglong bin disease (greening disease) is observed.

2.5.14.4 Tropical conditions of South Vietnam: Although the yield in the South is considerably higher than the North the quality is poorer. This is noticeable in the lack of acid, less aroma, tough texture and no orange coloration.

2.5.15 Prospects: As the demand for orange is always high in Vietnam (such that it has to import fruit from China), the prospect of large-scale cultivation of orange is good. However, greening and other virus-transmitted diseases must be controlled. A possible approach is to grow oranges in large areas, in complete isolation under strict quarantine, starting with disease-free material obtained from shoot tip grafting technique, and eradication of all diseased trees in the area. Specific prospects in the North and the South are discussed below:

2.5.15.1 In the North: Having favourable climatic conditions for the growth of orange trees, together with the nearness to large market, both domestically (in Hanoi and other large cities) and abroad, i.e. southern China where large population can absorb a large quantity of Vietnamese oranges, growing oranges in the North should be promoted. However, in order to enjoy such advantages, cultural management improvement of orchards must be undertaken.

2.5.15.2 In the South: Although climatic conditions are not favourable to produce high quality oranges for fresh consumption, tropical climate favours vigorous growth, and sweet fruit. This would be ideal for orange juice production and Cam Sanh (King madarin) produced from the South supplies high demand of people in the North.

2.6 Pummelo

(Rutaceae, 2n = 18)

2.6.1 Names:

Scientific: Citrus maxima (Burm.) Merr.(Syn.: C. grandis (L.) Osbeck, C. decumana L.)

Vernacular:

English: Pummelo, shaddock
French: Pamplemousse
Indonesia: Jeruk Besar, Jeruk Bali
Malaysia: Jambua, Limau Betawi, Limau Bali
Philippines: Lukban, Suha (Tagalog, Ilokano)
Thailand: Som O
Vietnam: Buoi

2.6.2 General: Pummelos are the largest of all citrus fruits. They also have the thickest rind, which is beneficial in keeping and transport. Main production areas in Vietnam are Long Vinh, Ben Tre, Dong Nai, Binh Duong, Hue, Phu Tho, and Ha Tinh provinces and Hanoi. The total area under pummelo cultivation in Vietnam is 8 170 ha, of which 5 000 ha produce fruit with annual production of 50 000 tonnes.

2.6.3 Origin and distribution: The origin of the pummelo is uncertaine as many authors have cited different places such as Polynesia, the foothills of the Himalayas, Southern China, and Malasia. The author, together with the IBPGR Citrus Collecting Mission in Southeast Asia during 1983-87 came to an agreement that the areas in southern Thailand and northern Malaysia, which have the highest diversity of pummelos, are most likely the centre of origin of pummelos. The fruit has spread through Thailand and Malaysia and the rest of SE Asia namely Indo-China, the Philippines, Indonesia into other Asian countries such as southern China, Japan, Myanma, Bangladesh, India, the Mediterranean and tropical America and Australia However, it remains a fruit of the Orient, where it is very popular. The best match of cultivars, environmental niche and growing skills appears to be found in Thailand.

2.6.4 Description: A medium-sized tree, 5 to 10 m tall with low spreading branches. Pummelo has the largest leaves, flowers and fruits among all citrus. Fruits are sub-globose to pyriform in shape, 10 to 20 cm in diameter with pale yellow or pink pulp filled with sweetish mild acid juice. In contrast to most citrus, pummelo is monoembryonic, thus it is a true zygotic seedling that exhibits variation due to hybridization of the parents.

2.6.5 Ecology: Pummelo prefers hot and humid tropical climates. It cannot grow well in high altitude (above 400 m) or in cooler climates. It tolerates a wide range of soils from coarse sand to heavy clay, but deep, medium-textured, fertile soil free from injurious salts is preferred. In Thailand, which has the best cultivars, the most popular areas of production are located on the riverbanks and former river courses. In Vietnam production is mostly in the South, although some cultivars can be grown in the Central areas and even the North (as in Ninh Binh province). There is a little difference in coloration among the cultivars grown at different sites; for instance, those grown in the North give yellow color but those grown in the South remain green. The composition of fruits grown in any location of the country is the same.

In Vietnam, pummelos are locality specific; a given cultivar is popular in a certain locality but almost unknown in other localities. The following are examples of major cultivars in their area of cultivation: ‘Doan Hung’ in Vinh Phu province; ‘Phuc Trach’ in Ha Tinh province; ‘Thanh Tra’ in Thua Thien - Hue province; and ‘Nam Roi’ in Vinh Long province.

2.6.6 Genetics and improvement: There has been practically no genetic improvement of pummelos. All existing cultivars were obtained from selection of chance seedlings and somatic mutations.

2.6.7 Major cultivars in Vietnam: Vietnamese term for pummelos is ‘Buoi’, which is often used in front of some cultivars of pummelos, for example ‘Buoi Nam Roi’, ‘Buoi Da Xanh’, ‘Buoi Duong Da Lang’, ‘Buoi Duong La Cam’, ‘Buoi Ca Tu’, ‘Buoi Oi’, ‘Buoi Bap’, ‘Buoi Tac Suu’, ‘Buoi Thanh’, ‘Buoi Xiem Vang’, ‘Buoi Chua’ and ‘Buoi Kha Linh’. However, some cultivars of pummelos do not bear the term ‘Buoi’ in front of their names (for unknown reasons) such as ‘Thanh Tra’ (Table 8).

In Vietnam, pummelos are locality specific; a given cultivar is popular in a certain locality but almost unknown in other localities. The following are examples of major cultivars in their area of cultivation: ‘Doan Hung’ in Vinh Phu province; ‘Phuc Trach’ in Ha Tinh province; ‘Thanh Tra’ in Thua Thien - Hue province; and ‘Nam Roi’ in Vinh Long province.

Table 8. Pummelo cultivars in Vietnam, their characters and location of cultivation

* No data available

2.6.8 Propagation: Although pummelo can be propagated by seed, this is not commonly practiced by the growers as the plants are not uniform, do notcome true to type, and have many long spines which become hard and sharp before fruiting begins. Propagation is normally done by air layering (marcotting) in Vietnam. In the South, it was recently found that some pummelo scions could be grafted successfully on Volkamer lemon rootstock. Pummelo seedlings of sufficiently uniform population can also be used as rootstocks for grafting. In the Philippines shield budding is a standard practice for propagation, using rootstock of calamandarin (possibly a hybrid of calamondin and mandarin).

2.6.9 Planting: Spacing varies from 6 to 8 m x 8 to 10 m, depending on the vigor of the cultivars as well as the fertility and moisture of the soil. During their young stage after planting, pummelos require shade and frequent watering until established.

2.6.10 Pests and disease: Compared to other citrus, pummelos are relatively tolerant to most pests and diseases. Leaf miner (Phyllocnistus citrella), leaf-eating caterpillar (Papilio demoleus, P. polytes), fruit-borer caterpillar (Citripestis sp.), scale insects (Chloropulvinaria psidii), red mites (Panonychus citri), fruit flies (Bactrocera dorsalis), nematodes (Pratylenchus sp., Tylenchulus semipenetrans, and Meloidogyne sp.) and rats (Rattus sp.) can be a problem.

Bacterial canker (Xanthomonas campestris p.v. citri) is common to most citrus but is not so serious in pummelos. Root rot (Fusarium solani, Phytophthora sp., Pythium sp.), gummosis (Phytophthora nicitianae var. parasitica, P. citropthora, P. palmivora or due to viroid: Cachexia xylopose), and brown rot (Phytophthora sp.) appear to be serious in some locations, but generally they are not a problem. Pummelo tolerates greening disease, but yields are reduced somewhat and the fruit may apprear lop-sided rather than symetrical.

2.6.11 Fruiting season: In the South, September to February (peak season November to January). In the North, August to November (peak season in October). There is some difference in cultivars with respect to fruit maturation, for example Phuc Trach (a popular cultivar of Ha Tinh province in the North), matures in August to September.

2.6.12 Harvesting and yield: Pummelos are potentially heavy bearers because they do not have a severe problem with pests and diseases Normally about 70 to 100 fruits per year are expected from a five-year old or older tree. This is equivalent to about 20 t/ha. However, the national average yield as given by NIAPP (1994) in Vietnam was 8 to 10 t/ha.

2.6.13 Post-harvest operations: At maturity, the color of the rind changes as the oil glands become more prominent and shiny. The fruit should be harvested immediately in order to obtain the highest quality and longest shelf life, which may be as long as several months. Having a rather thick rind, little post-harvest treatment is considered necessary.

2.6.14 Problems: Although pummelos havethe potential to be developed for large-scale production in Vietnam, there seems to be very little advance in this area. This is probably due to the lack of good cultivars and modern techniques of cultivation.

2.6.15 Prospects: Since Vietnam has large areas which are suitable for pummelo cultivation, there should be no problem in initiating large-scale production in the future, provided that a market outlets are clearly defined.. Having excellent keeping quality due to the presence of very thick rind, pummelos are ideal for long transportation, either domestically or export.