Prota 2: Vegetables/Légumes
Cent. pl. II: 10 (1756).
2n = 24
Solanum gilo Raddi (1820), Solanum incanum auct. non L.
African eggplant, garden egg, scarlet eggplant, bitter tomato (En). Aubergine africaine, aubergine écarlate, tomate amère, djakattou (Fr). Jiló, jagatú tunga (Po). Ngogwe, nyanya chungu (Sw).
Origin and geographic distribution
Solanum aethiopicum was domesticated from the wild Solanum anguivi Lam., via the semi-domesticated Solanum distichum Schumach. & Thonn. Both are found throughout tropical Africa, Solanum anguivi in disturbed vegetation and Solanum distichum in gardens. Solanum aethiopicum is grown throughout tropical Africa and South America (mainly Brazil), and occasionally elsewhere, e.g. in southernmost France and Italy. It is one of the leading vegetables in tropical Africa. In the humid zone of West Africa it is mainly grown for its immature fruit (garden egg), in the savanna area frequently for both its leaves and immature fruits (often called ‘djakattou’), and in East Africa, especially Uganda, mainly as a leaf vegetable (called ‘nakati’).
The immature fruits of Solanum aethiopicum are used as cooked vegetables in stews, and sometimes eaten raw. The leaves and shoots are used as a cooked vegetable. They are picked from the same plants that provide the fruit vegetable or from special leafy cultivars. Fruits of bitter cultivars are used as medicine in many African countries. Medicinal applications include the use of roots and fruits as a carminative and sedative, and to treat colic and high blood pressure; leaf juice as a sedative to treat uterine complaints; an alcoholic extract of leaves as a sedative, anti-emetic and to treat tetanus after abortion; and crushed and macerated fruits as an enema. Igbo people in south-eastern Nigeria traditionally welcome visitors into the family house by offering fruits. Solanum aethiopicum is sometimes cultivated as an ornamental. Some cultivars (Aculeatum Group) are occasionally used as a rootstock for tomato and eggplant.
Production and international trade
African eggplant is one of the most commonly consumed fruit vegetables in tropical Africa, in quantity and value probably the third, after tomato and onion, and before okra. Reliable statistics for sub-Saharan Africa are not available. A rough estimate for a few countries indicates an annual fruit production of 8000 t in Senegal, 60,000 t in Côte d’Ivoire and 4500 t in Burkina Faso. Commercial production to supply cities is increasing, as is export to Europe, e.g. from Uganda, Côte d’Ivoire and Senegal. Small-scale growers account for at least 80% of the total production. Leaves of Solanum aethiopicum are especially important in south-eastern Nigeria, Cameroon and Uganda. It is the most popular leafy vegetable of the market in Kampala. Bitter Solanum aethiopicum fruits, called ‘jiló’, are important as a market vegetable in tropical Brazil, where at least 7000 ha are cultivated.
Solanum aethiopicum fruits contain per 100 g edible portion: water 90.6 g, energy 135 kJ (32 kcal), protein 1.5 g, fat 0.1 g, carbohydrate 7.2 g, fibre 2.0 g, Ca 28 mg, P 47 mg, Fe 1.5 mg, β-carotene 0.35 mg, thiamin 0.07 mg, riboflavin 0.06 mg, niacin 0.8 mg, ascorbic acid 8 mg. The composition is comparable with that of eggplant. The composition of fresh leaves per 100 g edible portion is given as: water 82.1 g, energy 215 kJ (51 kcal), protein 4.8 g, fat 0.3 g, carbohydrate 10.3 g, fibre 2.4 g, Ca 523 mg, P 94 mg, Fe 6.0 mg, β-carotene 6.40 mg, thiamin 0.23 mg, riboflavin 0.44 mg, niacin 1.8 mg, ascorbic acid 67 mg (Leung, W.-T.W., Busson, F. & Jardin, C., 1968). This composition is within the range of other dark-green leafy vegetables.
Betulin and sterolin (sitosterol glucoside) have been isolated from the fruits and several sesquiterpenoids from the roots. Among these compounds are lubimin and epilubimin, which have antifungal activity. The leaves contain oxalate and alkaloids, e.g. solasodine, which has glycocorticoid effects. Their concentration is reduced by cooking. The characteristic bitter taste has been attributed to furostanol glycosides.
Adulterations and substitutes
In dishes African eggplant fruits can be replaced by eggplant (Solanum melongena L.). The leaves can be replaced by other Solanum leafy vegetables, mainly Solanum americanum Mill., Solanum scabrum Mill. and Solanum villosum Mill.
Shrub to perennial or annual herb, up to 200 cm tall, often much-branched; root system extending both vertically and laterally; branches and leaves with or without prickles and stellate hairs. Leaves alternate, simple; stipules absent; petiole up to 11 cm long; blade broadly ovate, (6–)12–30 cm × (4–)7–21 cm, obtuse or cordate at base, acute to obtuse at apex, slightly to deeply lobed at margin, pinnately veined; upper leaves smaller, narrower, less lobed and often subopposite. Inflorescence a lateral, racemose cyme, up to 5(–12)-flowered; peduncle often short or even absent, rachis short to long. Flowers bisexual, regular, (4–)5–8(–10)-merous; pedicel (2–)4–12(–15) mm long, up to 27 mm long in fruit; calyx campanulate, lobes 4–10 mm long; corolla stellate, 6–15 mm long, white, sometimes pale purple; stamens inserted near the base of the corolla tube and alternate with corolla lobes, filaments short and thick, anthers connivent, yellow, opening by terminal pores; ovary superior, 2–6-celled, style as long as or slightly longer than stamens, stigma small, obtuse. Fruit a globose to depressed globose, ellipsoid, ovoid or fusiform berry 1–6 cm long, smooth to grooved, red or orange, usually many-seeded. Seeds lenticular to reniform, flattened, 2–5 mm in diameter, pale brown or yellow. Seedling with epigeal germination; cotyledons thin, leafy.
Other botanical information
The genus Solanum comprises over 1000 species and is almost cosmopolitan, with at least 100 indigenous African species. Solanum aethiopicum belongs to subgenus Leptostemonum section Oliganthes, which comprises about 45 species. Four cultivar-groups are recognized within Solanum aethiopicum, three of which are important for Africa:
– Gilo Group. Mature leaves covered with stellate hairs, generally not prickly; fruit subglobose to ellipsoid, 2.5–12 cm long. The fruits are consumed. This is the most important cultivar-group, which includes cultivars with smooth fruits that are popular in West and East Africa, and cultivars with more or less strongly ribbed fruits. Depending on the location, preference is given to cultivars with pure white, creamy white, pale green, dark green, brown or purple fruits, or cultivars with fruits striped in two or more colours. Cultivars of Gilo Group are grown throughout tropical Africa in the more humid areas.
– Kumba Group. Mature leaves glabrous apart from minute glandular hairs, not prickly; fruit depressed globose, deeply furrowed, frequently multilocular, 5–10(–15) cm broad. The fruits are consumed, as well as occasionally the leaves. Locally the same plants are used for fruits and leaves, whereas other cultivars are only used as a leafy vegetable. Cultivars of Kumba Group are mainly found in hot, semi-arid regions of the Sahel. They are frequently referred to as ‘djakattou’, ‘djakhattou’ or ‘jakhatou’ in francophone countries, but these names can also refer to cultivars of Gilo Group.
– Shum Group. Mature leaves glabrous apart from minute glandular hairs, not prickly; fruit subglobose, 1–3 cm in diameter. It is mainly a leafy vegetable, and occasionally the ripe fruits are also consumed. It is most widespread in Central Africa, popular in Cameroon and Nigeria and still more so in Uganda where it is called ‘nakati’. It is mainly found in warm, high-rainfall areas or under irrigation.
– Aculeatum Group. Stems and leaves prickly, mature leaves covered with stellate hairs; fruit subglobose, furrowed, 3–8 cm in diameter. It is not eaten, mainly cultivated as an ornamental or as rootstock for tomato or eggplant, but not cultivated in Africa.
Intermediate forms between the three African cultivar-groups or between Solanum aethiopicum and its wild ancestor Solanum anguivi occur. Forms that do not fit in any of the cultivar-groups occur in humid regions of south-western Congo and northern Angola. These are shrublike with sweet fruits having the size of Gilo Group but hollow inside; their leaves are eaten. Further investigations are needed into these cultivars.
Plants of these four cultivar-groups can be crossed mutually as well as with Solanum anguivi and Solanum distichum to produce fully fertile hybrids, and therefore might be considered as a single biological species. The non-prickly semi-domesticated Solanum distichum may well be treated as a cultivar-group of the prickly wild progenitor Solanum anguivi.
Growth and development
Germination is epigeal, after which the cotyledons expand and the first true leaves form a rosette. New leaves rapidly increase in size and flowering starts (40–)70–100 days after sowing. As the first flowers are initiated, branching and subsequent production of smaller leaves occurs. Most cultivars of Kumba Group in dry hot savanna areas have a shorter growth cycle and flower earlier. Unlike Solanum melongena, all flowers are functionally bisexual and can set fruit. They are bee-pollinated, mainly by the genera Exomalopsis and Apis. Growth and flowering may continue indefinitely, but are suppressed once sufficient fruits have set. The small fruits of Shum Group ripen rapidly, turning red; they are eaten by birds, which disperse the seeds. The much larger fruits of Gilo Group and Kumba Group ripen more slowly and stay firm even when red or yellow, and can be stored for weeks or even months. In the dry season, plants may become dormant and appear to be dead, but can revive in the next rainy season, although they are then not very productive.
Solanum aethiopicum Gilo Group thrives in full sun in woodland savanna on fairly deep and well-drained soils with pH 5.5–6.8, and in temperatures of 25–35°C during the day and 20–27°C at night. Kumba Group grows in hotter conditions (up to 45°C day temperature) with air humidity sometimes as low as 20%, especially if irrigated. Shum Group thrives under warm, humid conditions. It will drop its leaves when it gets dry. In Uganda it is grown in swamps during the dry season. None of these cultivar-groups survive cold or very wet conditions. Waterlogging is not tolerated. Some tolerance of irrigation-induced salinity is reported from Senegal.
Manual preparation of the soil and hand weeding are sufficient, but large-scale production in Senegal necessitates mechanized soil preparation. Plants do not need staking. If possible, 15–15–15 or 10–10–20 NPK fertilizer may be applied at 150 kg/ha 10 days after transplanting and at 50 kg/ha at first flowering, and then at monthly intervals. Soluble fertilizers may be fed by drip irrigation. Farm or poultry manure can be applied at a rate of 10–20 t/ha. Plants grown as leaf vegetables (Kumba and Shum Groups) require extra nitrogen to be applied as top dressing at a rate of 50 kg/ha NPK 15–15–15 every two weeks. In the dry season, the crop needs about 5 mm water per day; irrigation twice per week is appropriate. Plants of Shum Group especially need a regular water supply. The growing season of Gilo Group and Kumba Group is extended when irrigation is applied during dry spells and towards the end of the rainy season. In addition, the fruit quality can be much improved by maintaining adequate soil moisture.
Propagation and planting
Seeds should be taken from fully ripe fruits, washed, and then dried on cloth or paper. They should not be exposed to direct sunlight. Seeds stored dry and cool are viable for years. Seeds also store well inside air-dried fruits, which is the traditional form of seed storage by farmers. The 1000-seed weight is 2–4 g. Germination takes 5–9 days for Gilo Group and Shum Group, but only 3–5 days for Kumba Group, although the latter may show seed dormancy and tends to have few seeds per fruit. Seeds are sown in sandy soil in nursery beds or boxes. Seedlings are transplanted to the field after 30–35 days, when they have 5–7 leaves and are 15–20 cm tall. Plants of Kumba Group grown in dry savanna regions are often spaced at 1 m × 1 m, whereas those of Gilo Group can be spaced at 50–100 cm in the row and 75–100 cm between rows, depending on the cultivar. They can be grown either on flat land or on ridges. The cultivation of Shum Group is rather different. Cultivars of the latter cultivar-group are grown for their young shoots, which are frequently harvested, and the crop can thus be spaced at 20–30 cm in the row and 60–75 cm between rows. An alternative is to broadcast seeds of Shum Group, whereby thinned plants are used as the first harvest. Seed is sometimes broadcast together with amaranths (Amaranthus spp.) and spider plant (Cleome gynandra L.), where the latter two crops are harvested early by uprooting and the plants of Solanum aethiopicum Shum Group remain.
Diseases and pests
Solanum aethiopicum is susceptible to several diseases and pest, although much less than eggplant. The most serious soil-borne diseases are wilt caused by Ralstonia solanacearum, collar rot and wilting caused by Sclerotium rolfsii and Verticillium dahliae, and root-knot nematodes (Meloidogyne spp.). These soil-borne diseases and pests can be controlled by crop rotation, e.g. with cereals or other starch crops or with amaranth, groundnut or onion, adequate drainage and a good soil structure. Root-knot nematodes are particularly problematic in areas where vegetables are grown year-round and unless crop rotation is taken seriously, production of African eggplant fruits, tomatoes and capsicums may not be economic. A serious disease, e.g. in Côte d’Ivoire, is Stemphylium floridanum; it causes small brown angular leaf spots causing a devastating defoliation. In Tanzania the chilli veinal mosaic virus (ChiVMV) spread by the green peach aphid (Myzus persicae) caused considerable damage. Spider mites (Hemitarsonemus and Tetranychus) are a serious problem in drier regions; acaricide sprays can control them. Other serious pests include grasshoppers (Zonocerus sp.), fruit and flower borers (Leucinodes and Scrobipalpa), leaf hopper (Jacobiasca lybica) and caterpillars (Selepa docilis). In Uganda the leaves of Shum Group are sometimes eaten by monkeys.
The fruits are collected when full sized but still immature; some sweet cultivars are harvested when the fruit colour changes to orange but at the stage when seeds are still immature and soft. The fruits are picked once per week. It is important to continue harvesting fruits even when the market is slack. Fruits that are left on the plant will change colour from pure white to creamy white or pale yellow, fill with seeds, and become less palatable. They will also hamper the development of new fruits. Leaves of Kumba Group are picked from young plants before flowering. Shoots with several leaves and flower buds of Shum Group are picked repeatedly throughout the wet season. Ratoon cropping whereby the main shoots are harvested and allowing new shoots to develop is practised frequently for Shum Group and less often for Kumba Group. This process can be repeated up to six times, as long as there is adequate irrigation and a top dressing with nitrogen is applied. Alternatively, Shum Group plants are uprooted when they are 40–50 cm tall when the first flowers tend to open. They are marketed as young plants with their roots attached. Irrigated crops may be harvested year-round.
The preferred weight for fruits of Gilo Group and Kumba Group is 30–40 g. One plant may produce from 500 g to about 8 kg of fruits, depending on the cultivar and growing conditions. Without irrigation, yields are 5–8 t/ha, and with irrigation 12–20 t/ha. Improved cultivars grown under favourable conditions may yield 50–80 t/ha. Improved cultivars of ‘jiló’ in Brazil yield 20–30 t of marketable fruits. Fruits of Kumba Group have mean weights of 70–120 g, sometimes even over 200 g; yield is 10–20 t/ha. Under good management, farmers growing cultivars of Shum Group can get up to 75 leaf bundles of 30 kg each per 100 m2. This means that the crop has a yield potential of 225 t/ha. The average leaf yield during the dry season for a once-over harvest, however, is only 30 t/ha.
Handling after harvest
Fruits free of rot or damage can be transported long distances, and stored for several days or even weeks under well-ventilated conditions. Provided that fruits are of good quality, standardization is not necessary; market women often mix different batches of fruits to make an attractive display. During periods of shortage, the market pays higher prices for small fruits because there are more of them per unit of weight whereas during periods of oversupply, larger fruits are appreciated. Mixtures of small and large fruits are common, but not mixtures of fruits with different colour. However, fruits for export are sorted and graded after collection from local markets, and then stored in cold rooms. Fruits and leaves are not normally processed or preserved for long periods. Fresh leaves are taken immediately to the market. The roots of uprooted young plants of Shum Group are washed and left on the plant, which helps to keep the crop fresh at the market when left standing in water. Shoots should preferably be harvested early in the morning or late in the afternoon. The product should be dry during transport and be dipped in cold water on arrival at the market to reabsorb enough moisture to retain its fresh appearance. The leaves can be sun dried and ground into powder for use in soups during the off-season. Ripe fruits of Shum Group are collected and dried in a similar way to those of Solanum anguivi. Fruits are dried and ground into powder for use as a medicine against high blood pressure.
A wealth of genetic diversity is maintained by small-scale farmers throughout Africa. Little work has yet been done on the characterization of the genetic resources of Solanum aethiopicum and allied species, however, in particular with respect to disease and pest resistance. The large-scale commercialization of a few cultivars, such as ‘Sodefel’ (Gilo Group) in Côte d’Ivoire and ‘Soxna’ (Kumba Group) in Senegal, often in monoculture, has led to the disappearance of some local cultivars but many local landraces can still be found throughout the African continent. Variation in Kumba Group is less than in Gilo Group, possibly due to less cross-pollination in the former. Extensive collections of genetic resources were made under the aegis of International Plant Genetic Resources Institute (IPGRI) from 1981–1986. In 2001, efforts were started to regenerate and evaluate the collections as part of the EGGNET project, an international project for management of the genetic resources of eggplant. A rich germplasm collection is held at INRA in Montfavet, France. A collection is kept at AVRDC in Arusha, Tanzania, and plant breeders in Ghana, Côte d’Ivoire, Nigeria, Senegal and elsewhere in Africa also maintain some accessions.
Although small-scale farmers have selected many diverse cultivars of Solanum aethiopicum and germplasm collections exist, little breeding has been undertaken. Selection criteria are consumers’ preference for fruit size, form, colour and taste (sweet or bitter), skin toughness, shelf life, as well as yield potential, disease resistance, earliness and duration of harvest season, plant architecture and fruit location, and for Shum Group ease of leaf harvest. In Senegal, mite-resistant very hairy cultivars of Kumba Group have been bred. Resistance to nematodes and insects, such as flower borers, is urgently needed. Tolerance or resistance to various pests and diseases is being sought amongst wild relatives of Solanum aethiopicum, and more especially Solanum melongena, and crosses between these two and other species have produced many hybrids, some of which are fertile. In Brazil, crosses of local cultivars of Gilo Group with Solanum melongena were more successful when the latter was used as female parent; the F1 was self-sterile but could be backcrossed with plants of Gilo Group. For breeding purposes, flowers must be bagged to prevent cross-pollination, and must be emasculated to prevent self-pollination. Molecular markers have revealed relatively little diversity within Solanum aethiopicum or its immediate progenitors, although morphological diversity and its inheritance have been studied intensively; wild-type characters such as prickles, hairs and long racemose cymes are often dominant. The Crops Research Institute in Ghana, jointly with the Natural Resources Institute (United Kingdom), selected ‘Dwomo’, the fruits of which have the shape and size of an egg. ‘Dwomo’ was crossed with Solanum anguivi and some promising high-yielding cultivars were selected. Technisem Seed Company in Senegal commercializes improved African eggplant cultivars. Three popular cultivars of Kumba Group are ‘Ndrowa’ (flat ribbed green-yellow fruits of 70–80 g, diameter 5 cm, with a mild taste, plant height 60–100 cm, harvestable 50–70 days after planting during 3–4 months, yield potential 25 t/ha, resistant to mites), ‘Ngalam’ (flat strongly ribbed pale green to white fruits of 120–180 g, diameter 7 cm, slightly bitter taste, plant height 60 cm, harvestable 50–60 days after planting during 6 weeks, yield potential 10 fruits per plant, resistant to mites), and ‘Jaxatu Soxna’ (flat ribbed pale green to white fruits of 40–50 g, diameter 5–6 cm, bitter taste, plant height 50 cm, harvestable 40–60 days after planting during 6 weeks, yield 20–25 fruits per plant or 30 t/ha, or used as leafy vegetable, resistant to mites, drought, high rainfall and high temperatures). In Japan ‘Iizuka’ of Aculeatum Group was selected as a rootstock for tomato and eggplant in protected cultivation because of its resistance to wilt.
All three cultivar-groups Gilo, Kumba and Shum are important vegetables in tropical Africa. As the fruits can be easily stored and transported, they are increasingly important products for the urban markets. Hitherto, Solanum aethiopicum has been rather neglected in research and breeding, except in breeding programmes for Solanum melongena. With more attention to breeding, agronomy and crop protection (IPM-research) in a cooperative effort by the public and private sector, its potential as a market vegetable can be further developed. Cultivars are needed for different ecological areas and consumer preferences in taste and degree of bitterness, and with resistances, e.g. to Stemphylium. The popularity of improved cultivars has led to increased risk of genetic erosion in the local cultivars, making the need for germplasm collections more urgent.
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Sources of illustration
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Correct citation of this article:
Lester, R.N. & Seck, A., 2004. Solanum aethiopicum L. In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, flowering branch; 2, flower; 3, different fruit types; 4, fruit in cross section; 5, seed
Redrawn and adapted by M.M. Spitteler
plant of Gilo Group with elongated fruits
fruit of the Gilo Group
genetic diversity in harvested fruits
fruits of cultivar ‘Soxna’, Kumba Group, Senegal
flowering and fruiting plant of Shum Group
fused petals in a Solanum macrocarpon flower (left) and free petals in a Solanum aethiopicum flower