Prota 2: Vegetables/Légumes
Gard. dict. ed. 8: Solanum No 6 (1768).
2n = 72
Solanum guineense (L.) Mill. (1768), Solanum nigrum auct. non L.
African nightshade, black nightshade, garden huckleberry (En). Morelle de Guinée, morelle noire (Fr). Erva moura (Po). Mnavu (Sw).
Origin and geographic distribution
Solanum scabrum occurs as a cultivated vegetable from Liberia to Ethiopia, and south to Mozambique and South Africa. It is very common in lowland as well as highland regions in West and East Africa. It is also reported from Réunion and may well occur on other Indian Ocean islands, where its status needs to be confirmed. The wide range of diversity of Solanum scabrum found especially in Nigeria and Cameroon suggests that its origin is likely to be in the warm humid forest belt of West and Central Africa. Outside Africa, Solanum scabrum can be found in Europe, Asia (India, China and the Philippines), Australia, New Zealand, North America and the Caribbean.
Leaves and fresh shoots of Solanum scabrum are widely used as a cooked vegetable. They are served with corn ‘fufu’, plantains, sweet potatoes, potatoes, yams, maize or pounded cocoyams. Solanum scabrum is popular in Côte d’Ivoire (known as ‘fouet’), Benin (‘ogomoh’), Nigeria (‘ogunmo’ or ‘odu’) and Cameroon (‘osan’ or ‘zom’). As it has a bitter taste, some people prefer not to use salt. Contrary to what is reported in older literature, fruits of Solanum scabrum are not eaten in Africa. Reports on its edible fruit from South Africa probably refer to Solanum retroflexum Dunal, and from North America, Asia, Australia and New Zealand refer to types or cultivar-groups that do not occur in Africa. In south-western Nigeria the inflorescence with buds, flowers and small fruits is normally removed before cooking; it can be very bitter in taste but this is appreciated by elderly people who may add them to their soup. Bitterness is reduced by discarding the cooking water and replacing it with fresh water. The cooking water may be very dark, which is not appreciated. Some people add milk or salt to further reduce the bitterness.
Solanum scabrum is widely used as medicinal plant. Leaf extracts are used to treat diarrhoea in children and certain eye infections and jaundice. In East Africa the raw fruit is chewed and swallowed to treat stomach ulcers or stomach-ache. Infusions of leaves and seeds are rubbed onto the gums of children who have developed crooked teeth. In the literature many other medicinal uses for Solanum species with black fruits have been recorded, but it is not likely that these refer to Solanum scabrum.
Solanum scabrum is used as fodder for cattle and goats. Both the leaves and fruits are a source of dyes. The anthocyanin pigments in the purple to black fruits are used as a dye or as a kind of ink.
Production and international trade
Solanum scabrum is commonly cultivated on smallholder plots, in kitchen gardens and increasingly near the major cities for market supply. No reliable statistics on production are available. It is one of the most important leafy vegetables in West and especially Central Africa, less important in East Africa. The crop is exported from Cameroon to Nigeria and Gabon. Yaoundé retail price statistics show that Solanum scabrum prices are lowest from May to October and rise to a peak price towards the beginning of the rainy season in March. The price per bundle remains constant, but the quantity and quality per bundle vary considerably.
The composition of 100 g edible portion of African nightshade leaves is: water 87.8 g, energy 163 kJ (39 kcal), protein 3.2 g, fat 1.0 g, carbohydrate 6.4 g, fibre 2.2 g, Ca 200 mg, P 54 mg, Fe 0.3 mg, β-carotene 3.7 mg, ascorbic acid 24 mg (Leung, W.-T.W., Busson, F. & Jardin, C., 1968). The dry matter content varies greatly, from 6–18 % depending on plant age, soil moisture and fertilizing. The protein is rich in methionine.
Green fruits contain comparatively high amounts of the glycoalkaloid solanine and the less poisonous solanidine. The initial effect of solanine poisoning includes diarrhoea and vomiting, and frequent consumption of this compound may lead to accumulation in the liver, causing dizziness, mental confusion and loss of speech, and it can even result in blindness. The leaves contain only low levels of these alkaloids, which are probably associated with its bitter taste. Unfortunately, heating or frying will not reduce the toxic effects of solanine and solanidine. The acceptable limit for these alkaloids is 20 mg per 100 g fresh weight of the edible portion. Most research stations in Africa have no facilities to analyse these alkaloids and are thus not able to screen accessions for this important characteristic. The degree of bitterness is easier to establish, and research is currently ongoing to determine how the glycoalkaloids relate to bitterness.
Adulterations and substitutes
In dishes Solanum scabrum leaves can be replaced by those of other species of the section Solanum, e.g. Solanum americanum Mill. or Solanum villosum Mill. with a comparable taste and bitterness, and sometimes also by Solanum aethiopicum L. or Solanum macrocarpon L. leaves.
Annual or short-lived perennial herb, erect and widely spreading, up to 100(–150) cm tall, unarmed; stem rounded or narrowly winged with more or less toothed wings, glabrous or sparsely pubescent, young stem more or less pubescent with short, simple hairs. Leaves arranged spirally, sometimes almost opposite, simple; stipules absent; petiole 2–10 cm long; blade rhomboid to ovate-lanceolate, up to 4.5–22 cm × 3–16 cm, cuneate at base and decurrent along the petiole, acute to acuminate at apex, sometimes obtuse, entire to sinuate or slightly toothed, glabrous or sparsely pubescent. Inflorescence an extra-axillary, umbel-like cyme, 3–10(–12)-flowered; peduncle 1–2.5 cm long, elongating up to 4 cm in fruit. Flowers bisexual, regular, 5-merous; pedicel 4–9 mm long, elongating to 12 mm in fruit, erect or nodding; calyx cup-shaped, 2–4.5 mm long, lobes triangular, becoming reflexed in fruit; corolla stellate, 7–16 mm in diameter, white or flushed purple with basal yellow-green star, lobes ovate-elliptical, 3–6 mm long; stamens inserted on corolla throat, filaments c. 1 mm long, with hairs on inner side, anthers connivent, 2–3 mm long, usually brown-yellow, opening by terminal pores; ovary superior, conical to ovoid, c. 1.5 mm long, style 3–4.5 mm long, hairy in the lower part, stigma capitate, pale green. Fruit a globose berry 10–16 mm in diameter, glossy deeply purple to purplish black at maturity, many-seeded. Seeds discoid, 2–3 mm long, creamy coloured, often tinged with purple. Seedling with epigeal germination; hypocotyl 4–5 mm long; cotyledons leafy, elliptical, 4–6 mm × 2–3 mm.
Other botanical information
Solanum scabrum belongs to the subgenus Solanum and section Solanum, formerly known as section Maurella, or section or subsection Morella. Currently about 30 species are included in this section of which 10–12 are known to occur in Africa. Research is still needed to better understand the species within section Solanum and their diversity. In Africa the name Solanum nigrum is often used for almost all species of section Solanum with blackish fruits, including Solanum scabrum. This confusion is probably aggravated by the use of vernacular names whereby one name can apply to several species, or several names to the same species. Solanum scabrum is often confused with Solanum americanum, but more slender stems, narrower leaves and smaller flowers and fruits distinguish the latter.
Growth and development
Seed germination can be problematic because of low vigour caused by improper seed extraction and therefore inadequate removal of sugars and germination inhibitors present in the fruit. Other causes of problematic germination are that seeds are not dried and stored properly, or that the seed is dormant. The seeds can remain viable for several years when kept dry and cool.
After seed emergence, growth is fast. The first flowers appear 8–11 weeks after sowing.
Flowering occurs earlier when the seeds are sown directly than when seedlings are transplanted. The plant continues to produce new flowers for several months. The flowers are mainly self-pollinated. Solanum scabrum has low levels of out-crossing, which is mainly done by honeybees, bumble bees and black syrphid flies.
The optimum temperature for seed germination is 15–30°C and for growth it is 20–30°C. Solanum scabrum grows from sea-level to well over 2000 m but does not tolerate night frost. The rainfall during the growing season should be at least 500 mm; it grows well under conditions with much higher rainfall but then becomes susceptible to leaf diseases. It prefers fertile soils, with high nitrogen content and rich in organic matter. Sandy loams to friable clay soils with a pH of 6.0–6.5 are suitable. The plants tolerate some shade, but grow better when exposed to full sun as long as they have adequate access to water.
Daily irrigation is necessary for the first week after transplanting, especially during the dry season. The irrigation frequency can later be reduced to two or three times per week, depending on temperatures, cloud cover or possible rains. Watering can be carried out through the paths between raised beds or by using a hose or a watering can. Overhead irrigation is less appropriate because of the potential spread of foliar diseases. Weeding is needed during the early stages of development. Nightshades require large amounts of nitrogen and other nutrients. Consequently, they do well in soils that are rich in organic matter, and also grow well on land covered with ash from recently burnt vegetation. Farmers use NPK 20–10–10, urea or ammonium sulphate when there is no poultry or farmyard manure available. Poultry manure is recommended at a rate of 15 t/ha alone or in combination with 400 kg/ha NPK 10–10–20. Side dressing is practised after every second harvest. However, high nitrogen levels reduce the dry matter content of the plants and make the crop more vulnerable to diseases unless the balance with potassium is correct. They also increase the level of unwanted nitrates in the leaves.
Propagation and planting
Propagation of Solanum scabrum is by seed and, less commonly, by cuttings. Most farmers produce their own seed and some buy their seed or seedlings from specialized producers. For a subsistence crop seeds are sown directly at the beginning of the rainy season. There are about 1000 seeds per g. A few (3–10) seeds are used per hole when sown among other crops in an intercropping system. The strongest plants are kept and the others removed as the first harvest or for transplanting. Direct sowing during the wet season results in taller plants and, when there is adequate room, in more and larger leaves and branches and higher dry matter content than with transplanting.
Sowing in nurseries and transplanting is normally practised when the crop is cultivated commercially. The seed can be mixed with ash, sand, soil, or dry poultry manure before broadcasting to spread the fine seeds evenly. The nursery requires manure for a good emergence of seedlings. Seeds are sown in lines 10–20 cm apart or seeds are broadcast. The soil of the nursery bed should be loosened to facilitate rooting. After sowing, the beds should be covered with a thin layer of soil, which also helps to prevent ants from carrying off the seeds. Sometimes the weed vegetation in the field is burnt to provide a layer of ash that is rich in nutrients, especially potash, and also to kill soil-borne pathogens and weeds. Transplanting takes place 4–6 weeks later, depending on prevailing temperatures, when the seedlings are at least 6–8 cm tall and have 5–6 true leaves, but are not more than 15 cm tall to avoid weak and thin plants. The seedlings are selected for their strength and freedom from diseases and planted late in the afternoon or early in the morning. Adequate water is needed just before and immediately after transplanting since roots are sensitive to drought.
When propagation by cuttings is practised, cuttings of 20–30 cm long are taken from the main stem and are trimmed before they are inserted into the soil. The spacing is 40 cm × 40 cm or even 40 cm × 60 cm, considering that plants may reach 1 m in height (if not trimmed). The advantage of this propagation method is that the first harvest can start early (3–4 weeks after planting). However, the total yield is lower than from transplanted seedlings or from plants sown directly.
Usually farmers use sole cropping. The spacing may differ, depending on cultivar and season. It is usually wider during the rainy season, when ventilation is required to reduce the incidence of diseases. Spacing is normally between 15–25 cm × 15–40 cm. A wider spacing is used when the crop is to be kept for a long period, encouraging stronger branches and an extended harvest period for which additional fertilizing is needed. Branching is stronger at a wider spacing, making up for the lower number of plants. Close planting is mainly used when the growing season is expected to be short or with once-over harvesting.
Diseases and pests
Many pests and pathogens of tomato also occur in Solanum scabrum. A major disease of African nightshades grown in the wet season of tropical highlands is late blight, caused by Phytophthora infestans. Yield losses of up to 100% have been reported for susceptible cultivars in the nursery and up to 45% in the field. Control is possible with three-weekly sprays of metalaxyl + mancozeb (Ridomil MZ, 2.5 kg/ha). Some Solanum scabrum cultivars in Cameroon are resistant to late blight. Another important disease is early blight (Alternaria solani), which occurs more in lowland regions. Other diseases include greyish-green leaf mould (Cladosporium oxysporum), eye spot (Cercospora nigrescens), powdery mildew (Leveillula taurica), bacterial wilt (Ralstonia solanacearum) and root-knot nematodes (Meloidogyne spp.). Bacterial wilt has been recorded in Kenya and Tanzania. Prominent viral diseases are yellow-vein virus (observed in Cameroon and Nigeria), probably transmitted by whitefly (Bemisia tabaci), and leaf curl and leaf mosaic viruses.
Common insect pests are black ants, elephant grasshopper (Zonocerus variegatus) and beetles (Lagria spp., Podagrica spp. and Epilachna hirta). Insect pests that are occasionally encountered include Cletus and Bathycoela species (Heteroptera) and larvae of an unidentified lepidopterous species. Black aphids (Aphis fabae) cause leaf curl. A traditional cure for pests is wood ash spread onto the leaves. Insect problems can generally be controlled with deltamethrin sprays. Some consumers in Cameroon regard insect damage on leaves as proof that insecticides have not been applied.
It takes about 4–5 weeks from transplanting to the first harvest for Solanum scabrum, when stems are cut down to about 15 cm from the ground, allowing new side shoots to develop. The length of harvested shoot varies from 15–50 cm, depending on the cultivar. Further harvests take place at 7–14 day intervals, on average 3–5 times/plant if there is no additional manure or fertilizer, but large-scale commercial farmers will harvest up to 10 times.
Optimum yields are obtained during the third or fourth harvest, which is about two months after planting. Yields of 7–27 t/ha are reported per harvest. Yields decline significantly after the sixth harvest unless adequate fertilizer is applied. Cumulative yields may reach 40 t/ha.
Handling after harvest
The produce is tied in bundles and sent to the market, where traders will split those bundles into smaller retail units. It is important to place the bundles in an upright position, especially when the time between harvesting and the final sale is more than six hours. If long stems are laid down horizontally, their heads are likely to turn upwards, making them less attractive. Traders at the market sprinkle some water on the leaves to keep them fresh. Harvested plants with their roots still attached need to be cleaned thoroughly. When they are placed in a bucket with water, they will remain fresh for a much longer time than the shoots alone.
At present all African nightshades are grown from local cultivars, and there is no danger of genetic erosion. Besides, Solanum scabrum occurs in many different countries. The largest germplasm collection is maintained at the University of Dschang, Cameroon. Another substantial collection is maintained at the Botanical Garden of Nijmegen University (Netherlands), and a small collection of local cultivars is maintained at NIHORT, Ibadan, Nigeria.
Solanum scabrum and other African nightshades are predominantly self-pollinating, although there are differences among species. In breeding, the absence of a self-incompatibility system is useful in stabilizing any crossings made, whereby the new population will be sufficiently uniform after only 2–3 generations. Cultivars can thus be created within a short period. Seed crops should be planted in blocks and not in lines; the outer rows should be discarded and fruits only collected from the inside of the block because pollinating insects may cause some cross-fertilization. For a seed crop the planting distance should be 50–100 cm × 50–100 cm, depending on the cultivar. Interspecific crosses of Solanum scabrum with Solanum macrocarpon or Solanum aethiopicum produced mature fruits but no viable seed.
It is worth promoting Solanum scabrum as it is an excellent leaf vegetable and a major source of income for many vegetable farmers and traders in urban and rural areas. Selection and breeding of improved cultivars with high growth vigour, strong resistance to pests and diseases and good consumer quality should be enhanced.
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Sources of illustration
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Correct citation of this article:
Fontem, D.A. & Schippers, R.R., 2004. Solanum scabrum Mill. In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
1, flowering and fruiting branch; 2, infructescence.
Redrawn and adapted by Achmad Satiri Nurhaman
ripe berries are not shiny