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Solanum tarderemotum Bitter

Protologue
Feddes Repert. 10: 547 (1912).
Family
Solanaceae
Chromosome number
2n = 48
Synonyms
Solanum eldoretianum auct., Solanum eldoretii auct., Solanum nigrum auct. non L.
Vernacular names
Black nightshade, African nightshade, Eldorets nightshade (En). Morelle noire (Fr). Erva moura (Po). Mnavu (Sw).
Origin and geographic distribution
Solanum tarderemotum is indigenous in Central and East Africa and is found in DR Congo, Rwanda, Burundi, Ethiopia, Kenya, Uganda and Tanzania. As a cultivated vegetable it is only known from a limited area in highland regions in Western and Central Province, Rift Valley and the south-western highlands in Kenya, and in northern Tanzania bordering these regions. Its occurrence was recently confirmed in the Mt Meru area of Tanzania, and plant populations found in south-western Tanzania may also belong to this species. The purple-fruited types found in the wild possibly represent the ancestor of the cultivated plants, which often have larger green fruits.
Uses
Leaves and young tender shoots of Solanum tarderemotum are cooked and used as spinach or fried directly without boiling, mainly as accompaniment to the local staple food. Fruits are removed and milk is usually added to the cooked leaves and left overnight to reduce bitterness. The leaves may be mixed with other leafy vegetables (e.g. amaranth) to enhance their palatability. The purple-fruited types are usually more bitter than green-fruited ones. Children often eat ripe fruits of the green-fruited types raw.
Infusions of leaves, roots and young fruits are used medicinally to treat duodenal ulcers, stomach upsets, boils, swollen glands and teething problems. Cooked Solanum tarderemotum is recommended for malaria patients. The plants are used as fodder when there is no market for the fresh produce.
Production and international trade
Solanum tarderemotum used to be mainly grown in kitchen gardens in mixed stands together with other vegetables. It is also picked from the wild. Of late there is an increasing demand for this crop that has led to its current commercial production. In local markets people rarely distinguish this species from other leafy Solanum species, that are all called ‘mnavu’. There is a lack of reliable data on production and prices, but Solanum tarderemotum has probably overtaken Solanum villosum Mill. as the most popular leafy Solanum vegetable in Kenya. Occasional cross-border trade occurs between Kenya and Tanzania. The price at local markets is lowest during the rainy season in April and highest in the warm and dry period before the rains start.
Properties
The composition of Solanum tarderemotum leaves is comparable to other dark green leafy vegetables.
Adulterations and substitutes
In dishes Solanum tarderemotum can be replaced by other species of the section Solanum, e.g. Solanum americanum Mill., Solanum scabrum Mill. or Solanum villosum Mill.
Description
Annual or short-lived perennial herb up to 150 cm tall, usually widely spreading with erect or prostrate branches, unarmed; stem narrowly winged with finely toothed wings, glabrous or sparsely pubescent, green to greenish purple with purplish nodes. Leaves arranged spirally, sometimes almost opposite, simple; stipules absent; petiole c. 2 cm long, winged; blade ovate to lanceolate, up to 10(–18) cm Χ 6(–7) cm, cuneate at base and decurrent along the petiole, acute to acuminate at apex, entire to undulate, sparsely pubescent, pale to medium green. Inflorescence an extra-axillary, simple, raceme-like cyme, 7–12-flowered; peduncle c. 2 cm long. Flowers bisexual, regular, 5-merous; pedicel 3–8 mm long, elongating up to 10 mm in fruit; calyx cup-shaped, c. 3 mm in diameter, up to 6 mm in fruit, with lanceolate to ovate or broadly triangular lobes reflexed or adherent in fruit; corolla stellate, 6–11 mm in diameter, white to pale purple, with yellow basal star, lobes 3–6 mm Χ 1–2 mm; stamens inserted on corolla throat, filaments 0.5–1 mm long, with hairs on inner side, anthers connivent, 1.5–2 mm long, yellow, opening by terminal pores; ovary superior, style 2–3 mm long, hairy in the lower part, stigma capitate. Fruit a globose berry 4–6 mm in diameter, pale green to purplish at maturity, dull, many-seeded. Seeds lens-shaped, c. 1.5 mm long. Seedling with epigeal germination.
Other botanical information
Solanum tarderemotum 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. Solanum tarderemotum is close to Solanum florulentum Bitter, but this species has a forked, many-flowered inflorescence. Cultivated plants of both species have been called Solanum eldoretianum or Solanum eldoretii in the literature, but these names have not been published validly. However, the section Solanum contains a number of species that are highly diverse and the same could well apply to this species. This could be so especially if it has been cultivated over a long period of time, and this appears to be the case in contrast to Solanum villosum, which has probably been brought under cultivation more recently. Solanum scabrum is a similarly diverse species, which must have been cultivated over a long period.
Growth and development
Normally, seed germination takes place in 5–7 days. Initial seedling growth is rapid. Flowering starts when plants are about 6 weeks old, but harvesting of leaves can continue thereafter. When after several months the crop comes to its end, the farmers collect the fruits that have dropped on the ground for extraction of seeds for the next season.
Ecology
Solanum tarderemotum occurs in a wide range of habitats from sea-level up to 3000 m altitude. It appears to tolerate drier conditions than the related Solanum villosum and Solanum scabrum. It grows best in soils high in N and P, with a high organic matter content. It is reported to grow wild in disturbed areas or as a weed on agricultural land, probably often as escapes from cultivation.
Management
Plots are kept free of weeds by hoeing, but this becomes unnecessary after closing of the canopy. Farmers in western Kenya often use foliar sprays with macro and micro plant nutrients. When these are not available, top dressing with a nitrogen fertilizer is given after every harvest. These regular fertilizer applications allow the farmer to lengthen the harvest period considerably. Irrigation is needed during the dry season and this is often the limiting factor for production. A crop rotation with e.g. maize or amaranth is recommended to control nematodes.
Propagation and planting
Most farmers produce their own seeds rather than buying those from the market. Seed germination is sometimes problematic because of low vigour caused by improper seed extraction and therefore inadequate removal of sugars and germination inhibitors present in the fruit. Other reasons for 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.
It is common practice to sow seeds in a nursery and transplant the seedlings when they are 10–15 cm tall and have at least 5 true leaves. Many growers mix the tiny seeds with fine soil, ash or dried manure before sowing. Nursery beds are mulched and irrigated daily. Well-fermented farmyard manure is incorporated into the plant beds whenever available because this gives better results than fertilizers only. Most farmers plant randomly with a spacing between plants of 15–20 cm. Other farmers may plant in lines with a spacing of 10–12 cm in the line and 30–45 cm between lines. The spacing is usually wider during the rainy season than during the dry season.
Diseases and pests
Solanum tarderemotum seems to be more susceptible to diseases than Solanum scabrum and Solanum villosum. Bacterial wilt (Ralstonia solanacearum) and the fungus Cladosporium oxysporum are the main diseases, but several other fungal diseases can also be noticed such as late blight (Phytophthora infestans), which may become problematic when farmers continue planting this crop closely during the rainy season. Some farmers use a fungicide, e.g. Ridomil-copper mixture, for control. The main pests are black aphids (Aphis fabae), flea beetles (Epilachna hirta), cutworms and nematodes. Aphids, flea beetles and cutworms are sometimes controlled by chemical spraying.
Harvesting
Young shoots of 10–15 cm in length are picked starting 3–4 weeks after transplanting or 5–6 weeks after sowing. The preferred harvest method is ratoon cropping where the apical shoot is picked at the first harvest, allowing side shoots to grow. Subsequent harvests are at intervals of 1–2 weeks for the following 8–12 weeks. The harvest period for a rainy season crop without additional fertilizing is 4–6 weeks, but a well-managed crop may be harvested for up to 4 months. Regular harvesting of young shoots causes a prolonged vegetative period, whereas undisturbed plants may flower 8 weeks after seedling emergence.
Yield
The average size of a plot is about 1000 m2, where the first harvest will be about 7 sacks of 90 kg, the equivalent of 6 t/ha. The second harvest may yield 11 t/ha, the third 14 t/ha or more. Harvesting at intervals of about 2 weeks can go on for 3 months or even longer as long as there is proper pest and disease control, and regular fertilizing and irrigation. Farmers may thus get up to 80 t/ha, but those who do not provide these inputs can expect an average cumulative leaf yield per season of 12–20 t/ha.
Handling after harvest
Harvested Solanum tarderemotum shoots are highly perishable and need to be sold as soon as possible before they wither. The best harvest time is either late in the afternoon for marketing the following day, or early in the morning on the day the produce is to be sold. Shoots may be dipped in water to keep them fresh, but more often they are wrapped in polythene sheets or banana leaves as small bundles to keep them fresh for some time.
Genetic resources
In Kenya germplasm of Solanum tarderemotum is kept at Maseno University. Other collections held at the National Seedbank and the University of Nairobi are not well labelled and represent the 4–5 species of nightshades with edible leaves that are known from Kenya. A few accessions are also maintained at the Botanical and Experimental Garden of Nijmegen University, Netherlands. The largest depository of germplasm is with the communities that utilize Solanum tarderemotum as a vegetable. There is a need for further collection, study and documentation concerning the ethnobotanic, agronomic and ecological aspects of this species and to carry out more surveys to gain a more complete picture of its distribution throughout East Africa.
Breeding
The characteristics to be selected and bred for should specifically include disease tolerance and improved storability of the product. Currently, no known attempts to select and breed Solanum tarderemotum are being made.
Prospects
With the recently witnessed increase in the popularity of indigenous vegetables in East Africa and the fact that it is already popular among some communities, Solanum tarderemotum has good prospects of becoming an economically important leafy vegetable for medium-elevation and highland regions. Biosystematic studies of Solanum tarderemotum and related species of the so-called Solanum nigrum complex are needed to understand the connections between wild and cultivated plants and to establish a clear and sound system of taxonomic nomenclature.
Major references
• Bukenya, Z.R. & Carasco, J.F., 1995. Solanum (Solanaceae) in Uganda. Bothalia 25(1): 43–59.
• Edmonds, J.M., 1977. Taxonomic studies on Solanum section Solanum (Maurella). Botanical Journal of the Linnean Society 75: 141–178.
• Edmonds, J.M. & Chweya, J.A., 1997. Black nightshades. Solanum nigrum L. and related species. Promoting the conservation and use of underutilized and neglected crops 15. Institute of Plant Genetics and Crop Plant Research, Gatersleben, Germany/International Plant Genetic Resources Institute, Rome, Italy. 113 pp.
• Gray, J.M., 1968. The taxonomy of the Morella section of the genus Solanum L. within South America. PhD thesis, University of Birmingham, United Kingdom.
• Schippers, R.R., 2002. African indigenous vegetables, an overview of the cultivated species 2002. Revised edition on CD-ROM. National Resources International Limited, Aylesford, United Kingdom.
Other references
• Bukenya, Z.R. & Carasco, J.F., 1999. Ethnobotanical aspects of Solanum L. (Solanaceae) in Uganda. In: Nee, M., Symon, D.E., Lester, R.N. & Jessop, J.P. (Editors). Solanaceae 4: Advances in biology and utilization. Royal Botanic Gardens, Kew, United Kingdom. pp. 345–360.
• Chirchir, J.K., 2001. Survey of Solanum species in the highland regions of central and western Kenya, 31 May–4 June 2001. KENRIK, National Museums, Kenya.
• Imbumi, M., 2001. Cultivation of Solanum eldoretti in Kisii-Kilgoris areas of Western Kenya. Records of a field trip during a workshop on indigenous vegetables held at Maseno from 14–16 August 2001, organised by the Kenyan Ministry of Agriculture and by DFID/ASSC project No ZX0099.
• Mwai, G.N., 2001. Preliminary report on collaborative taxonomic study of the African nightshades (Solanum L. section Solanum) at the Botanical Garden. University of Nijmegen, Netherlands.
• Mwai, G.N., 2002. Collection of Solanum leafy vegetables germplasm in the Western Kenya region. Student report for Nijmegen University, Netherlands.
• Onyango, M.A., 1993. Effect of plant density and harvesting frequency on the yield and vegetable quality of four variants of black nightshade (Solanum nigrum L.). MSc thesis, University of Nairobi, Kenya.
• Schippers, R.R., 2002. The Solanum nigrum complex in East Africa. Progress report for IPGRI's African Leafy Vegetables Programme.
• West, C.E., Pepping, F. & Temaliwa, C.R. (Editors), 1988. The composition of foods commonly eaten in East Africa. Wageningen Agricultural University, Netherlands. 84 pp.
Author(s)
• G.N. Mwai
Department of Botany, Maseno University, Kenya, P.O. Box 333, Maseno, Kenya
• R.R. Schippers
De Boeier 7, 3742 GD Baarn, Netherlands


Editors
• G.J.H. Grubben
Prins Hendriklaan 24, 1401 AT Bussum, Netherlands
• O.A. Denton
National Horticultural Research Institute, P.M.B. 5432, Idi-Ishin, Ibadan, Nigeria
Associate Editors
• C.-M. Messiaen
Bat. B 3, Rιsidence La Guirlande, 75, rue de Fontcarrade, 34070 Montpellier, France
• R.R. Schippers
De Boeier 7, 3742 GD Baarn, Netherlands
General editors
• R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
• L.P.A. Oyen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article:
Mwai, G.N. & Schippers, R.R., 2004. Solanum tarderemotum Bitter In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Lιgumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
Distribution Map wild and planted