Research on African vegetables at the Horticultural Development Centre (CDH), Senegal

Meissa Diouf
Centre pour le Développement de l'Horticulture, Institut Sénégalais de Recherches Agricoles (CDH/ISRA), Dakar, Senegal

Abstract

Research has been carried out by the Horticultural Development Centre (CDH) on the following traditional African vegetables: cassava, sweet potato, okra, African eggplant and bissap (Hibiscus sabdariffa). Technical pamphlets have been produced describing the genotypes that are adapted to our ecological, climatic and cropping conditions. These plant genetic resources are conserved in field genebanks (cassava and sweet potato) or as seed samples stored in freezers (African eggplant, okra and bissap). The main uses of these vegetables are: leaf vegetable and flour (cassava); chips, leaf vegetable and jam (sweet potato); fruit vegetable (okra, African eggplant, bissap - green calyx); juice (bissap - red calyx). The objectives of current research are: widening the range of available adapted varieties, improving the productivity of the varieties that have already been developed, and identifying varieties resistant to nematodes, the main pests in Senegal.

Introduction

Because of their nutritional value and their suitability for the ecological and climatic conditions of Senegal, traditional African vegetables could contribute significantly to food security, which is a prerequisite for all development. Unfortunately, however, in Senegal, as in most African countries, cultivation and use of these vegetables is becoming marginalized, and in some cases planting material has almost disappeared. Senegal realized the importance of traditional African vegetables as early as the 1950s, and attempts have been made to stimulate and revive their production. Government policy was translated into action by the Senegalese Institute of Agricultural Research (ISRA), established in 1974. Research at ISRA's Horticultural Development Centre (CDH) has mainly focused on cassava, sweet potato, African eggplant, okra and bissap.

After 20 years of this research work, the main achievements have been:

· the development of new varieties that are well adapted to our climatic conditions
· the development of husbandry systems, described in the technical pamphlets of the CDH
· the development of control methods against important pests
· improved storage techniques.

Major achievements in the production of traditional African vegetables in Senegal

Cassava and sweet potato

Both cassava and sweet potato are traditionally planted in lowland areas, in home gardens or in small plots, and receive little care. They constitute an important component of the Senegalese diet. However, they are being edged out of traditional agriculture; their cultivated area and output are now below those of cash crops such as groundnuts (Mbaye 1987), although production and cultivation gained momentum when they were included in the research programmes of the CDH.

Cassava (Manihot esculenta)

Between 1960 and 1970, the cultivated area under cassava was in excess of 35 000 ha, with a total output of 160 000 tonnes. In 1981/82, the production dropped to about 25 000 t from 8000 ha. The main factors responsible for this dramatic decline in production were drought, damage by the mealybug (Phenococcus manihoti) - a pest first reported in 1978 - and the African mosaic virus transmitted by Bemisia tabaci.

Since 1984, CDH has been carrying out a programme to introduce and screen high-yielding cassava varieties that are resistant to these major pests. This screening was carried out on 40 clones, 20 of them elite varieties from IITA (International Institute of Tropical Agriculture), Ibadan, and 20 from various ecological zones in Senegal.

The clonal selection method targeted the following characteristics:

· early maturity and high productivity
· resistance to the mealybug (Phenococcus manihoti)
· resistance to the African mosaic virus
· drought tolerance.

Since 1987, the three varieties Kombo 2, 30555 and 30572 - as well as another IITA variety (30786) - have been propagated at the Root and Tuber Crop Pre-Propagation Centre in Djibélor in the Ziguinchor area. After being propagated a second time by selected farmers, plant material produced at the Centre is then more widely distributed for cultivation (Mbaye 1987).

Sweet potato (Ipomoea batatas)

Sweet potato is produced all year round in Senegal. However, its production conditions are best toward the end of the dry season (March-May), as its yield is adversely affected by cold weather.

The programme's objective was to obtain high-yielding varieties that were resistant to the various pests and that could produce over a longer period. Of the 361 clones obtained by natural hybridization among local clones, 84 were selected on the basis of qualitative traits.

These 84 clones went through a second screening and 60 were retained. After a third screening, 47 clones were selected on the basis of a single quantitative trait, namely the mean tuber yield per plant. However, the clones had to meet marketing standards and a minimum level of resistance to pests. Based on these conditions, 20 clones were selected according to the following criteria:

· productivity
· resistance to nematodes (Meloïdogyne spp.)
· resistance to the mole cricket (Gryllotalpa africana)
· resistance to the weevil (Cylas puncticolis).

Okra (Abelmoschus esculentus)

Although okra production is well established in Senegal, yields as well as nutritional value and disease resistance could be improved. The total production is 8200 t for a cultivated area of 600 ha (Direction de l'Horticulture 1994).

One of the objectives of CDH was to collect as much germplasm as possible to identify highly adapted and productive genetic material. The okra breeding programme carried out in CDH has resulted in the development of two varieties: Population 12 was obtained through mass selection from a local variety, and Puso was developed through mass selection from the variety Emerald. Both varieties produce light green fruits, angular capsules, gradually become woody and are susceptible to nematodes, fusarium wilt, powdery mildew and cladosporium leaf spot.

CDH keeps a collection of 180 okra accessions to support its research work, which is currently aimed at developing a variety resistant to cold conditions. This collection includes local cultivars and accessions from the Sudan and the United States. Forty-eight varieties developed from this collection are intended for testing during the cold season.

Jaxatu or African eggplant (Solanum aethiopicum)

The African eggplant is a fruit and leaf vegetable widely produced in Africa, especially West Africa (de Bon 1984). The production of this plant is expanding in Senegal because of its economic and nutritional importance (Seck 1984). Total production is 4200 t from a cultivated area of 350 ha (Direction de l'Horticulture 1994).

Research carried out at CDH has led to the development of several varieties, notably Soxna and Keur Mbir Ndaw. The former is a high yielder despite the small size of its fruits, whereas the latter produces big fruits but yields less. Unfortunately, the prevalence of several insect pests and diseases makes it difficult for the crop to be produced all year round. The major pests are mites (Tetranychidae and Tarsonemidae). The extent of damage, particularly during the hot humid season from July to September - about 70% losses according to Benvenuti (1983) - is such that spraying against these pests is necessary to protect the crop (Seck 1986b).

Of the control methods proposed, selective breeding is the most promising given the great variability of the species and its allies. The Soxna variety, which is very susceptible to mites but is in great demand on the market, was included in the selective breeding programme carried out by CDH (Seck 1986a). The breeding programme on Soxna started with the screening of material, and led to the identification of resistant genotypes. This resistance trait was linked to the presence of hairs on the lower side of the leaves. Two of these hairy genotypes (Bot 2 and Bot 10e) were selected and crossed with Soxna to produce three lines: L10 (Bot 10e X Soxna), L16 and L18 (Bot 2 X Soxna). These three lines are currently being evaluated in farmers' fields and are promising. In addition, it has been observed that glabrous Solanum macrocarpon is also resistant to mites. Solanum sissymbriifolium, which is slightly hairy, is also highly resistant to mites. However, these two species have not been crossed successfully with Soxna because of interspecific incompatibility.

Investigations into the resistance mechanisms followed. Laboratory, greenhouse and field studies led to the conclusions that:

· Hairs probably represent a physical barrier against tetranychids, which are large (500 µm), but not against the smaller tarsonemids (200 µm). The latter can easily make their way between the hairs to reach the leaf blade, but without causing any significant damage.

· Mite resistance in S. macrocarpon is probably linked to the thickness of its epidermis. In hairy varieties, the hair base is reinforced with additional cell layers which increase the thickness of the epidermis. This phenomenon could partially explain the resistance to tarsonemids observed in hairy genotypes.

· The hypothesis that S. macrocarpon and S. sissymbriifolium produce repellent and/or antibiotic biochemical substances has been suggested. Hybridization tests are now being conducted to transfer one of these forms of resistance to susceptible but high-yielding genotypes or to combine two forms of resistance in one genotype (Diouf 1994).

Bissap (Hibiscus sabdariffa)

It is estimated that about 100 ha of bissap are cultivated annually. This crop, whose production is expanding, has traditionally been planted during the rainy season along field borders, and several varieties can be found around the country, including both green and red calyx types.

In 1976, two local types were collected from farmers' fields and two varieties were developed through two successive selfings: a red calyx variety (Koor) and a green calyx variety (De Lannoy 1979). A cropping system trial was carried out in 1979, mainly to study this crop further and to determine the yield parameters. Some seeds were produced by 1982. Generally, red calyx varieties produce more than 1 t seeds/ha, while green calyx varieties produce 400-600 kg seeds/ha. The maximum production of calyx on a dry weight basis is 500 kg/ha.

Selective breeding work carried out at CDH has focused on increasing the anthocyanin content of the calyx. A new red calyx variety was then developed, called 'High pigment content CDH line'. However, bissap still suffers pest and disease problems which may significantly hamper its production. Leafhoppers are a particularly serious problem in June-October and Oïdium abelmoschi in October-November, at the end of the growing period. Bissap is only moderately susceptible to Meloïdogyne spp.

Germplasm conservation and propagation

Cassava and sweet potato

Cassava and sweet potato are vegetatively propagated species. Plant material is stored as cuttings in the propagation plots for sweet potato and in greenhouses for cassava.

CDH uses a rapid multiplication technique for cassava in order to disseminate improved plant material to farmers. This multiplication technique uses 2-node cuttings. Under greenhouse conditions, it can produce 4 to 5 plantlets from a single 2-node cutting without using growth-promoting substances or sterilizing the growth medium. Between 30 and 50 cassava plantlets can be produced from a single 15-20 cm cutting. Superior sweet potato clones developed at CDH are kept as cuttings in the field and further propagation is then done from these cuttings. Although these cassava and sweet potato rapid multiplication techniques yield acceptable results, their multiplication rate is well below that of in vitro techniques, which can produce larger quantities of healthier plant material.

Okra, African eggplant and bissap

The seed production unit of CDH only produces base and pre-base seed of okra. Seed production is carried out in multiplication plots. Pre-base seed is stored in refrigerators, but space is limited. Base seed is stored at ambient conditions, which results in a decrease of the germination capacity as well as possible contamination.

For African eggplant, the seed unit again deals only with base and pre-base seed. Specialized public or private companies must therefore multiply the plant material to make it available to the farmers. However, CDH multiplies its own working germplasm. For all these species, refrigerators are used for short-term storage of seed and freezers for longer-term storage.

Uses of traditional African vegetables

Cassava

Several processing methods have been tried in Senegal. As early as 1972, the Food Technology Institute (ITA) started work on the processing of cassava into flour as part of a programme to promote local products. A blend of wheat flour and up to 15% cassava flour gave good results in breadmaking.

At the industrial level, a blend of wheat and cassava flours, of which the cassava flour was 15-20%, was used to make biscuits in the 1960s. However, these achievements had little impact on the population and today the major use of cassava is as a vegetable in Senegalese dishes.

Sweet potato

The consumer's preferences as to the characteristics of the sweet potato vary according to how he or she intends to use the tubers. Where the tubers are consumed raw, tender varieties with a high water content and a sweet taste are preferred, such as Ndargu, a variety that cannot withstand cooking. Tubers for cooking are chopped and boiled in water or in a soup. For this type of preparation, sweet varieties that remain firm during cooking, such as Walo and some IITA introductions, are preferred.

Sweet potato chips become popular whenever there are shortages of Irish potato. In 1972, IITA in collaboration with CDH developed a simple method for processing sweet potato into jam. Varieties that produce tender tubers with coloured flesh like Ndargu are preferred for this purpose.

Okra

The okra fruit is consumed mainly as a fresh vegetable. In Senegal, it is used in traditional dishes, especially soups. Sometimes, its leaves are consumed in the same way as spinach in the southern part of the country. It is highly nutritious as it contains fibres and carbohydrates as well as minerals and vitamins. However, fruits and leaves are not the only useful parts of okra. It is a secondary source of oil and gum and the fibres of its stem are used for rope-making.

Jaxatu or African eggplant

The African eggplant has many medicinal uses (treatment of colds, lowering the level of blood cholesterol, etc.) but is mainly used in traditional dishes as a fruit-vegetable in Senegal.

Bissap

The young leaves of bissap are consumed as a vegetable and the red calyxes are used to produce a refreshing, slightly acidic drink. There are export opportunities for the red calyx varieties, particularly for use in the cosmetic, textile and pharmaceutical industries.

Conclusion

Research conducted on traditional African vegetables at ISRA/CDH has led to the development of varieties that are well adapted to ecological and climatic conditions in Senegal, can be produced all year round and are resistant to some important pests. The production of these vegetables is now expanding. To further develop these crops, ex situ germplasm conservation facilities are needed. The wild relatives of some indigenous vegetable species are under threat or poorly known and more research work should be aimed at preserving them in order to exploit their genetic potential. Currently, CDH aims at using these genetic resources to develop varieties that are high-yielding and pest resistant, particularly to the nematodes which are a major problem in Senegal. Improvement of processing techniques should receive more attention from scientists for better utilization of food crops in the population's diet as well as in the various industries.

References

Benvenuti, G.C. 1983. Rapport semestriel d'activités (Juil-Nov), pp. 2-5. CDH/ISRA, BP 3120, Dakar, Sénégal.

de Bon, H. 1984. Description et culture d'une Solanacée légumière de l'Ouest africain: le Jaxatu (Solarium aethiopicum L). Agron. Trop. 39-1:67-75.

De Lannoy, G. 1979. Quelques notes sur la culture du Gombo et ses perspectives d'amélioration au Sénégal. CDH/ISRA, BP 3120, Dakar, Senegal.

Diouf, M. 1994. Etude des mécanismes de tolérance aux acariens du jaxatu (Solarium aethiopicum L.) et d'autres espèces du genre Solarium non-tubérifères. Mémoire de titularisation. CDH/ISRA, BP 3120, Dakar, Senegal.

Direction de l'Horticulture, Ministère de l'Agriculture du Sénégal. 1994. Estimation de la production légumière au Sénégal, campagne 1993-1994. Dakar, Senegal.

Mbaye, A. 1987. Contribution à la sélection de nouvelles variétés de manioc (Manihot esculenta Cranz) dans la zone des Niayes. CDH/ISRA, BP, 3120, Dakar, Senegal.

Seck, A. 1984. Contribution à l'amélioration génétique du jaxatu (Solanum aethiopicum L.) pour la culture en saison chaude et humide (Mémoire ESAT, 2^ème année).

Seck, A. 1986a. Sélection généalogique du jaxatu (Solarium aethiopicum L. subsp. Kumba) pour son adaptation aux conditions chaudes et humides: Etudes et sélection des descendances F2 et F3 obtenues par hybridation entre Soxna et 3 génotypes des sous-espèces aculeatum. CDH/ISRA, BP 3120, Dakar, Senegal.

Seck, A. 1986b. Synthèse des travaux de sélection créatrice réalisés sur le Jaxatu entre avril 1984 et juillet 1986. CDH/ISRA, BP 3120, Dakar, Senegal.