IPGRI-SSA Specific crops

Specific crops - Tubers

IPGRI activities in SSA

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Tuber crops provide an important source of food in most humid and subhumid tropical countries of SSA. All are clonally propagated and therefore slow to multiply and difficult to conserve. IPGRI is working to establish an integrated strategy to conserve and use tuber crops by combining different ex situ and in situ approaches to the problem. Yam (Dioscorea spp.), cocoyam (Xanthosoma sagittifolium) and frafra or Hausa potato (Plectranthus rotondifolius) have been selected as model crops for this work. Cultivated yams constitute a multi-species crop, including the traditional varieties of the D. rotundata-cayenensis complex, of D. bulbifera and D. dorietorum as well as introduced species such as D. alata and D. esculenta. Approximately 93% of world yams are produced in the ’yam belt‘ of West and central Africa. A considerable amount of the crop is cultivated by small- to medium-scale farmers and a large part of the community depends on it for food security and generation of income. Cocoyam is the third most important root and tuber crop in West and central Africa and probably the most important leafy vegetable. Frafra or Hausa potato, once an important crop in SSA, has been almost totally neglected by research, resulting in a significant decrease in its cultivation and use, although its area of distribution is still wide.

Work began with the study of the distribution of diversity and of farmer practices for the management of tuber diversity. A survey of ethnobotanical practices of cocoyam cultivation looked at farmers’ knowledge and description of genetic diversity, morphogenesis, reproduction of the crop and agronomic practices; the level of on-farm conservation of the germplasm; and uses to which farmers put the crop. RAPD markers were used to characterize collected germplasm. The activities were carried out in collaboration with the Botany Department, University of Ghana and the Plant Genetic Resources Centre of Ghana, University of Cocody in Côte d’Ivoire, Cameroon and the National Centre for Genetic Resource and Biotechnology in Nigeria.

IPGRI and two Ghanaian plant genetic resources institutions, the Savannah Agricultural Research Institute and the NPGRC carried out a survey of frafra potato conservation practices among farmers in northern Ghana. The study found differences in whether farmers used seed tubers or stem cuttings as planting material and recorded storage techniques and cultivation practices. The farmers’ classification system and emphasis on different traits depended on which propagation system they used.

Germplasm was collected and is being characterized at the Plant Genetic Resources Centre at Bunso, Ghana.

A joint IPGRI–IITA project is studying farmers’ practice of yam domestication and its contribution to improvement of the crop in West Africa in collaboration with Bariba, Nago and Fon farmers, Université National du Benin with IRD, Institut National des Recherches Agricoles du Benin, and the CIRAD-IITA Yam Research Coordination Unit. The current diversity in the traditional yam landraces, principally D. rotundata-cayenensis, is attributed to the availability of wild yams with cropping potential, different selection pressures, successive domestication, culture-derived modifications and somatic mutations. Wild species believed to have produced cultivated forms in West Africa include D. burkilliana, D. abyssinica and D. praehensilis. This process takes the wild species D. abyssinica and D. praehensilis through a farmer-developed technique of clonal propagation that transforms the material physiologically to produce cultivated varieties similar to members of the D. rotundata-cayenensis complex. The activity aims to move towards a process of farmer-led participatory plant breeding.

Field collections are the most common way of conserving tuber crops. This is a very expensive and labour-intensive approach, generally requiring annual regeneration. The fields are also very vulnerable to pests and diseases, made worse by the clonal nature of propagation of these crops and the diverse origins of the material. They are, in effect, often collections of diseases as diverse as the collection itself and are a potential source of further spread as material is distributed. In vitro cultivation is one approach that can be used both for cleaning planting material and keeping it in a state free from contamination. IPGRI and the University of Ghana are testing procedures for in vitro slow growth conservation in yam and developing new methods for cocoyam and frafra potato. The ultimate goal is cryopreservation in liquid nitrogen at –196° C, which will allow almost indefinite storage and will not require a steady supply of electricity. Dehydration and encapsulation protocols for yam cryopreservation developed by IRD are being tested at the university and new methods are being developed for cocoyam and frafra potato.

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