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Published in Issue No. 130, page 77 to 80 - (16129) characters

Underexploited tuber crops in Zimbabwe: a study on the production of Livingstone Potato (Plectranthus esculentus)

Patient D. Dhliwayo  Introduction
Root and tuber crops form a significant part of the diet of many Zimbabweans (COOPIBO Report, 1996). Although it can be stated that sweetpotato, potato and to some extent cassava play a major role in the diet, the contribution of other minor species should not be underestimated. The Livingstone Potato (Plectranthus esculentus) locally known as Tsenza in the Shona language is one of the edible indigenous tuber crops commonly grown in both the dryland and wetland areas in parts of the eastern districts of Zimbabwe (Chin’onzo 1994). Despite its acceptance, and nutritional value (Alleman 1996), the Livingstone Potato has received little or no attention from researchers in Zimbabwe and consequently its potential is not being fully realized. The Livingstone Potato could become one of the important horticultural tuber crops of the rural areas if its production is expanded. Production of Tsenza on a large scale will ensure conservation of genetic resources for biodiversity, agricultural development and benefit to rural farmers. Besides its contribution to food security and diversification of the local food base, production of the Livingstone Potato provides income-generation opportunities for rural Zimbabweans, especially women who are actively involved in the cultivation of this crop. Although P. esculentus could be considered of minor importance globally in terms of total production and commercial value, it does play a significant role in the food systems of particular communities in some countries including Zimbabwe.
Description of plant
Plectranthus esculentus (Coleous dazo, Coleous esculentus), generally referred to as the Livingstone Potato or Tsenza in Shona, is a dicotyledonous perennial shrub belonging to the family Labiatae (Purseglove 1968).
The plant has various forms or types: it may be erect, scrambling, woody or succulent with stems arising from a branching tuber, which is referred to as a stoloniferous rhizome. The shrub grows to approximately 0.6 m to about 2 m in height (Tredgold 1986; Schippers 2000). The plant bears yellow flowers which are about 0.015 m long (Purseglove 1968). The Livingstone Potato is understood to be distributed from equatorial Africa southward to Angola, the eastern Transvaal and Coastal Natal and is found as a cultivated crop in many countries including Zimbabwe (Schippers 2000). However, its origin is believed to be from two centres of dispersal: one is South Central Africa and the other Ethiopia (Greenway 1944).
In Zimbabwe, the Livingstone Potato is considered an untapped indigenous tuber crop (Mutenhabundo, pers. comm.; Chin’onzo 1994). Far less or no attention has been paid to this crop which contributes significantly to nutritional security in some rural communities. A study was carried out on P. esculentus in the major producing areas of Zimbabwe with the following objectives:
l to investigate the status of cultivation and importance of production of this crop
l to collect germplasm for characterization and preservation and development of propagation techniques for multiplication and distribution of high-quality planting material
l to strategize methods of promoting and increasing the production and utilization of the crop.
Production areas in Zimbabwe
Cultivation of Tsenza is commonly but not exclusively practised in the eastern districts of Zimbabwe. The major producing areas include Nyanga (rainfall >1000 mm/annum), Makoni and Mutasa (rainfall 750–1000 mm/annum).
Individual farmers in a few localized villages in Seke, Hwedza and Chiota (rainfall 750–l000 mm/annum) also grow this crop. In these areas, the Livingstone Potato is partly grown for household consumption and at least 70% is marketed. The roots are principally sold on the markets of the nearest towns but a considerable fraction of the produce also reaches the markets in the larger cities. Production facts and figures and official statistics on this crop are for the most part not available.
Cultural practices
Propagation
Tsenza is propagated vegetatively using the edible parts, i.e. the tubers. The tuber pieces (which can be planted as sprouted or unsprouted) are principally obtained from the previous crop and through farmer-to-farmer exchange. Several methods are practised for the storage and maintenance of planting material. Seed tubers are commonly kept in the production beds. However material also can be stored in underground pits or on the ground covered with a grass or leaf mulch. No other forms of propagation have so far been reported.
Land preparation>
The Livingstone Potato is commonly grown on raised beds in wetlands or vleis, although in parts of the eastern highlands cultivation on the flat is practised for dryland production. In both systems, virgin lands are considered the best for maximum crop performance and sandy loam soils are considered the most ideal. Field preparation usually commences around the month of June after the completion of all other major field jobs. In wetland production, land preparation starts with clearing of vegetation and then construction of beds, which are preferably laid along the slope to facilitate good drainage.
Where level culture is the choice, field preparation involves clearing of vegetation and then burning. In this system of cultivation, deep ploughing is done to encourage development of shapely tubers and to allow easy harvesting.
Planting
Generally, planting of seed tubers takes place from as early as July up to October. During this exercise, no planting specifications (i.e. spacing or seed rates) are followed although high planting densities are used. Active growth of the material only becomes apparent after the onset of the rainy season.
Use of fertilizers
Circumstantial evidence suggests that, in general, Tsenza production does not require the use of exogenously applied nutrients. Producers do not apply any fertilizers but the crop still gives satisfactory yields. The use of fertilizers has been notoriously associated with the production of poor-quality unmarketable tubers. Fertilizers are said to reduce shelf life, affect taste and texture and encourage development of oversized tubers. There is, however, no scientific evidence to support these claims.
Harvesting
The crop is harvested 6-7 months after planting. Bulk harvesting can be done but the crop can also be harvested as needed. The process involves completely pulling up or digging out the plants. The long finger-like tubers which are brittle and turgid are then easily broken off. Most growers have adopted a system of grading whereby the high-quality tubers are selected for the market and the rest is reserved for home consumption and for seed. Pest or disease-infected tubers are unacceptable. Generally, harvested tubers are kept in cool areas and underground pits. A grass or leaf mulch is normally used for the open storage; ashes and grass are used for the underground storage.
In either system of storage, tubers will normally keep for at least 2 months.
Pests and diseases
Weeds
Control of weeds is done at all stages of crop development. Hand-weeding is the common practice employed to discourage weed proliferation.
Pests
Production of Tsenza is seriously constrained by nematodes. These produce tiny galls on the tubers that adversely affect their marketability. In areas where Tsenza is produced, control is achieved by careful rotation, long fallow periods and careful selection of planting material.
Field observations also indicate that leafeaters of various species of caterpillars and insects can cause some damage to the crop. Rodents, especially moles, can severely affect the total yield and quality of tubers. Traditional control methods, which include the use of a herbaceous repellant called ‘tsinya’, were practised some years ago. Such traditional technologies are gradually being ignored, nor are the modern methods of pest and disease control being practised in the production of this crop. Other antagonistic organisms include crickets, springhares and porcupine but these do not appear to constrain production.
Diseases
Some diseases have been associated with Tsenza production. A fungal disease of importance is a leaf blight, which causes complete drying up of the plant. Fungal and bacterial rots may also be very serious, causing significant yield losses both in the field and during storage.
In conditions of high soil moisture, growers open up canals from the production beds to facilitate drainage of excess water as a means of disease control.
Varieties
At least 15 Tsenza varieties are grown in Zimbabwe (Table 1). It is believed that almost all of them have been selected by farmers and domesticated from the wild. There is, however, evidence of ongoing genetic erosion, which has resulted in the complete loss of some elite varieties. There are no documented descriptors of Zimbabwean Tsenza varieties available, but some of the interesting distinguishing characteristics include: plant habit (erect to creeping); tuber skin colour; tuber fresh taste, texture and shape; leaf morphology–shape, colour, smell, pubescence, etc.
Utilization
The primary product of Tsenza is the tubers, which are eaten raw or cooked. Cooked Tsenza is consumed as part of a major meal, especially in the rural areas. The use of Tsenza for domestic consumption is generally determined by dry matter content, sugar content and taste. Varieties with a high starch content require cooking for better digestion and palatability. Preparation involves boiling peeled or unpeeled tubers. ‘Chikone’, a dish of mashed Tsenza, is considered a traditional delicacy which is held in high esteem among the societies. The sweet-tasting varieties with low dry matter content are peeled and eaten raw, they function as an anytime snack. Although raw Tsenza is said to discolour the teeth, this characteristic is not a deterring factor. There are speculations that Tsenza tubers have medicinal properties. Some of the ailments known to be cured by the Livingstone Potato include stomach ache, nausea, backache and problems associated with the female reproductive system. Cooked Tsenza is recommended as a special food for infants, the elderly and the sick people, probably owing to its high digestibility. It is also believed that the Livingstone Potato increases potency in men and is especially useful to elderly men in polygamous marriages. For these therapeutic purposes it is recommended that the tubers be eaten raw and unpeeled. No other subsidiary uses of this crop have been reported.
Production-related constraints
Some constraints are common to Tsenza production across all the producing areas.
A neglected crop with no agronomic recommendations
Little or no attention has been given to this crop with the resulting in no agronomic recommendations being developed for growers. This general lack of information has led to the loss of interest in its cultivation and, consequently, a gradual loss of germplasm.
Limited utilization methods
The potential for diversification of the use of Tsenza even at household level has not been widely recognized by the farmers or researchers. Currently there are limited culinary uses of this crop and this might also seriously limit production and demand of this crop. The role that cultural change and social factors have played in pushing this crop to the edge should not, however, be dismissed.
Lack of improved varieties and unavailability of planting material
The absence of better-performing varieties preferred by farmers and the lack of adequate planting material are some of the important constraints to increased production in some areas where this crop is grown for commercial purposes. Farmers lack improved and efficient techniques of maintaining planting material. This has led to farmers growing a mixture of varieties that may not necessarily be the preferred ones.
Prejudice against indigenous crops and prohibition of vlei cultivation
In parts of Nyanga (Eastern District), a widely held belief suggests that a lobby by potato seed growers against production of ‘potentially nematode-infested’ indigenous crops almost wiped out Tsenza germplasm in these areas. This action has resulted in gross underutilization and neglect of this genetic resource. Prohibition of vlei cultivation in most communal areas has also severely limited production of the Livingstone Potato.
In these areas, it is very likely that the communities have never been exposed to the alternative of level culture in dryland production.
Future focus and needs
A number of constraints limiting the breakthrough for Tsenza as an attractive alternative to traditional root and tuber crops may be overcome if several important requirements are focused on this crop.
Policy and developmental issues
Despite their significance in several communities and regions, the role of minor root and tuber crops has often been taken for granted. Development in the area of underutilized root and tuber crops requires appropriate infrastructural development and capacity-building at both agricultural research and institutional levels. In addition, a policy framework in favour of such crops will serve to firmly support and successfully propel all the relevant activities pertinent to the development of underexploited root and tuber crops.
Research needs
Collecting and evaluation of Tsenza varieties would be the first step in germplasm research and development for improved production of the Livingstone Potato. This would be followed by development of techniques for rapid multiplication and distribution of improved varieties. Research on agronomic practices, pest and disease control, nutritional studies, post-production handling and processing will also be done. A project on Tsenza has been initiated at the Horticultural Research Centre, Marondera, Zimbabwe to improve food security and income-generation for rural communities through the maximum utilization and conservation of P. esculentus. More research activities will follow the germplasm-collecting exercise which has already been undertaken in Seke, Nyanga and Makoni some of the major production districts in the eastern regions of Zimbabwe. Similar activities are being undertaken in Malawi and South Africa, countries that together with Zimbabwe form a network whose vision is to have P. esculentus recognized as a valuable food crop.
Through this network and in close collaboration with other countries, the vision on the Livingstone Potato may be realized and P. esculentus will attain the merit and status it deserves.
Acknowledgements
I would like to thank the UNESCO/BAC Program for funding the germplasm-collecting mission during which this important information was gathered. I also thank my colleagues at the Horticultural Research Centre, Marondera, Zimbabwe for all their assistance in this work.
References
Alleman J. 1996. Two indigenous tuber species being evaluated as possible food crops. ARC Roodeplaat Vegetable and Ornamental Plant Institute. Pretoria, South Africa.
Chin’onzo SMT. 1994. Edible Vlei Tubers (Tsenza) Interesting Information. Department of Agricultural Extension Services. Harare, Zimbabwe.
COOPIBO Report. 1996. Study on the Feasibility of a Micropropagation Program For Selected Root and Tuber Crops. Zimbabwe.
Greenway PJ. 1944. Origins of some East African Food Plants. Part 1. East African Agricultural Journal 10: 34-39.
Purseglove JW. 1968. Tropical Crops Dicotyledons. Longman Hanow, UK.
Schippers RR. 2000. African Indigenous Vegetables: An Overview of the Cultivated Species. Natural Resources Institute / PCP EU Technical Centre for Agricultural and Rural Cooperation, Chatham, UK.
Tredgold MH. 1986. Food plants of Zimbabwe. Mambo Press, Gweru, Zimbabwe.

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