SWEET POTATO

Introduction

Sweet potato can be grown with little or no fertilizer and is widely grown by smallholder farmers nearly in all parts of the country. It is environmental friendly (reduces erosion and has low chemical use).

It has a short maturity period of 3 to 7 months and because of its short duration, it is very strategic for addressing food insecurity. Sweet potato has many uses, ranging from consumption of fresh roots or leaves to processing into animal feed, starch, flour, candy and alcohol.

Vines and roots can be used as livestock feed. Sweet potato roots are sold locally and there is potential for export market. Sweet potato can be grown twice a year in medium and high potential areas.

Description

Sweet potato root is long and tapered, with a smooth skin whose colour ranges between cream, yellow, orange, red, brown, purple and beige. The flesh may be cream, white (highest in starch) to orange (high in carotene) to purple.

Sweet potato plant has trailing long stems or vines bearing funnel-shaped pink or rose violet flowers. Sweet potatoes are drought tolerant perennial vines, with one main season. It is widely grown on a small scale, mainly in subsistence farming.

The root tuber is long and tapered with a smooth skin whose colour ranges between red, purple, brown and white. Its flesh ranges from white through yellow, orange and purple They are used as food, boiled ore roasted and eaten either alone or with other foods such as milk, porridge, soups or grains

Sweet potato is a food security crop that can be grown nearly in all parts of the country. 

It is an inexpensive crop because it does not need much labour or fertilizer and that can be grown by the rural poor

The orange-fleshed variety is  a good source of vitamin A for nursing mothers and children

The sweet potato vines are useful and nutritious as fodder crop.

Notable nutritional attributes of sweet potatoes include-

• Consumption of sweet potatoes protects against obesity. Although they are starch rich food with a good deal of calories, they produce a sensation of satiety reducing appetite.
• They are rich in Beta carotene making them appropriate in cases of arteriosclerosis (thickening and hardening of arteries)
• Recommended for individuals with increased need for energy e.g. athletes and convalescence. 

Altitude

0 –2100 M ASL and occasionally found up to 2400M ASL. The crop prefers lower and mild elevation zones.

Soils

Sweet potatoes is grown in a wide range of soil type, but does best on soils of friable/loose nature, which permit expansion of tubers.Sweet potatoes grow best in fertile sandy loams and do poorly in clay soils.

The crop does poorly in water logged, too shallow or stony soils.  Poorly aerated and bulky soils retard tuber formation and reduced yields. The crop is sensitive to saline and alkaline soils and they should be avoided. Too high fertility may result in excessive vegetative growth at the expense of tuber and starch formation.

Temperatures

It grows best at 24 0C, when temperatures fall below 120C or exceeds 350C growth is retarded.

Rainfall

750 – 1000mm ideal but crop can withstand drought though under drought conditions, yield s are drastically reduced if drought occurs in the first 6 weeks after planting and also during root formation and development.

Recommended Varieties

Recommended varieties low and high rainfall areas.

New varieties

They are early maturing, high yielding, good virus resistance and  contain increased levels of  vitamin A.  Examples include Zaparo, Naspato, Tainain, Kemp 36( a fodder)

Land Preparation

Sweet potato grows best and produces smooth, well-shaped storage roots in a well-prepared soil. Good land or soil preparation involves removal or incorporation of crop debris and any vegetation that may compete with the sweet potato crop, and deep manual or mechanical cultivation. Making ridges or mounds is recommended because of:

• Higher yields achieved
• Mechanization possible – during harvesting you can use a ridger
• Conserve soil moisture
• Reduces soil erosion
• Intercropping with other crops possible

Planting on flat bed or fields should be discouraged as it reduces yields.

Propagation

Propagated is by use of vines cuttings or storage roots.

1. Storage roots   

Storage roots are used when there are insufficient stem cuttings available, or when the level of pest and disease infestation is high so that few healthy vines are left.

The sets must be healthy, robust and avoid using the rinds. Cover with little soil 3 cm of soil to avoid rotting. The bed should be covered with straw to help retain moisture. This method is not recommended as the resulting yields are low.

2. Vines cuttings 

The use of vine cuttings is the recommended practice. This is because:-

• They are free soil borne diseases
• Use of vines leaves tubers for consumption or selling
• Yields are higher
• They produces roots with better shape and size

Preparation of planting materials to achieve higher yields

1. Select ‘clean’ planting material that are free of insects, soil, and any symptoms of viruses or fungal diseases.
2. The apical (tip) portion of the vine is better than the middle or basal portions. This portion is less likely to carry sweet potato weevils and fungal pathogens, and has been found to establish faster than other portions.
3. A length of 30 cm is recommended because cuttings of much greater length tend to wasteful of planting material while much shorter cuttings establish more slowly and give much lower yields.
4. The delay between cutting and planting may affect yield depending on the storage conditions for the cuttings. Storing cuttings for one to two days in humid conditions may be beneficial, promoting rooting at the nodes. Longer storage may adversely affect establishment by exhaustion of the cuttings\’ energy reserves.
5.  To minimize losses, leaves should be stripped from the lower portion of the cutting, and bundles of cuttings wrapped in a wet cloth or sack and kept in a cool, shady place away from wind. If roots develop during storage, they should be planted carefully to minimize damage to the roots. 
6. In drier areas with only one main rainy season, leave some tubers in the soil over the dry season. When the rain comes the tuber sprout and the new vines are used for planting.
7. If planting material is to be maintained in a multiplication plot before planting of the next crop, plant cuttings at approximately 15 x 20 cm spacing. New growth may be ready for cutting after 45 days.

Planting Methods

Sweet potato vine cuttings are planted at an angle with vine ends towards the centre of ridge or mound. 2/3 of the vine cutting is buried in the soil. Depth of planting is 4-6 cm deep

Planting on mounds

Make mounds 1M apart (i.e. centre of the mound to the next) each with a base diameter of 30-40cm.

Plant 3-4 vines in single stand at equal distance from each other.

Planting on ridges

Make ridges 1M apart (i.e. centre of the ridge to the next). The base of one ridge to the other is 10-15 cm. Planting is done either in single or double rows.

• Single row – Vines are planted at the middle of the ridge at 30cm between plants within the row. This is recommended in drier areas.
• Double rows – The vines are planted on left and right position of the ridges at 30cm between plants within rows and 50-60 cm between rows. 

Seed rate

The recommended no. of cuttings is 27,000 per ha (11,000 per acre)1 gunny bag of cuttings has approximately 2000 cuttings implying that 5 ½ bags – 6bags are needed for an acre.

Time of planting

It is done at the onset of short and long rains when there is sufficient moisture to enhance establishment.

Source of Planting Materials

Certified seeds are available from the following KARI Station:-

• KARI – MTWAPA
• KARI EMBU
• KARI KATUMANI
• KARI KIBWEZI
• KARI KISII KARI KAKAMEGA

Maturity period 

Leaf harvesting for vegetable use starts 2 months after planting. Generally maturity period ranges between 3-6 months after planting depending on variety and environmental conditions.

Maturity indicators 

• Yellowing and drying of leaves
• Mature tubers are recognized by the sap that comes out when detached from the root which does not darken easily or readily
• Cracking of the soil indicates maturity and location of the tuber

How to Harvest

Locate the large tuber by cracks on the ground. Loosen the soil around the tuber with a sharp tool like a fork, sharpened stick, metal rods or matchets or pangas and lift the tuber from the ground. Use a jembe or a ridger if you want to harvest the whole plot at once. Care should be taken to avoid damaging the tubers.

Piece meal

The tubers are harvested as needed and hence there is no fixed time for harvesting. 

Complete harvesting

The whole crop is harvested at once. Both the mature and immature tubers are harvested at the same time.

Yields

With good management and depending on variety you can achieve up to 38 tons/Ha although average yields ranges between 15-30 tons/Ha.

Storage

Sweet potato can be stored in the field in fresh form and harvested as needed (piecemeal). Tuber stored for long periods in the field are usually attacked by weevils and large mammalian pests.Fresh tubers can also be stored in covered pits under shade. Before being placed in the pit, the tubers are covered with banana leaves to avoid contact with soil. Sweet potatoes tubers can also be stored as dry chips.

Pests and Diseases

PESTS

Pests have been shown to affect production of sweet potatoes.The most important pest of the crop are the sweet potato weevil and moth.

Sweet potato weevil

Symptoms

• Thickening and cracking of the vines due to feeding by the adult weevil
• Larvae bores into the tuber leaving holes. This lead to bitter taste

Control

• Integrated pest management for this insect is recommended, consisting of the following measures: 
• Crop rotation 
• Use of clean planting material, deep planting and regular hilling to fill soil cracks around plants 
• Early planting and prompt harvesting
• Practice good field sanitation i.e. weeding, burn infested material
• Plant away from last year crop
• If seriously infested spray with dimethoate
• Organically incorporate a good amount of lantana camara before planting to repel the weevils.

Sweet potato moth (Omphisa anastomasalis) 

Symptoms

• Caterpillar bore into the main stem leading to the roots.
• Vines with severe tunnelling show weak growth and poor foliage development; later yellows, wilts and dies. 
• Infested plants show poor storage root formation. 
• In some cases caterpillars may bore directly into storage roots. 

Control

•  Handpick caterpillars or attacked vines and destroy them. This is feasible in small plots. 
• If seriously infested spray with dimethoate

Whiteflies (Bemisia tabaci) 

Whiteflies feed on the lower leaf surface. Direct damage by adults and nymphs sucking sap from the plant, is generally not economically important. However, high numbers of whiteflies may affect plant development, particularly during period of water stress and drought. They are more damaging as vector of virus diseases. 

Symptoms

• Large numbers of whiteflies are visible on the underside of the leaves and flies away in swarm when the leaves are disturbed.

Control

• Conserve natural enemies. Parasitic wasps and predators such as predatory mites, ladybird beetles, and lacewings are important in natural control of whiteflies 
• Spray neem extracts.

Aphids (Aphis gossypii and other species) 

Aphids suck sap from leaves and stems. They may cause considerable damage during periods of water stress. Aphids are vectors of virus diseases. 

Symptoms

• Leaf curl on the tender leaves
• Adult wingless females are oval-bodied, 1.2-2.1 mm in body length, of very variable colour.

Control

• Conserve natural enemies. Aphids are attacked by a wide range of natural enemies, which are very important in natural control of these pests like wasps
• Spray with dimethoate

Mole 

Moles occasionally feed on sweet potato storage roots either by digging through the ridges or accessing the exposed roots. They often spoil more roots than they actually eat. 

Control 

• Keep the field and surrounding areas clean. 
• Dig a deep ditch around the perimeter of their field to deter rodents from digging tunnels straight into their fields. 
• Set up traps
• Make a mixture of cow dung and pepper and place it in the burrows and then burn to smoke the rodents out. 
• Mole damage can be reduced by planting on mounds rather than ridges
• Planting the deep rooted, poisonous shrub Tephrosia vogelii in the field (CIP, the VITAA Partnership)

DISEASES

Sweet potato virus

It is spread by aphids and white flies

Causal agent – Sweet potato feathery Mottle virus and Sweet Potato Chlorotic Stunt virus 

Symptoms

• Dwarfing of plant
• Yellowing of vines and young leaves
• Excessive branching
• Dark, brown to blackish corky spots in the roots.

Control

• Use resistant or tolerant varieties where available 
• Use disease-free planting material 
• Practice proper field sanitation 
• Control the white flies and aphids
• Crop rotation

Alternalia disease

Causal agent – Alternaria spp.

Symptoms

• Blackened lesions on the stems and leaves.
• Leaves becomes shriveled, blackened, die and fall off in severe cases

Control

• Use resistant or tolerant varieties 
• Use health, clean disease-free planting materials

Marketing

Tubers should be free of surface wounds and bruises which reduces marketability and shelf life. The tubers are mainly sold fresh in local markets. They fetch better prices in major urban markets like Mombasa and Nairobi.

Utilisation and Value Addition 

Sweet potatoes can be consumed whole- boiled, roasted, deep fried or mixed with cooked beans, mashed and consumed as a complete meal. The tuber can be made into chips, crisps or dried and ground into flour.

The flour can be composited with wheat flour to make bread, cakes, biscuits, mandazis, doughnut, chinchin, crackers and chapatis. Sweet potato flour can also be composited with sorghum or millets and used to prepare uji or ugali. 

The leaves can be used as fresh vegetables or preserved. The vines are used as livestock fodder.

Starch is produced from sweet potatoes in much the same way as from cassava, except that the solution is kept alkaline (pH 6.8) by using lime, which helps to flocculate impurities and dissolve the pigments’

Other Opportunities in the Sweet Potatoes Enterprise

• Seed bulking  – contract farmers to produce vines/cuttings
• Livestock feed – growing of sweet potatoes as fodder also manufacturing livestock feeds
• Industrial use – Starch is produced from sweet potatoes for industrial use (garment factories)
• Production of composite flours

Key Policy Issues

Key areas of policy concerns are:-

• Promote the production and utilization of food security crops
• Increasing agricultural productivity and incomes especially for smallholder farmers
• Encouraging diversification into non- traditional agricultural commodities and value addition to reduce vulnerability
• Enhancing the food security and reduction in the no. of those suffering from hunger and hence the achievement of the MDGs
• Encouraging private-sector led development of the sector
• Ensuring environmental sustainability

Stakeholders

• MINISTRY OF AGRICULTURE (MoA)

The Ministry of Agriculture coordinates the implementation of agricultural, cooperative and rural development policies. The specific functions which will be pursued by the Ministry include: rural development policy; agricultural policy; crop production and marketing; land use policy; pests and disease control; agricultural research; phytosanitary services; information management for agricultural sector; cooperatives; regional development authorities; among others.

• KENYA AGRICULTURAL RESEARCH INSTITUTE (KARI)

A parastatal established under the Science and Technology Act (Cap 250) with the national mandate to carry out research activities covering agricultural and livestock development through its 17 research centers countrywide. KARI seeds units need to multiply enough planting material for farmers in their mandate areas.

• KENYA PLANT HEALTH INSPECTORATE SERVICE (KEPHIS)

KEPHIS was established by the Kenya Plant Health Inspectorate Service Order, 1996 under the State Corporations Act (Cap 446). It is mandated to : co-ordinate all matters related to pests and disease control; monitor the quality and levels of toxic residue in plants, their soils and products; administer Plant Breeders Rights; undertake inspection, testing, certification, quarantine control, variety testing and description of seeds and planting materials; establish the machinery for educating public on safe use of agro-chemicals; approve import application for seeds, plants and appropriate phytosanitary requirements and importation of such material; and be responsible for inspection of produce for export and import.

• KENYA BUREAU OF STANDARDS (KEBS)

It is a parastatal established under the Standards Act ( Cap 496). Its primary function is to promote standardization in commerce and industry through development of standards, quality control, certification and metrology. It has the mandate of establishing and enforcing quality standards of all products on the Kenyan Market, both locally produced and imported. It is evident that the roles of KEPHIS and that of KEBS overlap thereby creating a conflict. Given that KEPHIS specializes in plants, the Standards Act should be amended to exclude seeds, planting materials, horticultural and other agricultural produce.

• KENYA INDUSTRY RESEARCH AND DEVELOPMENT INSTITUTE (KIRDI)

It is a parastatal established under the Science and Technology Act (Cap 250). It is mandated to undertake research and development in industrial and allied technologies. KIRDI collaborates with Ministry of Agriculture and other stakeholders in technology development and transfer processing of tubers.

• UNIVERSITIES AND COLLEGES OF AGRICULTURE

Among the universities and colleges, Nairobi, Jomo Kenyatta, Moi, Egerton, Baraton and Bukura provide courses at degree and diploma levels related to agriculture, environment and are also involved in research.

• MINISTRY OF HEALTH (MOH)

Liaises with stakeholders in agriculture industry to ensure hygiene in market and public places. It also protects Kenyan consumers from the health risks of contaminated food. The Ministry ensures regular inspection of food premises to ensure they conform to health requirements. Also, inspection of food imports at ports of entry is made in order to detect foreign diseases. It participates in promotion of food hygiene curricula in schools. Sweet potatoes especially orange fleshed varieties should be promoted by the ministry in nutritional management of HIV/AIDS, high blood pressure, obesity.

• MINISTRY ENVIRONMENT AND NATURAL RESOURCES

The Ministry is responsible for environmental policy, environmental impact assessment, catchment area conservation, water conservation, among others.

• MINISTRY OF ROADS AND PUBLIC WORKS (MoR&PW)

The Ministry is responsible for construction, maintenance and rehabilitation of classified roads. The road network accounts for 80 percent of the country\’s total passenger and freight traffic.

• MINISTRY OF INFORMATION, TRANSPORT AND COMMUNICATION (MoIT&C)

The Ministry is responsible for postal and telecommunication services as well as transport by railways, road, sea, lake and air in Kenya. It is also in charge of harbors, ports, and metrological services, among others.

• LOCAL AUTHORITIES (CITY, MUNICIPAL, TOWN, URBAN AND COUNTY COUNCILS)

They develop market and market infrastructure for agricultural produce among others. They are also responsible for collection and disposal of garbage, provision of sanitary facilities and land allocation for marketing facilities.