Sweet Potatoes

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.

Soil should be cultivated when damp but not too wet. Cultivation of dry or very wet soil can break up the soil structure, leading to poor drainage and aeration, surface crusting, cracking allowing entry of weevils, and greater susceptibility to erosion.

After cultivation, the land is usually formed into ridges. Mounds are preferred by farmers working entirely with hand tools. In some areas, broad raised beds are used. On deep, well-drained soil, planting may be done on flat fields. Ridges are typically about 30-45 cm high, but may be higher in wet areas to maximize soil drainage.

To plant the crop, one prepares beds and plants the vines of 30 cm length with spacing of 30cm between plants by 80cm from one row to another.

1. Weeding and ridging

After crop establishment, further cultivation may be undertaken to control weeds, or to reform ridges (hill up) after fertilizer side dressing or to prevent weevil infestation. Weeding is done every time it is deemed necessary. However, disturbance of soil in the ridges should be minimized to avoid root damage.

2. Fertilization

The farmer uses organic farm yard manure to grow the tubers. However, in cases where inorganic fertiliser has to be used, phosphorus and potassium-based fertiliser are applied. Nitrogen-based fertiliser is discouraged as it enhances growth of leaves, neglecting the tubers.

a.Diseases

1. Sweet potato virus disease: Sweet potato virus disease (SPVD), caused by dual infection with the whitefly-borne Sweet potato chlorotic stunt virus (SPCSV) and the aphid-borne Sweet potato feathery mottle virus (SPFMV), is the most serious disease of sweet potato in Africa. Its symptoms are stunted plant growth and leaves exhibiting pale coloration, mosaic patterns, abnormal smallness or narrowness, distortion or crinkling. Yield of these plants is reduced significantly.

Control measures:

Control measures for sweet potato virus disease include:

• Planting virus-free vines from resistant varieties,
• Diseased plants must be removed from the field and burnt.

2. Sweet potato alternalia blight: (Alternaria solani).

Its symptoms are the presence of brown necrotic lesions on older leaves, with a typical bull’s eye appearance of concentric rings, 1 to 5 mm wide with well defined margins,  is the most evident symptom of the disease. As the disease develops, light brown lesions are formed with concentric rings. Several lesions can fuse and cover a great area of the leaf. When this occurs, the leaf drops.

Symptoms

Brown lesions on leaves with concentric rings resembling a target; lesions are usually restricted to the older leaves and may be surrounded by a yellow halo; small gray-black oval lesions with lighter centers may occur on stems and leaf petioles and occasionally on leaves; stem and petiole lesions enlarge and often coalesce resulting in girdling of the stem; defoliation may occur

Control measures:

• Destroy all sweet potato crop residue immediately following harvest;
• Plant resistant or tolerant sweet potato varieties where available;
• Plant only disease-free seed material.

b)Pests

• Sweet potato butterfly (Acraea acerata)  and sweet potato weevil (Cylas sp.).

Symptoms: larvae of A.acerata feed on leaves of sweet potatoes. Young caterpillars feed on the upper leaf surface whereas the older larvae eat the whole leaf lamina except the midrib. Heavy infestation can result in complete defoliation of the vines. Sweet potato weevil damage the crop by tunneling into the vines and roots.

Control measures:

Sweet potato butterfly: hand pick the nests of young caterpillars and burn them. In heavy infestation, insecticides can be used: 1 liter of Thiodan 35% E.C. or Sumithion 50% E.C, or  Malathion 50% E.C. per hectar (20 cc in 20 l of water per Acre).

• Ernose caused by Eriophyid mites ( Acaria sp.)

It causes extreme hairiness to vines and leaves of sweet potatoes and may lead to heavy losses in yield.

Control measures:

• Selection of mite free planting material,
• Good field sanitation.

Harvesting

Depending on grown variety, Sweet potato has the shortest growing cycle of the root crops and it is harvested at 4-5months after planting.  In traditional farming systems, where the crop is mainly intended for consumption by subsistence farmers, harvesting may be spread over several months. The crop is lifted by hand; great care being taken to avoid damage to the tubers, with only the amount needed for immediate consumption being dug. If the crop is grown for sale or when there is a pronounced dry season, the whole crop may be harvested at once.

• Storage

Apart from leaving sweet potato in the field during dry periods, sweet potatoes are perishable crops. They can be stored at normal room temperature ( 12-14^oC), in a dry place away from light.

In more advanced countries, sweet potatoes can be stored in what is called ” banking –  This required circular beds with foot high earthen walls to be prepared. The base of the circle was covered with straw and the potatoes piled up in a cone structure. The key with this type of sweet potato storage was to provide ventilation, prevent water from entering and keep the tubers cool You can store them in baskets to allow aeration but will not stay longer to avoid rotting. Sweet potato storage period cannot exceed two months.

• Post harvest processing

Sweet potatoes are perishable and storage is a bit difficult. However, they can be processed into other food stuffs which can be stored for a long period. Those products are mainly made of sweet potato flour. Those are biscuits, bread, doughnuts, juices, chips, etc…