Field Management Practices

Fertilizer application 

Normally sweet potato does not require fertilizer application though in extremely poor soils a farmer can apply compound fertilizer e.g 17:17:17 at a 100 Kg per Ha in two splits, one at planting and the other after 2 weeks.

Excessive nitrogen should be avoided since it encourages vegetative growth at the expense of tubers growth.

  • Farm yard Manure is the only manure found to give a good response in sweet potatoes – it is spread over the bed just before ridging or mounding is done

Weeding  

The 1st weeding must be done within 2 weeks after planting. The second one should come up two weeks later when earthing- up is being done

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 

 

Sweet potato weevil. Adult female, body length 6-8 mm

Sweet potato weevil. Adult female, body length 6-8 mm

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

 

Sweet potato weevil symptoms on tuber

Sweet potato weevil symptoms on tuber

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

 

White flies on the underside of the leaf

White flies on the underside of the leaf

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
Aphids. Adult wingless females are oval-bodied, 1.2-2.1 mm in body length, of very variable colour.

Aphids

  • 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)

 

Tephrosia at different stages of growth

Tephrosia at different stages of growth

Farmers in Wangige, Kenya chase away mole rats by pouring fermented cattle urine (one week old) into their burrowing holes and this has proved to be very effective. 

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