Diseases and pests of sweet potato

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.