Question: I’m producing sweet potatoes. Which pests and diseases should I guard against?

Sweet potato weevil and sweet potato moth are the pests that most often affect sweet potatoes. The most important field diseases are sweet potato feathery mottle virus, fusarium wilt, alternaria leaf spot, soft rot, sclerotium wilt, scurf and foot rot. During storage soft rot, surface rot and dry rot may occur.

SWEET POTATO WEEVIL

Adult weevils feed on foliage. The larvae tunnel into the sweet potato tubers and stems.

Preventive measures include the following:

• Crop rotation, which can help reduce the weevil population.
• Earthing-up tubers, which generally reduces infestation.
• Removing all tubers at harvest and never using cuttings from volunteers; in other words, from plants emerging in a field where sweet potatoes were planted the previous season.
• Burning all infested plants after harvesting.
• Using plant cuttings which are free from weevils or larvae.
• Not planting cuttings close to the previous sweet potato crop.
• Reducing soil cracking.
• Adjusting planting times so that there are no tubers in the dry season.
• Harvesting at the right time, as in-ground storage during the dry season promotes damage.
• Spraying with registered chemicals.

LARVAE OF SWEET POTATO MOTH

Large brown larvae (worms), that have a horn on the back of their bodies, feed on the leaves of sweet potatoes. Control them with registered chemicals at an early stage. Hand-picking larvae is usually sufficient if numbers are not too high.

FIELD DISEASES

Fusarium wilt

Fusarium wilt can cause severe yield loss. When cutting through the stem of an infected plant, a reddish-brown discolouration of vascular tissues can be seen. The leaves and stem turn yellow, wilt and die. Infected plants also infect the soil.

Preventive measures include the following:

• Using wilt-resistant cultivars.
• Using disease-free planting material.
• Practising field sanitation – removing infected and old plant residues.
• Rotating crops.
• Limiting any stress such as water deficiency combined with high temperatures during the growing season.

The same measures apply to alternaria leaf spot, soft rot, sclerotium wilt, scurf and foot rot. Also consider these additional measures to control soft rot:

• Do not harvest when soil is wet.
• Do not damage roots during harvesting.
• Wash off soil and store tubers in a cool, dry place.

Sweet potato feathery mottle virus

Although symptoms are seldom seen on the leaves, the yield decreases severely and there are more cracked tubers.

Here are some preventive measures:

• Planting virus-free material – obtain cuttings from registered vine growers.
• Controlling weeds in and around the field, especially wild ipomoea species.
• Using healthy-looking, vigorous material.
• Removing volunteer sweet potato plants, debris and weeds from the previous season before planting.
• Renewing plant material every two to three years.
• Cleaning all cutting equipment with a strong bleach solution.

STORAGE DISEASES

Try the following preventive measures against storage diseases such as soft rot, surface rot and dry rot:

• Do not store damaged sweet potatoes and those showing signs of disease.
• Remove and destroy infected tubers from the storing place.
• Store them in a cool, dry place.
• Clean containers with bleach.
• This article was written by Alistair Thompson and Diedrich Visser while employed by the Agricultural Research Council’s Vegetable and Ornamental Plant Institute (ARC-VOPI) in Roodeplaat, Pretoria, South Africa. It first appeared in Farming SA.