Identifying and controlling sweet potato viruses

Sweet potatoes are an important commercial crop and contribute significantly to food security in many poorer South African households. Dr Julia Mulabisana and Dr Sunette Laurie of the Agricultural Research Council’s Vegetable and Ornamental Plants division explain how farmers can recognise and control the most serious viruses that affect this crop.

Sweet potato (Ipomoea batatas Lam.) is produced commercially, as well as grown by subsistence and smallscale farmers, as an important and reliable food source. Unfortunately, like several other crops, sweet potato is prone to virus infections, which are associated with substantial yield and quality loss. Recent research at the Agricultural Research Council (ARC) has shown that mixed infections of begomoviruses, such as the sweet potato leaf curl virus, and potyviruses such as the sweet potato feathery mottle virus, have a detrimental effect on sweet potato production.

Virus-infected sweet potato vines used as planting material act as sources of infection and can transmit viruses from one place to another.

More than 30 viruses occur naturally in sweet potatoes worldwide. Some of the more important ones in South Africa are described below.

Sweet potato feathery mottle virus and sweet potato virus c

Sweet potato feathery mottle virus and sweet potato virus C are transmitted by the aphid species Myzus persicae and Aphis gossypii. An aphid can acquire the virus from infected plants in less than a minute while feeding, and transmit it to a healthy plant in seconds. Leaf symptoms vary with cultivar, climatic conditions and plant age.

Sweet potato virus C was previously called the C strain of sweet potato feathery mottle virus. However, comparison of the coat protein amino acid sequences revealed that it is distinct from other strains of sweet potato feathery mottle virus.

•  Symptoms Leaf symptoms include a diffuse mottled effect, faint to distinct chlorotic spots, which may have purple pigmented borders, and irregular chlorotic patterns (feathering) along the main veins (see Figures 1A, 1B and 1C). Symptoms are usually found in older leaves. Severely infected plants are stunted and their leaves are smaller.