Sweet potato (Ipomoea batatas Lam.) is produced commercially, as well as grown by subsistence and small-scale 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.
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. Symptoms are seldom seen on the leaves, and their appearance depends on the growth stage, variety, and the degree of stress, but yield can decrease and cracked storage roots can occur.
Sweet potato virus 2 is also referred to as sweet potato virus Y or sweet potato vein mosaic virus. This disease and sweet potato virus G are also transmitted by M. persicae and A. gossypii. Although single infections occur occasionally, the viruses are detected mostly in crops already infected with other viruses.
Symptoms induced by a combination of sweet potato feathery mottle virus, sweet potato virus 2 and sweet potato virus G are more severe than in the case of a single infection. Sweet potato virus 2 symptoms are generally chlorotic spots with diffuse mottling on the leaves (see Figure 2A). Symptoms of sweet potato virus G include mild mottling (see Figure 2B), vein yellowing or ring spots, and leaves often distorted with distinct vein clearing. The size and number of storage roots can also be affected.
Sweet potato leaf curl virus
Sweet potato leaf curl virus is a begomovirus transmitted by the whitefly species Bemisia tabaci. Sweet potato leaf curl São Paulo virus, sweet potato leaf curl and sweet potato mosaic virus are begomoviruses reported in South Africa. The three viruses can be found infecting a single sweet potato plant and have been observed in Limpopo, Mpumalanga and Gauteng.
On susceptible varieties, the leaves curl or roll upwards (see Figures 3A and B). In severe cases, plants can be stunted. Drastic yield loss has been reported following infection by begomoviruses.
Sweet potato chlorotic stunt virus
Sweet potato chlorotic stunt virus is a crinivirus transmitted by the whitefly species B. tabaci. It is the most serious disease affecting sweet potato in East and West Africa, and has also been reported in isolated areas in Southern Africa.
A single infection of sweet potato chlorotic stunt virus induces mild symptoms, which are often confused with nutritional deficiencies. When found in association with sweet potato feathery mottle virus, it causes sweet potato virus disease, a serious problem characterised by stunting (see Figure 4A), leaf narrowing and distortion, vein clearing and chlorotic mosaic (see Figure 4B).
Impact of sweet potato viruses on yield
A study conducted at the ARC’s Vegetable and Ornamental Plants (ARC-VOP) division to determine the impact of single, co- and mixed virus infections on the yield of sweet potato varieties, revealed that graft-inoculated plants with a mixed infection of sweet potato feathery mottle virus, and begomoviruses such as sweet potato leaf curl São Paulo virus and sweet potato mosaic virus reduced yield by an average of 68% in the first season of planting (see Graph 1).
Reductions in yield by 58% and 52% on average were recorded when varieties were inoculated with a single infection of sweet potato feathery mottle virus, and a co-infection of sweet potato leaf curl São Paulo virus and sweet potato mosaic virus, respectively, in the first season of planting. With virus infection, cracks can also increase by between 41% and 82%, adversely affecting the quality of marketable storage roots.
Healthy planting material can yield between 20t/ ha and 25t/ ha, while material infected with multiple virus infections can have a yield as low as 1t/ha to 4t/ha.
Prevention and control
Farmers are strongly discouraged from reusing sweet potato planting material. Instead, they should obtain cuttings from vine growers who replace their stock annually with virus-indexed planting materials obtained from the ARC-VOP.
Other control measures include the following:
- Isolate the new season crop from old sweet potato lands, preferably by more than 500m.
- Control weeds in and around the land. Focus particularly on wild Ipomoea species, as these harbour viruses when sweet potato is not in season.
- Remove volunteer sweet potato plants that may have survived from previous crops.
- Spray preventatively to limit the spread of the virus. Chemical control of aphids, however, has little effect due to the rapid nature of transmission.
- Use registered insecticides to control whitefly and thereby minimise the spread of viruses.
The science behind the virus detection
Viruses are detected by grafting sweet potato cuttings onto the indicator plant I. setosa in order to observe for symptoms development, followed by serological techniques such
as nitrocellulose enzyme-linked immunosorbent assay (NCM-ELISA) and double antibody sandwich ELISA (DAS-ELISA), using a broad spectrum antiserum.
Nucleic acid-based molecular techniques such as reverse transcription polymerase chain reaction (RT-PCR) and PCR are also used as rapid detection tests for the most common viruses of sweet potato.
The ARC-VOP has kept the sweet potato industry profitable over many years by running a disease-tested scheme. This provides healthy mother stock plants, enabling vine growers to start each season with planting material of disease-tested origin.
The Guide to Sweet Potato Production in South Africa can be purchased from the ARC-VOP by emailing Tsholofelo Tselapedi at [email protected].