Sweet potato chlorotic stunt virus and Sweet potato feathery mottle virus interact to cause sweet potato virus disease. There are different strains of both viruses in East Africa, and differences between the strains there and those elsewhere in Africa. The abbreviation of the disease is SPVD, and for the two viruses the abbreviations are SPCSV (see Fact Sheet no. 375) and SPFMV (see Fact Sheet no. 258), respectively. SPCSV belongs to the genus Crinivirus; SPFMV belongs to the genus Potyvirus.

Sweetpotato virus disease occurs throughout sub-Saharan Africa; it is particularly common in the Great Lakes region. Both SPCSV and SPFMV occur worldwide in all the sweet potato regions of Asia, Africa, North and South America and Oceania.

Sweet potato virus disease was first reported in the 1940s, it was not until 30 years later that the cause was known. The two viruses involved do relatively little damage on their own (see Fact Sheet nos. 375, 258).

Severe SPVD symptoms occur because the presence of SPCSV turns offf the resistance mechanism which normally keeps SPFMV in check, allowing SPFMV to reach much higher concentrations (up to 600 times) than if it were alone. Symptoms may become even worse if a third virus infects the plants: there are over 30 different types of virus that infect sweet potato.

When the two viruses occur together in susceptible sweet potatoes, symptoms can be highly variable with infected plants showing yellow blotches, mosaics and mottles, on leaves which are often deformed, and vines which are stunted (Photos 1&2). Yields of storage roots are generally low, but it depends when infection occurs; if it is early, storage roots fail to develop.

By contrast, infection by SPCSV alone results in a mild yellowing or reddening of older leaves and stunting, as its name suggests. Infection by SPFMV alone results in yellow spots or purple ringspots and, occasionally, feather-like patterns bordering the major leaf veins. On the storage roots, some strains, on certain varieties, cause networks of small cracks in the skin, or longitudinal fissures, in circular bands one or more centimetres wide (Photos 3&4). Inside, the storage roots may show black spots. These symptoms are known as russet crack and internal cork, respectively.

Often, neither virus produces symptoms in sweet potato when alone, or the symptoms occur only when plants are under stress and growing slowly; for instance, when there is not enough rain. If growing conditions improve, the rapidly growing vines often appear healthy.

Before planting:

• Encourage farmers to consider the choice of planting material carefully:
  • Use varieties with resistance (see below). Farmers should be encouraged to use these varieties, not only for their tolerance to disease, but also because growing large areas will reduce infection of susceptible varieties if grown nearby.
  • If unsure that cuttings are disease free, grow cuttings in nurseries for later planting in the field; this avoids farmers taking cuttings from crops that may have unseen SPVD infections and cuttings that may develop the disease when planted. Infected plants should be checked in the nursery and any with SDVD discarded.
• Plant new plots at least 15 m from older crops where diseased plants might be present; this gap may limit whiteflies with SPCV from invading new crops. It is better if this gap is filled with a barrier crop, especially if the barrier is tall, e.g., maize.
• Do not plant new plots download from older plots; this is to prevent aphids and whiteflies from carrying the viruses on the wind to new crops.

During growth:

• Remove plants with SPVD, especially during the first month or so after planting; this practice can benefit the use of tolerant and also susceptible varieties. The reason for this is complex: SPCSV allows SPSMV to multiply many time more than if present alone, so preventing its spread is important. Whiteflies, spread SPCSV, but they move only short distances - about 15 m - from their host plants. Therefore, early removal of infected (source) plants can have a large impact.