Virus infection (unspecified)

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Disease type

Viruses are the smallest living things, and they live and multiply inside the cells of their host. They cause disease in the host by diverting the host's resources away from its own essential functions, in order to multiply the virus. They may be concentrated in a particular tissue, such as in the veins, and hence cause specific symptoms there.

A large number of viruses can infect sweetpotato. As the crop is vegetatively propagated, viruses get passed on from one generation to the next. It is normal for a sweetpotato crop to be infected by several viruses at the same time. Most viruses cause few if any symptoms when present alone. Little information is available on their effect on yield, but some depression is common and will vary with growing conditions. Some combinations of viruses are particularly harmful - the most significant of these being Sweetpotato Virus Disease (SPVD).

Economic importance

The economic impact of virus infection is rarely measured, but can be quite considerable. When virus-free planting material is compared with "normal" crops, the yields are usually at least 20% greater. In some situations, and with some cultivars, the yield benefit of virus removal is much greater.

Geographical distribution

Worldwide. Some viruses have limited geographic distribution. Often the strain (genotype) of a virus may vary from one place to another.

Symptoms

Symptoms are often not obvious, but a problem may be suspected due to yield decline over several years. Typical virus symptoms are slower growth, possibly with paler leaves or interveinal chlorotic mottling. However, these symptoms are often overlooked if there are no uninfected plants for comparison. Vein clearing (paler colour on the leaf veins) or chlorotic spots or ring-spots might be induced. Some viruses cause deformities in expanding leaves, including puckering, "fan-leaf", or curling of margins.

As symptoms are usually mild, they are often unnoticed until most or all plants are infected, and yield depression is observed. However, where symptoms are more conspicuous or severe, they will appear on individual plants.

It is difficult to diagnose specific viruses from symptoms alone. A positive diagnosis is usually achieved through laboratory tests, including antibody response (serological tests), electron microscopy, and grafting onto a test host (usually Ipomoea setosa) which may show stronger and more specific symptoms than sweetpotato.

Biology and ecology

Most sweetpotato viruses are spread from infected to uninfected plants in the saliva of sucking insects, such as aphids and whiteflies. Once a cell is infected, it multiplies the virus and releases virus particles to infect adjacent cells. Plants do not have an immune system comparable with that of humans, and generally they can not get rid of the virus. However, they can suppress its reproduction and spread within the plant to some extent, and this may vary among cultivars resulting in some virus tolerance.

Within the host, many viruses are concentrated in the phloem cells, in the vascular tissue of the plant, where they can move fairly freely from one cell to the next. Outside the phloem they may spread relatively slowly from one cell to another. Hence the tip of an actively-growing shoot (which does not yet have phloem cells) is usually free of viruses. Virus-free plants can be obtained by tissue-culture of these tips (meristem culture). Usually the plants are grown at a constant high temperature for some time before harvesting meristems, to suppress viral reproduction.

Host range

Most viruses are specific to sweetpotato and its closest relatives. Within the crop ecosystem, infection sources are usually only other sweetpotato crops. Most infections are through planting material, from one generation to the next.

Management

Clean seed programs

Viruses can be removed by heat treatment and meristem culture. This process usually results in a yield increase of both vines and roots, from 20% to 200%, depending on the severity of the original virus infestation. The higher yield may be maintained for several years in the field, before the virus load has built up again.

A program in China has significantly boosted regional yields by supplying farmers with clean planting material regularly. Clean seed programs are also expanding in USA, South Africa and Australia. Such a system is the standard practice for Irish/white potato production, but is not yet widely used for sweetpotato.

Host plant resistance

Cultivars may vary in either their resistance to infection in the field, or their tolerance of the infection. However, no immunity is available. Breeding programs generally do not specifically address virus tolerance, but field-based selection tends to discard genotypes most sensitive to local virus strains. Cultivars selected in one area may perform poorly in another, if different strains of virus are present.

Cultural control

Selection by farmers of symptomless planting material can help control the most damaging viral diseases.

Traditionally, growers have restored their crop yields by selecting and propagating from volunteer seedlings within their crops. As the seeds rarely carry viruses, seedlings often yield well at least for a few years. This practice has contributed to the rapid diversification of sweetpotato genotypes in all areas where it is grown.

References

Clark, C.A. and Moyer, J.W. 1988. Compendium of sweet potato diseases. APS Press. 74 p.

Schaefers, G.A. and Terry, E.R. 1976. Insect transmission of sweet potato disease agents in Nigeria. Phytopathology 66:642-645.

Contributed by: Jane O'Sullivan

Disease type

Economic importance

Geographical distribution