Over 30 viruses have been identified in sweetpotatoes. The most important ones belong to the potyvirus family and the geminivirus family.
Four viruses in the genus potyvirus are found essentially wherever sweetpotatoes are grown in the United States. Although they spread at different rates in the field and may occur at different incidence, they all are very common. They share similar symptoms and are transmitted by many species of aphid non-persistently. While each of these four viruses has minimal impact on sweetpotato yields when they infect plants by themselves, as plants become infected by two, three, or all four of the viruses, yields may be reduced by up to 25-40%. These thus represent a chronic problem that our clean plant programs try to overcome. Three tests are available for sensitive detection of these viruses: two PCR tests that amplify a single product for any member of the genus, and a multiplex polymerase chain reaction (PCR) testing procedure developed by Li et al. (2012) that allows sensitive, specific detection of all four viruses in one test. The viruses in descending order of frequency of detection are:
Sweet potato feathery mottle virus (SPFMV) – SPFMV is the most common sweetpotato virus and can re-infect clean plants rapidly in the field, in many cases re-infecting 100% of plants in one season when a source of SPFMV is near clean plantings. Several strains of SPFMV have been recognized: ordinary (O), East African (EA), and russet crack (RC). The RC strain has been shown to cause the russet crack disease on storage roots of the cultivar Jersey and it has been presumed but not definitively proven that the same strain causes russet crack on contemporary cultivars such as Beauregard.
Sweet potato virus G (SPVG) – although it occasionally spreads more rapidly than SPFMV, SPVG is overall second most common. It is not known to have multiple strains.
Sweet potato virus C (SPVC) – was considered to be a strain of SPFMV until 2010 but is now recognized as a separate species. Information on spread of SPVC and its effect on yield is lacking.
Sweet potato virus 2 (SPV2) – is found less frequently than the other three potyviruses above. Isolates from Louisiana have not been transmitted by aphids under experimental conditions, but isolates from other countries and possibly other regions of the U.S. have been transmitted by aphids experimentally.
This is a group of viruses that has proliferated on several important crop plants over the past 25 years. Since the first description of Sweet potato leaf curl virus in 1999, between 10 and 23 species of geminivirus have been found in sweetpotato, but SPLCV is the only one for which there is significant biological information. Since the Li et al. 2004 PCR test has so far reliably detected a broad range of geminiviruses, it is not clear that the distinctions among the numerous species is biologically meaningful, thus we intend to treat these viruses as a group.
Diffuse chlorotic leaf spots are a symptom of SPFMV, a common potyvirus.
SPLCV does not cause foliar symptoms on most sweetpotato genotypes, but can reduce yield of Beauregard by about 30% despite the lack of symptoms. In some growing conditions, storage roots may have darker than normal skin color and appear lobed or fluted. These viruses are transmitted by whiteflies and can thus be potentially difficult to control in greenhouses where whiteflies are often hard to manage.
Although geminiviruses are not known to occur commonly in commercial production of table stock sweetpotatoes, there is a lack of recent survey data and their true incidence has not been reliably determined. They are common in older cultivars of purple ornamental sweetpotatoes. Because they represent a significant risk for contamination, especially in greenhouses, and a risk for yield reduction, and because it would be difficult to recognize contamination because there are no consistent foliar symptoms, geminiviruses represent a group for which we should maintain vigilance.
Sweet potato chlorotic stunt virus SPCSV has only been detected on three isolated occasions in the U.S. since serological or molecular methods became available to specifically identify it. However, historical reports of symptoms suggest the unproven possibility that it may have occurred more commonly in the U.S. before these methods became available. SPCSV is thought to have originated in Africa, where it interacts with SPFMV to cause a devastating disease known as Sweetpotato virus disease (SPVD) that caused 80-90% yield reductions when American cultivars were grown there. By itself, SPCSV causes very mild symptoms on sweetpotato that could easily be confused with nutrient deficiency (especially phosphorus) symptoms. There are two major strains of this virus, the West African, which has been found in the U.S., and the East African which is more severe but has not been found in the U.S. They are transmitted by whiteflies. Since sweetpotatoes growing in the greenhouse during winter months often show symptoms similar to those caused by SPCSV, and since it is whitefly transmitted, it represents a potential problem that could become economically devastating and thus requires vigilance.