Real-Time PCR Assays for Detection and Quantification of Sweetpotato Viruses

Sweetpotato (Ipomoea batatas (L.) Lam.) is the third most important root crop in the world, ranking seventh among all crops in global production and fourth in developing countries after rice, wheat, and corn (10). In Africa, sweetpotato plays a vital role in peoples’ ability to sustain their families, especially during periods of hardship. However, sweetpotato plants, like other crops, are significantly affected by viral diseases. The cultural practice of vegetative propagation provides an efficient way for viruses to be perpetuated and disseminated between cropping seasons or growing areas (22). Several viral diseases have been reported in sweetpotato, but only a few have been well studied and characterized (3,22). Sweetpotato viruses are difficult to transmit mechanically, they are not transmitted through seed, and their host range often is restricted to the family of Convolvulaceae (18,26). The lack of progress in sweetpotato virus identification and classification is due to two important factors: the difficulty in isolating and purifying viruses from sweetpotato and the high frequency of mixed infections and synergistic complexes (7). The most common sweetpotato virus is Sweet potato feathery mottle virus (SPFMV), a member of the Potyvirus genus and Potyviridae family. It occurs virtually everywhere sweetpotato is grown, including countries in tropical and subtropical areas as well as in temperate regions (22). The universal presence of SPFMV often has masked the presence of other viruses in sweetpotato, especially those belonging to the same family, such as Sweet potato virus G (SPVG) and Ipomoea vein mosaic virus (IVMV), making the effort to detect or isolate them specifically very difficult (24). Sweet potato chlorotic stunt virus (SPCSV), a whiteflytransmitted member of the genus Crinivirus and the family Closteroviridae, interacts synergistically with SPFMV to cause sweet potato virus disease (SPVD), the most serious disease of sweetpotato in Africa, where it was first reported, or in South America, where is has been found more recently (5,21,25). Sweet potato leaf curl virus (SPLCV), a whitefly-transmitted geminivirus, and related begomoviruses, have been reported from a handful of locations around the globe, including Taiwan (2), Japan (20), Israel (6), and the United States (14). However, the geographic range of this virus is largely unknown and it may well be present in regions from which it has not yet been reported (4). Further understanding of the biology of these viruses is the most important prerequisite to their proper and effective control. The objective of this work was to develop real-time polymerase chain reaction (PCR) assays for the detection and quantification of selected sweetpotato viruses directly from sweetpotato plants; quantify and compare titer levels of SPFMV, IVMV, and SPVG in different host plants; and evaluate this method as an alternative detection assay for SPCSV and SPLCV.