Properties of Strains of Sweet potato feathery mottle virus and Two Newly Recognized Potyviruses Infecting Sweet Potato in the United States

Sweet potato has been identified as a crop that has great potential for alleviating food security concerns (28). However, because it is vegetatively propagated, it is prone to accumulate viruses and other pathogens. Sweet potato viruses are difficult to transmit mechanically, they occur in mixed infections, and their host range often is restricted to the family Convolvulaceae (24). These factors cause problems in detection, isolation, and identification of sweet potato viruses. Except for the most common strains of Sweet potato feathery mottle virus (SPFMV), the potyviruses infecting sweet potato have not been well characterized (24). Two strains of SPFMV originally were described and are distributed widely (23). The common strain, SPFMV-C, causes foliar symptoms but not root symptoms. Russet crack, a disease induced by a strain of SPFMV (SPFMV-RC) was thought only to be a problem on the ‘Jersey’ type of sweet potato (dry-fleshed), but not thought to affect the orange, moist-fleshed types including ‘Beauregard’. However, in recent years, observations of a disease similar in appearance to russet crack have come from North Carolina, Virginia, California, and from isolated instances in Louisiana, Mississippi, New Mexico, and Texas (19; C. A. Clark, unpublished data). A SPFMV strain was reported in a sweet potato clone from Zimbabwe (10), but sequence data suggests that it could be regarded as a distinct potyvirus. Kreuze et al. (20) constructed a phylogenetic tree based on the coat protein (CP) nucleotide sequence and recognized four distinct clusters; RC, comprising russet crack isolates form the U.S., Japan, and Korea; O, comprising isolates from several countries in Asia; EA, comprising isolates from East Africa; and the most divergent group, C, for the common strain isolates. In addition, several other potyviruses have been reported recently from sweet potato, including Sweet potato chlorotic fleck virus (SPCFV), Sweet potato virus G (SPVG), and Sweet potato mild speckling virus (SPMSV) (27). It is clear that a complex of potyviruses affects sweet potato, but it is not clear how these various potyviruses relate to one another. A reverse transcription polymerase chain reaction (RT-PCR) assay utilizing degenerate primers corresponding to the border sequences of potyvirus nuclear inclusion b (NIb) protein and CP genes was used for specific detection of a severe strain of SPFMV in Japan (25). Moreover, SPFMV, Sweet potato latent virus (SwPLV) and SPVG could be detected by RT-PCR in 18 sweet potato clones from China (14). The sensitivity of PCR offers a potential advantage for detection of these potyviruses that are unevenly distributed and present at very low levels in infected plants. The universal presence of SPFMV often has masked the presence of other viruses in sweet potato, especially potyviruses, and hindered efforts to isolate and identify these viruses. Although they can be detected by RTPCR assays, it is not yet possible to differentially detect each of the different potyviruses quickly, inexpensively, and efficiently enough to permit studies on their incidence and spread in the field. Among the sweet potato virus indexing methods, graft inoculation onto the Brazilian morning glory (Ipomoea setosa Ker.) is the most reliable (24). However, the procedure is time consuming and requires a great amount of labor and greenhouse space. The objective of this work was to compare some biological and molecular properties of six virus isolates of the family Potyviridae obtained from sweet potato from different regions in the United States and to evaluate the potential of RT-PCR for detection of SPFMV.