Sweetpotato propagation in Uganda usually involves the use of vine cuttings from mature crops obtained from a farmer’s own gardens or from neighbors’ (Bashaasha et al., 1995, Gibson et al., 2009). This, however, may not be the case in areas with a long dry season in which the existing sweetpotato fields dry out, resulting in lack of planting material at the onset of the rain (Gibson et al., 2011). Most farmers in such areas tend to wait for the surviving roots in the soil to sprout to obtain planting materials, leading to late planting (Gibson et al., 2009; Namanda et al., 2011, 2012; Yanggen and Nagujja, 2006). The health status of such sprouts is not known; but, if infected by virus, they may act as a local source of inoculum from which insect vectors can quickly spread it. This may result in virus accumulation and perpetuation over generations, thus reducing sweetpotato quality and yield (Gibson et al., 2009; Karyeija et al., 1998).
To overcome the challenge of late planting by ensuring timely access to planting material in areas prone to drought, a method of sprouting storage roots has been developed (Namanda, 2012) and is being promoted in Uganda, Tanzania, and Malawi by the International Potato Center (CIP). The source of seed roots used by farmers for this technology is from their commercial fields and, in most cases, farmers do the selection after harvesting the entire field, implying that the health status of the mother plants of selected roots is not known. Some farmers may even get roots from neighbors or marketplaces and use them to generate planting materials. A major limitation with this method of using storage roots as a source of planting material is that little or no information is available on the virus status of the storage roots, and therefore the decision about which root to save as seed is difficult.
SPFMV (family, Potyviridae; genus, Potyvirus) and SPCSV (family Closteroviridae; genus, Crinivirus) are the major economically important sweetpotato viruses in Uganda, although other viruses such as Sweet potato mild mottle virus (family, Potyviridae; genus, Ipomovirus), Sweet potato caulimo-like virus (family, Caulimoviridae; genus, Cavemovirus), Sweet potato chlorotic fleck virus, and Sweet potato leaf curl Uganda virus (family, Geminiviridae; genus, Begomovirus) have also been reported (Aritua et al., 2000, 2007; Mukasa et al., 2003; Ndunguru et al., 2009; Wasswa et al., 2011). Single infection by these viruses causes mild or no foliar symptoms, and this has made selection against infected material difficult, leading to their continued use. However, vines infected by a single virus, especially SPFMV, have been reported to revert to a healthy status (Gibson et al., 2014).
Reversion is a phenomenon whereby virus-infected plants naturally become healthy through the mechanism of gene silencing (Gibson et al., 2014). Reversion has been reported in cassava from Cassava mosaic geminiviruses (family, Geminiviridae; genus, Begomovirus) and has been exploited in the management of this virus (Fargette et al., 1994; Gibson and Otim-Nape, 1997). In sweetpotato, however, reversion from SPFMV has been reported In landraces from East Africa (e.g., Tanzania and New Kawogo) and locally bred varieties (e.g., ‘NASPOT 11’), but not with exotic varieties (Aritua et al., 1998; Wasswa, 2012, Gibson et al., 2014). This could be the reason why sweetpotato farmers in Uganda have continued to use symptomless vines successfully from their fields without serious sweetpotato virus disease (SPVD) infection. In all the reversion studies done so far, the focus has been on SPFMV, and they are limited to vine materials only. It is not clear whether plants infected by SPCSV alone or sprouts from infected roots can recover and/or revert from virus infection.
Coinfection of SPFMV and SPCSV results in severe disease symptoms characterized by a combination of vein clearing, chlorosis, leaf purpling, mosaic, leaf distortion, mottling, and complete plant stunting, which is described as SPVD (Gibson et al., 1998; Gutierrez et al., 2003; Milgram et al., 1996). These clear symptoms make it easy for a farmer to select against such vines when sourcing for planting materials. It is, however, not known whether the storage roots derived from infected vines among the Ugandan cultivars can produce symptomatic sprouts to enable farmers to rogue during the time of sprouting in screenhouses before field planting or, if infected, whether they too can revert to a healthy status. In some US sweetpotato cultivars such as Beauregard, when infected singly with the russet crack strain of SPFMV, the roots show external cracking and internal “corkiness,” and when planted, the virus symptoms on sprouts are very clear, making it easier to select against such roots (Clark and Moyer, 1988; Gutierrez et al., 2003). Infection by other strains of SPFMV, as well as Sweet potato virus C, Sweet potato virus G, and Sweet potato virus 2 are also common in the United States and cause yield losses without causing readily discernible symptoms on roots. Thus, the problem is common wherever sweetpotatoes are grown. This study was designed to determine whether viruses (single or mixed infection) can move from infected vines to storage roots, then express symptoms on the sprout, and to determine whether the infected sprout can revert to a healthy one.
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