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.
Adikini, S., Mukasa, S.B., Mwanga, R.O.M. & Gibson, R.W. 2016 Effects of Sweet potato feathery mottle virus and Sweet potato chlorotic stunt virus on the yield of sweetpotato in Uganda J. Phytopathol. 164 242 254
Aritua, V., Alicai, T., Adipala, E., Carey, E.E. & Gibson, R.W. 1998 Aspects of resistance to sweet potato virus disease in sweet potato Ann. Appl. Biol. 132 387 398
Aritua, V., Bua, B., Barg, E., Vetten, H.J., Adipala, E. & Gibson, R.W. 2007 Incidence of five viruses infecting sweet potatoes in Uganda: The first evidence of Sweet potato caulimo-like virus in Africa Plant Pathol. 56 324 331
Aritua, V., El-BedeOwy, E., Lanya, M.O. & Ewel, P. 2000 Yield and reaction of non-indigenous sweetpotato clones to sweetpotato virus disease in Uganda, p. 48–54. In: Y. Nakazawa and K. Ishiguro (eds.). Proceedings of the 1st International Workshop on Sweetpotato Cultivar Decline Study, 8–9 Sept. 2000. Miyazaki, Miyakonojo, Japan
Bashaasha, B., Mwanga, R.O.M., Ocitti p’Obwoya, C. & Ewell, P.T. 1995 Sweet potato in the farming and food systems of Uganda: A farm survey report. International Potato Center (CIP) and National Agricultural Research Organization (NARO), Lima, Peru
Clark, C.A. & Hoy, M.W. 2006 Effects of common viruses on yield and quality of Beauregard sweet potato in Louisiana Plant Dis. 90 83 88
Clark, F.M. & Adams, A.N. 1977 Characteristics of the microplate method of enzyme linked immunosorbent assay for detection of plant viruses J. Gen. Virol. 34 475 483
Fargette, D., Thresh, J.M. & Otim-Nape, G.W. 1994 The epidemiology of African cassava mosaic geminivirus: Reversion and the concept of equilibrium Trop. Sci. 34 123 133
Gasura, E., Mashingaidze, A.B. & Mukasa, S.B. 2009 Occurrence, prevalence and implications of sweetpotato recovery from sweetpotato virus disease in Uganda Afr. Crop Sci. Conf. Proc. 9 601 608
Gibson, R.W., Mpembe, I., Alicai, T., Carey, E.E., Mwanga, R.O.M., Seal, S.E. & Vetten, H.F. 1998 Symptoms, aetiology, and serological analysis of sweetpotato virus disease in Uganda Plant Pathol. 47 95 102
Gibson, R.W., Mwanga, R.O.M., Kasule, S., Mpembe, I. & Carey, E.E. 1997 Apparent absence of viruses in most symptomless field-grown sweetpotato in Uganda Ann. Appl. Biol. 130 481 490
Gibson, R.W., Mwanga, R.O.M., Namanda, S., Jeremiah, S.C. & Barker, I. 2009 Review of sweetpotato seed systems in East and Southern Africa. Integrated Crop Management Working Paper 2009-1/ International Potato Center (CIP), Lima, Peru.
Gibson, R.W. & Otim-Nape, G.W. 1997 Factors determining recovery and reversion in mosaic-affected African cassava mosaic virus infected cassava Ann. Appl. Biol. 131 259 271
Gibson, R.W., Wasswa, P. & Tufan, H.A. 2014 The ability of cultivars of sweetpotato in East Africa to ‘revert’ from sweet potato feathery mottle virus infection Virus Res. 186 130 134
Gutierrez, D.L., Fuentes, S. & Salazar, L. 2003 Sweetpotato virus disease (SPVD): Distribution, incidence, and effect on sweet potato yield in Peru Plant Dis. 87 297 302
Hall, A.J., Bockett, G.N. & Nahdy, S. 1998 Sweet potato post-harvest systems in Uganda: Strategies constraints and potentials. International Potato Centre (CIP) Social Science Department Working Paper Series No. 1998-7. CIP, Lima, Peru
Karyeija, R.F., Gibson, R.W. & Valkonen, J.P.T. 1998 The significance of sweetpotato feathery mottle virus in subsistence sweetpotato production in Africa Plant Dis. 82 4 15
Miano, W.D. 2008 Replication of viruses responsible for sweet potato virus disease in resistant and susceptible sweetpotato genotypes and identification of molecular markers linked to resistance. Louisiana State University and Agricultural and Mechanical College, PhD Diss
Milgram, M., Cohen, J. & Loebenstein, G. 1996 Effects of sweet potato feathery mottle virus and sweet potato sunken vein virus on sweet potato yields and rates of reinfection of virus-free planting material in Israel Phytoparasitica 24 189 193
Mukasa, S.B., Rubaihayo, P.R. & Valkonen, J.P.T. 2003 Incidence of viruses and virus-like diseases of sweet potato in Uganda Plant Dis. 87 329 335
Mwanga, R.O.M., Odongo, B., Niringiye, C. & Alajo, A. 2007 Release of two orange-fleshed sweetpotato cultivars, ‘SPK004 (‘Kakamega’) and ‘Ejumula’, in Uganda Hort. Sci. (Prague) 42 1728 1730
Mwanga, R.O.M., Odongo, B., Turyamureeba, G., Alajo, A., Yencho, G.C., Gibson, R.W., Smit, N.E.J.M. & Carey, E.E. 2003 Release of six sweet potato cultivars (‘NASPOT 1’ to ‘NASPOT 6’) in Uganda Hort. Sci. (Prague) 38 475 476
Mwanga, R.O.M., Yencho, G.C., Gibson, R.W. & Moyer, J.W. 2013 Methodology for inoculating sweetpotato virus disease: Discovery of tip dieback, and plant recovery and reversion in different clones Plant Dis. 97 30 36
Namanda, S. 2012 Current and potential systems for maintaining sweetpotato planting material in areas with prolonged dry season: A biological, social and economic frame work. University of Greenwich, PhD Diss
Ndunguru, J., Kapinga, R., Seruwagi, P., Sayi, B., Mwanga, R.O.M., Tumwegamire, S. & Rugutu, C. 2009 Assessing the sweetpotato virus disease and its associated vectors in northwestern Tanzania and central Uganda Afr. J. Agr. Res. 4 334 343
Rukarwa, R.J., Mashingaidzel, A.B., Kyamanywa, S. & Mukasa, S.B. 2010 Detection and elimination of sweetpotato viruses Afr. Crop Sci. J. 18 223 233
Villordon, A.Q. & LaBonte, D.R. 1996 Genetic variation among sweetpotatoes propagated through nodal and adventitious sprouts J. Amer. Soc. Hort. Sci. 121 170 174
Wasswa, P. 2012 Sweetpotato viruses in Uganda: Identification of a new virus, a mild strain of an old virus and reversion. University of Greenwich, PhD Diss
Wasswa, P., Otto, B., Maruthi, M.N., Mukasa, S.B., Monger, W. & Gibson, R.W. 2011 First identification of a sweet potato begomovirus (sweepovirus) in Uganda: Characterization, detection and distribution Plant Pathol. 60 1030 1039
Yanggen, D. & Nagujja, S. 2006 The use of orange fleshed sweetpotato to combat vitamin A deficiency in Uganda: A study of varietal preferences, extension strategies and post-harvest utilization. Social Sciences Working Paper No. 2006-2. The International Potato Centre, Lima, Peru