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Background: Cassava (Manihot esculenta Crantz) is staple food and major source of calories for over 500 million people in sub-Saharan Africa. The crop is also a source of income for smallholder farmers, and has increasing potential for industrial utilization. However, breeding efforts to match the increasing demand of cassava are impeded by its inability to flower, delayed or unsynchronized flowering, low proportion of female flowers and high fruit abortions. To overcome these sexual reproductive bottlenecks, this study investigated the effectiveness of using red lights to extend the photoperiod (RLE), as a gateway to enhancing flowering and fruit set under field conditions. Materials and methods: Panels of cassava genotypes, with non- or late and early flowering response, 10 in each case, were subjected to RLE from dusk to dawn. RLE was further evaluated at low (LL), medium (ML) and high (HL) red light intensities, at ~ ≤ 0.5; 1.0 and 1.5PFD (Photon Flux Density) in µmol m−2 s−1 respectively. Additionally, the effect of a cytokinin and anti-ethylene as plant growth regulators (PGR) and pruning under RLE treatment were examined. Results: RLE stimulated earlier flower initiation in all genotypes, by up to 2 months in the late-flowering genotypes. Height and number of nodes at first branching, particularly in the late-flowering genotypes were also reduced, by over 50%. Number and proportion of pistillate flowers more than doubled, while number of fruits and seeds also increased. Number of branching levels during the crop season also increased by about three. Earlier flowering in many genotypes was most elicited at LL to ML intensities. Additive effects on flower numbers were detected between RLE, PGR and pruning applications. PGR and pruning treatments further increased number and proportion of pistillate flowers and fruits. Plants subjected to PGR and pruning, developed bisexual flowers and exhibited feminization of staminate flowers. Pruning at first branching resulted in higher pistillate flower induction than at second branching. Conclusions: These results indicate that RLE improves flowering in cassava, and its effectiveness is enhanced when PGR and pruning are applied. Thus, deployment of these technologies in breeding programs could significantly enhance cassava hybridizations and thus cassava breeding efficiency and impact. [ABSTRACT FROM AUTHOR]

The most important potato viruses in Uganda are Potato virus Y (PVY), Potato leafroll virus (PLRV), Potato virus X (PVX), Potato virus S (PVS), Potato virus A (PVA) and Potato virus M (PVM). Utilization of R genes in breeding for resistance to viruses has not been explored in Uganda due to limited information on the prevalence of R genes in the available genotypes. This study aimed at identifying potato genotypes with R genes for resistance to potato viruses important in Uganda. The study screened 71 potato accessions from the National Potato Breeding Programme at Kachwekano Zonal Agricultural Research and Development Institute for the presence of resistance genes to viruses using diagnostic molecular markers. The results indicated that 21 out of 71 genotypes had resistance markers, of which nine genotypes, NKRN59.58, Derby, Markies, Sifra, 395017.229, Nakpot 5, 20108.5, Royal and 393220.54, had Ryadg gene for PVY resistance and two genotypes, Kimuri and 319919.3, had Rysto gene for resistance to PVY and PVA. Nine genotypes, 395011.2, Markies, Nakpot 5, 20108.5, Sifra, 20157.6, Royal, 2015.8 and Ambition, had the Nbtbr gene for resistance to PVX. In addition, 14 genotypes, 395011.2, Markies, Nakpot 5, Sarpouna, 393220.54, 391046.14, Sarpomira, 395077.12, Sifra, 20157.6, Royal, Ambition, Kimuri and Caruso, had the Nsadg gene conferring resistance to PVS. Four genotypes, Markies, Sifra, Nakpot 5 and Royal, had the Ryadg, Nbtbr and Nsadg genes for combined resistance to PVY, PVX and PVS. The resistant genotypes could be used as parents to introgress resistance genes into susceptible cultivars. [ABSTRACT FROM AUTHOR]

Sweetpotato is usually propagated in Uganda by vine cuttings from mature crops, but sometimes sprouts from storage roots are used, especially in drought-prone areas. No information is available on whether the storage of roots of Ugandan cultivars are infected with the viruses and whether the sprouts on them express symptoms so that farmers can eliminate diseased ones. Information on root sprout reversion from virus infection is also lacking. The storage roots of five sweetpotato cultivars was sourced either by random selection of roots from already harvested roots or obtained from symptomless plants selected before harvest at Makerere University Agricultural Research Institute, Kabanyolo (MUARIK), and the National Semi Arid Resources Research Institute (NaSARRI). Roots were also generated in a screenhouse after being inoculated with Sweet potato feathery mottle virus (SPFMV) and/or Sweet potato chlorotic stunt virus (SPCSV). More than 70% of sprouts from roots of all the cultivars selected after harvest at MUARIK and NaSARRI were infected with the viruses. For roots obtained from symptomless plants, 64% and 21% of the sprouted roots from MUARIK and NaSARRI were infected with the viruses, respectively. Most of the root samples from MUARIK had visible virus symptoms on sprouts and tested positive for both SPFMV and SPCSV, whereas those from NaSARRI did not show symptoms and were infected primarily with SPFMV. Plants graft-inoculated with either SPCSV or SPFMV alone produced both infected and noninfected roots, whereas all the root sprouts from dually infected plants showed virus symptoms. Reversion from virus infection was observed on root sprouts infected singly with SPFMV, whereas those infected with SPCSV showed recovery only, and none of the root sprouts infected by both viruses showed recovery. This study proves that roots are good reservoirs for viruses, and reversion occurs only when singly infected with SPFMV. Therefore, there is a need to establish seed channels in which seedstock is cleaned continuously and made available to farmers. [ABSTRACT FROM AUTHOR]

Viruses infecting wild flora may have a significant negative impact on nearby crops, and vice-versa. Only limited information is available on wild species able to host economically important viruses that infect sweetpotatoes (Ipomoea batatas). In this study, Sweet potato chlorotic fleck virus (SPCFV; Carlavirus, Betaflexiviridae) and Sweet potato chlorotic stunt virus (SPCSV; Crinivirus, Closteroviridae) were surveyed in wild plants of family Convolvulaceae (genera Astripomoea, Ipomoea, Hewittia and Lepistemon) in Uganda. Plants belonging to 26 wild species, including annuals, biannuals and perennials from four agro-ecological zones, were observed for virus-like symptoms in 2004 and 2007 and sampled for virus testing. SPCFV was detected in 84 (2.9%) of 2864 plants tested from 17 species. SPCSV was detected in 66 (5.4%) of the 1224 plants from 12 species sampled in 2007. Some SPCSV-infected plants were also infected with Sweet potato feathery mottle virus (SPFMV; Potyvirus, Potyviridae; 1.3%), Sweet potato mild mottle virus (SPMMV; Ipomovirus, Potyviridae; 0.5%) or both (0.4%), but none of these three viruses were detected in SPCFV-infected plants. Co-infection of SPFMV with SPMMV was detected in 1.2% of plants sampled. Virus-like symptoms were observed in 367 wild plants (12.8%), of which 42 plants (11.4%) were negative for the viruses tested. Almost all (92.4%) the 419 sweetpotato plants sampled from fields close to the tested wild plants displayed virus-like symptoms, and 87.1% were infected with one or more of the four viruses. Phylogenetic and evolutionary analyses of the 3′-proximal genomic region of SPCFV, including the silencing suppressor (NaBP)- and coat protein (CP)-coding regions implicated strong purifying selection on the CP and NaBP, and that the SPCFV strains from East Africa are distinguishable from those from other continents. However, the strains from wild species and sweetpotato were indistinguishable, suggesting reciprocal movement of SPCFV between wild and cultivated Convolvulaceae plants in the field. [ABSTRACT FROM AUTHOR]

Sweet potato feathery mottle virus ( SPFMV) and Sweet potato chlorotic stunt virus ( SPCSV) are the most common viruses infecting sweetpotato in Uganda. Field plots planted with graft inoculated plants of virus-free cultivars Beauregard, Dimbuka, Ejumula, Kabode and NASPOT 1 were used to assess the effect of SPFMV and SPCSV on yield and quality of sweetpotatoes in two agro-ecologies. SPFMV spreads rapidly to control plots at Makerere University Agricultural Research Institute Kabanyolo ( MUARIK), and these plots had similar yields to those singly infected with SPFMV but at the National Semi Arid Resource Research Institute (Na SARRI) where SPFMV spreads slowly, plots infected with SPFMV yielded 40% less than the control. Recovery from SPFMV appeared to be more frequent at Na SARRI than at MUARIK. Infection by SPCSV alone resulted in yield losses of 14-52%, while mixed infections of SPFMV+ SPCSV resulted in yield losses in both locations of 60-95% depending on the cultivar. SPCSV and mixed infections of SPFMV+ SPCSV also reduced the number of roots formed as well as the diameter of the roots, resulting in a greater length to diameter ratio compared to the healthy control. This study, therefore, confirms that both SPFMV and SPCSV, both singly and when mixed, can reduce yield, the extent depending on the cultivar. To mitigate the effect of these viruses, farmers should use clean planting materials of resistant varieties. [ABSTRACT FROM AUTHOR]

Sweet potato is a vegetatively propagated crop where vine cuttings from previous crops or volunteer plants are used as planting material. This practice can lead to the accumulation of systemic pathogens, especially viruses. However, the contribution of this practice to degeneration of sweet potato cultivars in Uganda has been only speculative, hence the need to document the rate of cultivar degeneration in high and low sweet potato virus disease pressure zones. Four cultivars of sweet potato – ‘Beauregard’, ‘Dimbuka’, ‘Ejumula’ and ‘NASPOT 1’ – were planted in a series of field trials in central (Kabanyolo) and Eastern (Serere) Uganda over five generations (G1, G2, G3, G4 and G5). The trials started with virus-free planting material and each succeeding trial retained planting material from the previous one, as well as receiving fresh clean material. Data were recorded on virus incidence and severity monthly for 4 months, root yield and vine weight at harvest after 6 months. Virus symptoms were observed 1 month after planting in all the plant generations, withSweet potato feathery virusandSweet potato chlorotic stunt virusbeing the most prevalent viruses detected. The cultivars ‘Beauregard’ and ‘Ejumula’ had highest disease incidence and severity, with the latter collapsing after a single season in both locations. Storage root yields and numbers were greatest in G1 but remained similar although less in all subsequent generations (G2, G3, G4 and G5) for each cultivar. Since it is impractical to provide fresh planting material each year for farmers, the focus should be on breeding more resistant varieties of sweet potato. [ABSTRACT FROM PUBLISHER]

Sweetpotato (Ipomoea batatas) is a widely grown food crop, in which the most important diseases are caused by viruses. Genetic variability of three widely distributed sweetpotato viruses was analysed using data from 46 isolates ofSweet potato feathery mottle virus(SPFMV), 16 isolates ofSweet potato mild mottle virus(SPMMV) and 25 isolates ofSweet potato chlorotic stunt virus(SPCSV), of which 19, seven and six isolates, respectively, are newly characterized. Division of SPFMV into four genetic groups (strains) according to phylogenetic analysis of coat protein (CP) encoding sequences revealed that strain EA contained the East African isolates of SPFMV but none from elsewhere. In contrast, strain RC contained ten isolates from Australia, Africa, Asia and North America. Strain O contained six heterogeneous isolates from Africa, Asia and South America. The seven strain C isolates from Australia, Africa, Asia, and North and South America formed a group that was genetically distant from the other SPFMV strains. SPMMV isolates showed a high level of variability with no discrete strain groupings. SPCSV isolates from East Africa were phylogenetically distant to SPCSV isolates from elsewhere. Only from East Africa were adequate data available for different isolates of the three viruses to estimate the genetic variability of their local populations. The implications of the current sequence information and the need for more such information from most sweetpotato-growing regions of the world are discussed in relation to virus diagnostics and breeding for virus resistance. [ABSTRACT FROM AUTHOR]

The use of certified sweetpotato (Ipomoea batatas) seed (CSS) was reported to unlock the yield potential of the crop leading to an over 80% yield increase. Despite the availability of elements of a formal CSS system, uptake in Uganda has remained dismal. Interdependence between farmer knowledge, perceptions, and purchase behaviors (PBs) is not well understood in a way that could allow the design of meaningful policies and interventions. This study aimed at unveiling factors underlying PB through purchase intention (PI) and willingness to pay (WTP) by interviewing 289 farmers from Omoro and Mpigi districts in Uganda, representing the low and high virus disease pressure zones, respectively. A modified theory of planned behavior (TPB) incorporating awareness as independent and farmer innovativeness as mediating variables expounded the relationships. Data were collected on perceptions, knowledge, innovativeness, and PI using a five‐point Likert scale plus WTP as a price, subjected to descriptive and structural equation modeling analysis. Results indicated significant locational differences in most socioeconomic characteristics. Middle income, CSS information access, and farming experience of 7–30 years conferred more likelihood of high PI. Conversely, high PI, group membership, and being female and middle‐aged increased the likelihood of high WTP. The effects of attitude, perceived behavioral control, and awareness were significant in high disease zone, while subjective norm was significant across disease zones. PI significantly affects WTP in high disease zone (p = 0.000) but its effect is insignificant in low disease zone (p = 0.569). An interplay of socioeconomic, perception, and knowledge attributes influences CSS PB calling for social marketing strategies with location‐specific interventions. Core Ideas: Farmer‐certified sweetpotato seed purchase intentions differ by disease pressure zone.A weak relationship exists between farmers' purchase intentions and their willingness to pay for certified sweetpotato seed.A modified theory of planned behavior predicts purchase decision‐making parameters for certified sweetpotato seed.Perceptions and awareness levels influence farmers' certified sweetpotato seed purchase behaviors.Area‐specific social marketing strategies can be adopted to spur certified sweetpotato seed purchases. [ABSTRACT FROM AUTHOR]

Cassava brown streak disease (CBSD), caused by cassava brown streak virus (CBSV) and; Ugandan cassava brown streak virus (UCBSV) causes the most destructive cassava disease in Tanzania. Thus, breeders urgently need CBSD-resistant cultivars to combat the impact of this disease, safeguard cassava production, improve the yields and secure food supply for communities reliant on this staple crop. In this study, we used four cassava cultivars; Albert, KBH 2002/135, KBH 2006/026 and Kiroba to explore how viral small interfering RNA (vsiRNAs), an antiviral silencing mechanism, confers CBSD resistance. Cassava plants were inoculated by grafting method using the single infection of CBSV, UCBSV and mixed infection of these viruses (co-infection). Total RNA was extracted from leaves of each cultivar followed by deep sequencing. The result showed that high level of vsiRNAs was produced in inoculated plants, with the most prominent class being 21 and 22 nucleotides. Kiroba produced the highest level of vsiRNA in both CBSV and UCBSV, whereas KBH 2006/026 produced high level of vsiRNA only with UCBSV infections. In contrast to UCBSV, inoculation with CBSV stimulated severe symptoms but relatively low levels of vsiRNA. Co-infection treatment showed a more complex interaction between the host and virus, with variations in the severity and amount of vsiRNA produced. We conclude that resistance to CBSD varies depending on the type of cultivar and virus species, and the occurrence of CBSD is suppressed in plants with elevated vsiRNA. Therefore, a good understanding on the resistance status of parental materials for breeding is recommended to breeders as a basis for improving cassava production. • CBSV and UCBSV pose a major threat to cassava production in Tanzania. • The occurrence of CBSD is suppressed in plants with high levels of vsiRNA. • CBSD resistance varies depending on the type of cultivars and virus species. • A good selection of cassava cultivars is important for successful cassava production. [ABSTRACT FROM AUTHOR]

This study employed an extended theory of planned behaviour to assess the socio-psychological factors influencing farmers' willingness to continue participating (WTCP) in collaborative activities of community-based innovation platforms (CB-IPs). Using a structured questionnaire, data were obtained from a random sample of 231 farmers who had participated in collaborative activities of the CB-IPs. Data were analyzed using partial least squares-structural equation modelling. Findings showed that although farmers' past-experience did not directly influence their WTCP in collaborative activities of CB-IPs, it was positive and significant when mediated by attitudes and perceived behavioural control (t-value = 6.840; p-value = 0.000). Therefore, it can be concluded that farmers' WTCP in collaborative activities of CB-IPs is enhanced by three socio-psychological factors: favourable past-experience, positive attitudes towards collaborative activities of CB-IPs, and farmers' perceived capacity to participate. Hence, conveners of collaborative activities of CB-IPs ought to improve farmers' attitudes through organizing beneficial activities and building their capacity to participate. This study contributed to the body of knowledge by using a behavioural perspective to examine the socio-psychological factors that influence farmers' continued participation in collaboration activities of CB-IPs. [ABSTRACT FROM AUTHOR]

Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) is globally the most common pathogen of cultivated sweet potatoes ( Ipomoea batatas; Convolvulaceae). Although more than 150 SPFMV isolates have been sequence-characterized from cultivated sweet potatos across the world, little is known about SPFMV isolates from wild hosts and the evolutionary forces shaping SPFMV population structures. In this study, 46 SPFMV isolates from 14 wild species of genera Ipomoea, Hewittia and Lepistemon (barcoded for the matK gene in this study) and 13 isolates from cultivated sweet potatoes were partially sequenced. Wild plants were infected with the EA, C or O strain, or co-infected with the EA and C strains of SPFMV. In East Africa, SPFMV populations in wild species and sweet potato were genetically undifferentiated, suggesting inter-host transmission of SPFMV. Globally, spatial diversification of the 178 isolates analysed was observed, strain EA being largely geographically restricted to East Africa. Recombination was frequently detected in the 6K2-VPg-NIaPro region of the EA strain, demonstrating a recombination 'hotspot'. Recombination between strains EA and C was rare, despite their frequent co-infections in wild plants, suggesting purifying selection against strain EA/C recombinants. Positive selection was predicted on 17 amino acids distributed over the entire coat protein in the globally distributed strain C, as compared to only four amino acids in the coat protein N-terminus of the EA strain. This selection implies a more recent introduction of the C strain and a higher adaptation of the EA strain to the local ecosystem. Thus, East Africa appears as a hotspot for evolution and diversification of SPFMV. [ABSTRACT FROM AUTHOR]

Seed set in East African Highland Cooking bananas (EAHBs) is extremely low and therefore hampers breeding. Pollen–pistil interaction is a key contributing factor. We assessed the effect of pollen germination media (PGM) on seed set in EAHBs. Five EAHB cultivars were pollinated with pollen from the wild banana 'Calcutta 4'. Glucose-based PGM sprayed on freshly emerged stigmas significantly increased seed set per 100 fruits per bunch. Increases were 73.5% in 'Enzirabahima', 39.9% in 'Mshale', and 302.4% in 'Nshonowa'. However, PGM did not increase seed set in the female sterile 'Mlelembo' and 'Nakitembe'. As larger bunches were more fertile, good field management practices are also recommended to get more seed to improve breeding efficiency. [ABSTRACT FROM AUTHOR]

Low female fertility in bananas is the biggest hurdle for banana breeding. The aim of this study was to determine seed set patterns in East African Highland Cooking bananas (EAHBs) to inform future decisions on a more targeted approach of increasing seed set and subsequently banana-breeding efficiency. Matooke (AAA) and Mchare (AA) bananas are genetically distinct but belong to the same genetic complex, referred to as EAHBs. Seed set patterns in "Enzirabahima" (AAA), "Mshale" (AA), and "Nshonowa" (AA), all with residual fertility, were examined after hand pollination with a highly male fertile wild banana "Calcutta 4" (AA). Seed set in "Enzirabahima" is predominant in distal hands. Mchare cultivars have a slightly more even distribution of seeds in their hands compared to "Enzirabahima". There is a gradual increase in seed set from proximal to distal hands with a slight drop in the last hand. This pattern is more definite in "Enzirabahima" and "Mshale", while "Nshonowa" has a somewhat inconsistent pattern. There is also a drop in seed set per 100 fruits per hand from small to larger bunches. However, larger bunches have a higher pollination success compared to smaller bunches. They therefore set more seed on 100 fruits per hand and per bunch basis, if bunches without seed are accounted for. Pollination success rate increases from smaller to larger bunches of EAHBs. Seed set is biased toward the distal third part of fruits of examined EAHBs, as well as tetraploid Matooke hybrid "401K-1" (AAAA), and improved diploid "Zebrina" GF (AA) that were used for comparison. In comparison, in the highly female fertile "Calcutta 4", seed set is along the entire length of the fruit. Seed set bias in the distal hands and distal end of fruits suggests a systematic mechanism rather than a random occurrence. It is expected that this information will provide a foundation for increased crossbreeding efficiency in bananas. [ABSTRACT FROM AUTHOR]

Viruses limit sweetpotato (Ipomoea batatas) production worldwide. Many sweetpotato landraces in East Africa are, however, largely virus‐free. Moreover, some plants infected by the prevalent Sweet potato feathery mottle virus (SPFMV) may be able to revert to virus‐free status. In this study, we analysed reversion from SPFMV, Sweet potato virus C, Sweet potato mild mottle virus, Sweet potato chlorotic stunt virus (SPCSV) and Sweet potato leaf curl Uganda virus using the indicator plant I. setosa and PCR/reverse‐transcriptase PCR. We also investigated environmental factors (temperature and soil nutrients) that may influence reversion from virus infection. We tested reversion in the East African cultivars New Kawogo, NASPOT 1 and NASPOT 11, and the United States cultivars Resisto and Beauregard. Reverted plants were asymptomatic and virus was undetectable in assayed parts of the plant. After graft inoculation, only the East African cultivars mostly reverted at a high rate and from most viruses though cultivar Beauregard fully reverted following sap inoculation with Sweet potato virus C. None of the tested cultivars fully reverted from single or double infections involving SPCSV, and reversion was only observed in co‐infections involving potyviruses. Root sprouts derived from SPFMV‐reverted plants were also virus free. Reversion generally increased with increasing temperature and by improved soil nutrition. Overall, these results indicate variation in reversion by cultivar and that the natural ability of sweetpotato plants to revert from viruses is malleable, which has implications for both breeding and virus control. [ABSTRACT FROM AUTHOR]

The complete positive-sense single-stranded RNA genome of Cassava brown streak virus (CBSV; genus Ipomovirus; Potyviridae) was found to consist of 9,069 nucleotides and predicted to produce a polyprotein of 2,902 amino acids. It was lacking helper-component proteinase but contained a single P1 serine proteinase that strongly suppressed RNA silencing. Besides the exceptional structure of the 5'-proximal part of the genome, CBSV also contained a Maf/HAM1-like sequence (678 nucleotides, 226 amino acids) recombined between the replicase and coat protein domains in the 3'-proximal part of the genome, which is highly conserved in Potyviridae. HAM1 was flanked by consensus proteolytic cleavage sites for ipomovirus NIaPro cysteine proteinase. Homology of CBSV HAM1 with cellular Maf/HAM1 pyrophosphatases suggests that it may intercept noncanonical nucleoside triphosphates to reduce mutagenesis of viral RNA. [ABSTRACT FROM AUTHOR]

The article presents a study which intends to summarize, integrate knowledge from temperature and tropical production systems, and develop management programs for sweetpotato chlorotic stunt virus (SPCSV) in sweetpotatoes. The study used polymerase chain reaction (PCR) detection and enzyme-linked immunosorbent assay (ELISA) methods. Results show that the awareness of siRNA analysis-based approach detecting unknown viruses, provides possibilities for virus control enhancement.

The yield and protein performance in a soybean genotype result from its interaction with the prevailing environmental conditions. This makes selecting the best genotypes under varied target production environments more complex. This study's objectives were to determine protein content and protein stability of 30 elite soybean genotypes in major soybean-growing areas of Uganda, assess the yield performance and stability in soybeans and determine the relationship between the protein content and grain yield in soybeans. The genotypes were planted in a randomized complete block design of three replications for six seasons across eight locations in Uganda. Genotype and genotype-by-environment (GGE) biplot analyses classified the test locations into three mega-environments for soybean protein and grain yields. Genotype NII X GC 20.3 had the highest mean protein content of 43.0%, and BSPS 48A-9-2 and BSPS 48A-28 were superior for the mean grain yield (1207 kg ha−1). Bulindi was the most discriminating and representative test environment for soybean yield. A weak and negative correlation (r = −0.1**, d.f. = 29) was detected between the protein content (%) and yield (kg ha−1). The highest-yielding genotypes BSPS 48A-9-2, BSPS 48A-31, and Nam II × GC 44.2 are recommended for further evaluation under farmers' production conditions for selection and release as new soybean varieties in Uganda. [ABSTRACT FROM AUTHOR]

Background:The bipartite single-stranded RNA genome of Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus; Closteroviridae) encodes a Class 1 RNase III (RNase3), a putative hydrophobic protein (p7) and a 22-kDa protein (p22) from genes located in RNA1. RNase3 and p22 suppress RNA silencing, the basal antiviral defence mechanism in plants. RNase3 is sufficient to render sweetpotato (Ipomoea batatas) virus-susceptible and predisposes it to development of severe diseases following infection with unrelated virus. The incidence, strains and gene content of SPCSV infecting wild plant species have not been studied. Methodology/Principal Findings:Thirty SPCSV isolates were characterized from 10 wild Ipomoea species, Hewittia sublobata or Lepistemon owariensis (family Convolvulaceae) in Uganda and compared with 34 local SPCSV isolates infecting sweetpotatoes. All isolates belonged to the East African (EA) strain of SPCSV and contained RNase3 and p7, but p22 was not detected in six isolates. The three genes showed only limited genetic variability and the proteins were under purifying selection. SPCSV isolates lacking p22 synergized with Sweet potato feathery mottle virus (SPFMV, genus potyvirus; Potyviridae) and caused severe symptoms in co-infected sweetpotato plants. One SPCSV isolate enhanced accumulation of SPFMV, but no severe symptoms developed. A new whitefly-transmitted virus (KML33b) encoding an RNase3 homolog (<56% identity to SPCSV RNase3) able to suppresses sense-mediated RNA silencing was detected in I. sinensis. Conclusions/Significance:SPCSV isolates infecting wild species and sweetpotato in Uganda were genetically undifferentiated, suggesting inter-species transmission of SPCSV. Most isolates in Uganda contained p22, unlike SPCSV isolates characterized from other countries and continents. Enhanced accumulation of SPFMV and increased disease severity were found to be uncoupled phenotypic outcomes of RNase3-mediated viral synergism in sweetpotato. A second virus encoding an RNase3-like RNA silencing suppressor was detected. Overall, results provided many novel and important insights into evolutionary biology of SPCSV. [ABSTRACT FROM AUTHOR]