R. W. Gibson

Sweetpotato Viruses: 15 Years of Progress on Understanding and Managing Complex Diseases

Sweetpotato is a member of the morning glory family that is thought to have originated in Central or South America but also has a secondary center of diversity in the southwest Pacific islands. It is grown in all tropical and subtropical areas of the world and consistently ranks among the 10 most important food crops worldwide on the basis of dry weight produced, yielding about 130 million metric tons per year on about 9 million hectares. Sweetpotato is an important crop for food security. It has been relied on as a source of calories in many circumstances. Vines and/or storage roots can be used for direct human consumption or animal feed. Growing awareness of health benefits attributed to sweetpotato has stimulated renewed interest in the crop. Orange-fleshed cultivars, a source of vitamin A, were introduced to developing countries with hope that they would replace the white-flesh varieties and help alleviate vitamin A deficiencies. In East Africa, sweetpotato virus disease, which is caused by the synergistic interaction of the whitefly-transmitted crinivirus and the aphid-transmitted potyvirus, can cause losses of 80 to 90% in many high-yielding genotypes. During the past 15 years, as molecular methods have been adopted, much has been learned about the composition of the sweetpotato virus complexes, the effects of virus diseases on production systems, the biology of the virus–plant interaction, and management approaches to sweetpotato virus diseases. This article is intended to summarize what has been learned since earlier reviews, integrate knowledge gleaned from experiences in tropical and temperate production systems, and suggest courses of action to develop sustainable management programs for these diseases.

Comparisons of coat protein gene sequences show that East African isolates of Sweet potato feathery mottle virus form a genetically distinct group

Summary. Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) infects sweet potatoes (Ipomoea batatas) worldwide, but no sequence data on isolates from Africa are available. Coat protein (CP) gene sequences from eight East African isolates from Madagascar and different districts of Uganda (the second biggest sweet potato producer in the world) and two West African isolates from Nigeria and Niger were determined. They were compared by phylogenetic analysis with the previously reported sequences of ten SPFMV isolates from other continents. The East African SPFMV isolates formed a distinct cluster, whereas the other isolates were not clustered according to geographic origin. These data indicate that East African isolates of SPFMV form a genetically unique group.

Development of superior cassava cultivars in Ghana by farmers and scientists: The process adopted, outcomes and contributions and changed roles of different stakeholders

SummaryA participatory breeding programme involving farmers in two Ghanaian communities and scientists from CRI (Ghana) and NRI (UK) to develop superior cassava cultivars is described. Initial situation analyses of the communities indicated that cassava is increasing in importance both as a food and a cash crop. Most farmers utilised landraces of cassava; modern varieties were scarcely mentioned. Seeds of 16 half-sib families obtained from a crossing block in Nigeria at the International Institute of Tropical Agriculture were planted in a field in each community. During seedling and subsequent clonal generations, accessions selected either by farmers or scientists were retained to the next generation. This selection process has identified 29 superior accessions from amongst 1350 original seedlings. Farmers were relatively consistent in their selection from year to year and their selections corresponded with their stated criteria. Official variety release requires additional multilocational and inspection trials and postharvest assays but otherwise seems harmonious with a participatory breeding approach; our early involvement of farmers may facilitate early release, an important factor in cost-effectiveness. A stakeholder workshop confirmed the need for improved markets for cassava; surveys of current and potential markets have led to field trials with cassava processors. Adoption of a participatory approach, with farmers and scientists taking on new roles and decentralisation of activities, implies a concomitant transfer of influence and resources.

Working with farmer groups in Uganda to develop new sweet potato cultivars: decentralisation and building on traditional approaches

Scientists and farmers in Uganda identified preferred sweet potato: (1) varieties through participatory varietal selection (PVS); and (2) new clones from seedling populations through a participatory plant breeding (PPB) approach. During these two processes, farmers identified 51 attributes of their landraces and of released varieties and used 21 criteria to select clones from amongst the seedling populations. Scientists had, in publications, listed attributes (11 main attributes identified), morphological descriptors (11) of released varieties and varietal needs (23) of sweet potato farmers. One released variety (NASPOT 1) was selected by farmers during PVS, mostly for its high and early yield of large, sweet and mealy roots, and several clones were selected through PPB amongst the seedling populations for a wider range of attributes. Some varietal attributes needed by farmers were not included by scientists either because they were very laborious, for example, selecting on-station for clones suitable for sequential piece-meal harvesting, or because occurrence of important abiotic or biotic stresses such as drought or pest damage were difficult to predict. Farmers seldom mentioned disease resistance but did mention pest resistance, consistent with easy visibility of both the causes of and the damage due to pests. Unlike scientists, farmers made no mention of a need for cultivars to have perceptually distinct features, despite this being a common attribute of landraces of most crops.

Two Serotypes of Sweetpotato feathery mottle virus in Uganda and Their Interaction with Resistant Sweetpotato Cultivars.

ABSTRACT Isolates of Sweetpotato feathery mottle virus (SPFMV, genus Potyvirus, family Potyviridae) were obtained in several districts of Uganda from sweetpotato plants infected with the sweetpotato virus disease (SPVD), the most important disease of this crop in Africa. A monoclonal antibody (MAb 7H8) raised against the coat proteins (CP) of a mixture of the SPFMV strain C (United States) and the isolate SPV-I (West Africa) distinguished Ugandan SPFMV isolates into those detectable and not detectable by the MAb. These two serotypes differed in prevalence in different districts of Uganda and in two common sweetpotato cultivars. Both serotypes could be transmitted simultaneously by single aphids. The serotypes differed in their ability to systemically coinfect sweetpotatoes that were infected with Sweetpotato chlorotic stunt virus (SPCSV, genus Crinivirus), the virus required to induce SPVD in SPFMV-infected plants. One sweetpotato breeding line, resistant to SPFMV from the New World, was infected by graft-inoculation with all SPFMV isolates from Uganda. Another SPFMV-resistant sweetpotato line became infected with SPFMV and developed SPVD only following coinoculation with SPCSV.

A Review of Perceptual Distinctiveness in Landraces Including an Analysis of How Its Roles Have Been Overlooked in Plant Breeding for Low-Input Farming Systems

A Review of Perceptual Distinctiveness in Landraces Including an Analysis of How Its Roles Have Been Overlooked in Plant Breeding for Low-Input Farming Systems. Traits providing perceptual distinctiveness (PD), which allow less commercial farmers in developing countries to recognize and name individual landraces, enable the creation and management of their diversity and the transfer of knowledge of each to other farmers and succeeding generations. Worldwide examples illustrate how PD traits on seeds and vegetative propagules help maintain genetic purity and provide markers at planting time, identifying landraces suitable for planting at particular locations and times and for future household and market needs. PD traits on the yield also enable household members and customers to identify and value landraces for different uses. To fulfill these roles, they are generally highly salient, restricted in number, environment-independent, qualitatively inherited, generally with expression based on one or a few genes, and often culturally significant. Even so, they are seldom mentioned as varietal selection criteria by farmers, who may be unaware of their importance, or in plant breeding programs and in situ conservation of plant genetic resources projects; the need for national variety release committees and policymakers in developing countries to include them is emphasized.

Benefits of participatory plant breeding (PPB) as exemplified by the first-ever officially released PPB-bred sweet potato cultivar

NASPOT 11 is a recently released sweet potato cultivar, bred by participatory plant breeding (PPB) in Uganda. It is already grown extensively by farmers who call it Tomulabula. In on-farm and on-station yield trials, Tomulabula yielded as well as the researcher-bred variety NASPOT 1 and sometimes more than the local landraces Dimbuka and New Kawogo, which have also been released. Farmers were asked to what extent Tomulabula, NASPOT 1 (the most popular station bred cultivar in Uganda) and the local indigenously bred cultivar they were currently growing satisfied 52 attributes previously identified by farmers as beneficial in sweet potato. Those cultivars whose breeding involved farmers (Tomulabula and the local cultivar) were perceived mostly to satisfy a broad range of attributes (i.e. had few ‘Very Bad’ scores) while those which involved researchers (Tomulabula and NASPOT 1) were the most frequently rated as ‘Very Good’ for specific attributes. Instances were observed and accounts given of how Tomulabula is sold at a premium and how it had improved farmers’ lives. These outcomes are attributed to PPB combining the strengths of farmers and researchers. The involvement of the Ugandan National Sweetpotato Program (UNSP) ensures that planting material will be conserved and also available in adequate amounts for official distribution.

How sweet potato varieties are distributed in Uganda: actors, constraints and opportunities

Sweet potato varieties are distributed in Uganda by three systems: formal, project-based and informal. In a partial formal system, the National Sweetpotato Program (NSP) breeds and tests both orange (O)- and white (W)-fleshed sweet potato (FSP) varieties. The NSP and other national institutions provide vine stocks of mostly OFSP varieties to project-based seed systems involving large private sector multipliers, which are found predominantly in east and central Uganda. These are mainly either cooperatives derived from smallholder groups organized by projects or are individual farmers with large holdings who had hosted NSP variety trials. The private sector multipliers sell planting materials of released varieties to projects, which distribute them free to selected households. Projects buy hundreds and occasionally thousands of sacks of vines but they distribute them to only a limited number of needy households and for only one or a few seasons. The informal system functions predominantly in areas with a long dry season, in which vines cannot survive. Its vine multipliers use the wetter lowlands to maintain mostly landraces, which they sell as small bundles of vines to many smallholders at the onset of the rains. Very few multiply released varieties. Thus, with a ready market, the system is sustainable but quantities sold are price limited. All vine multipliers supplied healthy planting material. It is proposed that the NSP should aim their trials of new varieties at informal vine multipliers and projects should supply vines to these multipliers in order to improve their access to them. This would promote the sustainable distribution of vines, including new varieties, over a greater area and to a larger number of farmers.

The ability of cultivars of sweetpotato in East Africa to 'revert' from Sweet potato feathery mottle virus infection.

Asymptomatic field plants are the normal source of the vine cuttings used as sweetpotato planting material in Africa. Previous and new tests of such East African material, mostly using the very sensitive method of graft inoculation to the indicator plant Ipomoea setosa, showed that a majority tested virus-negative. This was despite their never having undergone any science-based therapy. To investigate how this occurs, in a replicated greenhouse experiment, plants of susceptible cultivars from the USA and Peru and three resistant Ugandan cultivars were graft-inoculated with Sweet potato feathery mottle virus (SPFMV), the commonest virus infecting sweetpotato. When the grafts were established, cuttings were taken, rooted and proved to be infected. The health status of each of these new plants was then followed over a 10-week period using a quantitative polymerase chain reaction assay. Most of the plants of the Ugandan cultivars eventually tested SPFMV-negative whereas those of the USA and Peru seldom did. Furthermore, in subsequent graft-inoculations of scions from the tip, top, middle and base of the vine of every plant to I. setosa plants, again, most of the scions of the Ugandan cultivars tested SPFMV-negative whereas those of the USA and Peru seldom did. These tests demonstrate the phenomenon of reversion in the Ugandan cultivars and can explain how most unprotected Ugandan sweetpotato field plants tested SPFMV-negative.

Sweet potato cultivar degeneration rate under high and low sweet potato virus disease pressure zones in Uganda

Abstract 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, with Sweet potato feathery virus and Sweet potato chlorotic stunt virus being 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.