Robert O.M. Mwanga

Chemical basis for resistance in sweetpotato Ipomoea batatas to the sweetpotato weevil Cylas puncticollis

The aim of this work was to determine the basis of resistance in a sub-Saharan sweetpotato variety, New Kawogo, to the African sweetpotato weevil Cylas puncticollis. This insect feeds on the roots, reducing quality and yield, and is the most important production constraint of sweetpotato in Africa. Laboratory bioassays were designed to determine how the performance of weevils differed on susceptible and resistant roots. Subsequently, liquid chromatography-mass spectrometry (LC-MS) analysis of the root surface and root latex identified quantitative and qualitative differences in the chemical profiles with higher levels of octadecyl and hexadecyl esters of hydroxycinnamic acids reported in the resistant variety. The compounds were synthesized to confirm their identity and incorporated into artificial diets for bioassays on C. puncticollis. High levels of mortality and developmental inhibition were recorded for larvae feeding on treated diets, and the effect was dose-dependent. Thus, in contrast to previous work on resistant African sweetpotato cultivars, resistance in New Kawogo is not only active, but is quantifiable and manageable for breeding. Work is underway to determine what effect these compounds have on the weevils at a molecular level. The inheritance of the root latex esters will be studied in new crosses and mapped in new populations using quantitative trait loci (QTLs) that are currently being developed.

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.

Sweetpotato weevil (Cylas spp.) resistance in African sweetpotato germplasm

Host-plant resistance could be a useful tool for managing the weevils Cylas puncticollis and C. brunneus, which are major insect pests of sweetpotato in Africa. There is currently little information on existing resistance mechanisms against Cylas spp. in African cultivars, except where lower levels of weevil damage were attributed to escape due to deep rooting and reduced soil cracking, limiting the exposure of roots to weevils. Here, we evaluate weevil resistance in 134 sweetpotato cultivars and landraces over two seasons in two agroecologically diverse locations. Several sweetpotato cultivars, including New Kawogo, expressed resistance to Cylas spp. The resistance characteristics have been demonstrated in previous laboratory experiments to be quantifiable and thus potentially useful in targeted plant-breeding against Cylas spp. We showed external root and stem base damage to be an accurate quantitative indicator of internal root damage, offering rapid and accurate evaluation of resistance in field trials for screening. Moreover, weevil resistance can be assessed earlier in plant development, so saving time in the selection of the progeny from breeding programmes.

Toxicity of seven Bacillus thuringiensis Cry proteins against Cylas puncticollis and Cylas brunneus (Coleoptera: Brentidae) using a novel artificial diet.

ABSTRACT “Sweetpotato weevils” Cylas puncticollis (Boheman) and Cylas brunneus F. (Coleoptera: Brentidae) are the most important biological threat to sweetpotato, Ipomoea batatas L. (Lam), productivity in sub-Saharan Africa. Sweetpotato weevil control is difficult due to their cryptic feeding behavior. Expression of Cylas-active Bacillus thuringiensis (Bt) Cry proteins in sweetpotato could provide an effective control strategy. Unfortunately, Bt Cry proteins with relatively high toxicity against Cylas spp. have not been identified, partly because no published methodology for screening Bt Cry proteins against Cylas spp. in artificial diet exists. Therefore, the initial aim of this study was to develop an artificial diet for conducting bioassays with Cylas spp. and then to determine Bt Cry protein efficacy against C. puncticollis and C. brunneus by using this artificial diet. Five diets varying in their composition were evaluated. The highest survival rates for sweetpotato weevil larvae were observed for diet E that contained the highest amount of sweetpotato powder and supported weevil development from first instar to adulthood, similar to sweetpotato storage roots. Seven coleopteranactive Bt Cry proteins were incorporated into diet E and toxicity data were generated against neonate C. puncticollis and second-instar C. brunneus. All Bt Cry proteins tested had toxicity greater than the untreated control. Cry7Aa1, ET33/34, and Cry3Ca1 had LC50 values below 1 µg/g diet against both species. This study demonstrates the feasibility of using an artificial diet bioassay for screening Bt Cry proteins against sweetpotato weevil larvae and identifies candidate Bt Cry proteins for use in transforming sweetpotato varieties potentially conferring field resistance against these pests.

Resistance to the weevils Cylas puncticollis and Cylas brunneus conferred by sweetpotato root surface compounds.

Seven resistant varieties of sweetpotato were compared with three susceptible varieties in field trials and laboratory bioassays and showed that resistance was an active process rather than an escape mechanism, as field resistant varieties also had reduced root damage and oviposition compared with susceptible varieties in the laboratory. Liquid chromatography-mass spectrometry (LC-MS) of root surface and epidermal extracts showed significant variation in the concentration of hexadecyl, heptadecyl, octadecyl, and quinic acid esters of caffeic and coumaric acid, with higher concentrations correlated with resistance. All compounds were synthesized to enable their positive identification. Octadecyl coumarate and octadecyl caffeate applied to the surface of susceptible varieties in laboratory bioassays reduced feeding and oviposition, as observed on roots of resistant varieties, and therefore are implicated in weevil resistance. Segregating populations from breeding programs can use these compounds to identify trait loci for resistance and enable the development of resistant varieties.

Tackling vitamin A deficiency with biofortified sweetpotato in sub-Saharan Africa

Orange-fleshed sweetpotato (OFSP) is a rich plant-based source of beta-carotene, which the body converts into vitamin A. In sub-Saharan Africa, sweetpotato is known as a food security crop but most varieties grown are high dry matter white-fleshed types, lacking beta-carotene. In 1995, researchers recognized the potential of OFSP varieties to address widespread vitamin A deficiency in SSA using an integrated agriculture-nutrition approach. With their partners, they confronted conventional wisdom concerning food-based approaches and institutional barriers, to build the evidence base and breed 42 OFSP varieties adapted to farmer needs and consumer preferences. Subsequently, a multi-partner, multi-donor initiative, launched in 2009, has already reached 2.8 million households. This review summarizes that effort describing how the changing policy environment influenced the process.

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.

Sweetpotato (Ipomoea batatas L.).

Sweet potato has traditionally been viewed as a “poor person’s crop” or “orphan crop,” and it has attracted limited attention compared to other staple crops. However, during the last decade, this perception has changed, and it is widely acknowledged that sweet potato has great potential to contribute to the alleviation of malnutrition and hunger in the developing world. Orange-fleshed sweet potato, in particular, with its high provitamin A content, has become a prominent example of the effectiveness of biofortified staple crops to combat vitamin A deficiency. Similarly, increasing awareness of the nutritional value of sweet potato is driving consumer demand among health-conscious consumers globally, and its potential use in a wide range of value-added human and animal products is widely recognized. As the public and private sectors learn more about the benefits and opportunities of sweet potato, they have invested more in crop improvement; thus our understanding of the importance and potential of the crop is increasing.

Segregation of hydroxycinnamic acid esters mediating sweetpotato weevil resistance in storage roots of sweetpotato

Resistance to sweetpotato weevils (Cylas spp.) has been identified in several sweetpotato (Ipomoea batatas) landraces from East Africa and shown to be conferred by hydroxycinnamic acids that occur on the surface of storage roots. The segregation of resistance in this crop is unknown and could be monitored using these chemical traits as markers for resistance in F1 offspring from breeding programs. For the first time in a segregating population, we quantified the plant chemicals that confer resistance and evaluated levels of insect colonization of the same progeny in field and laboratory studies. We used a bi-parental mapping population of 287 progenies from a cross between I. batatas ‘New Kawogo,’ a weevil resistant Ugandan landrace and I. batatas ‘Beauregard’ a North American orange-fleshed and weevil susceptible cultivar. The progenies were evaluated for resistance to sweetpotato weevil, Cylas puncticollis at three field locations that varied climatically and across two seasons to determine how environment and location influenced resistance. To augment our field open-choice resistance screening, each clone was also evaluated in a no choice experiment with weevils reared in the laboratory. Chemical analysis was used to determine whether differences in resistance to weevils were associated with plant compounds previously identified as conferring resistance. We established linkage between field and laboratory resistance to Cylas spp. and sweetpotato root chemistry. The data also showed that resistance in sweetpotato was mediated by root chemicals in most but not all cases. Multi-location trials especially from Serere data provided evidence that the hydroxycinnamic acid esters are produced constitutively within the plants in different clonal genotypes and that the ecological interaction of these chemicals in sweetpotato with weevils confers resistance. Our data suggest that these chemical traits are controlled quantitatively and that ultimately a knowledge of the genetics of resistance will facilitate management of these traits, enhance our understanding of the mechanistic basis of resistance and speed the development of new sweetpotato varieties with resistance to sweetpotato weevil.

Genetic analysis and association of simple sequence repeat markers with storage root yield, dry matter, starch and β-carotene content in sweetpotato

Molecular markers are needed for enhancing the development of elite sweetpotato (Ipomoea batatas (L.) Lam) cultivars with a wide range of commercially important traits in sub-Saharan Africa. This study was conducted to estimate the heritability and determine trait correlations of storage root yield, dry matter, starch and β-carotene content in a cross between ‘New Kawogo’ × ‘Beauregard’. The study was also conducted to identify simple sequence repeat (SSR) markers associated with these traits. A total of 287 progeny and the parents were evaluated for two seasons at three sites in Uganda and genotyped with 250 SSR markers. Broad sense heritability (H2) for storage root yield, dry matter, starch and β-carotene content were 0.24, 0.68, 0.70 and 0.90, respectively. Storage root β-carotene content was negatively correlated with dry matter (r = −0.59, P < 0.001) and starch (r = −0.93, P < 0.001) content, while storage root yield was positively correlated with dry matter (r = 0.57, P = 0.029) and starch (r = 0.41, P = 0.008) content. Through logistic regression, a total of 12, 4, 6 and 8 SSR markers were associated with storage root yield, dry matter, starch and β-carotene content, respectively. The SSR markers used in this study may be useful for quantitative trait loci analysis and selection for these traits in future.