Enhancing yam improvement for West Africa

Hiroko Takagi

EDITS Project: JIRCAS International Collaborative Research for West African crops

In the past, most agricultural investments and international agricultural research in Africa were focused on developing major cereals and crops for export. Recently, however, the focus has shifted to approaches to diversify agricultural innovations in defined locations to contribute to productivity and profitability increase and achieving sustainable food security to overcome poverty and malnutrition. In addition to so-called “major global crops”, attention has also been placed on many more crops that are regionally or locally important for nutrition and income and that are often underresearched but are nutritious, valued culturally, adapted to local environments, and contribute to diversifying regional agriculture systems.

The Japan International Research Center for Agricultural Sciences (JIRCAS), together with several Japanese research institutions and IITA, initiated in 2011 a 5-year collaborative research project called “Evaluation and Utilization of Diverse Genetic Materials in Tropical Field Crops (EDITS)”. The project focuses on yam (EDITS-Yam) and cowpea (EDITS-Cowpea), and aims to generate a solid understanding of the available wide genetic resources in these West African traditional crops, and develop efficient evaluation techniques for effective crop improvement. The outputs from these collaborative efforts are expected to contribute to breeding programs in West Africa.

JIRCAS is playing a key role by linking the Japanese scientific capacities to African communities through IITA, which is the entry point for many overseas research institutions to overcome the various constraints in African agriculture. The knowledge and techniques gained from the collaborative research project is expected to enhance the development of improved yam and cowpea varieties that can help promote rural livelihoods in West Africa.

EDITS-Yam

Yam is a traditional staple crop of significant economic and sociocultural importance in West Africa. The demand for yam is projected to increase, mostly due to population growth in the region. However, little improvement of farm yields has been registered in this crop in the last few decades, indicating an urgent need for more investment in yam research and development. To increase its productivity and enhance the income generation capacity of small-holder farmers, research-for-development should focus on increasing productivity through improved varieties and production technologies to meet the regional needs.

The last couple of years saw a breakthrough in genome sequencing technologies, and in the application genomic information to plant breeding. Genome analysis and improved molecular techniques would tremendously facilitate germplasm characterization, genetic mapping and tagging, and functional genomics of yam. These new tools, if incorporated into the breeding program, will pave the road for effective genetic improvement of yam. Since April 2011, JIRCAS together with the Iwate Biotechnology Research Center (IBRC) and IITA, has been implementing EDITS-Yam to develop and use advanced genomic and molecular tools to enhance germplasm evaluation and improvement for D. rotundata in West Africa.

EDITS-Yam is designed to strengthen genotyping using molecular tools and develop phenotyping protocols to facilitate yam breeding. The project aims to (1) generate the first reference genome of D. rotundata (Guinea yam), (2) develop and apply genomic information and molecular tools in yam breeding, (3) provide improved tools for biodiversity analysis and identification of potentially useful germplasm, and (4) develop phenotyping protocols for important agronomic traits. The outputs from this collaborative research are expected to contribute to the enhancement of yam breeding activities in the region. Consequently, new improved varieties will provide better food security and income for the small-holder farmers in West Africa and beyond.

Progress in 2011-2013

Sequencing of Guinea yam genome

To enhance Guinea yam breeding by fully exploiting modern genomics tools, generating a reliable reference sequence is a prerequisite. To this end, we have been gathering efforts to obtain the first whole genome sequence (WGS) of D. rotundata. The de novo assembly is currently in its final stage. The reference of genome will be completed soon, and the finding will be shared with the global yam community (Fig. 1).

Whole-genome sequencing-based analysis of diversity in Guinea yam

Next generation sequencing (NGS) allows large-scale genome-wide discovery of genetic markers that are important for genomic and genetic applications such as construction of genetic and physical maps, and analysis of genetic diversity. As a component of the on-going effort to construct the first draft sequence of D. rotundata and accelerate the breeding program, WGS-based genetic diversity analysis of D. rotundata accessions is under way. So far, 10 D. rotundata breeding materials, including five landraces and five breeding lines, have been resequenced. These materials are diverse with respect to traits such as maturity time, yield, tuber quality, and resistance to nematode and Yam mosaic virus (YMV), and have been extensively used as parental lines in the IITA yam breeding program.

Aligning the Illumina paired-end short reads obtained from resequencing of the breeding materials to D. rotundata scaffold sequence allowed genome-wide extraction of single nucleotide polymorphism (SNP) and insertion/deletion (indel) markers, which are being used to estimate the genetic relatedness among the lines/accessions studied and reveal the genetic diversity available to breeders. Findings of this study will have huge implications for genetic and genomic studies in yams, including among others, the application of SNPs, the most abundant genetic markers in genomes, for the development of high throughput genotyping platforms and for marker-assisted breeding. More accessions will be considered for resequencing in the future to mine the diversity in D. rotundata in detail.

Diversity Research Set (DRS) as a tool for diversity evaluation of D. rotundata germplasm

The availability of genotypic and phenotypic tools is critical to understand the diversity present in germplasm collections and enhance the active use of genetic resources. IITA currently holds over 2,000 accessions of D. rotundata. Of these, we selected a subset of experimental materials called Diversity Research Set to develop genotyping and phenotyping tools and protocols for germplasm evaluation.

In principle, DRS should be small in size for ease of handling and to allow a detailed analysis of diversity, but retain most of the diversity present in the original collection both at molecular and morphological levels. Accordingly, 106 accessions have been selected as the DRS-EDITS based on 21 key morphological traits, ploidy level, and SSR polymorphisms. The materials are currently being used for (1) detailed genotyping using DNA markers generated from the ongoing WGS, (2) morphological characterization and identification of key descriptors for regional D. rotundata collection, and (3) detailed phenotyping of economically important traits (Fig. 2)

Developing phenotyping protocols

In yam, as well as other root and tuber crops, phenotyping remains the major bottleneck to fully use genotyping information in germplasm evaluation and breeding. EDITS-Yam is also aiming to develop phenotyping protocols on key traits such as tuber yield, earliness of tuber growth and maturation, starch content and properties in collaboration with agronomists and food science specialists. These protocols, once developed, will be used for large-scale phenotyping applied to genetic and diversity studies (Fig. 3).

The information generated and tools developed in the framework of the EDITS-Yam project are expected to contribute immensely to broadening the knowledge base in yam, thereby facilitating the management of available genetic resources and aiding efficient use of yam germplasm for future improvement of the crop. This project and the collaboration it forged are expected to contribute to raising the profile of yam, and trigger the initiation of more and concerted international approaches to yam research for development. The preliminary outputs from the EDITS-Yam project suggest that there is a need for complementary studies to effectively use genetic and genomic tools being generated for yam improvement. To this effect, possibilities for additional resources are being explored.

Viral disease threats to yam in West Africa

Lava Kumar, l.kumar@cgiar.org

Virus diseases pose serious challenges to seed and ware yam production and also impede international exchange of yam planting material in West Africa, which is home to about 91% of the global edible yam production. Current efforts to control virus threats are directed towards propagation of virus-free seed yam. However, deployment of genetic resistance in farmer-preferred cultivars is critical for sustainable virus disease control in West Africa.

One disease and several viruses

‘Mosaic disease’ is a common disorder caused by several different viruses infecting yam in West Africa. About 17 viruses have been identified in edible and medicinal yam in different parts of the world1. The virus species belonging to the genera Potyvirus and Badnavirus are most widespread. Two potyviruses, Yam mosaic virus (YMV) and Yam mild mosaic virus (YMMV), and several badnaviruses—generically referred as yam badnaviruses (YBVs) —are frequently detected in farmer-grown yam in the West African yam belt that stretches from western regions of Cameroon to Cote d’Ivoire, including Nigeria, Bénin, Togo, and Ghana. About 88% of the global yam production area and 91% of the global production is confined to this region. Six Dioscorea species, viz., D. rotundata (white yam), D. alata (water yam), D. cayenensis (yellow yam), D. dumetorum (bitter yam), D. bulbifera (aerial yam), and D. esculenta (lesser yam) are widely cultivated for food use in West Africa. Virus infections are known in all these species but most prevalent in D. rotundata and D. alata, the two most predominant species covering >70% of the cultivated area in West Africa.

Different viruses cause almost similar symptoms and are difficult to distinguish from one another based on symptoms alone. In general virus symptoms in yam consist of deformation of leaf lamina, mottling, yellowing, vein banding, mosaic pattern on leaves, stunting, and poor growth (Fig. 1). Symptom expression can differ based on the genotype, time of infection, environmental conditions and cultivar. Mixed infection with more than one virus often results in severe symptoms. Although effects of virus infection on tuber size have not been accurately quantified, data from published and unpublished studies suggest about 20 to 50% reduction in tuber yield. In addition, ‘internal brown spot disease’ reported from Côte d’Ivoire is known to cause dry corky necrosis in tubers (Fig. 1b). The cause of this disease is not known, but based on symptoms it is regarded as viral in nature. In addition, badnavirus sequences have been found to be integrated in the yam genome. These are termed as endogenous pararetrovirus sequences (EPRVs) or endogenous yam badnaviruses (eYBVs) and have been detected in almost all yam species grown in West Africa, the Caribbean, and South-Pacific regions2. However, the pathological significance of yam EPRVs is not known.

Virus spread along with plant parts and insect vectors

Viruses infecting yam are systemically distributed in all plant tissues, including tubers. Consequently, tubers, setts, or any plant tissue from infected plants serves as a source for virus spread through vegetative propagation (Fig. 2). In addition, some of the yam viruses are transmitted from plant to plant by insect vectors. For instance, YMV and YMMV are transmitted by aphids and YBVs are transmitted by mealybugs. Insect vectors play an important role in spreading virus from infected plants to uninfected plants. A recent study demonstrated YMV transmission through botanical seeds of yam, albeit only a small percentage of seed serves as virus carriers.

Infected seed yams contributing to high virus incidence in West Africa

Farmers in West Africa mainly cultivate yam by planting small tubers (seed yam) or pieces of tubers (setts and minisetts) derived from larger tubers, which are sourced from their own harvest, brought from neighbors, or markets. This practice contributes to the accumulation and perpetuation of tuber-borne viruses. Considering that about one quarter of the yam tuber harvest each year in West Africa is used for propagation, the risk of virus perpetuation dramatically increases through infected tubers from generation togeneration. Historical data and recent surveys conducted as part of the Yam Improvement for Income and Food Security in West Africa (YIIFSWA) project estimated an average virus incidence of >70% in almost all the farmers’ fields, reflecting the perpetual use of infected tubers due to lack of availability of virus-free seed yams. Lack of virus resistance in the popular landraces and improved cultivars, poor awareness about viral diseases, and severe shortage of virus-free planting material are other factors that continue this prevailing situation in West Africa. Coordinated action is required to control the unabated spread of yam viruses through use of virus-infected seed yams.

Multipronged approach for yam virus disease control

Virus disease management of clonally propagated crops which are also transmitted by insect vectors requires a multipronged strategy: (i) reduce virus inoculum in the field by phytosanitation (removal and destruction of infected sources) and replacement of infected seed stock with virus-free propagation material, (ii) use resistant cultivars to prevent infection, and (iii) control insect vectors to prevent further spread. Unfortunately, many of these tactics are not being practiced due to lack of appropriate resources such as virus-free seed stock or highly resistant varieties.

YIIFSWA impetus

In the ongoing Bill & Melinda Gates Foundation-funded YIIFSWA initiative, surveys were conducted in the major yam production regions in Ghana and Nigeria to determine the situation of virus incidence and severity during the 2012-13 seasons. Mean virus disease incidence in both countries was greater than 85% and mean severity was 3, based on a 1 to 5 rating scale (1 = no symptoms and 5 = most severe symptoms). Local landraces were dominantly used by farmers compared to released cultivars. Virus diagnostics tests by reverse-transcription polymerase chain reaction (RT-PCR) assays detected YMV in D. rotundata in all the locations, sometimes in mixed infection with YMMV. YBVs were also detected in all the locations, however it was not clear if these positive results were from episomal? infection or eYBVs. Knowledge on virus diversity determined by sequencing of portion of viral genomes was incorporated into the diagnostic test development to further enhance sensitivity and specificity of yam virus diagnostic tools.

Efforts are also ongoing to produce virus-free yam stocks of the most popular farmer-preferred cultivars (landraces and improved cultivars) in Nigeria and Ghana. A suite of macro and micropropagation technologies combined with thermo- and chemotherapy techniques have been employed to generate stocks free of YMV, YMMV, and CMV. Such virus-free stocks have been established for the cultivars Adaka, Aloshi, Alumaco, Ame, Amula, Danachia, Gbangu, Kemi, Makakusa, Obiaturugo, Ogini, Ogoja, TDr 89/02475, and TDr 89/02665. They are being mass propagated for use as nucleus stock for breeder-class seed yam production. Similar efforts are ongoing to generate virus-free stocks of a wider range of D. rotundata and D. alata cultivars. This, in combination with YIIFSWA activities on strengthening the seed yam systems through improved seed production techniques and capacity development is anticipated to regularly infuse stocks of high quality seed yams produced from virus-free sources by specialist seed growers and contribute to productivity gains.

‘Positive selection’ (PS) is another approach piloted as part of YIIFSWA and allied initiatives to prevent reuse of tubers from severely infected plants for seed purpose. PS is a simple on-farm method of selectively harvesting seed yam tubers from healthy looking plants or plants showing mild symptoms, when asymptomatic plants are not available. This eliminates tubers with high virus concentration and infected with multiple viruses, the two conditions responsible for severe symptoms, poor plant performance, and degeneration of seed yams. Implementation of PS over several seasons is expected to reduce virus inoculum in the fields, improves the quality of farmer-saved seed yam, and reduces the need for regular seed replacement. However, this approach requires additional effort in the form of monitoring crops before senescence, tagging, and separate harvesting of tubers from selected plants. Awareness creation among growers about the benefits of PS and training in selection of healthy looking plants is critical to the sustainable implementation of this approach.


Resistant varieties required for sustainable management

Resistant varieties offer the most convenient, economical, and sustainable option for controlling virus diseases. In addition, they are easy for dissemination and adoption. Almost all the popular landrace cultivars were found to be susceptible. Some were found to have tolerance showing mild symptoms at the later crop growth stage (e.g., Amula). Germplasm sources with high levels of host plant resistance to virus diseases have been identified in the Dioscorea landraces3. However, all the improved varieties released as of 2013 were found to be susceptible. Limited breeding efforts for virus disease resistance demonstrated dominantly inherited resistance to YMV in certain D. rotundata crosses4,5, indicating the promise of breeding for developing cultivars with high levels of virus resistance with end-user preferred traits.

Conclusions

Virus diseases pose a major threat to West African yam production, affecting tuber yields and seed yam quality. Reuse of farmer-saved seed in successive seasons has contributed to high virus incidence and seed yam degeneration. In the absence of high levels of virus resistance in farmer-preferred varieties, it is imperative to infuse clean stocks of popular cultivars through seed systems, coupled with approaches such as phytosanitation and positive selection to reduce virus inoculum in the fields. Concerted efforts in this direction started recently through initiatives such as YIIFSWA. However, these efforts need to be complemented with breeding programs to develop cultivars with high levels of resistance and end-user preferred attributes for sustainable control of virus diseases and also to ensure sustainability of quality seed yams.

References

1. Kenyon et al. 2003. An overview of viruses infecting yams in sub-Saharan Africa. In: Eds. Hughes, J. d’A and Odu, B.O. Plant virology in sub-Saharan Africa, Proceedings of a conference organized by IITA, IITA, Nigeria. pp432-439.

2. Seal et al. (2014). The prevalence of badnaviruses in West African yam (Dioscorea cayenensis-rotundata) and evidence of endogenous pararetrovirus sequences in their genomes. Virus Research (in press) [doi:10.1016/j.virusres.2014.01.007]

3. Asiedu R. 2010. Genetic improvement of yam. In: Yam Research for Development in West Africa – Working Papers. IITA-BMGF Consultation Documents, IITA. pp 81-108.

4. Mignouna J. et al. 2002. Identification and potential use of RAPD markers linked to Yam mosaic virus resistance in white yam (Dioscorea rotundata Poir.). Ann. Appl. Biol. 140: 163­169.

5. Odu et al. 2011. Analysis of resistance to Yam mosaic virus, genus Potyvirus, in white guinea yam (Dioscorea rotundata) genotypes. J. Agri. Sci. 56: 1-13.

A new paradigm for improving yam systems

N. Maroya, R. Asiedu, P. Lava-Kumar, D. Mignouna, T. Abdoulaye, B. Aighewi, M. Balogun, U. Kleih, D. Phillips, A. Lopez-Montes, F. Ndiame, J. Ikeorgu, E. Otoo, N. McNamara; S. Abimiku, Sara Alexander, and R. Asuboah

In West Africa, yam (Dioscorea spp.) plays a very important role as a source of income, food security, and livelihood systems for at least 60 million people. The crop also makes a substantial contribution to protein in the diet, ranking as the third most important source. Farmers engage in yam cultivation for cash income and household food supply. Yam traditionally plays a significant role in societal rituals such as marriage ceremonies and annual festivals, making the crop a measure of wealth. Yams therefore have significance over and above other crops in the region. At the regional level, yam seems to be a superior economic good in all countries. As incomes increase, consumers shift from cassava to yam. This is related in part to regional cultural values and consumer preferences, which is mainly due to the relative ease in consumer food preparation.


Despite its importance in the economy and lives of many people, yam faces many constraints that significantly reduce its potential to support rural development and meet consumers’ needs as an affordable nutritional product. Unavailability and high cost of high quality disease-free seed yam is a major constraint in West Africa. This is followed by high levels of on-farm losses of tubers during harvesting and storage, low soil fertility, and high labor costs associated with land preparation and staking. Other constraints include losses due to diseases caused by viruses and fungi and nematode attack. Scale insects, tuber beetles, and termites affect the tubers in some areas. These effects are experienced more in the dry savannah agroecologies where yam cultivation is rapidly expanding due to the shrinking arable land in the traditional moist humid areas. In addition, the seed yam system in West Africa is mainly informal and entirely market driven.


Yam Improvement for Income and Food Security in West Africa (YIIFSWA) was initiated to increase yam productivity of 200,000 smallholder farmers (90% with less than 2 acres) in Ghana and Nigeria by 40% (2011 to 2016), and deliver key global goods research products that will contribute to the sustainable development of the yam sector.


Early gains

The project started by identifying yam production systems with partners (Fig. 1).

The yam value chain surveys with farmers, marketers (including exporters), transporters, and processors helped to estimate the cost of production of ware and seed yam, analyze costs and benefits of yam transaction, and identify major ware yam supply and distribution routes in both Nigeria and Ghana. Detailed value chain analysis has shown that yam production is a profitable business and yam farmers are able to generate substantial income from the production of tubers. But at the same time, production costs tend to be high (in particular for seed yam and hired labor) and selling prices depend on the season. There is significant price variability between the new yam season (August to October), the peak season (November to April), and the slack season (May to July). During peak seasons there is much yam in the markets but because of unavailability of good storage facilities, yam are sold at the lowest prices (Fig. 3). The gross margins can be negative if farmers get the timing of their harvest wrong, or are unable to sell at times when prices are higher.


YIIFSWA baseline studies conducted in 600 and 800 households, respectively in Ghana and Nigeria, indicated that only 3% and 10% households are headed by females in Nigeria and Ghana. Land was by far the major natural capital for small-holder farmers in yam-growing areas. The average farmland available was about 2.4 ha in Nigeria and 2.7 ha in Ghana. Priority has been given by households to yam over other food and cash crops. The areas under yam cultivation are generally small and the primary objective of small-holder farmers is to meet subsistence needs.

To develop the capacity of farmers organizations (FOs) by linking them to service providers (SPs) that would offer demand-driven services, a profiling exercise was conducted on 77 and 44 FOs and 40 and 17 SPs in Nigeria and Ghana, respectively. Overall, the performance indicators revealed that the selected FOs in Nigeria performed better than the ones in Ghana, in terms of quality of governance, internal management, value chain management, and marketing strategy. However the selected Nigerian FOs performed poorly, compared to Ghana counterparts, in the internal management and operations indicators.


Over 90% of the farmers use tubers harvested from the previous season as ‘seed yam’ or sourced from local markets, which are of poor quality due to pest and disease attack and lack of seed yam replacement. To improve the quality of farmer-saved seed yam, two NGOs—the Missionary Sister for Holy Rosary (MSHR) in Nigeria and Catholic Relief Services (CRS) in Ghana—have taken on the responsibility to train yam growers on seed yam multiplication using minisett technique combined with seed treatment to protect them from nematodes and fungal attack. So far, about 16,784 farmers were trained in Nigeria and Ghana.


The seed and ware yam sanitation challenges were also tackled. Surveys were conducted to identify pest and disease prevalence to establish appropriate strategies to control biotic threats to seed yam and ware yam. Virus diagnostics has been simplified to detect major yam-infecting viruses, Yam mosaic virus (YMV), Yam mild mosaic virus (YMMV), and Cucumber mosaic virus (genus, Cucumovirus), through a multiplex PCR-based assay.


Breeder seed yams produced in 2012 by Crops Research Institute (CRI) in Ghana and National Root Crops Research Institute (NRCRI) in Nigeria were handed over respectively to the Grain and Legume Development Board (GLDB, Ghana) and National Agricultural Seeds Council (NASC, Nigeria) for generation of foundation seed yam.


The breeder seed yam under production in 2013 is 0.8 ha and 0.5 ha, respectively for Nigeria and Ghana. GLDB has taken the challenge in Ghana and has a 1.5-ha foundation seed production site at Afraku (Ashanti region); while in Nigeria NASC selected two private seed companies (Greengold Construct Nigeria Ltd. and Romarey Ventures Nigeria Ltd.) that were engaged in foundation seed yam production for the first time.


New techniques such as aeroponics and temporary immersion bioreactors systems (TIBS) were effectively established at Ibadan. Results of experiments on the use of aeroponics system were encouraging for both pre-rooted planted and direct planting of vine cuttings of D. rotundata and D. alata. The successful growth of yam on the aeroponics system is reported for the first time with production of microtubers and mini-bulbils.


The TIBS is reported on yam in general, and in only one article for D. rotundata. YIIFSWA established a TIBS running on automated computer system with remote control through Internet. The 128 units of TIBs can produce a minimum of 12,800 plantlets per cycle. The running of the TIBs and the aeroponics systems for breeder or foundation seed yam production will speed up the generation of initial stocks of seed yam to supply the formal seed system.


Integrating available technologies, local and improved varieties to increase yam productivity is also another key objective of the project. Improved varieties with good performance in low soil fertility and drought stressed environments, and in staked and no staked system have been identified (table). The integration of landraces with seed selection and treatment, effective weed control, fertilizer application under no-stake system has indicated yields of 50% above the local technology. Studies have also determined that choice of yam varieties could be same for both men and women farmers and sometimes preferences differ.


For an effective operational seed system the capacity building of the players is key. To that effect national training workshops were organized: breeder and foundation seed production training workshop in Ghana and Nigeria; seed yam quality management protocol (QMP), yam virus disease diagnostics. In addition many partners (NASC, GLDB, CRI; SARI, CRS, MSHR, etc.) have organized training workshops for different stakeholders mainly on minisett technique for yam propagation.


Conclusions

In 24 months of project implementation, significant results were achieved on baseline studies, value chain analysis, farmers’ organization profiling, farmers training, and participatory selection of new genotypes. New techniques on high ratio propagation (aeroponics and TIBS), novel methods to develop virus-free planting materials, and the multiplex RT-PCR test for simultaneous detection of major viruses infecting yam, were successfully established. The formal seed yam system has been initiated and training have started. These initial successes are expected to pave a way to tackle greater challenges confronting the seed yam sector in West Africa.




Unlocking the diversity of yam

IITA scientists inspect yam plants in the field gene bank. Photo by O. Adebayo, IITA.
IITA scientists inspect yam plants in the field gene bank. Photo by O. Adebayo, IITA.

The International Year of Biodiversity (IYB) has emphasized the need for global action that will unravel the genetic diversity of yam, a root crop that provides food security to 300 million people in sub-Saharan Africa.

Yam is grown in about 51 countries in the tropics and subtropics, with yields averaging about 11 t/ha in the major producing countries of West Africa (Nigeria, Cote d’Ivoire, Ghana, and Bénin). However, little is known about the tuber crop’s diversity.

“This aspect is important for yam improvement to meet the demand of people depending on this crop for food and livelihood,” says Ranjana Bhattacharjee, IITA Scientist working on fingerprinting the yam germplasm collection.

Yam provides calories and puts money in the pockets of farmers. The tuber-bearing climbing plant from the genus Dioscorea also plays a major role in sociocultural activities in West Africa including traditional marriages and the New Yam Festival.

Globally, there are over 600 species of yam but only a few are cultivated for food or medicine. Scientists fear that some species are threatened and might become extinct as a result of climate change and genetic erosion. This prompts the calls for conservation.

The major edible species of African origin are white Guinea yam (D. rotundata Poir.), yellow Guinea yam (D. cayenensis Lam.), and trifoliate or bitter yam (D. dumetorum Kunth). Edible species from Asia include water or greater yam (D. alata L.), and lesser yam (D. esculenta [Lour.] Burkill). Cush-cush yam (D. trifida L.) originated from the Americas. White Guinea yam and water yam are the most important in terms of cultivation and use.

Yam tuber. Photo by IITA.
Yam tubers. Photo by IITA.

This preferred staple is usually eaten with sauce directly after boiling, roasting, or frying. The tubers may also be mashed or pounded into dough after boiling, or cooked with sauces and oils. They can be processed into yam balls, chips, and flakes.

Fresh yam tubers are peeled, chipped, dried, and milled into flour that is used in preparing dough called amala (Nigeria) or telibowo (Bénin). Commercial products based on dry flakes or flours from the tuber are produced in Nigeria, Ghana, and Côte d‘Ivoire for export and sale in urban areas.

Though millions depend on the crop, especially in sub-Saharan Africa, not many outside of Africa know about the tuber’s potential for commercialization, and its role in enhancing food security in the region, according to Robert Asiedu, Director of the Program on Root and Tuber Systems at IITA.

“We talk about yam tubers as a food staple of millions of Africans to donors or investors who don’t even know what yam is, how it looks or tastes. So the question is: How would they even think of investing in research in a ‘little-known’ staple like yam?”

Perhaps yam’s low profile in the developed countries or in the West is the major limitation in attracting funding for research, but this hardy tuber is an important “part of man” especially in Africa, the Caribbean, Asia, and the South Pacific Islands where it is widely eaten. According to Asiedu, it is the “preferred and most appreciated staple food and calorie source” in areas where it is grown.

Yam faces constraints that include the high costs of planting material and of labor, decreasing soil fertility, the inadequate yield potential of varieties, and increasing levels of field and storage pests and diseases associated with intensive cultivation.

To tackle some of these constraints, work at IITA for the last few years has focused on improving the tuber, primarily white and yellow Guinea yam, and water yam.

Man with huge yam tuber. Photo by IITA.
Man with huge yam tuber. Photo by IITA.

The breeding program uses the 2,216 accessions of Guinea yam and 816 of water yam in IITA’s genebank to study resistance to anthracnose and virus diseases. Improved populations have been developed with partners in the national agricultural research and extension systems (NARES), who have released varieties in Nigeria (National Root Crops Research Institute, 7) and Ghana (Crops Research Institute, 3).

Despite the success in yam improvement, new challenges keep on coming, prompting researchers to use other tools, such as molecular characterization to unlock the genetic diversity of yam.

Recently, the Global Crop Diversity Trust funded a project in IITA to duplicate, document, and distribute the germplasm of yam to other partners in accordance with the International Treaty on Plant Genetic Resources for Food and Agriculture. Such support is indeed a milestone in yam research. The project also aims to fingerprint the entire germplasm collection at IITA. This will help in understanding the extent of genetic diversity present in the collection. From this, the genes for important traits can be determined through association mapping, a tool that could be used successfully to improve and sustain the crop.

As the world marks the IYB, serious attention from other donors is necessary to keep the crop as a “part of man.”