From yam production and postharvest constraints to opportunities

D.B. Mignouna, d.mignouna@cgiar.org, T. Abdoulaye, A. Akinola, and A. Alene

Food insecurity remains a huge concern in West Africa. Agriculture, without doubt remains the main source of food and livelihood. Over the past two decades, agricultural yields have stayed the same or declined. Although there has been a recent rise in agricultural productivity, it derived more from expanded planting areas for staple crops than from yield increases. Thus, increasing and sustaining agricultural productivity should be a critical component of programs that seek to reduce poverty and attain food security in the region.

Yam (Dioscorea spp.), a vegetatively propagated crop cultivated for its underground edible tubers, is the mainstay for about 300 million people in West Africa. It is a very important food and income source for millions of producers, processors, and consumers in the region. About 48 million tons are produced annually in this subregion on 4 million ha. The five major yam-producing countries (Bénin, Côte d’Ivoire, Ghana, Nigeria, and Togo) account for 93% of the world’s production, with Nigeria alone accounting for 68% of global production (36 million t on 3 million ha) with 31.8% of the population depending on yam for food and income security. The crop contributes substantially to the amount of protein in the diet, ranking as the third most important source, much more than the more widely grown cassava, and even higher than some sources of animal protein. Hence, yam is important for food security and income generation with a domestic retail price of US $0.49/kg. Yam is also integral to the sociocultural life in the subregion.

In present-day Nigeria, yam is still culturally significant because it plays an important role in betrothal ceremonies or traditional marriages. It is one of the significant items a suitor presents to his in-laws to obtain their approval to marry their daughter. Some grooms are compelled to present as many as 40 pieces of long and fat yam tubers, aside from gallons of palm oil, baskets of kola nuts, bags of salt, and other sundry items, the nonprovision of which could invalidate the union. The cultural importance of yam is higher in some regions in Nigeria as it is a crop celebrated annually during the New Yam Festival, with rituals to thank the god of agriculture, to seek its blessings for a bumper harvest in the forthcoming years. Yam is produced more in the middle belt zone of Nigeria and is consumed more in the South, but those making commercial gains from its sales are core northerners from the North West, the North Central, and the North East.

Despite its importance in the economy and lives of many people, the crop faces several constraints that significantly reduce its potential to support rural development and meet consumers’ needs for improved food security and enhanced livelihood. Constraints limiting yam production and postharvest handling need to be identified to provide a basis for appropriate interventions. This was the reason behind the interventions through the Yam Improvement for Income and Food Security in West Africa (YIIFSWA) project. YIIFSWA was initiated to work with other stakeholders in West Africa to identify the opportunities of interventions that could potentially help to increase productivity in the region. This report documents production and postharvest constraints and opportunities in yam.

Using Nigeria and Ghana as cases, important worldwide yam-producing countries, a study was carried out using a multistage, random sampling procedure in selecting a total of 800 and 600 households, respectively. All surveyed households were interviewed using a structured questionnaire.

Survey results indicated that a range of factors limited yam production and storage. These include insect pests, diseases, water-logging, drought, rodents, low soil fertility, shortage of staking material, inadequate input supply and storage facility, land shortage, high cost of labor, lack of improved varieties, and others such as theft (Fig. 1).

High cost of labor stands out as the most pressing problem in all the surveyed zones, both in Nigeria and Ghana. For instance, mounding as a seedbed preparation method, is laborious, and hence expensive. But apart from mound making all yam production operations are labor intensive because they are performed with hand hoes, machetes, and digging sticks without any form of a labor-saving technology.

Another main constraint are insect pests and diseases. The unavailability and high cost of good quality disease-free seed yam had been on one hand a result of pests and diseases and on the other hand a serious hidden constraint due to the fact that farmers do not purchase seed yam. Other important constraints mentioned were the inadequate input supply that was very pronounced in Ghana, low soil fertility more reported in Nigeria, rodents and drought (Ghana), water-logging (Nigeria), lack of improved varieties more prominent in Ghana, shortage of land and staking material (Ghana), and others such as theft that were not negligible in both countries.

It is clear that there are shared priority constraints in the two countries, indicating no specificity of problems by country. The YIIFSWA research agenda needs to be informed by the constraints facing yam farmers and based on these the following interventions were identified: (i) Key investments for lowering farmers’ production cost using agricultural research (breeding, agronomy) and extension (improved agronomic and management practices; and (ii) Managing pests and diseases.

As regards opportunities, yam could be be a formidable force in the fight against poverty, hunger, and deadly diseases if research and development measures are implemented to develop and disseminate technologies that can bring the crop into central focus in national food policies. This will enable it to benefit from policy programs that can drive down production costs. Yam is a preferred food in the region; some varieties, especially yellow varieties, are sources of betacarotene. The crop is produced mostly for sale, and it is increasingly becoming a major source of foreign exchange in the region as an export crop.

Therefore, YIIFSWA, through its initiatives, should ensure that all constraints are turned into opportunities for all the yam value chain players in general and farmers in particular.

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.




NRM in cassava and yam production systems

Stefan Hauser, s.hauser@cgiar.org

Cassava has the potential to produce roots even under poor soil conditions. Photo by IITA
Cassava has the potential to produce roots even under poor soil conditions. Photo by IITA

Why are yields of cassava in Thailand and India three times higher than in Africa and production costs in Brazil only one-third of those here? Although Africa suffered from the Cassava Mosaic Disease pandemic and currently faces the threat of Cassava Brown Streak Disease, breeding tolerant and resistant germplasm has contributed to yield gains over the last three decades. Thailand, India, and Brazil have been successful in commercial cassava production with yields between 25 and 40 t/ha. The question arises: how can African farmers realize more of the >80 t/ha yield potential of cassava?

Natural resource management (NRM), agronomy, and crop husbandry have hardly ever been credited with “breakthrough” solutions to hunger and poverty. However, when more than 50-75% of the cassava yield potential is not being realized, major improvements are clearly possible through NRM, agronomy, and appropriate crop husbandry.

Agronomy and crop husbandry
For West Africa there is still a dearth of agronomic information on cassava. Currently a density of 10,000 plants/ha is the standard, while further increases are being recommended without concrete data on the yield responses to increased density by different growth types. Cassava varieties vary widely in their branching height and level of ramification, leading to different levels of ground cover by single plants and of the start and intensity of intra-specific competition. Cassava yield distribution within the same variety is highly biased (Fig. 1), raising questions on the optimum plant density and issues such as genetic uniformity and crop responses to edaphic (soil) factors.

Figure 1. Relative contribution to yield of individual plants of cassava variety Zizila in DRC.
Figure 1. Relative contribution to yield of individual plants of cassava variety Zizila in DRC.

One future effort will be to determine optimum plant densities for monocrops by major cassava growth types.

Intercropping cassava with maize or grain legumes is still widely practiced and needs to be improved. The short-term intercrops are cleared from the field, leaving space unused that can be invaded by weeds, thus there needs to be a follow-up either with weed control or a second crop to occupy the open space. Various crops will be tested for their capacity to perform between developed cassava plants and their contribution to total system productivity.

Weed control remains a problem as there are no postemergence herbicides that cassava would tolerate. Combinations of preemergence herbicides with appropriate planting techniques have the potential to reduce weed competition and labor. For instance, a preemergence herbicide can be applied before planting cassava. The herbicide kills germinating seeds as they break through the herbicide-sealed soil surface. The cassava stakes need to be planted in a vertical position and the orientation needs to be correct so that no emerging cassava leaves touch the soil surface. Such technologies combined with the follow-up use of postemergence herbicides with shields can drastically reduce labor and increase productivity as weeding can be done at the most efficient time.

Fertilizer
Fertilizer use is low in Africa yet it appears certain that fertilizer or other forms of soil nutrient replenishment can contribute to yield increases, higher farm incomes, possibly to lower consumer prices, and thus to better livelihoods. Using average nutrient uptake into all cassava plant parts (dry matter basis) of 6.2 kg N/t, 1 kg P/t, and 5.3 kg K/t, a total supply of 165–25–145 kg N-P-K/ha is required to attain 50% of the current potential yield (45 t/ha fresh roots). Such amounts are unlikely to be supplied by the soil and thus nutrient supply is a crucial factor in achieving higher cassava yields.

There are no recent fertilizer response curves for cassava and yam in West Africa, hence, farmers do not know the composition and amounts of fertilizer to apply. The nutrient(s) most limiting to cassava production have not been quantitatively determined. The replenishment of any most limiting nutrient would lead to substantial yield increases. Depending on the limiting nutrient, productivity and profitability increases may be possible at a very low cost and risk. IITA uses a stepwise approach, first determining the most limiting nutrient(s) followed by elaborating the optimum quantity required and the construction of recommendations for optimal nutrient composition and quantities. IITA is currently working with the International Fertilizer Development Center on testing special fertilizer blends for cassava, addressing the augmentation of neglected nutrients such as sulfur, magnesium, zinc, and boron.

Use of other nutrient sources
Compost, manure, mulch, and rock phosphate have all been proposed as means to improve soil nutrient status and crop production. However, none of these sources has had a major impact as farmers need land to produce biomass or else infrastructure is required to mine, process, and distribute rock phosphate. Although the biological sources are important, constraints in biomass production need to be overcome first.

Figure 2. Crop yield response to planted herbaceous fallow in West and Central Africa.
Figure 2. Crop yield response to planted herbaceous fallow in West and Central Africa.

Mineral fertilizers alone cannot sustain crop production on degraded land. Soil organic matter and  soil micro-, meso-, and macro-fauna are important in maintaining soil quality and health. Traditionally, fallow phases between crops were replenishing the soils’ production capacity. With increased population densities, fallow phases have been shortened or no longer exist. Thus, soils do not recover but continue to lose their production potential. Farmers do not seem to invest in soil fertility but look for ways of coping with ever less fertile soils, thereby degrading them to a stage where cropping becomes unprofitable.

Such situations have been encountered in southern Bénin. Soil fertility and quality management techniques, such as cover crops, manure application, or any other form of organic matter and nutrient recycling have not been adopted at larger scales. In retrospect, there have been constraints to the adoption that were not considered in the process of technological development. Today, with more options available and a stronger and earlier involvement of farmers in research for development, such approaches are worth reconsidering. One such technology, using leguminous cover crops, had little if any success in cassava (Fig. 2).

Controlling the cover crop was a major problem. Consequently IITA works today on efficient and effective control methods. Pueraria phaseoloides was introduced to smallholders in southern Cameroon but it was not readily accepted as farmers immediately identified it as an aggressive weed, able to destroy crops. However, two years into the use of Pueraria, fallow farmers noticed that the weeds most difficult to control had disappeared and that it was easier to clear Pueraria than the natural fallow. Some farmers burned the Pueraria only to find the land ready to crop without major labor input. Yields of cassava, maize, and groundnut were generally higher after Pueraria, whereby the labor-saving burning produced the highest yields (Fig. 3).

Figure 3. Cassava fresh root yield in burned and mulched maize-cassava and burned maize-cassava-groundnut intercrop.
Figure 3. Cassava fresh root yield in burned and mulched maize-cassava and burned maize-cassava-groundnut intercrop.

Considering farmers’ needs
Pueraria was introduced for soil fertility replenishment but was adopted for its labor-saving effects. Soil fertility was not perceived as a problem and thus positive effects on the soil could be compromised (by burning) without compromising yields. Effects such as weed suppression and the reduction of soil-borne pests and diseases may contribute to the yield increases after Pueraria.

Livestock integration and the search for synergies
Few farmers adopted the use of green manures for soil fertility improvement because they have no direct benefits from it. Herbaceous legumes have rarely been used to feed livestock, although there is (anecdotal) evidence that livestock feed on them and that they are beneficial to growth and reproduction. In the IITA-led CRP on Humidtropics, livestock integration will be a major aspect. It will add value to green manure species when these are used to feed livestock that will also benefit from the canopies of root and tuber crops (cassava leaves) remaining at root harvest. Thus, there will be an increase in returns of animal manure to fields, and to crop yields through the combined use of green and animal manures for improved food security and farm incomes.

Outlook
Efficient combinations of agronomic practices, nutrient supply, and soil management practices will be developed to increase the productivity of cassava and yam while improving the status of the natural resource base. Synergistic effects between these measures and the integration of livestock or fish farming will increase resource use efficiency and income generation as well as the quality of the farm food supplies. Due consideration of social and gender aspects in farm household operations will identify the entry point best suited for IITA’s interventions. Farmers’ feedback and innovations will be integrated into approaches on sustainable intensification to increase food production and improve rural livelihoods while enhancing the capacity of the agroecosystems to deliver essential services.

References
Hauser, S. and C. Nolte. 2002. Biomass production and N fixation of five Mucuna pruriens varieties and their effect on maize yields in the forest zone of Cameroon. Journal of Plant Nutrition and Soil Science 165: 101–109.
Hauser, S., C. Nolte, and R.J. Carsky. 2006. What role can planted fallows play in humid and sub-humid West Africa? Nutrient Cycling in Agroecosystems 76: 297–318.

Yam breeding at IITA: achievements, challenges, and prospects

Antonio Lopez-Montes (a.lopez-montes@cgiar.org), Ranjana Bhattacharjee, and Gezahegn Tessema
A. Lopez-Montes, Yam Breeder; R. Bhattacharjee, Molecular Geneticist; G. Tessema, Associate Professional Officer, IITA, Ibadan, Nigeria

Yam is an impotant staple food in West Africa. Photo by IITA.
Yam is an impotant staple food in West Africa. Photo by IITA.
Yam—an integral part of the West African food system
Yam (Dioscorea spp.) is a multi-species, clonally propagated crop cultivated for its starchy tubers. About 10 species are widely cultivated around the world, but only D. rotundata, D. alata, and D. cayenensis are the most widely cultivated species in West Africa, accounting for 93% of the global yam production. Since its inception, IITA R4D efforts have focused on developing new varieties of yam with desired agronomic and quality traits and to improve yam-based cropping systems.

Largest collection of yam genetic resources
IITA maintains the largest world collection of yam, accounting for over 3,000 accessions mainly of West African origin. The collection represents eight species: D. rotundata (67%), D. alata (25%), D. dumetorum (1.6%), D. cayenensis (2%), D. bulbifera (2%), D. mangenotiana (0.25%), D. esculenta (0.7%), and D. praehensilis (0.3%). The passport data and characterization information on these accessions are maintained in databases accessible at http://genebank.iita.org/. On request, these germplasm accessions are distributed following Standard Material Transfer Agreements (SMTA). As in many other crops, the request for gene bank accessions has been low for use in national and international yam improvement programs. Of a total of 3170 accessions, only 1077 accessions have been distributed in the last 10 years.

To increase the use of yam germplasm, which are a wealth of rare alleles for target traits, a core collection (391 accessions) was established in 2006 representing 75% of genetic diversity of the entire collection using data on 99 morphological descriptors and country of origin. The germplasm collection is being genotyped using 18 DNA-based markers. Presently, research efforts are under way in collaboration with CIRAD for cryopreservation, using liquid nitrogen, to reduce the cost of maintenance of such a large collection. Efforts to improve yam germplasm conservation and use will be continued under the framework of the CGIAR Research Program (CRP) on Roots, Tubers and Bananas (RTB) for Food Security and Income. As part of this program efforts will be made to (a) optimize ex situ and in situ yam conservation methodologies; (b) increase coverage of yam gene pools; (c) evaluate, genotype, and phenotype yam collections for important traits; (d) enrich databases with information on yam collections and make it freely accessible to users; and (e) improve procedures for safe exchange of RTB genetic resources.

Making the difference
IITA’s yam breeding program has mainly focused on clonal selection from landraces and hybridization of elite clones of D. alata and D. rotundata. Conventional breeding efforts in yam have resulted in substantial achievements leading to release of high-yielding and disease-resistant cultivars. For instance, through collaborative evaluation of IITA-derived breeding lines with national research institutes (National Root Crop Research Institute, Umudike, Nigeria, and the Crops Research Institute, Ghana), 10 varieties of D. rotundata (10 during 2001–2009 in Nigeria and 1 in 2007 in Ghana) and 5 varieties of D. alata (during 2008–2009 in Nigeria) were released. More lines are in the pipeline to be released by these institutions in Nigeria and Ghana, and also in Benin, Burkina Faso, Côte d’Ivoire, Sierra Leone, Togo, and Liberia. The released varieties have multiple pest and disease resistance, wide adaptability, and good organoleptic attributes.

Novel vertical sacs method for seed yam production using vine cuttings. Photo by L. Kumar.
Novel vertical sacs method for seed yam production using vine cuttings. Photo by L. Kumar.
Some work has also been carried out in interspecific hybridization, but it is faced with a lot of challenges, including cross-compatibility and synchronization of flowering. For instance, D. rotundata can be crossed to D. cayenensis, but crossing either of the two to D. alata has not been successful. Research effort in interspecific hybridization has been geared towards the genetic improvement of yam, primarily on D. rotundata, D. cayenensis, and D. alata by transferring complementary traits from one to the other, e.g., higher carotenoid in D. cayenensis transferred to D. rotundata by interspecific hybridization.

Besides success in hybridization, efforts of the breeding program resulted in identification of resistance to nematodes (D. dumetorum), fungi and viruses (D. alata and D. rotundata); selection of germplasm for their response to soil nutrients and nutrients use efficiency; physicochemical characterization of D. alata for food quality, sensory evaluation of ‘amala’ (yam flour paste) and pasting characteristics of fresh yam as indicators of textural quality in major food products. Studies are ongoing to determine the variation in nutrient retention during processing of yam into food products; characterization of tuber micronutrient density, specifically for iron, zinc, total carotenoids, ascorbic acid (vitamin C), phytate, and tannin content. Traits, such as photoperiod response, flowering, and dormancy are also being studied in D. rotundata.

The future thrust will be on reducing the breeding period required to develop improved varieties with consumer-preferred traits, as well as increased participation of stakeholders for improved efficiency and impact of the yam breeding program. Developing participatory value chain strategy will set priorities not only for research and development but also for a consistent value chain articulation and low risk models to link farmers to markets. Yam for food security, food industry (flour, pasta, noodles, pancakes etc.), and pharmacology (drugs, cosmetics) needs prioritized by stakeholders will drive the development of new varieties, that are high yielding, resistant to diseases and pests, and with good adaptability to specific production systems, low fertility soils, and dry environments. GIS-based characterization of yam production systems, yam growth models and genome sequencing will provide strategic knowledge for the success of the yam breeding program. Rapid and high-ratio seed yam propagation systems will support the variety development and dissemination efforts to breeders and other stakeholders. The implementation of the new scheme is expected to reduce the time to develop and recommend new varieties from 9 to 3.5 years and facilitate rapid release of consumer-preferred varieties by the national programs.

Genomic resources for yam improvement
Research on biotechnology of yam includes tissue culture, genetic transformation, and development and use of molecular markers. However, no genetically modified yam has been produced so far although this approach could be used to transfer resistance to virus and anthracnose diseases into popular commercial varieties. Progress on yam genomics and transformation is covered in Bhattacharjee et al.

Researchers in accelerated yam breeding trial plot. Photo by L. Kumar.
Researchers in accelerated yam breeding trial plot. Photo by L. Kumar.
Future prospects
Review of constraints in yam production in West Africa identified the high cost of planting material, high labor costs, poor soil fertility, low yield potential of local varieties, pests and diseases (on-farm and in storage), and shortage of quality seed yam of popular landraces and released varieties as major limitations. To overcome these challenges, in the next five years under the CRP-RTB framework, yam breeding efforts will focus on (a) development of new breeding tools and strategies, (b) trait capture and gene discovery, (c) pre-breeding for new traits, (d) development of new varieties incorporating consumer-preferred characters, and (e) aligning research with farmer and end-user priorities.

These efforts will be supported by the ongoing R4D programs on developing efficient phenotyping protocols for nutrient use efficiency, moisture stress tolerance and biotic stresses in different yam species; regeneration protocol for transformation of various species (D. rotundata, D. alata, and D. cayenensis); methods for efficient interspecific hybridization among D. alata, D. rotundata, D. bulbifera, D. cayenensis, and D. dumetorum; establishment of marker-assisted breeding platform; techniques for rapid propagation of high quality seed yam; protocol for double haploids from yam microspores; and adoption of stakeholder participatory approaches in development and release of new varieties. Ongoing efforts to strengthen seed yam systems for ensuring sustainable production and supply of quality seed yam in West Africa, and communication and promotional strategies for the dissemination of breeding materials and improved varieties underpin the success of these efforts.

Genomics for transforming yam breeding

Ranjana Bhattacharjee (r.bhattacharjee@cgiar.org), Melaku Gedil, and Antonio Lopez-Montes
R. Bhattacharjee, Molecular Geneticist; M. Gedil, Head, Bioscience Center; A. Lopez-Montes, Yam Breeder, IITA, Ibadan, Nigeria

Breeding challenges in yam
Yam (Dioscorea spp.), a multi-species, polyploidy, and vegetatively propagated crop, is an economically important staple food for more than 300 million people in West Africa, Asia, Oceania, and the Caribbean. The five major yam-producing countries in West Africa (Bénin, Côte d’Ivoire, Ghana, Nigeria, and Togo) account for 93% of worldwide production. Dioscorea rotundata and D. alata are the species most commonly cultivated in West Africa1.

Scientists strategizing genomics for precision breeding. Photo by L. Kumar.
Scientists strategizing genomics for precision breeding. Photo by L. Kumar.
The genetic improvement of yam is faced with several constraints, including the long growth cycle (about 8 months or more), dioecy, plants that flower poorly or not at all, polyploidy, vegetative propagation, heterozygous genetic background, and poor knowledge about the genetics of the crop2. Progress has been made in breeding to develop F1 full-sib mapping populations from crossing male and female parents of D. rotundata for traits such as multiple tuber production, improved cooking quality, and virus disease resistance; and of D. alata for resistance to anthracnose, improved cooking quality, and reduced tuber oxidation3. These are valuable sources of populations for genetic analysis in yam for its improvement.

Current status of yam genomics
There is no convenient model system for yam genomics. In recent years, some progress has been made in the development of molecular markers to assess their potential for germplasm characterization and phylogenetic studies in D. rotundata-cayenensis and their wild progenitors, such as D. abyssinica and D. prahensilis. Two framework linkage maps were constructed using D. alata that included 338 AFLP markers on 20 linkage groups with a total map length of 1055 cM; and D. rotundata in which 107 AFLP markers were mapped on 12 linkage groups (585 cM) for the male and 13 linkage groups (700 cM) for the female. Three quantitative trait loci (QTLs) on the male and one QTL on the female were identified for resistance to yam mosaic virus (YMV). Similarly, one AFLP marker was found to be associated with anthracnose resistance on linkage group 2, explaining about 10% of the total phenotypic variance.

Another linkage map was generated for D. alata based on 508 AFLP markers that covered a total length of 1233 cM on 20 linkage groups, accounting for about 65% of the entire genome. Genes conferring resistance to YMV have been identified in D. rotundata and to anthracnose in D. alata by the successful use of bulked segregant analysis (BSA). Two RAPD markers, OPW18850 and OPX15850, closely linked in coupling phase with the dominant YMV-resistance locus Ymv-1 were identified. Similarly, two RAPD markers, OPI171700 and OPE6950, closely linked in coupling phase with anthracnose resistance gene, Dcg-1, were identified2.

Designing molecular markers using a bioinformatics platform. Photo by A. Alonge, IITA.
Designing molecular markers using a bioinformatics platform. Photo by A. Alonge, IITA.
Enriching the repertoire of molecular markers
In an effort to develop additional genomics resources, IITA was involved in sequencing ESTs from a cDNA library constructed from floral tissue. However, the first several hundred sequences were predominantly housekeeping genes. Recently, in a collaborative project with University of Virginia through USAID-Linkage funds, several thousand ESTs were generated using cDNA libraries from yam leaf tissues challenged with Colletotrichum gloeosporioides, the fungal pathogen responsible for yam anthracnose disease. This resulted in the identification of >800,000 EST sequences, from which about 1152 EST-SSRs were generated in D. alata for use in a yam improvement program. Although AFLP markers have been used for generating linkage maps so far, efforts are under way to saturate the maps with these EST-SSRs to identify the genomic regions associated with resistance to anthracnose disease.

DNA barcoding
Species identification in the genus Dioscorea has remained a challenge when active domestication is continuing in several parts of West Africa. Research on DNA barcoding is under way using chloroplast markers (rbcL, matK, and trnH-psbA) to understand the phylogenetic relationship between different species and also to get an insight into the ongoing domestication process.

Whole genome sequencing
Important considerations for the whole genome sequencing of yam include the genome size, ploidy level, and availability of homozygous clones. Estimation of the genome sizes of various Dioscorea species showed widely variable figures: D. alata and D. rotundata have genome sizes of about 800 mega base pairs (Mbp). Recently, an initiative was launched at IITA in collaboration with the Japan International Research Center for Agricultural Sciences (JIRCAS) to complete the whole genome sequencing of D. rotundata. Preliminary data yielded reasonable sequences. Further work is in progress to generate additional sequence data from the BAC library to facilitate the assembly of the genome which will culminate in producing the first draft genome sequence of Dioscorea species. Additional genomic information produced by resequencing several breeding materials and a parallel project in transcriptome analysis are poised to result in the discovery of a large number of molecular markers and help in the annotation of the genome.

Transcriptome analysis
In contrast to the genome sequence, which is fixed and uniform in all cells of a particular organism, transcriptome refers to the study of the total set of transcripts (expressed genes) in a given cell/tissue at a particular developmental stage or external environmental condition that could influence the physiology of the cell/tissue. IITA, in collaboration with USDA-Agricultural Research Service, Stoneville, embarked on RNAseq, the latest revolutionary tool for transcriptome profiling, based on differential gene expression for anthracnose disease. One of the expected outcomes of this project is to enrich the genomic resources available for yam improvement, including the discovery of SNPs. The latest informatics and statistical methods will be applied to saturate the available linkage map and high resolution mapping of the QTL(s) for anthracnose resistance in different genetic backgrounds.

Genotyping-by-sequencing
With advances in the next generation technologies, the costs of DNA sequencing have come down to such an extent that genotyping-by-sequencing (GBS) is now possible in almost all crops. IITA has recognized the potential of such innovative techniques in accelerating the breeding of clonally propagated crops, such as yam. Hence, in an ongoing USAID-Linkage project, a diverse panel of D. alata genotypes, including parents of available mapping population progenies segregating for anthracnose disease will be genotyped by sequencing to identify a large set of SNPs and determine the divergence among the parents.

Yam roots. Photo by IITA.
Yam roots. Photo by IITA.
Conclusions
To meet the steadily increasing demand, the viable approach is to adopt the innovative plant breeding strategies for yam that integrate the latest innovations in molecular technologies with conventional breeding practices. As efforts are under way to obtain the complete genome sequences and the development of additional genomic resources, the groundwork for deploying yam molecular breeding has been laid. With the availability of new genomic markers and GBS, it would be possible to fingerprint yam germplasm to identify duplicates/mislabeled accessions, to conduct diversity analysis and association mapping. As the genus Dioscorea contains several other useful species, comparative genomic tools can be used to transfer or deduce genetic and genomic information in other species.

References
1 Gedil, M. and A. Sartie. 2010. Aspects of Applied Biology 96:123–135.
2 Mignouna, H.D., M.M. Abang, and R. Asiedu. 2007. Yams. Pages 271–296 in: Genome mapping and molecular breeding Vol. 3: Pulses, Sugar and Tuber Crops, edited by C. Kole. Springer, Heidelberg, Berlin, New York, and Tokyo.
3 Sartie, A. and R. Asiedu. 2011. Development of mapping populations for genetic analysis in yams (Dioscorea rotundata Poir. and Dioscorea alata L.). African Journal of Biotechnology 10: 3040–3050.

Boosting yam productivity in Ghana and Nigeria

IITA and partners recently launched the Yam Improvement for Income and Food Security in West Africa (YIIFSWA) project, supported by a US$12 million grant from the Bill & Melinda Gates Foundation.

The project aims to boost yam productivity and double the incomes of three million yam small-holder farmers in West Africa. It will focus on increasing yields through better seed yam supply and improving markets for this underground, edible tuber.

A key priority of the project is to ensure that affordable higher-yielding pest- and disease-free seed yam are available to farmers, along with storage and handling technologies that can reduce postharvest loss. The project will also develop a host of new yam varieties that can address the challenges in yam production.

The project has major emphasis on training and capacity development of farmers and farmers’ organizations, and improvement of linkages of small-holder farmers to markets where a strong and steady demand for seed and ware yam allow them to realize the economic benefits of increased productivity.

IITA leads the 5-year project in collaboration with the national organizations of Ghana and Nigeria, the UK’s Natural Resources Institute (NRI), the Alliance for a Green Revolution in Africa (AGRA), and Catholic Relief Services (CRS).

For more information, visit www.iita.org/web/yiifswa.

Clean yam tubers from vine cuttings

Hidehiko Kikuno, h.kikuno@cgiar.org

Production of yam seed tubers using vine cuttings and in vitro micropropagation is quick, cost-effective, and results in clean planting material. This new propagation system for yam developed by IITA uses vine cuttings grown on carbonized rice husks combined with in vitro micropropagation (tissue culture).

The traditional system uses tubers as seeds, is inefficient and costly. High production costs are attributed to the use of seed yam tubers, which account for about 30% of the total yield and as much as 63% of the total variable cost incurred per season of cultivation. The multiplication rate in the field using the traditional system is also very low (1:5 to 1:10) compared, for instance, with some cereals (1:300). Low quality seed yam containing pests (nematodes) and pathogens (viruses) also result in a poor yield of ware yam tubers.

Clean seed tuber production system using vine propagation in combination with tissue culture techniques. Source: H. Kikuno, IITA.
Clean seed tuber production system using vine propagation in combination with tissue culture techniques. Source: H. Kikuno, IITA.

The use of vine cuttings as a planting material gives a higher multiplication rate that is about 20−50 times more than the traditional system. It also significantly lowers the risk of nematode infestation and promotes faster multiplication and better and more uniform crop quality. Although viruses are difficult to eliminate, planting materials (seedlings or tubers) produced by this approach are relatively clean compared with those from other propagation methods used in the open field.

An experiment conducted from 2009 to 2010 using seed tubers produced by vine propagation and planted at 25 cm × 1 m spacing resulted in the production of tubers both large (200−400 g) and small (<10−30 g). Large tubers are suitable for use as seed yam for planting in the field, whereas small tubers are resown to obtain appropriately sized seed yam (about 200−400 g) (Table 1).

Tubers from vine cuttings. Photo by. H. Kikuno, IITA
Tubers from vine cuttings. Photo by. H. Kikuno, IITA

Attempts are also being made to standardize the procedure for the direct use of vine cuttings as planting material using cv. TDr 95/18544. The success of this approach could change the way in which yam is propagated in the future and eliminate the dependence on seed yam for planting needs. It would also boost the availability of yam by ~30% (Table 1).

Another trial conducted to understand the appropriate time to excise vine cuttings established from tissue culture materials revealed that the best time for vine cutting is before the rapid tuberization stage. Vine cuttings taken after tuberization were poorly established (see figure below; Kikuno et al. 2010).

Correlation between rooting of vine cuttings and dry weight of tubers formed on mother plants. Time course of rooting of wild vine cuttings and growth of tubers of mother plants on yam (D. alata cv. 95/00361). Bars in each figure indicate % of vine cuttings with rooting. Source: H. Kikuno.
Correlation between rooting of vine cuttings and dry weight of tubers formed on mother plants. Time course of rooting of wild vine cuttings and growth of tubers of mother plants on yam (D. alata cv. 95/00361). Bars in each figure indicate % of vine cuttings with rooting. Source: H. Kikuno.

This new technology offers a rapid solution for a high-output production of seed yam or yam planting material. At the same time, it addresses the need for large numbers and the quick distribution of improved varieties to farmers. This knowledge would be useful for NARES, CSOs, and farmers involved in producing and distributing seed yam, and in maintaining and multiplying breeder and foundation seeds. The technologies can also be used as a research tool by scientists.

The project was funded by the Japanese Government (Ministry of Foreign Affairs), Sasakawa Africa Association, Tokyo University of Agriculture, and International Cooperation Center for Agricultural Education at Nagoya University in Japan under the Ministry of Agriculture, Forestry and Fisheries funded the project. Partners include the Tokyo University of Agriculture; National Root Crops Research Institute at Umudike, Nigeria; Crop Research Institute, Ghana; and Institute of Agricultural Research for Development, Cameroon.

Reference
Kikuno H, Matsumoto R, Shiwachi H, Toyohara H, and Asiedu R. 2007. Comparative effects of explants sources and age of plant on rooting, shooting and tuber formation of vine cuttings from yams (Dioscorea spp.). Japanese Journal of Tropical Agriculture 51, Extra issue 2.

Yam festival

The Yam Festival is a popular holiday in Ghana and Nigeria, two countries in the yam belt in West Africa. It is usually held in the beginning of August at the end of the rainy season. The festival is named after yam, which are the first crops of the season to be harvested. It marks the end of one farming season and the beginning of another, a season of plenty.

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

In West Africa, yam cultivation is associated with a wide variety of beliefs and taboos which govern planting, harvesting, and consumption. Sacrifices are offered to the gods at various stages of growth from planting to harvest. These are also performed in various yam-growing areas of the Pacific.

Sometimes cocoyam substitutes for yam in offering food sacrifices to earth deities. Raw yam is also used for forecasting harvest prospects.

Nigeria
The New Yam Festival is a 2-day cultural festival in southern Nigeria. Dancers wear masks that reflect the seasons or other aspects of nature. It is chiefly celebrated by two large cultural groups: the Ibo or Igbo of the southeast, and the Yoruba of the southwest. The Ibo call the festival Iri Ji; ji means yam. The Yoruba call it Eje.

Various communities celebrate Iriji in different ways. But all have a parade, songs, dancing, and drumming. Because a good yam harvest is important for survival, the people give thanks to the spirits of the earth and sky. The New Yam Festival is celebrated by gathering, blessing, and then feasting on the new yams.

Ghana
The Yam Festival is called the Homowo or “To Hoot at Hunger” Festival. The people hope for a good harvest so that no famine will hit in the coming year. This festival takes place in many rural communities. Women dig up the yam and carry them home in baskets on their heads. Villagers gather together as the women and young girls prepare the feast, with the yam as prized food. They choose a young boy to carry the best yam to the festival dinner, and another boy follows him beating a drum. Other young people from the village march to the beat of the drum and the sound of a woodwind instrument, and sometimes musket fire. Chiefs, under umbrellas and wearing robes made from the famous, brightly colored Ghanaian Kente cloth, follow the yam, and the young people dance. Other activities include singing, wearing animal masks, and displaying fetishes.

Outside Africa
In Indonesia, the traditional yam festival occurs once every 4 years. A big seed yam weighing 2-3 kg is planted near a tree which is stripped of its bark to provide the yam vine with sturdy support. The yam is watered during the dry season and harvested after 4 years for the festival. Similar festivals are celebrated in the Pacific Islands, especially in Papua New Guinea.

History and legend
The New Yam Festival in Nigeria also has religious meaning for those who still practice the native tribal religions. Although most Nigerians are either Muslim or Christian, many still honor the spirits of the land and the souls of their ancestors in their everyday lives and in their ceremonies.

According to Ibo myth, a man named Ibo, or Igbo, gave the tribe its name. A very old legend explains how the yam and the cocoyam, another starchy root vegetable, became such important foods for the Ibo.

During a time of terrible famine, a tribesman named Ibo was told by a powerful spirit that he must sacrifice his son Ahiajoku and daughter Ada to save his other children from starvation. After Ahiajoku and Ada were killed, the spirit told Ibo to cut their bodies into many pieces and to bury the pieces in several different hills of soil.

Ibo did these, and, in a few days, yam leaves sprouted from the hills containing pieces of Ahiajoku’s flesh, and leaves of the cocoyam sprouted from the hills where Ada’s flesh was buried. The spirit told Ibo and his living children to farm these two crops. They did so, and when the yam and cocoyam were harvested, they provided food that kept the family from starvation. Because of this, Ahiajoku is worshiped as the god of yam. He is greatly honored during the New Yam Festival.

Sources
http://www.novelguide.com/a/discover/jwwh_04/jwwh_04_00086.html
http://www.vivienne-mackie.com/articles/holidays/family/yam.html
http://en.wikipedia.org/wiki/Yam_Festival
http://www.familyculture.com/holidays/yamfestival.htm
Orkwor, G.C., R. Asiedu, and I.J. Ekanayake, editors. 1998. The importance of yams. Chapter 1 in Food yams: Advances in research. p. 10.