Cassava (Manihot esculenta Crantz) has played an important role on the Africa's agricultural stage. The CGIAR system has considered it the most important root crop in sub-Saharan Africa, and average consumption exceeds 300 kg per person per year in some areas. Its hardy nature also means that it is moving into more marginal areas. Nonetheless it is faced with production constraints of pest, diseases, and poor soil fertility everywhere it is grown on the continent. Some of the most severe disease problems, for example the cassava mosaic disease, are found in Africa. The CIAT project "Genetic Enhancement of Cassava", as part of its global responsibility for improving cassava, recognizes the importance of cassava in Africa and aids increased productivity through the provision of improved germplasm and training in conventional and new methods of cassava breeding.

Partnership with IITA

CIAT works closely with its sister institute, the International Institute of Tropical Agriculture (IITA) to ensure African farmers benefit from 30 years of cassava breeding and the wide range of germplasm on which it was based. In the late
eighties to early nineties, CIAT and IITA, with funding from the International Fund for Agricultural Development (IFAD), introduced hundreds of thousands of sexual seeds of improved, drought-tolerant cassava germplasm, which originally came from Northeast Brazil, but was improved at CIAT for productivity and root quality. This IFAD project extended cassava cultivation from its traditional areas in the humid and dry savanna ecologies of West Africa to the region's semi-arid areas.

Combining CMD Resistance, Resistance to Other Stresses and High Productivity

The presence of cassava mosaic disease (CMD), the most important production constraint in Africa, and its absence in Latin America have severely limited the usefulness of CIAT cassava germplasm in Africa. Drought-tolerant germplasm has had to undergo further breeding at IITA before it could be useful to farmers. This situation has now changed with CIAT having recently identified molecular markers that are closely associated with very high levels of resistance to CMD. CIAT can now develop, through these markers, CMD-resistant varieties for Africa.

A project for Africa has been proposed to combine high productivity and excellent resistance to three major biotic stresses (CMD, cassava green mite, and cassava
bacterial blight) with well-adapted varieties local to eastern and central Africa. This improved germplasm is expected to increase food security for small farmers, reduce cassava prices, increase farmers, profit margins, and make the crop more competitive with other crops in the region and on the global market.

Assessing the Genetic Diversity of African Cassava

CIAT also assesses the genetic diversity of cassava landraces in Africa, using molecular markers. This project is funded by the International Programme in the Chemical Sciences (IPICS, of Uppsala University, Sweden). Simple sequence repeat (SSR) markers are employed to obtain a quantitative estimate of diversity in local cassava varieties from Tanzania and Nigeria (see some results at http://www.ciat.cgiar.org/molcas). Further studies are being carried out in Uganda and Ghana.

The genetic diversity of African landraces will also be compared with that of Latin American landraces to identify heterotic pools. Activities include examining the high
genetic differentiation found in the SSR study between Guatemalan and Nigerian landraces for heterotic groups.

Germplasm Exchange

• Thirty-three elite parental lines were shipped as in vitro plantlets from CIAT to IITA for a joint line-by-tester experiment.
  
• More than 2000 sexual seeds from six wild Manihot relatives with high protein and dry matter content, and from advanced breeding lines with high dry matter yield were shipped from CIAT to IITA.
  
• In vitro plantlets of parental lines with CMD resistance were transferred from IITA to CIAT and the Central Tuber Crops Research Institute (CTCRI), India.

Molecular Marker-assisted Selection for CMD Resistance

• CMD markers developed at CIAT were tested for their fidelity in marker-assisted selection for CMD resistance on more than 2500 progenies that had already been established at IITA and field evaluated for CMD resistance.
  
• CIAT conducted marker-assisted breeding of lines to combine CMD resistance, high carotene content, and good dry matter yield. These lines will then be recombined (again using markers) with high protein lines currently being developed. The goal is to develop varieties with high contents of carotene, protein, and dry matter, and carrying high CMD resistance.
  

Joint Line-by-tester Experiment

IITA will cross 20 elite lines from each of CIAT and IITA to two common testers. The resulting sexual seeds will be shared between the two centers.

Improving Carotene Content in Cassava

Later this year, crosses will be made to combine high carotene content and CMD resistance, and high protein content and CMD resistance. The crosses will then be recombined to give lines high in all three traits.

Developing double haploids in cassava via wide crosses between cassava and castor IITA has made the crosses, and CIAT is now analyzing the DNA of the hybrids to
ascertain parental contributions.

New Partnerships

• A project proposal "Improving Cassava Germplasm in Uganda, Kenya, Tanzania, Rwanda and the DR Congo for Yield, Root Quality and Resistance to CBB, CGM, and CMD" has been developed by CIAT, IITA, and NARS for submission to the Rockefeller Foundation. The principal objective is the introduction of elite parents with high dry matter yield, drought tolerance, good culinary quality, yellow roots, and excellent resistance to the cassava bacterial blight (CBB), cassava green mites (CGM), and the cassava mosaic disease (CMD) to NARS in the target countries for improving local germplasm. The project also seeks to quicken the process using molecular marker-assisted selection.
  
• This year, CIAT received about 10 requests from African NARS and NGOs interested in receiving improved germplasm.
• A possible initiative is being explored between CIAT and the International Livestock Research Institute (ILRI) to use cassava foliage as feed component for small and large ruminants in Africa.

Training PhD Students from IITA

• Two NARS scientists from Ghana and Nigeria were trained at CIAT in the use of marker tools for assessing genetic diversity.
  
• Two Ugandan PhD student are working on QTL mapping of cyanogenic potential and dry matter content, respectively.
  
• A Nigerian PhD student has successfully completed his studies on QTL mapping of early bulking at CIAT.
  

Future Perspectives

• Germplasm development work with IITA will continue.
  
• In collaboration with IITA, CIAT will include African NARS as recipients of CIAT's advanced breeding materials with combined CMD resistance.
  
• CIAT will increase the number of participants in individual training and begin group training in the use of marker tools.