3. Varieties and planting material
The full potential of cassava will not be realized until production constraints are mitigated in higher-yielding varieties, and cassava grower shave reliable access to disease-free planting material.
Cassava is grown from cut stems
The cassava genepool consists of the cultivated species Manihot esculenta, and some 100 wild species. Both wild relatives and traditional landraces are the primary sources of genes and gene combinations for new varieties.
Biotechnologists and molecular breeders have used cassava accessions in genebanks to determine which genes control specific traits. With the decreasing cost of molecular biology and biotechnology, the time is right to begin the genome-wide characterization of cassava genetic diversity and to fill gaps in germplasm collections before valuable diversity is lost.
Wild relatives of cassava could make an important contribution to the breeding of varieties suitable for low-input intensification. However, wild Manihot species have been poorly collected and are threatened in many of their native habitats. Action is urgently needed to create in situ reserves for wild Manihot.
The harmonization of data on genebank accessions, and more efficient data generation and dissemination, are needed to facilitate the acquisition of germplasm that could be used to augment locally available heritable variations for cassava genetic improvement. Through the International Treaty on Plant Genetic Resources for Food and Agriculture, FAO can provide a neutral platform for synergistic cooperation.
The breeding of higher-yielding varieties with resistance or tolerance to biotic and abiotic stresses has contributed to big increases in cassava yields over the past 30 years. The International Center for Tropical Agriculture (CIAT) has released clones with resistance to cassava bacterial blight and insect pests, and tolerance to root rot. In Africa, the International Institute of Tropical Agriculture (IITA) has produced improved varieties with traits such as resistance to cassava mosaic disease and green spider mites.
With climate change threatening agriculture in many parts of the world, breeding efforts will focus increasingly on “stacking” multiple traits in elite varieties. There should also be greater focus on developing locally adapted varieties that can produce very good yields for a range of end-uses, with minimal need for agrochemicals or irrigation.
National breeding programmes should be encouraged to introduce the outputs of the pre-breeding activities of CIAT and IITA into their own breeding programmes that use landraces and other farmer-preferred genotypes as parents. Until now, the focus has been on evaluating breeding lines for wide adaptation; that work must now be complemented by introgressing traits from locally adapted materials.
Farmer participation in variety trials and selection criteria needs to become a key step in the development of new varieties. Farmers’ criteria must inform all stages of selection, and trials in farmers’ fields should begin as early as possible in the selection process.
The availability and use of high quality planting materials that maintain genetic purity and are free of diseases and pathogens are crucial to intensified cassava production. Although protocols have been developed for the rapid multiplication of cassava, few countries have a formal cassava seed system.
In the absence of such systems, cassava development programmes in Africa have used a 3-tier community-based approach for rapid multiplication. At the top level, material from breeders is multiplied on research stations and government farms to produce clean, healthy foundation seed. The secondary level involves further multiplication on farms often run by farmer groups and NGOs. Certified material is then distributed to tertiary multiplication sites in farming areas.
Elsewhere, the use of poor-quality planting material remains one of the major causes of low yields. Farmers can help to improve the situation with improved agronomic practices. They should cut stems from vigorous plants that show no symptoms of pests or diseases. Cut stems need to be stored upright in the shade, with the base of the stems resting on soil that has been loosened with a hoe and is watered regularly. Stakes cut from stems should be about 20 cm long, with 5 to 7 nodes, and soaked for 5 to 10 minutes in hot water to kill pests or disease-causing organisms that might be present.
Farmers can increase the size of their next cassava harvest by cutting stakes only from well-fertilized plants with high yields. That simple practice will markedly increase production, especially of traditional varieties that may be susceptible to pests and diseases.
Save and Grow: Cassava (FAO, 2013) can be purchased from email@example.com