Contributors to this page: IITA, Nigeria (Dominique Dumet), Bioversity International/ILRI, Ethiopia (Alexandra Jorge); INIA, Peru (Llerme Rios); independent consultant (Clair Hershey).
When are field banks used
Cassava and wild Manihot species collections have been traditionally maintained in field conditions. Many cassava collections are still kept under field conditions, despite having the highest risk of losses. However, the advantages of keeping cassava germplam in the field are the relative technical simplicity and to have vegetative material available for immediate use in breeding work, evaluation or dissemination within the country.
- Field genebanks are generally not considered to be an adequate conservation method due to the phytosanitary and other risks that accompany them.
- They are warranted when there is a regular demand for planting material for various purposes, such as characterization, evaluation and use in breeding.
- Field genebanks may be the only alternative for countries or institutions that do not have more advanced and more secure techniques of conservation available (such as in vitro conservation techniques).
- Nonetheless, most national programmes still use field collections as their main form of maintaining cassava germplasm. We aim to provide here recommendations to make the best use of field genebanks when this is the best practical option.
- Programmes that are able to maintain only field genebanks should make every effort to ensure that the genebank accessions are replicated in another institution, such as IITA or CIAT, in a more secure form of conservation.
- Field genebanks are particularly vulnerable to losses caused by poor adaption of material that has originated in environments that are very different from that of the genebank location. One solution to this common problem is to take a regional approach to genebank management, where accessions of similar adaptation are kept together in a regional genebank located in a similar environment. For example, accessions from high rainfall environments have a greater probability of susceptibility to mite and insect pests, while accessions from dry areas tend to have a greater probability of susceptibility to foliar diseases.
- Wild Manihot species are more difficult to regenerate in field genebanks than cultivated landraces (M. esculenta); most of them are perennial and difficult to regenerate through cuttings, so alternative conservation techniques should be used for these. In the few genebanks that maintain wild Manihot species, the typical practice is to propagate them through seeds.
- Cuttings from field banks can generally only be disseminated within a country, and sometimes only within certain regions of a country, due to pest and disease quarantine regulations. Other forms of propagation, e.g. in vitro or seeds, should be used to exchange germplam between countries or quarantine regions.
How to establish a field bank
Theoretically, a perennial collection of cassava could be maintained for many years without regeneration. However, maintenance problems often increase after a year or two, making replanting at more frequent intervals necessary. Common problems include lodging from excessive growth and the build-up of pests and diseases. Adaptation problems typically occur when the edapho-climatic characteristics of the genebank location are very different from the collection site, where the variety is presumably adapted well enough to be selected and propagated year after year by the growers. These adaptation problems are most likely to occur if the country where the accessions have been collected is geographically and climatically diverse.
The wild Manihot species comprise a wide range of plant types, from recumbent vines to medium-sized trees of 20 or more meters in height. Hence, there is no generalization that applies across the species for field conservation practices, nor are there published guidelines. There are four institutions world-wide with extensive experience in field genebank management of the wild Manihot species and these institutions should be consulted for advice with specific species: CIAT (Colombia), IITA (Nigeria), CENARGEN/EMBRAPA (Brazil) and the University of Brasilia (Dr Nagib Nassar).
- After about two years, there may be many problems with the less well-adapted cassava material and it is therefore more efficient to renew the entire collection than to give intensive care to a certain proportion of it. The optimum renewal time for each collection will need to be determined through experience in the specific location.
- If at all possible, a programme should maintain the collection in more than one site, especially if there is no in vitro collection of the same material.
- Ideally, this should be with staggered plantings, i.e., at each location the bank is maintained for two years, but material is planted in alternate years, so that there is always mature planting material available.
- If this system is used, plot size can be quite small — just four or six plants per plot, in each location.
- If it is not possible to manage two sites, then the same alternate-year planting/renewal system should be used for two different fields at the same site.
- While some evaluations can be made in the genebank, it is often preferable to plant separate experiments for that purpose, where appropriate spacing and management can be better tailored to the objectives of the evaluation.
- All operations in the genebank, and especially the renewal process, must be supervised by qualified personnel. It is very easy to introduce identification errors into the procedures unless carefully monitored.
References and further reading
Fukuda WMG. 1996. Banco de germoplasma de mandioca: manejo, conservação e caracterização. Cruz das Almas, BA: EMBRAPA-CNPMF. 103 p. (EMBRAPA-CNPMF, Documento, 68).
Hershey C. 2008. A global conservation strategy for cassava (Manihot esculenta) and wild Manihot species. Summary of stakeholder deliberations and recommendations prepared for the Global Crop Diversity Trust. Available from: http://isa.ciat.cgiar.org/urg/urgweb_folder/files/unitfiles/A%20Global%20Conservation%20Strategy%20for%20Manihot%20%20August%202008-2.pdf. Date accessed: 7 Oct. 2010.
IITA Genebank Manual Series, Cassava field bank operations at the International Institute of Tropical Agriculture (IITA). International Institute of Tropical Agriculture, Ibadan, Nigeria.
Mohd SS, Rao VR, editors. 2001. Establishment and Management of Field Genebank, a Training Manual. IPGRI-APO, Serdang. 121 p. Available here.
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