Field banks for cassava and wild Manihot species

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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Distribution of cassava genetic resources

Contributors to this page: CIAT, Colombia (Daniel Debouck); IITA, Nigeria (Dominique Dumet); Bioversity International/ILRI, Ethiopia (Alexandra Jorge); INIA, Peru (Llerme Rios); independent consultant (Clair Hershey).

An important role of in vitro genebanks is the distribution of germplasm to different users.

Click here to open a PDF document that shows how to disseminate germplasm in detailed steps.

Policies and regulations

Cassava dissemination (photo:IITA)

Common policies on distribution and access to plant material

• Follow the International Treaty on Plant Genetic Resources for Food and Agriculture (ITPGRFA) (www.planttreaty.org) for in-trust germplasm using the Standard Material Transfer Agreement (SMTA).

Policy exceptions

• None.

National laws and regulations

It is essential to follow the terms and conditions in the host country agreements:

• Export permits.
• Phytosanitary certificate.
• Certificate of origin.

International laws and regulations

The shipment of plant material should be sent with the SMTA under the International Treaty for Plant Genetic Resources for Food and Agriculture.

Phytosanitary regulations

• Phytosanitary certificates are needed for most countries.
• See the safe transfer of germplasm (STOG) section in this website (this is essential to avoid the spread of pests and diseases).

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User related issues for distribution

Feedback to users

Factors that can influence the delivery of plant material:

• Respond to requests with lists of material, forms and conditions of access and SMTA as soon as possible after receipt of request (users may not know about the conditions so it is better to inform them before proceeding with the request).
• Provide passport and germination data with requests.

Recommended procedures that ensure the material distributed matches the client request:

• On specific requests, match accession numbers with specified request.
• If accession numbers are not specified, match accessions to users needs. These could include:
  • Species.
  • Plant habit to fit the crop system.
  • Environment.
  • Specific traits required, such as disease resistance. 
  • Use (e.g. consumer-related traits).

Feedback from users

The most relevant information required to be received from users:

• Information on characterization and evaluation/use (information on performance in one area allows better selection of germplasm for similar areas).

Quantity of material recommended to be distributed

Cassava accessions typically consist of a single clone, i.e there is no genetic variation within an accession. A single plant can represent the full diversity of the accession. The main consideration on quantity of material to send is to assure that the recipient has a very high probability to successfully recover the accession from in vitro culture.

• Three to five seedlings per accession are normally adequate.

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Procedures for distribution

Check availability

Availability in stock

• Check availability of vegetative material in the field or laboratory.
• Distribution of the requested material should not cause the accession to fall below the minimum stock or distribution of vegetative cuttings cause plant death.
• Accessions with little vegetative material or those which are not healthy should not be distributed.

Checking passport data

• Passport and evaluation data can be checked to ensure that the accession meets the requestor’s needs (this avoids waste of germplasm by sending material that the requestor does not need or want).

Preparing vegetative material for distribution

• Register the request: track requests according to the dates they are received (this allows requests to be handled on a first come first served basis).
• Prepare a list of accessions available: generate lists of accessions that meet users' needs.
• Check requirements for Standard Material Transfer Agreements (SMTA).
• Micropropagate or multiply the necessary plant material.
• Transfer material into multiplication or rooting media to promote the aerial growth and roots, to facilitate acclimatization.
• Prepare shipment, after 4-6 weeks of growth.
• Check the vigour of the plants before packaging. Only seedlings showing well-developed root and shoot systems should be considered for distribution.
• Pack the tubes carefully in boxes, with all the necessary labels. Use labels with good adhesive and clear printing (this avoids errors and mixing during shipping).
• Acquire the Phytosanitary Certificate to include in the shipment.
• Acquire the Import Permit.
• Elaborate list of information to include in the shipment:
  • Accession number.
  • Accession identification.
  • Crop name.
  • Taxonomic identification.
  • Country of origin.
  • Biological status.
  • Collecting location.
  • Source.
  • Characterization data used to verify accessions should be provided upon request.
  • Cover letter, reminding users of the terms and conditions of access and requesting feedback (important to make contact with the user for future feedback).
• Include a handbook of procedures, explaining how the material should be handled. In vitro seedlings need special attention prior to transferring to field conditions.
• Register information into the plant genetic resources database.

Dispatching the plant material

• Pack the plant material for secure shipment,and to avoid damage or mixing the material.
• Attach documents e.g. the seed list, SMTA, Phytosanitary Certificate, Import Permit, GMO-free certificate in a strong envelope or a cardboard box.
• Attach a copy of the Phytosanitary Certificate, Import Permit and list of material to the outside of the box, if the material is being dispatched to another country.
• A copy of the SMTA must be attached to the outside of the envelope or box. Use of the material constitutes agreement with the terms of the SMTA.
• Label the envelope/box with the complete mailing address of the requester.
• Include a reply form (a reply form should be used to acknowledge that the material has been received by requester in good condition).
• Use courier or other rapid means of transit (method should avoid heating and delays in transport).
• Record date and method of shipping (shipping details are important for tracking during shipping).
• Update the genebank inventory.

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Recording information during distribution

System for tracking material/inventory system for distribution

• Update related data tables in the database management system (a database system allows easy and fast access to data and allows macros to be written for routine operations).
• Use labels with good adhesive and clear printing (this avoids errors and mixing during shipping).

References and further reading

Ceballos H. 2006. Cassava research at CIAT [poster]. Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia. Available from: http://webapp.ciat.cgiar.org/news/pdf/poster02_scmeeting_06.pdf. Date accessed: 26 August 2010.

Guevara CL, Mafla G. 1996. Manihot collections held at CIAT. In: Engelmann F, editor. Management of field and in vitro Germplasm collections. CIAT, Cali, Colombia. pp. 109-112.

Mafla G, Roa JC, Debouck DG. 2004. Observations about the distribution of cassava germplasm from an international collection. Poster presented at the CBN-V. Available from: http://isa.ciat.cgiar.org/urg/urgweb_folder/files/posters/CBN-V.%20G%20Mafla%20ppt.pdf. Date accessed: 30 August 2010.

Mafla G, Roa JC, Flor NC, Aranzales E, Debouck DG. 2006. Distribution of cassava germplasm from an international genebank: a service to the global agriculture. Poster presented at the first meeting of the Governing Body, ITPGRFA, Madrid, Spain, 12-16 June 2006. Available from: http://ciat-library.ciat.cgiar.org/Articulos_Ciat/CIAT40years.pdf. Date accessed: 30 August 2010.

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Viability and monitoring in cassava field banks

Contributors to this page: IITA, Nigeria (Dominique Dumet), Bioversity International/ILRI, Ethiopia (Alexandra Jorge);  INIA, Peru (Llerme Rios); independent consultant (Clair Hershey).

Health diagnosis in field banks

• Preferably, test the source plants to identify healthy plants as a source of cuttings.
• If possible, do random health tests during field monitoring and checks.
• Follow recommendations from the cassava health diagnosis menu for detailed procedures.

Routine monitoring methods

Assures the viability, adequate growth and good health of the plants during field growth.

• Secure and protect the field against theft, vandalism and damage by animals.
• Weekly or bi-weekly (maximum) surveying of the collection is essential, to be aware of any problems that arise and need to be corrected.

• Check regularly for plant vigour and survival rates.
• Check regularly for mixtures due to handling mistakes: look for uniform characteristics, such as colours of young apical leaves (expanded and non-expanded), petiole, cortex and external stem as well as pulp, cortex and external root colours; flowering and branching types, pubescence of young leaves, shape of the central lobe, internode length (leaf scars), storage root peduncle and surface texture.
• Check regularly for pests and diseases. Contact plant health experts to identify symptoms and recommended appropriate control measures.
• Weed the field regularly.

Recording information during routine monitoring in field banks

The following information should be recorded for each step:

• Site name and map/GPS reference.
• Name of collaborator.
• Field bank site name (a code to identify the site location).
• Plot reference (the plot number at the field site).
• Accession number; population identification.
• Date of monitoring (date when data is collected).
• Date of test (the date that the test was carried out).
• Results of tests and action taken (removal of plant or application of insecticide or quarantine measures).
• Name of staff (name of staff recording the data).
• Survival rates (number of plants alive).
• Details of plants removed (due to type mixtures or pests or diseases contamination).
• Damage [a score of 1-5 (where 5 is most damaged) or qualitative assessment of damage (insect, disease etc)].
• Vigour [assessment of vigour of the plants on a scale of 1-5 (where 5 is high)].
• Field management details (watering, fertilizer, weeding, pest and disease control, stresses recorded, others).

References and further reading

Fukuda WMG. 1996. Banco de germoplasma de mandioca: manejo, conservação e caracterização. Cruz das Almas, BA: EMBRAPA-CNPMF. 103p. (EMBRAPA-CNPMF, Documento, 68).

Hershey C. 2008. A Global conservation strategy for cassava (Manihot esculenta) and wild Manihot species. A consultancy report to CIAT, on behalf of the Global Crop Diversity Trust, Rome. (Final report under review).

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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Field management in cassava field banks

Contributors to this page: IITA, Nigeria (Dominique Dumet), Bioversity International/ILRI, Ethiopia (Alexandra Jorge); INIA, Peru (Llerme Rios); independent consultant (Clair Hershey).

Establishment

Choice of environment

• Cassava grows well between 30°N and 30°S in areas where annual rainfall is more than 750 mm a year, and mean daily temperatures are above 18°C (low to medium altitude (1500–2000 m) tropics, or low altitude subtropics).
• Well-aerated, loose and light sandy loam soil is recommended for cassava. It is sensitive to frost but tolerant of long dry periods, soils with low pH, high aluminium and low fertility.
• The area where material is maintained should be as free as possible of diseases and insect pests that could cause losses of material or create difficulties in the transfer of clean planting material to other sites.
• If water is available either through irrigation or well-distributed rainfall, the crop can be planted at any time of the year; preferably at the beginning of the warm season (growth slows during cool weather).
• In places where rainfall is seasonal and irrigation is not available, delay planting until rains are reliable.

Field preparation

• Cassava can be maintained in field plantings as a perennial plant, but periodic renewal every one or two years is desirable to avoid problems of excessive vegetative growth, cumulative disease and insect problems and to facilitate maintenance generally.
• Allow an overlapping period in the field of at least six months between the ‘old’ and the newly planted field to ensure that material that did not germinate can be replanted and provide a constant supply of planting material for research programmes.
• As a reference, approximately 0.3 ha is needed for the maintenance of 1000 accessions (0.4 ha when cuttings are planted for germplasm characterization purposes).

Traditional field methods

• Field plots should be uniform in fertility, with light textured, deep, well-drained soil and as free as possible from noxious weeds.
• Avoid stony, clay, shallow, hard or waterlogged soils, or manage them to correct the problems.
• In sandy soils, apply minimum tillage to conserve soil, organic matter and moisture and reduce soil erosion.
• In poorly drained soils, make ridges or mounds to reduce water logging.

Innovative method developed at CIAT

• In order to combine the benefits of lower space requirements with continual availability of planting material for experimental use, CIAT devised a slow-growth system based on restricting the root development in small planting pots (bonsai effect).
• Plants occupied only a small fraction of the space they would occupy if allowed unlimited growth in the field.
• Maintaining a cassava germplasm collection in containers has the potential advantages of space savings, better protection against pests, diseases and weather-related damage, and labour savings.
• Disadvantages can include difficulty in using plants as a source of planting material for field trials (generally small and weak stems), cost of infrastructure and cost of material.

Field planting

Plot size and spacing will depend on the size and purpose of the collection, land availability and demand for planting material.

Layout

• It may be beneficial for management purposes to group germplasm according to vigour, plant height, and branching habit, establishing at least three groups: high, intermediate and low vigour.
• Establish a distance of 2.2, 1.5 and 1.0 m between plots for high, intermediate and low vigour groups respectively, to minimize competition while making efficient use of land area.
• Space plants 1.0–1.5 m apart if evaluations are to be made simultaneously, or closer if the collection is solely for germplasm maintenance (0.75–1.0 m within the row and 1.0 m between rows) to minimize weed growth and land requirements.
• Maintain a minimum of five and an optimum of ten plants per accession to ensure adequate survival and supply of planting material.

Planting

• Plant the stakes directly in the ground (so that half or two-thirds of the stake is covered) or in soil ridges or mounds, vertically or at an angle, or even bury them horizontally about 5 cm below the soil surface. The local planting practice of experienced cassava growers in the area can also provide a good guide.
• Identify plots very carefully, putting a plastic tag in the first plant of the left hand row of the plot. Place an extra label on a plastic, metal or strong wood stake in front of the plot.
• Note that three to five extra cuttings are planted behind each peg as backup in case some of the main cuttings die.
• Draw a field map of the collection immediately after planting, with each accession located on the map, including both plot numbers and accession numbers.
• Replant missing plants one month after planting.

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 Maintenance and management

 Weed management

• Ensure adequate control of weeds pre-emergence by ploughing and harrowing the soil or applying pre-emergence herbicides before planting, and post-emergence with herbicide applications, inter-row weeders or regular manual weeding.
• Weed as often as necessary to avoid competition between plants. Weeding may be required up to four times per season, depending on the environment.
• Critical times are during the initial four months or until leaves form a canopy and weed growth is suppressed.

Irrigation

• The soil must be moist at planting; otherwise irrigation is required.
• If irrigation is not available, it is important to plant the collection at the beginning of the wet season when rain is reliable.

Fertilization

• Fertilization is usually not required for the sole purpose of germplasm maintenance. However, add manure at land preparation (e.g. cow dung or poultry manure) if necessary.
• Apply NPK 15:15:15, depending on the soil analysis, about eight weeks after planting, around the plant, not touching the stems or leaves.

Pruning

One of the problems of longer-term maintenance in the field can be excessive growth of some accessions.