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).
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.
- 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.
Plot size and spacing will depend on the size and purpose of the collection, land availability and demand for planting material.
- 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.
- 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.
- 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.
- 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 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.
One of the problems of longer-term maintenance in the field can be excessive growth of some accessions.
- It has been learned that periodic trimming back of these accessions often creates entry points for pests and diseases and is therefore counter-productive. Instead, follow the guidelines for plant spacing given above.
- Re-plant the genebank on new land every regeneration cycle.
- Rotate with grass or leguminous crops to break the cycle of certain root pathogens and prevent land degradation.
Common pests and diseases
The Americas have the greatest diversity of cassava pests, followed by Africa and then Asia.
Under natural conditions, pests and pathogens are often kept under control by a combination of natural enemies, host plant resistance, and management practices.
In genebanks, these controls are often absent or reduced and pest and disease management can become a major challenge.
- Damage in Africa is often high due to the lack of natural predators of pests.
- Damage tends to be seasonal. Often insect and mite pests are more damaging in the dry season and diseases more damaging in the wet season.
- Consult the cassava health diagnosis menu for detailed list of pests and diseases and procedures.
Pest and disease control
- Weekly or bi-weekly (maximum) surveying of the collection is essential, to be aware of any problems that arise and need to be corrected.
- Select healthy planting material. Do not take cuttings from plants that had leaf chlorosis, shoot tip die-back, cankers, fungus patches or streaks on the stems.
- Treat cuttings with pesticides and fungicides before planting, and the plants during the growth stage when necessary.
- Rogue and burn diseased plants regularly during the growth season (if it does not compromise the survival of a specific accession).
- After harvest, destroy discarded stems and roots that have disease symptoms or pest contamination.
- Use natural enemies against cassava pests as much as possible. Complement by applying appropriate pesticides as necessary.
- Weed the field regularly.
- In the worst situation, cuttings can be replaced with the backup stems.
- Insecticide or miticide treatments may be required to prevent widespread invasion of white flies, green mites, mealy bugs, termites, grasshoppers and other pests.
- To be effective, treatments need to be applied at an early stage of insect development (visible eggs or larvae).
- Pesticides should always be used according to label instructions.
- When working with pesticides, worker safety is always of utmost importance. Gloves, mask, (to cover the nose and mouth), safety goggles and rain boots are needed to give the sprayers full protection.
- Herbicides are applied both pre-emergence immediately after planting and post-emergence at three, six, and nine months after planting.
- For pre-emergence weed control, it is advisable to spray on a moist soil and prior to a forecast rain if possible, to facilitate diffusion of the chemical into the soil, in contact with weed seeds.
- At the earlier development stages (up to three months), plants are short and tender. Extreme caution is advised during spraying to avoid chemical contact with young plants by using a guard fitted to the nozzle of the spraying equipment.
- This precaution is not as important when the plant matures, but efforts should always be made to spray only on weeds.
- Herbicides should always be used according to label instructions.
- When working with herbcides, worker safety is always of utmost importance. Gloves, mask, (to cover the nose and mouth), safety goggles and rain boots are needed to give the sprayers full protection.
- Harvest the stakes at the end of the growing season (this guide does not refer to any root or seed harvesting, dealing only with the vegetative propagules), usually 12–18 months after planting, depending on the cultivars and environment. In some environments most of the leaves will have dropped, but in others, a leaf canopy remains at maturity.
- Be careful to identify the stem cuttings from each plot.
- Store the stakes in a well-ventilated and shaded cool place until planting or in case they need to be replanted (keep extra planting material for a while until the collection is established).
- Take care during the harvesting and subsequent handling of the stakes not to bruise them.
- Extend storage time (not recommended for collections) with longer uncut stakes tied in bundles pre-treated in pesticide, at 70–80% RH and 20–23°C.
- Stakes can also be stored (also not recommended for collections) buried in the ground for several months, with the basal side down, or laid horizontally; regular watering is required to avoid excessive dehydration.
- Stakes or cuttings also store well for weeks in polythene bags in drier areas and/or during the dry season.
System for tracking material/inventory system during field bank storage
- Use pegs, tags or barcodes for labelling.
- Use impermeable ink and write clearly.
- Plots must be well labelled to avoid errors.
- Barcodes help avoid errors in recording.
Recording information during field bank storage
The following information should be recorded for each step:
- Site name and map/GPS reference.
- Name of collaborator.
- Field genebank site name (a code to identify the site location).
- Plot reference (the plot number at the field site).
- Accession number; population identification.
- Name of staff (name of staff recording the data).
- Method of planting, date and spacing.
- Field layout used.
- Field management details (watering, fertilizer, weeding, pest and disease control, stresses recorded, others).
- Environmental conditions (altitude, precipitation, temperature, soil type and others).
- Number of plants established.
- Days from planting to flowering (note: this will only be important if seed collection is anticipated).
- Harvest date and method.
- Number of plants harvested.
- Quantity of cuttings harvested.
- Comparisons with reference materials (record any identification numbers or references of any samples taken from the plots).
- Any evaluation undertaken during the growing period or at harvest.
- Post harvest (describe any relevant procedures).
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).
Frison EA, Feliu E, editors. 1991. FAO/IBPGR Technical Guidelines for the Safe Movement of Cassava Germplasm. Food and Agriculture Organization of the United Nations, Rome/International Board for Plant Genetic Resources, Rome.
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.