Planting material preparation

Sweetpotato planting material is either obtained from vine cuttings, the most common source, or from storage roots.

Use of stem cuttings. Farmers obtain cuttings from an established crop before or just after the harvest of storage roots. The cuttings are either used to establish a maintenance field, or directly for planting the next sweetpotato crop.

Below are some factors affecting yield, when using stem cuttings:

• Care should be taken to select “clean” planting material. This means choosing cuttings that are free of insects, soil, and any symptoms of viruses or fungal diseases.

• Generally the apical portion of the vine is better than the middle or basal portions. This portion is less likely to carry sweetpotato weevils and fungal pathogens, and has been found to establish faster than other portions. For cultivars with long vines, the second or third cut is acceptable. Sometimes, the second cutting is better than the apical portion, if vine growth has been so fast that the stem has not matured in the apical portion.

• Length of cutting is less important than the number of nodes. Typical size is 20-40 cm, with 5-8 nodes. The conditions of the field may influence the relationship between cutting length and crop development. Farmers should experiment to decide what length is best under their conditions.

• Usually one-third to two-thirds of the cutting is buried. A minimum of 2-3 nodes, but up to about 8 nodes, is placed under the soil. The effect of number of buried nodes on the final yield is inconsistent among reported studies, but may depend on the plant density, with lower number of nodes being acceptable at higher plant density.

• The delay between cutting and planting may affect yield depending on the storage conditions for the cuttings. Storing cuttings for one to two days in humid conditions may be beneficial, promoting rooting at the nodes. Longer storage may adversely affect establishment by exhaustion of the cuttings’ energy reserves. To minimize losses, leaves should be stripped from the lower portion of the cutting, and bundles of cuttings wrapped in a wet cloth or sack and kept in a cool, shady place away from wind. If roots develop during storage, they should be planted carefully to minimize damage to the roots.

If planting material is to be maintained in a multiplication plot before planting of the next crop, plant cuttings at approximately 15 x 20 cm spacing. New growth may be ready for cutting after 45 days.

Use of storage roots. Storage roots are used when there are insufficient stem cuttings available, or when the level of pest and disease infestation is high so that few healthy vines are left. They may also be used in highly mechanized production, as the sprouts can be harvested mechanically from the seedbed.

• Healthy storage roots should be selected from plants that produced high yield.

• The roots are planted densely in a seedbed located away from other sweetpotato crops.

• Roots are covered with about 3 cm of soil, and the bed covered with straw to help retain moisture.

• When the sprouts have grown long enough, they are cut near their base and planted directly in the field.

• To maximize the number of cuttings, remove the tips of the sprouts when they are about 20 cm long, to promote branching.

Rapid seed multiplication. When large amounts of cuttings are needed, rapid multiplication may be done. Although the merit of this practice has not been fully acknowledged by sweetpotato growers, it can be the easiest way to produce large amount of planting materials. This method involves the following steps:

• Cuttings of about 30 cm are taken from either established plants or sprouted storage roots. These are then cut into single node cuttings, with the leaf attached. The tip of the vine is discarded.

• A seedbed is prepared with a mixture of loose, humus-rich soil and rice hull ash. The single-node cuttings are planted at a high density, with the stem section buried and the leaf upright.

• The seedbed is regularly watered and is prevented from drying especially during the first week of establishment.

• After about two weeks, the seedlings have developed enough roots to be transplanted into the field. They should be removed from the seedbed with care to avoid damaging the roots. Transplanting should be done in the late afternoon to avoid excessive evaporation and wilting.

When sweetpotato is vegetatively propagated for a number of generations, yield decline is often observed. This is usually due to a build-up of viruses, many of which show no obvious symptoms. This often gives the impression that a new variety (carrying few viruses) yields much better than traditional varieties, when in fact it may be no better after a year or two when viruses have accumulated.

Viruses can be removed by heat treatment and meristem culture. This process usually results in a yield increase of both vines and roots, from 20% to 200%, depending on the severity of the original virus infestation. The higher yield may be maintained for several years in the field, before the virus load has built up again.

A program in China has significantly boosted regional yields by supplying farmers with clean planting material regularly. Such a system is the standard practice for Irish/white potato production, but is not yet widely used for sweetpotato.

References

Rasco, E.T., Jr. and Amante, V. 2000. Sweetpotato in Tropical Asia. SAPPRAD-PCARRD. 230 p.

van de Fliert, E. and Braun, A. 1999. Farmer field school for integrated crop management  of sweetpotato. Field guides and technical manual. International Potato Center. Lima, Peru. 266 p.

Contributed by: Vilma Amante and Jane O'Sullivan

Land preparation

Planting

Soil management

Water management

Vine lifting

Integrated pest management

Harvesting

Postharvest practices

Stem cuttings for planting (V. Amante).

Storage roots are used as source of planting materials (E. Data).

Seedboxes planted from single node cuttings (E. van de Fliert).