Various unidentified cultivars are grown in Jamaica, but preference is given by consumers to smooth red skin types with white flesh.
The starchy tubers may range from a few cm to over 30 cm in length, and they may be spindle-shaped or almost spherical and weigh from a few grams to 3.0 kg.
Tubers may have smooth or irregular ribbed surface and the skin and flesh may range from pure white through cream , yellow, orange, pink, to very deep purple, depending on the amount of various carotenoid pigments present and its presence or absence of anthocyanins (plant pigments).
The crop is ready for harvest after approximately 3-6 months, when the leaves turn yellow and begin to drop, or when a tuber can be cut without the sap turning black.
In addition, the high natural sugar content pre-disposes the tubers to a pathogenic attack.
1. Tubers should be harvested intact, i.e. tips should not be broken, as these ends will act as entry points for pathogenic attack.
2. Do not throw roots onto heaps or from one row to the other, as the tubers will break, crack, or be bruised or cut.
3. Harvested tubers with damp soil adhering may be left in the field for approximately one hour or longer to dry, but not too long to suffer from sun scald. Sun scald is manifested into black heart disease.
Tubers should be gathered into boxes or ventilated crates in which they can remain until they reach delivery point.
Use of bags and loosely packed trucks should be discouraged as the tubers are easily damaged and the least amount of handling will result in less bruising of the skin.
Do not over-pack the boxes or field containers as this will lead to compression damage and bruising of the skin.
Java Black Rot (Botryodiplodia theobromae)
This is a storage and marketing disease which also causes heavy losses. The rot is brown in colour and is moderately firm. As the decay progresses, the tissues pass from light brown in colour and eventually turn black as the fruiting bodies form. Eventually the tubers become hard and black.
The pathogens (diaporthe and fusarium spp) enter through wounds or broken ends and cause firm dry regions of decay which lose water rapidly to become hard and dark.
CONTROL OF DISEASES
Prompt curing of wounded areas
Low temperature storage
Chemical control using TBZ, Benlate or Botran, used to control
Rizopus and Fusarium fungi.
Simple storage techniques
Tubers can be packed in moist saw dust or coir dust and stored for 3-5 days.
Tubers can be stored in thick perforated plastic bags at ambient temperature.
Cold storage at 13-16ºC and 85-90% RH can result in storage for up to 4-6 months.
Storage below 12ºC results in chilling injury. This affects the cooking quality, increase susceptibility to decay and internal breakdown of the tubers.
POST HARVEST DISEASES
The affected tissues of the tubers become soft and watery and yield a yellowish brown liquid when broken.
The tissue darkens and become withered as water is lost.
Black Rot (ceratocystis)
The early symptoms of this disease appear as round, slightly sunken spots which enlarge and become black with small black fruiting bodies like small black bristles.
Water loss and shrivelling are the most important causes of deterioration of sound intact tubers during storage. This results in the tubers appearing dull, stale , and unattractive.
Tubers that will be stored should be properly cured, I.e. healing of damaged areas by formation of new skin. This can simply be done by placing the tubers in containers covered with dried grass. Ventilation is restricted to allow a build-up of heat and moisture to about 27-34ºC and high moisture for 4-7 days.
Care must be taken to prevent the accumulation of carbon dioxide in the environment and free water on the tubers which would encourage rotting.
Selection & Grading
This can take place in the field, or at delivery.
Tubers must be firm or only slightly soft or shriveled. This is an indicator of a well matured tuber with the development of appropriate nutrients.
Tubers must be fairly well shaped; deeply indented tubers are difficult to peel and process.
Tubers must be clean; free from adhering soil or other foreign matter, e.g. pesticide residue.
The presence of excess soil may cover damaged areas, therefore causing
Tubers must be free from damages, e.g. growth cracks, cuts or bruises, and damage caused by insect, e.g. sweet potato weevil (cylas sp.) in the field.
Tubers can be sorted based on varietal characteristics, i.e. same skin colour, shape, and character of flesh. This may have implications for culinary use.
For more information contact:
Mrs. Janette Lawrence
Marketing Extension Unit
Str Line: 977-1151
Fax: 927-1592 or 970-4660