In Trinidad and Tobago, yams are not as popular a root crop as cassava and sweet potato. Its use declined due to an increase in anthracnose disease outbreaks that literally destroyed its cultivation. One of yams important attributes is its ability to synthesize natural estrogens, an important hormone for women in an age of increasing alternative medications. This bulletin is intended to provide agronomic information for the successful cultivation of yams.
There are more than 25 varieties of yams held at the Research Division, Centeno. Ten of the local and introduced varieties have been characterized and evaluated for yield, pest and disease tolerance and cooking quality.
Recommended Local varieties: Dioscorea alata (White Lisbon, 'cut and throw way', 'river yam'), rotundata, cayenensis, trifida (cush-cush), esculenta, Chinese yam.
Recommended Introduced varieties: Belep, Kinnibayo, Ashmore, Ebbo etc.
1. Soil type:
The best soils for growing yams are the sandy clay loams. However, yams can be cultivated on most soil types once the yam holes are properly dug and filled with organic material. Organic matter improves soil structure, acts as a slow release fertilizer and allows for optimal growth of the tubers. Adequate drainage, proper aeration, a friable soil and sufficient moisture (not waterlogged) are other critical requirements for yam growth.
2. pH: 4.5-6.5.
Yams are sensitive to shade leading to low yields and should be grown under full sunlight.
4. Temperature: 25-32o C is the range of temperature for optimum tuberization.
1. Clear land of all grass, brush and trees.
2. Practice maximum tillage operations and incorporate organic matter to ensure adequate drainage, aeration, nutrition and room for tuber growth.
3. Plough and rotovate.
4. Apply limestone at 2-4 t/ha before rotovating.
METHODS OF PLANTING
There are four main methods of planting yams:
1. RIDGES: Form ridges 1m apart and 25-40 cm high. Fill furrows with rotted organic matter then plants the yam setts (see Planting Material).
2. MOUNDS: Heap soil into a small mound and add organic matter evenly before planting.
3. HOLES: Dig hole 45cm x 45cm x 45 cm. and fill with rotted organic and sharp sand in the following ratio- 1:2. Plant one yam sett in hole.
4. FLATS: Minimum tillage is done and vines are allowed to grow flat on the ground without a trellis.
There are two local methods of planting yams that are practiced using minimum land preparation and crop care:
5. RIVER BANKS: The land is not tilled. Holes are dug using a cutlass and setts placed in each hole.
6. FOREST: Mounds are prepared using rotted trees, fallen leaves etc. are then planted in the mounds.
A. There are four ways of propagating yams:
1. Tubers: This is the most important method of propagation in the field. The planting material is called a "sett" and the size of each sett should be between 400 grams to 500 grams. Pre-germinate setts in moist sawdust or coconut coir.
2. Seeds: True seeds found on vines e.g. Cush- cush.
3. Cuttings: Basal vine cuttings are best. Use 6 cm-8 cm pieces, dip in a rooting hormone and place in a propagating bin.
4. Tissue culture: Used to produce "clean" plantlets i.e. free from diseases.
B. There are three types of setts (4 oz.pieces) that can be obtained from a whole tuber:
1.Head setts: These are the best yam setts to plant because of the presence of the primary nodal complex (eye) which gives rise to the new plants.
2. Middle setts.
3. Tail setts.
Whole tubers can also be planted. Dust all planting setts, especially those with an exposed cut surface, with a fungicide and anhydrous lime at storage and before planting. Dusting encourages wounds to heal and prevents the entry of pathogenic organisms that potentially could cause spoilage to the stored planting material.
Fill each planting hole with rotted organic matter mixed with sharp sand and one handful of urea. Plant yam stets 1m apart on the ridges and 10 cm deep in hole. An easy way to remember what month to plant yams is to spell YAM backwards: MAY.
Yams must be staked to expose the massive canopy to full sunlight throughout its growth.
1. Individual staking: One stake per plant e.g. bamboo stakes.
2. Pyramidal staking: The top of the stakes is slanted to form a peak.
3. Trellising: String wire between two strong posts and each stem is trailed along a string towards the wire support.
Controlling weeds for the first 6-8 weeks after planting is important. Use a contact herbicide in the furrows during growth and manually remove weeds from around the plant. Selective herbicides can be used to control grass weeds and manual methods can be used to control broad-leaved weeds.
A soil test should be done to determine fertilizer types and rates and also any limestone requirements.
1. Two months after sprout emergence apply 85 gm–114 gm of 16:8:24 NPK fertilizer placed 15 cm - 20 cm away from the base of the plant.
2. Six months after planting, a similar amount of potash fertilizer e.g. Muriate of Potash is recommended to encourage optimum tuber bulking.
Best yields are achieved using high levels of organic manures and high levels of potash.
PEST AND DISEASE MANAGEMENT
The major disease problem is anthracnose (Colletotrichum gloeosporioides Penz.). Anthracnose is normally seen as small, black spots between the leaf veins. Some infections can coalesce to form massive blighted lesions. This disease can be significantly managed using a number of cultural practices with a combination of timely chemical controls as listed hereunder:
1. Soak the planting material with recommended copper fungicides prior to planting and drench the planting holes with the fungicide after planting.
2. Spray the vines upon shoot emergence.
3. Alternate fungicides to prevent the build up to chemical resistance.
4. Encourage healthy plants by ensuring adequate levels of nutrients.
5. Sanitize the field by raking and removing fallen leaves prior to cultivation to reduce the source of the inoculum (spores of the fungus).
6. Practice crop rotation.
7. Inter-crop with corn. The corn crop helps to move the spores up and away from the yam leaves reducing the infection rate.
8. Use windbreaks on the windward side of the crop.
9. Inspect the field continuously and rogue out infected plants, especially at the windward side of the field.
Yams mature between 9-10 months after planting and this can be seen by the yellowing of the leaves and natural dieback of the vines. After removing the vines, lift the tubers using hand forks taking care to do as little damage to the tubers. Tubers should be dusted with anhydrous lime and a fungicide to accelerate wound healing.