Starfruit/carambola trees can be propagated generatively (seeds) and as well as vegetatively (budding and grafting onto seedling rootstocks, or air-layering), whilst tissue culture propagation has met only limited success, as shoots but not roots have been regenerated. Recently, gibberellic acid applied in a lanolin paste to the basal stem of young seedlings increased stem diameter growth rate, thus reducing the time by nearly half for plants to reach graftable stem.
However, both of the generative and vegetative propagation methods have been reported to have several advantages and disadvantages. Trees from seedlings are mostly not true to type in characters and take longer time to mature and bear fruits. Fruit yield can be inconsistent and sometimes low depending upon the cultural and eco-climatological conditions of the field. On the contrary, vegetatively propagated trees are more homogenous in nature and possess short immaturity period. As such, starfruit is more popularly propagated vegetatively among fruit growers in Taiwan, Malaysia, Indonesia and Florida (USA) and many other countries in the world. Inarching is successful in India, shield-budding in the Philippines and the Forkert method in Java, Indonesia. Trees can be top-worked by bark-grafting, a popular technique in Java. For mass production, side-veneer grafting of mature, purplish wood, onto starfruit seedlings gives best results for most workers. A grafted or seedling plants are ready for field planting when they are16 to 24 months old. Propagation by tissue culture has been only partly successful, as shoots but not roots have been regenerated.
1. Plant propagated from seeds
Those who are interested in planting star fruit from the seeds, firstly they have to select big fruits that are free from diseases and damages and must be fully matured/ ripe from vigorous and healthy trees. The seeds are carefully removed from the pulp and free of the mucilage. They are then rinsed with uncontaminated water, treated with 4% formalin or 1% sublimate to prevent rotting and then air-dried. Later, the seeds are evenly planted in the germination beds (see Figure 3A &B) or trays filled with washed river sand or soil mix.
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The germination beds or trays are sheltered from rain and direct sunlight (except for the morning or the evening sunlight). Watering of the beds or trays is carried out twice daily. The seeds will germinate in one week in tropical condition and require 14 to 18 days in winter. The germinated seeds are allowed to grow in situ for a period of 1.5 months. By this time, the seedlings have developed 2 to 4 leaves and ready to be transplanted into 15 x 25 cm polythene bags containing 2:1 soil mix. Transplanting must be carried out in the morning or evening to ensure that the roots are not injured. To prevent further root injury, the seedlings are lifted from the beds or trays with soil wrapping the roots. The seedlings are planted in the centre of the bags and soil around the plant base is lightly compacted. The planted bags are arranged in 4 x 25 polythene bags per block and kept under shade for a period of 6-12 months until the plants achieved 20 cm height before transplanting into the field.
2. Plants propagated vegetatively
(a) Change varieties or cultivars
(b) Optimize cross-pollination and pollination.
(c) Take advantage of particular rootstocks
(d) Benefit from interstocks.
(e) Perpetuate clones.
(f) Produce certain plant forms.
(g) Repair damaged plants.
(h) Increase the growth rate of seedlings and
(i) Indexing viruses.
Grafting and budding can be performed only at very specific times when weather conditions and the physiological stage of plant growth are both optimum. Large scale production of vegetatively propagated plants is achieved by two methods, namely air-layering/ marcotting and budgrafting/grafting. The budgrafting method has been tried on B1, B2 and B3 with some commercial clones B10 and B17 and has been proven to be very suitable. Air-layering has been practiced and advocated. However, root formation is slow and later performance is not wholly satisfactory.
i. Air-layering/ marcotting/gotee
A healthy one-year old branch acutely positioned is selected from a healthy mother tree. Two rings are cut at 3-4 cm apart around the branch. The bark between the rings is completely removed and the exposed cambium layer is gently scraped off from the wood. The cut is wiped with clean cloth or tissue paper. The edge of the bark towards the shoot is applied with root-promoting hormone (IBA). A ball of moist soil mix (2:1) is placed around the cut and the soil ball is wrapped up in a transparent polythene sheet and secured tightly with string at both ends of the wrapper. After several weeks the roots will develop sufficiently in the soil ball within the polythene wrap. The rooted branch/ branches are cut off from the mother tree at 3-4 cm below the wrap and then kept in hardening area after 6 - 12 months in the nursery are planted out at a spacing of 4 m x 6 m.
ii. Grafting
Budgrafting (see Figure 4)
It's the most popular method in star fruit propagation because of its practicality among fruit growers or nursery men. Almost all drawbacks of air-layering technique can be overcome. This is easily achieved by using the 6-8 month old seedlings as rootstocks and buds taken from healthy and perfect mother plants of high yield. Bud-grafting can be carried out as the following: Firstly, prepare the budding patch on the stem of rootstock at 10-15 cm height above the soil surface. A sharp and clean budding knife is required to make the patch. The two vertical cuts are made with this knife separately followed by two horizontal cuts at the top and bottom of the former cuts. The bark in the cut patch or window is carefully removed exposing the wet cambium layer. An identical cut is performed to remove the bud from the scion with the bud carefully centered. While removing the bud patch from the scion stock slight rotation of the bud patch around the budstick will assure the "eye" of the bud to remain attached to the bud patch. Carefully place the bud patch with the new bud in the cut patch or window ascertaining complete contact at both the top and bottom of the cut patch or window. Small openings along the sides of the cut patch or window are not important or critical. The newly placed bud patch should be securely tied with budding tape, parafilm or wide rubber band. Ensure that the bud is not covered by the budding tape. After 18 - 21 days, the graft can be checked for its success of grafting which can be easily recognized by the developing bud from the patch. The budding tape or parafilm is carefully removed from the graft and the top of the seedling is cut off at 10 cm above the graft. The exposed cut of the stem is applied with melted paraffin or fungicide mix to prevent rotting of the stem. Finally, the budgrafted plants are transplanted into bigger size polyethylene bags and kept the nursery until they are one-year old before planting in the field.
It should be noted that, budgrafting is not only done on young trees or seedlings but this propagation technique is sometimes applied to rejuvenate old unproductive and low yielding star fruit trees. Usually the unproductive branches of the old trees were budgrafted with scion buds from other high yielding and quality starfruit trees.
Figure 4 . Diagram illustrating the bud grafting technique
• Cut a slice of bud and bark from the parent tree.
• Cut a similar sliver off the rootstock, making a little lip at the base to slot the scion into.
• Join the two together and bind.
• In time, the scion bud will grow into a shoot , which will develop into the desired tree.
Wedge Grafting
This grafting technique (see Figure 4 ) is another mean of producing high quality starfruit trees. Its quite an easy technique to apply. Firstly, one has to get ready with a rootstock plant of 6-8 months old. The stem of the rootstock is cut at 10-15 cm height from the soil surface. A center cut of 3-5 cm (v-shaped cut) is made through the stem (see diagram). In the meantime a shoot scion of similar length with the rootstock containing at least 3 buds taken from a very productive and healthy mother plant. The cut end of the shoot must be shaped like a wedge or inverted V-form. This shoot is then inserted into the cut of the rootstock and the graft must be tied firmly with budding tape or elastic band. The grafted seedling is then covered with a translucent polythene cover moistened in the inside with water sprays. The seedling is then kept under a shelter (see diagram). New buds usually appear 15-20 days later. The plastic cover is then removed and the grafted seedlings are moved to the hardening area for several weeks before they are transplanted into the field.
3. Tree Management
When young, the starfruit tree is quite delicate and requires careful attention. Because it is a fast-growing tree, it requires pruning and thinning of excess fruit and water shoots at an early stage. Good crops are harvested from grafted varieties when they are 2-3 years old. Yields of up to 900 kg/year are common for 10-year-old budgrafted trees.
4. Germplasm Management