Propagation: Pomelo can be propagated by many methods including seeds, cuttings, air-layering, grafting, and tissue culture. In commercial practice, pomelo is commonly propagated by grafting an individual bud of a selected variety onto a rootstock seedling or by air-layering method as in Vietnam. In the Philippines, shield budding is a standard practice for propagation, using rootstock of calamandrin (possibly a hybrid of calamondin and mandrin).
Seed: Although some cultivars can be reproduced by seed, this method is considered inferior and not commonly practiced by the growers as the plants are not uniform, do not come true to type, and have many long spines which become very hard and sharp before fruiting begins. Varieties that are reproduced by seed take more time to produce fruit; they are more susceptible to diseases and tend to produce more thorns than grafted varieties. Their fruit is also harder to pick as a result of the upright and thorny growth.
Rootstock propagation: Seeds should be selected from fruit trees that are free from Citrus Canker (Xanthomanas citri) to avoid contamination on the seeds. Select fully developed seeds from mature/ripe fruits. Fruit that dropped or fall on the ground should not be picked up and use to extract seeds for rootstock.
Seeds removed from the fruit are cleaned, disinfected with CHLOROX at 5% solution, rinsed with tap water and air dry before sowing. Dried seeds can also be stored for a year in a cool, dry atmosphere (Williamson and Jackson 1994).
Extracted seeds should be sown immediately to avoid drying. Soil media should be light type (sandy loam), sterilized and free from any soil borne insect pests like root grubs and nematodes. Growth media should have good drainage and moderate water-holding capacity. Media that are high in organic material (>50%) can create problems and should be avoided if possible. Normally soil mixture of 3 parts top soil, 2 parts sand and 1 part compost (organic matter) is suitable for raising pomelo seedlings successfully in perforated polybags (size 18 cm x 30 cm - flat) as containers. Otherwise, seed beds (nursery) can also be used but should be isolated from existing citrus orchard to prevent possible contamination through insect vectors.
Seeds for rootstock should germinate within days after planting, and they are ready to be budded as soon as they have a stem of about 6 mm in diameter.
Potting and Transplanting of Seedling Rootstocks: Seedlings from seed beds are ready for potting in polyethylene plastic bags 21-28 days after germination and place them in the nursery. Avoid transplanting seedlings with deformed root system (goose-neck root) and should be eliminated. Water immediately the newly potted plants.
Fertilize the seedlings when signs of new shoots or growth have developed.
Care of Seedling Rootstocks: Water the seedbed to facilitate germination.
When seeds started to germinate, exposed the seedbed to sunlight. Do not put shades. Shades provide a microclimate which favors the growth of dumping-off pathogen. Seedlings that grow without shades are hardy and free from dumping-off disease.
Seedlings do not need too much water. Too much water on poor drained beds predisposes the seedlings to dumping-off disease. Spray chemicals to protect young seedlings from pests and diseases. Recommended minimum dosage should be used to prevent leaf burn.
Budwood collection: Select branches that are mature, vigorous, round (rather than angular), and close to or smaller than the diameter of the rootstock in which the bud will be grafted. Adequate maturity can often be identified by formation of lignified tissue in the green bark (lines of wood formation). Branches earmarked for budwood production should be regularly inspected and treated to keep them free of pathogens and insects. A round bud-stick gives more good buds that angular bud-stick. Cut the selected limbs from the parent tree so that 20-25 cm (8-10 in) of desirable buds will remain after removing the new flush and all leaves. When removing the leaves, a small portion of the petiole that is adjacent to the bud should remain attached to the bud as a handle until the budding process is completed (Williamson and Jackson 1994).
Bud preparation: Budding can be achieved with very little equipment. A sharp knife is needed to cut buds from the parent tree. In addition, polyethylene wrap is needed to keep the bud secured to the rootstock and to seal in moisture and seal out rain or irrigation water.
Budwood storage: Budwood cut from the parent tree is best used shortly after cutting. However, if stored moist in a sealed plastic bag in a cool place, budwood can survive for 2-3 months after cutting from the parent plant. Once individual buds are cut from the budwood, they should be used immediately for best grafting success, although individual buds may remain viable for a few hours when stored cool and moist.
Pre-grafting treatments: No pretreatments are necessary for successful budding of healthy scions.
Propagation area: Survival of budded plants is greater in protected environments. If available, shade houses or greenhouses with adequate water are essential.
Vegetative propagation known as "T-budding": Budding uses a bud (scion) cut from the parent tree that is grafted onto a seedling rootstock. Once the bud is in place, the foliage of the rootstock above is cut off or tied down to "force" the bud growth. Producing trees through budding allows the selection of rootstocks that can impart the characteristics of the parent tree such as disease tolerance and allow production in soils not suited for the scion (Williamson and Jackson 1994).
Asexual Propagation and Care of Budded Seedlings: Rootstocks are ready for budding in 6-8 months; or at least pencil-sized in stem diameter.
Apply nitrogenous fertilizer at 5-10 grams per rootstock plant 2-3 weeks before budding.
Budding should be done at a height of 13-20 cm above the ground level.
Do not fertilize newly budded plants; unless the bud eye has shown signs of growth.
Early growth: Buds that are successfully growing will be green and have callus forming around the edges 14-21 days after budding. Wraps can be removed at this time. When bud-eyes started to germinate; decapitate by "lopping" or "cripping" the top the rootstock at 5-7.5 cm above the bud-eye union is recommended to hasten the growth.
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| Left: A recent bud graft, showing callous formation around the edge. Photo: J. Williamson | Right: Grafting scion wood to the top of a seedling (wedge or cleft grafting) is also a common method of propagating citrus varieties. Photo: C. Elevitch |
Weeding: It should not be done when the scion are succulent and tender since they are susceptible to breakage when touch.
Other comments on propagation: Cuttings, especially from young branches, root well. This method can be used to preserve varieties until suitable rootstock is available.