Selection of good planting material

Most citrus orchards consist of grafted trees that combine favourable attributes of the scion and the rootstock. However, the choice of the rootstock should depend on the adaptability to local conditions and resistance to common soil borne infections. Sour orange (Citrus aurantium L.) is the most widely planted rootstock and is an excellent rootstock for areas free of Citrus Tristeza Virus (CTV). Rough lemon (Citrus jambhiri) is also a widespread rootstock, but should be avoided in areas known to have blight and nematodes.

When choosing cultivars for scion production, factors like disease resistance, drought resistance and quality as well as yield performance should be considered. Valencia is an important cultivar for orange juice and this cultivar is known to do well under organic management. Washington navel is used mainly as fresh fruit. Other varieties used in Africa include Hamlin, Tangelo, King Star ruby and Red blush. There are many other local variants, some of which are good for organic production. Although they may not be high-yielding, they may be adapted to local production conditions.

Purchased seedlings should be obtained from good nurseries, to make sure they are free of diseases (especially viruses) and pests and for which the authenticity of rootstock and scion cultivars is guaranteed.

Raising seedlings in the nursery

The use of virgin sites for nurseries is very important in organic production. It reduces the risk of soil-borne diseases like Phytopthora, Pythium and pests like nematodes to which citrus seedlings are very susceptible. The site can also be treated to make it safe by covering it with a black polythene sheet for at least two days of full sun or by burning dead plant material over the site.

Citrus trees are normally propagated by grafting (budding) a preferred citrus tree onto an appropriate rootstock. This ensures that the new tree has the same characteristics as the mother plant. T-budding is the most common method of grafting oranges. However, budding should be done during warm months (not during cold periods or during dry conditions). Budded plants are normally ready for transplanting 4 to 6 months after budding. Citrus rootstocks are ready for grafting 6 to 7 months from pricking. If budding is to be done, seedlings are budded 9 to 10 months from pricking.

Recommendations to farmers regarding preparation of rootstocks:

Choose seeds from healthy and vigorously growing mother trees, which are not grafted or budded. Extract the seeds and subject them to hot water treatment at about 55 °C for 10 minutes.
Sow the seeds immediately in seedbeds, mixed with well-prepared compost.
Water the seeds regularly, preferably twice a day until they germinate. Seeds germinate in 2 to 3 weeks. If seedlings appear congested, thin them out so as to produce sturdy and vigorous plants. Thinning also reduces chances of disease outbreaks.
Seedlings are ready for picking and potting in polybags when they have 2 pairs of leaves and a bud.
Seedlings are normally ready for budding when they reach pencil thickness or about 6 to 8 months after germination.
Dumping off is the most serious disease favoured by wet soil, dense shading, and humid environment due to overcrowding. To prevent dumping off, shelter the seedlings to keep rain off. This reduces leaf wetness also and can reduce fungal infections such as cercospora leafspot. Pests like aphids, leaf miners and orange dogs are likely to attack. These can be controlled by applying plant extracts, for example, from neem or tephrosia.

Recommendations to farmers regarding budding of rootstocks:

Select healthy trees from which to obtain bud graft branches (bud sticks). Cut a number of branches, about 30 cm long, using a very sharp knife.
Select the seedlings onto which the grafting will be done. They should be above 30 cm tall in order to ensure that the budding takes place at about 30 cm above the ground, not higher. The closer to the ground you insert the graft, the more the graft receives nutrients, water and sap that are undiluted. The higher up you place the graft, the greater the chance is of it either dying off or taking too long to become a strong branch. Before budding, it is good to check if the bark peels easily. A bark that is peeling indicates that the seedling is undergoing active growth and is ready for budding.
Make a 1½-inch cut in the shape of a ‘T’ at the grafting location using a sharp knife, opening the bark of the stem. Continue to cut well below the bark into the seedlings wood.
Pick up the first bud stick and choose the biggest bud. With a sharp knife, gently slice the bud together with the bark and a thick portion of the wood to which it is attached. Take care not to touch the cut side of the bud with your fingers as this may impair the connection of the bud to the rootstock.
Uncover the wood underneath the ‘T’ cut in your orange tree by gently lifting up the bark and gently insert the bud into the open wood under the ‘T’ slice, starting at the top of the ‘T’ and moving downwards. The bud will be held in place by the wood of the main seedling. Ensure that the bud is put exactly in the same direction as it was oriented on the bud stick.
Wrap budding tape underneath and above the graft in order to prevent any movement that might hinder the integration of the bud into the actual wood of the orange seedling. The tape also avoids moisture loss and callousing (tissue overgrowth) at the grafting site. The bud should also be entirely covered.
Remove the budding tape when the buds start sprouting, in about 2 to 3 weeks, and continuously remove all rootstock sprouts (including the terminal bud of the seedling). The new bud will then be able to grow to take over the entire crown of the orange tree.

The objective of budding is to align the cambium layer of both rootstock and bud as closely as possible. Thus, it is important that the bud is removed from the bud stick with a smooth cut, and the bud and rootstock to be tightly pressed together. Farmers can start by practising the budding process on branches of older trees. After they gain experience, then they can do it on young seedlings.