Oranges

CITRUS PRODUCTION IN KENYA

Introduction

Citrus spp. are natives of the subtropical and tropical regions i.e. it embraces tropical, subtropical and the intermediate zones.  Citrus is widely cultivated in tropical as well as subtropical African countries. While fresh fruit for the market is produced preferably in subtropical climates (e.g. South Africa) and Mediterranean climates (e.g. Tunisia, Egypt, Morocco, Libya), citrus for juice is predominant in tropical climates because of the possibility for higher sugar content.

The most important commercial species of citrus fruits are:
•    Sweet oranges (Citrus sinensis) e.g. Washington naval variety, Minneola variety, Valencia variety among others.
•    Limes (C. aurantifolia)
•    Lemons (C. limon)
•    Mandarins (C. reticulata). These are often called tangerines.
Economic importance

 Establishing a new citrus orchard
i.    Selecting suitable growing conditions
Citrus trees can be grown in a wide range of climatic and soil conditions. But proper site selection remains the key to successful organic production. Climate has a significant effect on nearly all aspects of citrus growth and quality development:

>> Extremely hot temperatures are damaging to citrus. Ideal temperatures range from 13 °C to 38 °C. At higher temperatures, flowers and leaves drop prematurely.

>> Rainfall or irrigation throughout most of the year is necessary for citrus, however, dry and hot temperatures during the day and cool temperatures at night are favourable conditions for good colour development.

Although citrus grows in a wide variety of soils, it grows best in deep soils of medium texture and moderate depth, with good drainage and high fertility. Moderate pH conditions between 5 and 7 are preferable for citrus, as they generally allow adequate availability of nutrients.

Adequate drainage is also needed, as tree growth is reduced in poorly drained soils or where compacted soil layers are present in the root zone. Furthermore, poor drainage causes problems with Phytophthora and other soil borne diseases.
 
Yields in subtropical climates peak at 20 to 25 years, while yields in tropical
regions reach a maximum at 10 to 15 years already because of the higher pest
and disease pressure in these regions.
Botanic description
Citrus sinensis is a small, shallow-rooted evergreen shrub or tree about 6-13 m high with an enclosed conical top and mostly spiny branches. Twigs angled when young, often with thick spines. Leaves smooth, oval, 5-15 x 2-8 cm, dark green above, glossy, with a distinctive smell often similar to the fruit, petiole winged. Flowers small, waxy greenish-white, fragrant; calyx broad saucer-shaped, petals 5, white elliptic, 1.3-2.2 cm long. Fruits orange, reddish-green to yellowish-green, round, 4-12 cm, consist of a leathery peel 6 mm thick, tightly adherent, protecting the juicy inner pulp, which is divided into segments that may not contain seeds, depending on the cultivar

uch as temperature, soil type, location, plant density and crop age influence the quantity of water required. Well-distributed annual rainfall of not less than 1000 mm is needed for fair crop. In most cases, due to dry spells, irrigation is necessary. Under rain-fed conditions, flowering is seasonal.
There is a positive correlation between the onset of a rainy season and flower break. With irrigation flowering and picking season could be controlled by water application during dry seasons. Irrigation systems involving mini sprinklers irrigating only soil next to citrus trees have been developed as an efficient and water conserving irrigation method.

Propagation

The most common method of citrus propagation is by budding. When old trees are top-worked, bark grafting is used. Citrus varieties grown from seed have numerous problems like late bearing, uneven performance due to their genetic variability and susceptibility to drought, root invading fungi, nematodes and salinity. Rootstocks are therefore used to meet all citrus requirements (tolerance / resistance to pests and diseases, suitability to soil and water conditions, as well as compatibility with scion variety selected). Rootstocks also improve the vigour and fruiting ability of the tree, as well as the quality, size, colour, flavour and rind-thickness of the fruit.

Citrus rootstocks have the following characteristics:

•    Rough lemon (C. jambhiri)
Seedlings produce a uniform and fast growing rootstock, which is easy to handle in the nursery. The plant develops a shallow but wide root system with a vigorous taproot. Trees budded on rough lemon produce an early, good yield but the fruit quality especially during the first years is not satisfactory. Trees are comparatively short-lived. Rough lemon prefers deep, light soil and do not tolerate poor drainage or waterlogging. It is tolerant to citrus tristeza virus but susceptible to Phytophthora spp., citrus nematodes and soil salinity. It is drought tolerant. Rough lemon can be budded with oranges, mandarins, lemons, limes and grapefruits. It is the most commonly used rootstock in East Africa.

•    Cleopatra mandarin (C. reticulate)
It is suited to soils of heavier texture. On this rootstock, trees are slow growing with low yields in early years. Trees are long-lived. Its influence on fruit quality is good. It is tolerant to soil salinity. It is susceptible to poor drainage, Phytophthora spp. and citrus nematodes. It can be budded with oranges, mandarins and grapefruits.

•    Citrus trifoliate (Poncirus trifoliate)
It is a dwarfing stock and is most suitable for heavy and less well-drained soils. Rootstock propagation is slow, but budded trees yield heavily and produce high quality fruits. The plants develop abundant roots and often several taproots, which penetrate the soil deeply. It should not be used in calcareous soils. It is tolerant to Phytophthora spp. and citrus nematodes. It can be budded with oranges, mandarins and grapefruits.

•    Carrizo / Troyer citrange (P. trifoliate x C. sinensis)
Rootstocks are somehow difficult to establish. In order to promote fibre roots, young plants should be undercut as long as they are in the seedbed. Citranges are not suitable for very light and strongly alkaline soils. They are sensitive to overwatering but once established produce high quality fruits. They are somehow tolerant to Phytophthora spp. and citrus tristeza virus but susceptible to Exocortis viroid and citrus nematodes. They can be budded with oranges, mandarins and grapefruits.

•    Citrumelo (P. trifoliate x C. paradise)
Plants produce an expansive root system and therefore have good drought tolerance. They can be used on a wide range of soils and produce an outstanding quality of fruit. They are tolerant to Phytophthora spp. but susceptible to citrus nematodes. They can be budded .oranges, tangarines and grapefruit.

•    Rangpur lime(C. aurantifolia)
This stock is suitable for various soil types, including deep sand. It prefers warm locations. It produces vigorous, well-bearing trees with a high degree of drought resistance. It is susceptible to Phytophthora spp. and citrus nematodes. It can be budded with oranges and grapefruits.

•    Sweet orange(C. sinensis)
This rootstock produces large and vigorous trees and is suitable for light to medium soils, which are well drained. It produces good quality fruits and the trees are long-lived. It has low drought tolerance and is very susceptible to Phytophthora spp. and citrus nematodes. It can be budded with oranges, mandarins and grapefruits.
•    Sour orange (C. aurantium)

An excellent rootstock in locations where citrus tristeza virus is not a problem since it is very susceptible to the disease. It is tolerant to poor drainage. It has low tolerance to drought. It produces very good quality fruits. It is tolerant to Phytophthora spp. but susceptible to citrus nematodes. It can be buddedwith oranges and grapefruits.

Planting

•    Select seeds from healthy mother trees for rootstocks
•    Hot water treat seeds at 50° C for 10 minutes
•    Seeds perform better when planted soon after they are extracted
•    Sow seeds in seedbeds or polybags (18x23 cm). Seeds germinate in 2 to 3 weeks
•    Water the seeds regularly, preferably twice a day until they germinate
•    Seedlings are normally ready for budding when reaching pencil thickness or 6 to 8 months after germination.
•    T-budding is the most common method.
•    Do budding during warm months. Avoid budding during cold periods and during dry conditions
•    Budded plants are ready for transplanting 4 to 6 months after budding
•    Alternatively, obtain budded plants from a registered fruit nursery. These budded plants should be ready for transplanting in the field.

Transplanting in the field

•    Transplant in the field at onset of rains.
•    Clear the field and dig planting holes 60 x 60 x 60 cm well before the onset of rains.
•    At transplanting use well-rotted manure with topsoil.
•    Spacing varies widely, depending on elevation, rootstock and variety. Generally, trees need a wider spacing at sea level than those transplanted at higher altitudes. Usually the plant density varies from 150 to 500 trees per ha, which means distances of 4 x 5 m (limes and lemons), 5 x 6 m (oranges, grapefruits and mandarins) or 7 x 8 m (oranges, grapefruits and mandarins). In some countries citrus is planted in hedge rows.
•    It is very important to ensure that seedlings are not transplanted too deep.
•    After transplanting, the seedlings ought to be at the same height or preferably, somewhat higher than in the nursery.
•    Under no circumstances must the graft union ever be in contact with the soil or with mulching material if used.

Tree management / maintenance

•    Keep the trees free of weeds.
•    Maintain a single stem up to a height of 80-100 cm.
•    Remove all side branches / rootstock suckers.
•    Pinch or break the top branch at a height of 100 cm to encourage side branching.
•    Allow 3-4 scaffold branches to form the framework of the tree.
•    Remove side branches including those growing inwards.
•    Ensure all diseased and dead branches are removed regularly.
•    Careful use of hand tools is necessary in order to avoid injuring tree trunks and roots. Such injuries may become entry points for diseases.
•    As a general rule, if dry spells last longer than 3 months, irrigation is necessary to maintain high yields and fruit quality. Irrigation could be done with buckets or a hose pipe but installation of some kind of irrigation system would be ideal.