General Information and Agronomic practices
Citrus spp. are natives of the subtropical and tropical regions of Asia and the Malay Archipelago. They have been cultivated since ancient times, and spread to other regions of the world, including the Mediterranean, South America and southern states of USA such as California and Florida, where suitable soil and climatical conditions exist. The citrus growing belt follows the equator, extending either side of it about 35 North and 350 South latitude. It embraces tropical, subtropical and the intermediate zones.
The most important commercial species of citrus fruits are:
- Sweet oranges (Citrus sinensis)
- Limes (C. aurantifolia)
- Grapefruits (C. paradisi)
- Lemons (C. limon)
- Mandarins (C. reticulata). These are often called tangerines.
Citrus varieties of commercial importance include the following:
- Oranges: 'Washington Navel' (alt: 1000-1800 m above sea level), 'Valencia Late', 'Hamlin' and 'Pineapple' (all alt from 0 - 1500 m above sea level)
- Mandarins: 'Kara', 'Satsuma' (0-1500 m above sea level), 'Clementine', 'Dancy' (0-1800 m above sea level), 'Pixie', 'Encore' and 'Kinnow'
- Tango/ Tangelo (hybrids of mandarins): 'Temple' a Tango (mandarin x orange) and 'Minneola' a Tangelo (mandarin x grapefruit)
- Grapefruit: 'Marsh Seedless', 'Duncan' and 'Ruby Red', 'Red Blush' (0-1500 m above sea level) and 'Thomson' (1000-1500 m above sea level)
- Lemons: 'Meyer', 'Eureka', 'Lisbon' and 'Villa Franca' (1000-1500 m above sea level), Rough lemon (0-1500 m)
- Limes: 'Mexican', 'Tahiti' and 'Bears' (0-1500 m)
Note: All varieties mentioned are available in Kenya, particularly in Kenya prison farms, albeit their commercial availability is a problem due to citrus greening disease, which is prevalent in Kenya in all areas above 900 m altitude. Since there is no citrus certification scheme in Kenya, there is no assurance that planting material derived from any Kenyan nursery is greening disease-free.
Nutritive value per 100 g of edible portion
|Raw or Cooked Citrus||
Energy (Calories / %Daily Value*)
|Carbohydrates (g / %DV)||Fat (g / %DV)||Protein (g / %DV)||Calcium (g / %DV)||Phosphorus (mg / %DV)||Iron (mg / %DV)||Potassium (mg / %DV)||Vitamin A (I.U)||Vitamin C (I.U)||Vitamin B 6 (I.U)||Vitamin B 12 (I.U)||Thiamine (mg / %DV)||Riboflavin (mg / %DV)||Ash (g / %DV)|
|Lemon raw||29.0 / 1%||9.3 / 3%||0.3 / 0%||1.1 / 2%||26.0 / 3%||16.0 / 2%||0.6 / 3%||138 / 4%||22.0 IU / 0%||53.0 / 88%||0.1 / 4%||0.0 / 0%||0.0 / 3%||0.0 / 0%||0.3|
|Lime raw||30.0 / 2%||10.5 / 4%||0.2 / 0%||0.7 / 1%||33.0 / 3%||18.0 / 2%||0.6 / 3%||102 / 3%||50.0 IU / 1%||29.1 / 48%||0.0 / 2%||0.0 / 0%||0.0 / 2%||0.0 / 1%||0.3|
|Orange raw||47.0 / 2%||11.7 / 4%||0.1 / 0%||0.9 / 2%||40.0 / 4%||14.0 / 1%||0.1 / 1%||181 / 5%||225 IU / 4%||53.2 / 89%||0.1 / 3%||0.0 / 0%||0.1 / 6%||0.0 / 2%||0.4|
|Grapefruit pink raw||42.0 / 2%||10.7 / 4%||0.1 / 0%||0.8 / 2%||22.0 / 2%||18.0 / 2%||0.1 / 0%||135 / 4%||1150 IU / 23%||31.2 / 52%||0.1 / 3%||0.0 / 0%||0.0 / 3%||0.0 / 2%||0.4|
|Grapefruit white raw||33.0 / 2%||8.4 / 3%||0.1 / 0%||0.7 / 1%||12.0 / 1%||8.0 / 1%||0.1 / 0%||148 / 4%||33.0 IU / 1%||33.3 / 56%||0.0 / 2%||0.0 / 0%||0.0 / 2%||0.0 / 1%||0.3|
*Percent Daily Values (DV) are based on a 2000 calorie diet. Your daily values may be higher or lower, depending on your calorie needs.
Climate conditions, soil and water management
Citrus species can thrive in a wide range of soil and climatic conditions. Citrus is grown from sea level up to an altitude of 2100 m but for optimal growth a temperature range from 2deg to 30deg C is ideal. Long periods below 0deg C are injurious to the trees and at 13deg C growth diminishes. However, individual species and varieties decrease in susceptibility to low temperatures in the following sequence: grapefruit, sweet orange, mandarin, lemon/lime and trifoliate orange as most hardy.Temperature plays an important role in the production of high quality fruit. Typical colouring of fruit takes place if night temperatures are about 14deg C coupled with low humidity during ripening time. Exposure to strong winds and temperatures above 38deg C may cause fruit drop, scarring and scorching of fruits. In the tropics the high lands provide the best night weather for orange colour and flavour.
Depending on the scion/ rootstock combination, citrus trees grow on a wide range of soils varying from sandy soils to those high in clay. Soils that are good for growing are well-drained, medium-textured, deep and fertile. Waterlogged or saline soils are not suitable and a pH range of 5.5 to 6.0 is ideal. In acidic soil, citrus roots do not grow well, and may lead to copper toxicity. On the other hand at pH above 6, fixation of trace elements take place (especially zinc and iron) and trees develop deficiency symptoms. A low pH may be corrected by adding dolomitic lime (containing both calcium and magnesium)
Zinc Deficiency in Citrus
A citrus orchard needs continuous soil moisture to develop and produce, and water requirement reaches a peak between flowering and ripening. However, many factors such 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.
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.
- Select seeds from healthy mother trees for rootstocks
- Hot water treat seeds at 50deg C for 10 minutes
- Seeds perform better when planted soon after they are extracted
- Sow seeds in seedbeds or polybags (18 x 23 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.
- Citrus is under irrigation in the major citrus world producing countries.
Manure and fertiliser
For normal growth development (high yield and quality fruits), citrus trees require a sufficient supply of fertiliser and manuring. No general recommendation regarding the amounts of nutrients can be given because this depends on the fertility of the specific soil. Professional, combined soil and leaf analyses would provide right information on nutrient requirements.In most cases tropical soils are low in organic matter.
To improve them at least 20 kg (1 bucket) of well-rotted cattle manure or compost should be applied per tree per year as well as a handful of rockphosphate. On acid soils 1-2 kg of agricultural lime can be applied per tree spread evenly over the soil covering the root system. Application of manure or compost makes (especially grape-) fruits sweeter (farmer experience).
Nitrogen can be supplied by intercropping citrus trees with legume crops such as mucuna, cowpeas, clover or dolichos beans, and incorporating the plant material into the soil once a year. Mature trees need much more compost/well rotted manure than young trees to cater for more production of fruit.Conventional fertilisation depend on soil types, so it is recommended to consult the local agricultural office.
In windy areas, a windbreak should be provided as citrus is sensitive to strong winds. A windbreak provides protection at orchard tree level for about 4-6 times its height.
- Plant the windbreak as close as possible and at right angles to prevailing winds.
Symptoms of mineral deficiency
|Nutrient Element||Leaves||Fruit||Tree growth|
|Nitrogen||Pale yellow to old ivory||Reduced crop||
May produce abundant bloom.Flower buds may fall without opening
|Phosphorous||Small, dull||Reduced crop. Large.Puffy, bumpy surface,enlarged core cavity and thick rind.||Reduced|
Yellow mottling along margin.
Developing a green wedge to "Christmas tree" pattern.
Eventual complete yellowing and defoliation.
Yellow veins, remain green until final stage of general chlorosis.
|Reduced crop||Eventually reduced|
Mottled yellow between main veins.Small narrow Early fall.
|Reduced crop,some pale yellow off types||Eventually reduced|
Normal green along main veins.
Rest of leaf pale green to light yellow
|Reduced crop||Eventually reduced|
|Potassium||Old leaves curl and loose their green colour||Small, smooth, thin rind,drop prematurely||Reduced|
|Copper||Deep green, oversized, then darkened||
Splitting and gumming.
Dark brown gum soaked eruptions.
May turn black.
Gum in centre core
Twigs enlarge at nodes,blister and die back.
Gum pockets."Cabbage head" growth
Intercropping with shallow rooted crops such as vegetables, herbs, green manure legumes sweet potatoes etc, is recommended in order to keep the soil cultivated around citrus trees.
- Harvest fruits when they are mature. Mature fruits change colour where night temperature is about 14degC coupled with low humidity
- In low altitude areas where fruits remain green, it is necessary to test a few fruits for maturity
- Harvest fruits using a sharp knife, taking care not to bruise the fruits
- Fruits can also be plucked. However, plucking causes the stem to break close to the fruit thus increasing the chance of it being infected
- Wash, sort and grade fruits under shade. Washing water must be clean or treated
- Discard deformed and infected fruits
- Pack fruits in aerated containers for transport to the market
Fresh Quality Specifications for the Market in Kenya
The following specifications constitute raw material purchasing requirements.
|(c) S. Kahumbu, Kenya|