Citrus (Citrus spp.)
The cultivated citrus species belong to the family Rutacea and are native to the tropical and subtropical regions of South-East Asia. These plants are large shrubs or small trees (5–15 m height), with spiny shoots. Some of the citrus grown in Kenya include Oranges (Valencia late, Washington navel, Mineola, Pixie), lemons, and lime, tangerine and grape fruit. Citrus juice contains a high quantity of citric acid giving them a characteristic sharp flavour. They are also good sources of vitamin C.
(Citrus sinensis )
Citrus thrives well under low-land conditions. It is grown from sea level up to an altitude of 2100 m asl. However production is best at 0-1500m asl. Individual species and varieties decrease in susceptibility to low temperatures in the following sequence: grapefruit, sweet orange, mandarin, lemon/lime and trifoliate orange. The upper altitude limit of cultivation is depends on the occurrence of frost. The highlands provide the best night weather for orange colour development and flavour. Oranges grown in areas with warm nights (altitude below 1200 m asl) usually remain green even when ripe.
Citrus are ever green and require readily available soil moisture at all times. Citrus require at least 900 mm of annual rainfall, well distributed over the growing period.
Citrus can be grown on a wide range of soils. However, a deep light loamy, fertile soil is ideal. Citrus does not tolerate water logged and saline soils. Good drainage is essential for sustained high yields. Citrus does well in soils with a pH of 5.0-6.0. In extreme acidic soils, citrus roots do not grow well and nutrients are leached out or may even become toxic e.g. copper. On the other hand, for pH above 6, trace elements are fixed (especially zinc and iron) and trees develop deficiency symptoms. A low pH can be corrected by adding dolomitic lime (lime which has magnesium) but may not be practicable for heavy soils
For optimal growth, a temperature range of 25-30°C is ideal. Growth is reduced if temperatures fall below 13°C and damage occurs if temperatures are below 3°C. Citrus, however, will tolerate high temperatures if the water supply is adequate. The orange colour of the fruit develops at a temperature of 14oC coupled with low humidity.
- The area to be planted should be cleared of bushes and tree stumps. Eliminate perennial weeds such as couch grass which can become troublesome later.
- Planting holes (size 60 x 60cm) should be prepared well in advance before the rains. The top soil should be separated from the subsoil during the digging of holes and then mixed with about 2 debes of well decomposed manure and 100g of DAP.
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 soil salinity. Citrus trees are therefore mainly propagated vegetatively. The most common method of propagation is by budding but old trees can be top-worked through bark grafting. 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 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.
Rootstocks are chosen on the basis of their adaptability to various soils and soils conditions, resistance to soil borne diseases, compatibility with different scions and tolerance to specific viral diseases as described below :-
- Rough lemon (C. jambhiri)
- It is the most common rootstock and can be budded with oranges, mandarins, lemons, limes and grapefruits.
- The plant develops a shallow but wide root system with a vigorous taproot and is therefore drought tolerant.
- Seedlings produce uniform and fast growing rootstock
- Trees budded on rough lemon produce early and yield well but the fruits are of low quality during the first years.
- Trees are comparatively short-lived.
- Rough lemon prefers deep, light soil but do not tolerate poor drainage.
- It is tolerant to Citrus tristeza virus but susceptible to Phytophthora spp, citrus nematodes and soil salinity.
- Cleopatra mandarin (C. reticulate)
- Trees are long-lived with good influence on fruit quality and can be budded with oranges, mandarins and grapefruits..
- It is suited to soils of heavier texture and tolerant to soil salinity.
- Trees on this rootstock grow slowly and the yields are low in the early years.
- It is susceptible to poor drainage, Phytophthora spp and citrus nematodes.
- Citrus trifoliate (Poncirus trifoliate)
- It is a dwarfing stock that can be budded with oranges, mandarins and grapefruits.
- 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 is most suitable for heavy and less well-drained soils but should not be used in calcareous soils.
- It is tolerant to Phytophthora and citrus nematodes.
- Carrizo / Troyer citrange (P. trifoliate x C. sinensis)
- They can be budded with oranges, mandarins and grapefruits although the rootstocks are difficult to establish.
- In order to promote development of fibre roots, young plants should be undercut when they are in the seedbed.
- Not suitable for very light and strongly alkaline soils.
- They are sensitive to overwatering
- Produce high quality fruits once established.
- Slightly tolerant to Phytophthora and citrus nematodes.
- Citrumelo (P. trifoliate x C. paradise)
- They can be budded with lemons and limes and can be used on a wide range of soils
- Plants produce an expansive root system and therefore have good drought tolerance.
- They produce an outstanding fruit quality.
- They are tolerant to Phytophthora but susceptible to citrus nematodes.
- Rangpur lime (C. aurantifolia)
- They can be budded with oranges and grapefruits
- This stock is suitable for various soil types, including deep sand
- It is suitable for warm areas.
- It produces vigorous, well-bearing trees that are highly drought tolerant.
- It is susceptible to Phytophthora and citrus nematodes.
- Sweet orange (C. sinensis)
- This rootstock can be budded with oranges, mandarins and grapefruits
- Can produce large and vigorous trees and is suitable for light to medium well drained soils.
- Produces good quality fruits
- The trees are long-lived.
- It has low drought tolerance and is very susceptible to Phytophthoraspp and citrus nematodes.
- Sour orange (C. aurantium)
- It can be budded with oranges and grapefruits
- It produces very good quality fruits.
- It is an excellent rootstock in locations where citrus nematode and citrus tristeza virus is not a problem since it is very susceptible to these diseases.
- It is tolerant to poor drainage.
- It has low tolerance to drought.
- It is tolerant to Phytophthora
Raising seedlings for rootstocks
- Use seeds extracted from freshly picked fruits of the above varieties.
- Plant seeds immediately after extraction for better performance.
- Plant seeds in seedbed or containers (18x23cm).
- Seeds germinate in 2-3 weeks.
- Water the seeds regularly preferably twice a day until they germinate.
- Nurseries should only be established in areas below 800m asl.
- Plants should be budded when they are 15-20cm tall or 6-8 months after germination.
- Budding should be done during the warm months and not during cold and dry periods.
- Budded plants are ready for transplanting 4-6 months after budding. Plant in the field at the onset of rains
Transplanting of budded plants should be done in the field at the onset of rains. The spacing for the different citrus trees is:
- 6 x 6m for oranges and limes
- 7 x 7 m for Lemons
- 8 x 8m for grape fruits.
- When planting, the budding union of young trees should be set slightly raised than in the nursery to allow for settling so that the union is well above the ground.
- Use of a planting board will help in planting the trees at the correct height.
- The sub-soil should be used to make a basin around the trees after planting, to retain water.
- The hole should then be filled with the manure/top soil mixture, firming gently all around the roots.
- In dry planting, it is necessary to water the plant well and regularly. Some 35 litres of water per week is required. Subsequently mulch the young trees.
In areas with inadequate rainfall, irrigation is recommended especially during flowering and ripening. The irrigation water should be of good quality with low salt content. Water requirement peaks are between flowering and ripening, and lack of water during this period will cause flower and fruits abortion.
The reasons for cultivation are to control weeds which compete with trees for fertility and moisture, and to incorporate cover crops and bulky organic materials into the soil. However, cultivation destroys surface roots, affects soils structure unfavorably and causes water to penetrate less readily. Shallow tillage or use of herbicides is recommended for weed control. With the soil bare and firm, more heat is absorbed and radiated, warming the orchards on cold nights. Fruit quality improves and sometimes better yields are obtained.
Intercropping in banana can be done before the banana canopy closes using the inter row space during the first months of establishment. This space can be used for with annual crops e.g. legumes such as groundnuts, vegetables, maize. The intercrop should be planted before or at the same time as bananas. Intercrop should not be planted too close to the banana plant.
Citrus is intolerant to strong dry winds and if a wind break is not provided, die back of the top branches may occur. Suitable wind breaks reduce damage to trees and fruits in areas subjected to occasionally strong winds. A wind break provides lateral protection at orchard tree level over a distance of about 4-6 times its height. The windbreak should be planted as near as possible and at right angles to damaging winds.
- Maintain a single stem up to a height of 80 cm
- Remove all side branches below this height (All side shoots growing from the tree trunk below the main branches and on the rootstocks should be removed).
- Pinch or break the top branches at a height of 90 cm to encourage side branching
- Allow 3-5 shoots to develop as branches and select 3-4 of the strong branches growing outwards to form the framework.
- Remove any other side branches, including those growing inwards.
- Ensure all the diseased and dead branches are removed regularly.
- Prop-up any sagging branches.
Apply manure at the rate of 1-2 debes (20-40kg) per tree during planting
The following rates of Nitrogen application per tree are recommended:
- 1st year 100gms
- 2nd year 250gms
- 3rd year 400gms
- 4th year 550gms
- 5th year 700gms
A subsequent yearly application of 1.2kg nitrogen in mature orchards will maintain a high level of production. The nitrogen quantities should be split and applied 1-2 weeks after the onset of the long and short rains.
Phosphorous should be applied as single superphosphate from the 3rd year. The following rates per tree are recommended:
- 3rd year 250g,
- 4-5th year 600g;
- 0-1.5kg per tree in subsequent years.
Potassium is important as a fruit sweetener. Applications of up to 600-750g, of potassium per year for a mature tree in split applications will improve fruit quality if there is deficiency. Soil testing will show whether a potassium application is necessary.
Trace elements deficiencies
Deficiencies of one or more minor elements may occur in citrus orchards. This is mainly seen in different degrees and patterns of leaf discoloration. Accurate diagnosis can be made and confirmed at recognized analytical laboratories to which leaf samples of suspected trees may be sent.
Symptoms of Nutrient/ element Deficiency
||pale yellow to old ivory
May produce abundant bloom. Flower buds may fall without opening
||Reduced crop. Large, puffy bumpy surface around, enlarged core cavity and thick rind
||Yellow, mottling along margins developing a green wedge “Christmas tree” pattern.
Eventually complete yellowing and defoliation
||Yellow veins; remain green until final stage of general chrolosis. Reduced size
||Eventually reduced growth
||Mottled yellow between main veins.
Small, narrow, early fall.
|Reduced crop, some
pale yellow off-type
||Normal green along the main veins and the other parts are pale green to light yellow.
||Eventually reduced growth
||Old leaves curl and lose their green colour
||Small, smooth thin rind.
||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 and ‘cabbage
Pests and Diseases
Periodic, careful orchard inspection is essential to check on plant disorders and infestation.
|Citrus Aphid (Toxoptera citricidus)
(Black Citrus Aphid) and (Brown Citrus Aphid) (Toxoptera aurantii)
|Clusters of small black or brown soft bodied insects found on young tender shoots and underside of leaves.
They cause growth distortion by sucking plant sap.
Honey dew and sooty mould is usually present.
Citrus also aphids transmit Tristeza and other viral diseases.
They feed on new growth and blossoms, thus high numbers are found on the leaf surface during flushing.
Stems of young shoots die back
|Natural enemies such as lady bird
Use recommended insecticide such as Bulldock Star, Calypso sc 480, Decis Tab, ALFIX 10 EC
(not on Lemon and or Saville orange)
|Citrus Psyllid (Trioza erytreae
||The nymphs are small brown scale like insects which settle on the underside of the leaves. They cause pits on the underside and raised bumps on the upper side of the leaf.
The leaf blades are cupped or otherwise distorted and yellowish in colour.
The psyllid is important as a vector of the greening disease.
Symptoms of psyllid attack on leaves; lower surface of leaves covered by waxy fine threads like cotton wool.
Galls are formed on the upper surface of leaves.
|Rogue infected plants
Water and fertilize trees in early stages.
Remove other Rutaceae e.g. Clausena anisate, Fagar capensis and vepris indulata which are wild hosts.
Use recommended insecticide such as Decis Tab , Bulldock star, Calypso sc 480, Actara 25WG, ALFIX 10 EC.
(not on Lemon and or Saville orange)
|Systates weevil (Systates spp)
||Defoliate leaves in nurseries
||Effectively controlled by natural enemies. Use of chemicals such as Actara 25 Wg, Confidor 70
|Citrus Black Fly (Aleurocanthus woglumi)
||Groups of shiny black scale-like insects surrounded by a white fringe of wax, found on the underside of the leaves
||Effectively controlled by natural enemies. Chemicals used for control of aphids as mentioned in aphids control.
Soft green scale (Coccus viridiis or Coccus alpinus)
Soft brown scale
|Scales are small motionless insects that seem to be fixed on the plant.
There are many species of scales on citrus which differ in shape and colour.
There are two main groups: hard (armoured) and soft (naked) scales.
They suck sap mainly on leaves and branches resulting in discolouration of foliage and branch die back.
Soft scales often secrete honey-dew inducing growth of sooty mould.
Fruits and leaves are then coated heavily with sooty mould turning them black
|Use recommended insecticide such as Basudin 600 Ew
|False codling moth (Cryptophlebia leucotreta)
|Difficult to control as eggs are laid throughout the fruiting period and it has a wide host range. The eggs are deposited on leaves and the fruits, hatching into caterpillars which are white and later turn pink. They penetrate into the pulp causing a yellow patch on fruit skin around the entry point
||All infested fruits both fallen and on trees, should be destroyed by burying not less than 50cm deep.
Avoid use of broad spectrum insecticides to allow egg parasitism by Trichogrammatoidea spp
Control as in Aphids
Orange dog (Papilio
|The adult lays whitish/grey eggs mainly on the underside of older leaves.
The larvae if disturbed show a bright, yellow orange, V-shaped organ from behind the head, which produces a strong smell. In the nurseries, their feeding can cause complete defoliation.
|Hand picking of the caterpillars and destruction of the eggs usually provides satisfactory control on small trees provided the plants are checked regularly
|Fruit Flies (Ceratitis cap itata) and
|The female fly lays eggs within the skin of ripening fruits.
Spots develop on the skin where eggs were laid and on hatching the larvae enter the fruit.
On immature fruits, the spots are yellow or brownish, but in older fruits soft rotting follows fruiting stage.
The attacked area becomes soft, turns brown and decays.
|Destroy all infested fruits
Spray alternative rows with plus protein hydrolysate during the fruiting stage.
|Citrus Bud mite(Aceria sheldoni)
|Are normally found under the bracts of leaf-axil buds and the bases of petioles and constantly spread to new growth.
The damage by this mite causes malformed twigs, leaves, flowers, fruits, and multiple bud formation.
|Numerous natural enemies such as predatory thrips, lady birds and predatory mites normally attack the mites.
Use recommended miticides such as Kelthane, Dynamec 1.8EC, Mitac EC, Binapacryl, as soon as the first sign of the attack is seen.
|Citrus Rust mite
|Lemon fruits attacked acquire silver colour while oranges and grape fruit acquire russet-colour. The skins of attacked fruits are thicker than usual
||As for citrus bud mite.
|Attacks young leaves and causes chlorosis and deformation of leaves. Attacked green fruits become bronze to dark brown. Infestation on ripe fruits causes yellow or silver discoloration
||As for citrus bud mite.
|Citrus thrips (Scirtothrip saurantii)
|Damage is caused by larvae and adults, which feed on young fruits producing brown blemishes on the rind.
If infestation is severe, the leaves are deformed.
Young twigs are retarded and misshapen.
Young leaves are underdeveloped and drop when touched.
|Thrips should be controlled with selective insecticides to avoid increase of other pest
Control as in Aphids
|Citrus woolly whitefly (Aleuroth
| The eggs are laid in spiral and circle on the lower surface of young leaves.
The young stages that resemble soft scale insects are covered with curly waxy filaments which give them a woolly appearance.
They produce large amounts of honey-dew which leads to the growth of thick layers of sooty mould, which cover the infested trees.
This leads to defoliation, loss of fruits and dwarfing of the trees. Small mottled fruits are produced on affected plants
|Keep the tree healthy by adding farmyard manure or fertilizer once at the onset of rains as follows.
-Apply 2 debes of farmyard manure per tree
-Apply DAP fertilizer:
i) For 1-2 year old trees, tree (200g per tree)
ii) For 2- 3 year apply 60 soda bottle tops per tree (300g)
iii) For trees over 3 year old, apply 500g per tree
Remove poorly placed branches since control is easier in pruned trees
Prune and apply farmyard manure or fertilizer at the onset of rains.
Chemical control as in aphids, ensure that you spray the underside of the leaves
|Stem-Pitting caused by
Tristeza virus (localised phloem tissues)
|The disease is graft transmissible.
This virus is found on young or old citrus grown on susceptible rootstocks of sour orange and grape fruit.
A stem-pitting (honey comb) pattern is observed when a small strip of the bark is removed just below the bud onion.
|Use healthy planting material
Use tolerant rootstocks e.g. Citranges, Citrumelo or Cleopatra Mandalin.
|Citrus scab(Elsinoe fawcetti)
|Yellow to orange corky spots occurring singly or grouped on the underside of leaves especially on rough lemon.
Infected leaves may become crinkled and rolled.
Similar pattern of spotting is observed on fruits and twigs.
|Use recommended such as Aliettte 80 WP, Amister 250 SC,
|Green mould(Penicillium dignitatum)
|Common disease of stored fruits although it may be observed in the orchard, mainly on fallen fruits
||When harvesting care should be taken in handling the fruits
|Leaf spot & fruits spot cause; fungus (Phaeoram angolensis
||On leaves, this is observed as circular, mostly solitary spots with a yellow halo. Disease is common at high altitude.
On young fruits, brown necrotic lesions form. These are usually circular, slightly sunken, with a surrounding ring of raised epicarp, giving the fruit a blistered appearance. During wet weather, the lesions produce spores and become black. Lesions on mature fruit are normally flat but sometimes have a slightly sunken brown centre. Diseased fruits ripen prematurely and drop or dry up on the tree.
|Use recommended fungicide e.g. Captan, Ranko 75WP, Rankonil 500SC, Merpan 83WP, Rodazim SC, Bavistin, Kocide DF, Cobox, copper nodox, Delan etc
|Root Rot & Gummosis cause: fungi (Phytopthora spp)
|Root rot causes a slow decline of the scion tip. The foliage turns yellow and drops while twigs die-back.
Gummosis is a rotting of bark anywhere on the tree. An early symptom is the sap oozing from small cracks in the infected bark giving the tree a bleeding appearance
|· Treat citrus seeds with hot water at 50° C for 10 minutes (just too warm to keep a finger in for any amount of time).
· Drench soil with recommended fungicide e.g. Allieta 80 WP
· Use tolerant or resistant rootstocks. Trifoliate orange is resistant. Swingle citrumelo, sour orange, rough lemon, and citranges (Carrizo and Troyer) are tolerant.
· Bud seedlings at a height of 25 cm above the soil, to keep the bud union well above ground level.
· Avoid transplanting on heavy or poorly drained soils.
· Do not heap soil around the tree base.
· Avoid basin and flood irrigation.
· Do not over irrigate and ensure water does not contact the bud union.
· Avoid injuries to roots and trunks when cultivating.
· Gummosis can be halted by bark surgery before 50% of the trunk is affected.
· Scrape away dead bark tissue, remove about 10 mm margin of healthy tissue and paint the wound with slurry of copper based fungicide (allowed under organic production)
· Do not replant citrus into planting sites where other citrus has been grown and proven unhealthy.
|Citrus greening caused by the bacterium (Candidatus Liberibacter africanus
|The disease is transmitted by citrus psyllids (Trioza erytreae) and through use of grafting materials obtained from diseased trees.
The disease is neither soil-borne nor seed-borne and is not mechanically transmitted.
Optimum temperature for symptom expression is 21 to 24°C; symptoms are masked above 26° C.
The disease is especially destructive to sweet oranges and mandarins but less severe on lemon, grapefruit, citron and West Indian lime.
Rootstocks have no effect on greening disease.
It can cause stunting, leaf and fruit drop, and twig die-back.
Usually, only one of a few branches is affected while the rest of tree appears normal.
Affected fruits are small, poor quality, lopsided, start to colour from the fruit stalk instead of the stylar end and drop prematurely.
There is interveinal chlorosis and mottling of leaves.
Out of season blooming frequently occurs.
|Use clean planting material
Control of citrus psyllid in nurseries and field
Remove tree with more than 50% infection from field.
Prune greening infected branches back to main trunk with less than 50% infection.
Nurseries should be established only in areas below 800m in altitude
|There are three anthracnose diseases of citrus caused by Colletotrichum spp.
i) Post bloom fruit drop
Description: C. acutatum infects petals and produces water-soaked lesions that eventually turn pink and then orange brown as the fungus produces spores. Infected fruitlets abscise at the base of the ovary, and the floral disk, calyx, and peduncle remain attached to the tree, forming structures commonly referred to as ‘buttons’. Leaves surrounding an affected inflorescence are usually small, chlorotic, and twisted and have enlarged veins. Warm, wet weather favours disease development.
ii) Lime anthracnose
Description: It affects only Mexican lime. It attacks flowers, young leaves, young shoots and fruits. Infected fruit develop abscise, and ‘buttons’ are produced as in post-bloom fruit drop. In severe cases, young leaves become totally blighted and drop, and shoot tips die-back, producing wither tip symptoms. The fruit lesions are often large and deep, and cause fruit distortion. The disease is favoured by warm, wet weather.
iii) Rind blemish on fruit
Description: The disease is caused by C. gloeosporioides. It is particularly severe on grapefruits.
The blemish appears as a superficial, reddish brown discolouration, often in the form of tear stains that usually appear following prolonged light rains in warm weather.
|Avoid overhead irrigation
Prune of dead tree tissues
Use recommended fungicides e.g. Captan, Ranko 75WP, Rankonil 500SC, Merpan 83WP, Rodazim SC, Bavistin, Kocide DF, Cobox, copper nodox, Delan
Scales and ladybird larvae on orange Damage by the citrus rust mite on an orange fruit
Citrus bud mite Mealy bugs on citrus
Citrus woolly whitefly Citrus blackflies
Citrus aphids Citrus psyllid
Greening disease Citrus thrips
Fruit flies Fruit and leaf spot
Citrus tristeza virus Anthracnose on leaves
Gummosis disease Anthracnose on fruit
The first economic crop can be expected 3-4 years after transplanting. There are usually two citrus picking seasons each year in Kenya; June-July and December-January. Yields vary according to the variety and the following yield ranges per tree may be obtained.
- Oranges 90-130kg.
- Lemons 130-180kg
- Grape fruit 180-250kg
- Limes 100-120kg
Oranges, grapefruits, tangerines and lemons are picked when fully mature and when uniform orange colour is observed. In areas where the fruit remains green, it is necessary to test a few for ripeness. The fruit is harvested individually either by clipping off the stalk or by bending and pulling with a light twist. Care should be taken not to injure or bruise fruit as that leads to rapid spoilage. Limes are picked when green.
Post harvest management
Oranges and grape fruits are graded according to fruit diameter (using a roller grader) into three respective sizes as follows:-
Limes and lemons for export should be clean and free from dirt, traces of pesticides and blemishes. They should be of uniform size and weight in each container. The container should be lined with tissue paper below and above the layer of the produce. The weight of each pack should not be more than 6kg.
Many citrus fruits, such as oranges, tangerine and grapefruits are eaten fresh. Lemonade or limeade is popular for beverages prepared by diluting the juices of lemon and lime and adding sugar. Lemons and limes are also used as garnishes or in cooked dishes. Their juice is used as an ingredient in a variety of dishes and is commonly found in salad dressings and squeezed over cooked meat or vegetables. Marmalade is a condiment derived from cooked orange and lemon, and is usually sweetened to cut the bitterness and produce a jam. Lemon or lime is commonly used as a garnish for water, soft drinks, or cocktails. Citrus juices, rinds, or slices are used in a variety of mixed drinks.
Oranges are mainly sold in the local market for fresh consumption. Grape fruit and lemons are bought by the canning factories for making marmalade and some are sold for fresh consumption. Limes are mainly exported although substantial quantities are used locally for domestic purposes