CITRUS PRODUCTION
The genus citrus comprises several species but the most important one are:
- Citrus aurantium - Sour orange
- Citrus sinensis - Sweet orange e. g W. Navel, Valencia, Kuno Navel
- Citrus Reticulata - Mandarin
- Citrus Paradisi - Grape fruit e. g Star ruby, red blush
- Citrus aurantifolia - Mexican lime
- Citrus latifolia - Tahiti lime
- Citrus Limon - Lemon
- Poncirus trifoliate - Trifoliate orange.
Citrus has its origin in Southern China Northern East India and South East Asia. The first introductions of citrus to Uganda were in 1900. Budded citrus (budlings) were imported into Uganda in 1929 from South Africa, Jamaica and Australia. Citrus bud wood was later imported from Florida, USA and some from Israel-which bud wood was used to establish the Kiige, Odia and Ongom Citrus schemes.
Current situation.
Citrus bud wood was imported into Uganda from Spain, Hawaii and Florida in 1987 by FAO. These included Washington Navel, Hamlin, Valencia, Tangelo, King Star ruby and Red blush. Eureka, Lisbon Meyer, and Villafranca were among the lemons introduced. Tahiti lime and Mexican lime were the two lines introduced. Other introductions (Nova, Minneola, Lamati and Kuno navel) were from South Africa in 1998.Introductions made in 1987 and 1998 were all planted out at KARI, evaluated and later released out to farmers.
Most recent introductions were brought in from Kenya in 2003. These were also evaluated and have been planted out for further evaluation.
Climatic requirements
Climate is one of the most important factors affecting the profitability of Citrus orchard. Temperature, rainfall and wind are the most important components of the climate affecting citrus production. Extremely hot or extremely cold temperatures are damaging to Citrus.
Citrus requires temperature ranges from 130 C–380 C. At high temperatures flowers and leaves drop.
For good citrus production, trees will require rainfall or irrigation through out the year. Where dry seasons occur, planning for irrigation is required. Favorable rain fall ranges between 125mm-500mm but are grown in areas receiving as much as 1000-1500mm of rainfall/annum.
A dry, hot at day cool at night climate favours good color development.
Soil requirements
Citrus trees grow in a wide variety of soils but they grow best in soils of medium texture and moderate depth, with good drainage and high fertility, and which are not high in soluble salts or alkaline. The best PH for citrus ranges between 5 and 6. In too acid soils, citrus roots do not grow well and nutrients are leached out or may be toxic. At a PH above 6, plants will develop deficiency. The water table must be well below the surface at least 5 feet.
Propagation
The best citrus commercial varieties do not reproduce true by seed; they must be propagated by budding or grafting.
These commercial varieties are budded/ grafted onto seedlings of species known to give good results when used as rootstock. It is highly important to use rootstocks known to grow and are adapted to the soil conditions on which the tree are to be grown .
Budding is by far the most generally used method of propagation. Grafting apparently gives just as good results, but it is slower and requires more scions as compared to budding.
The seed commonly used for raising citrus rootstock is rough lemon.
Raising citrus rootstock
Seed for rootstock is obtained from fresh fruit, which is picked from the tree. Select mature lemon fruits from healthy plants. The easiest way to extract seed is to cut shallowly through the rind all the way around the fruit and then twist the fruit a part. The pulp is then squeezed through a sieve and the seeds washed. Seeds are sown immediately after washing.
If seeds are to be stored, the surface of the seeds should be rapidly dried after washing to prevent drying of the inside. Seeds can then be treated with a seed fungicide e. g thiram. Seeds are then placed in plastic bags, sealed off and kept at temperatures between 6 0C and 10 0 C. Seeds stored under these conditions will survive up to six or eight months.
Sowing/ Planting seed
Seeds are planted in containers or more commonly in seedbeds. The soil should be at least 30cm deep, light in texture, uniform and well drained.
If only a few seedlings are to be grown, the seeds may be planted in boxes of convenient size. The boxes should be put in a warm, partly shaded place, and the soil should be kept moist but not wet.
If large numbers of seedlings are to be grown, seedbeds must be prepared. It is desirable to use a virgin soil, which has not for several years been used for growing citrus seedlings or vegetables.
Select a site at some distance away from other citrus seedlings in order to reduce the danger of infestation from pests.
Citrus seedlings are planted 3 – 5 cm deep and covered with sand or other porous materials, which will not compact. The seed if not freshly extracted from the fruits is often placed in moderately warm water and allow to soak for 24 or more before planting to hasten the process of the germination and to prevent the problem of goose neck in the seedlings.
Depending on the moisture content of the soil and the temperatures, seed will germinate in three to four weeks.
Care in the bed:
Before and after seedling emergency, the soil should be given frequent light watering to keep the surface moist. When the plants are 10 cm. high, the frequency of watering is decreased but the quantity increased.
Keep seedlings weed free. This promotes quick growth and vigour.
Pest and disease control. Dumping off is the most serious disease. Wet soil, dense shading, and humid environment and over crowding are conditions favorable to dumping off and hence should be avoided. Pests include; aphids, leaf miners and orange dogs.
Thinning. If seedlings appear congested, thin them so as to produce sturdy and vigorous plants. Thinning also reduces chances of disease outbreak.
Potting and picking:
Seedlings are ready for picking when they have 2 pairs of leaves and a bud
Grafting/Budding:
Citrus rootstocks are ready for grafting 6 – 7 months from pricking. If budding is to be done, seedlings are budded 9 –10 months from pricking.
T budding is the technique commonly used in propagating most citrus seedlings. Whip or splice grafting are also used.
Field establishment and management:
Dig holes 60 cm deep x 60cm wide. When digging separate topsoil from sub soil. Mix 1 tin full of well decomposed manure with top soil and put back into the hole to cover the first 30cm. Remove polythene sleeves before planting. Transplant at the start of or during the rain period. Make a basin around the plants for harvesting water. Mulch around each plant and provide a cage to avoid damage by animals. Citrus are spaced 6mx 6m giving 126 plants/acre.
Management in the field
Remove grafting tape a month after transplanting. If this tape is not removed it will cut into the plant tissues, retard growth and eventually kill the scion part of the plant.
.Remove shoots which grow below the graft union. These shoots if not removed will retard growth or kill the scion part of the grafted plant. Plants will also take long time to produce fruits, as the shoots below the graft union will take up most nutrients.
Weeding.
Farmers are advised to keep their fruit fields weed free. Ring weed around the trees and slash the rest of the orchard. At the young stage of the crop, intercrop with lowly growing crops like beans, ground nuts. The inter crop should be planted away from the basin around the plant. Keep the plants and the inter crop weed free.
Manure application. This is important for:
-faster growth
-helping to fend off some diseases and other stresses
-to build productive capacity
Mulching- Helps in conserving moisture and suppressing weeds. It is advisable to mulch the plants immediately after transplanting. Mulch should be added especially after every weeding and manure application. This encourages continous growth in plants and brings forth good out put. Place the mulch at least 1 foot away from the plant
Pruning:-It consists of the removal of dead, dry wood and water sprouts. All branches low on the trunk are also removed. Shoots below the graft / bad union are should also be removed. To have a tree, which is easy, to manage and to encourage lateral growth the apex is decapitated at 50 cm from the ground.
Maintain a basin around the plant-This basin is important especially in the districts that experience prolonged periods of harsh dry weather. Its purpose is to harvest and hold water for the plant so that plants have enough water for production.
Irrigation:
Many farmers in Uganda cannot afford irrigation equipment so they should use cultural practices such as mulching, use of organic manure and maintaining a basin around each plant, to conserve water. There are irrigation methods that are relatively cheap. An example is filling jerrycans with water and perforating them at the bottom and in the lids. A jerrycan is placed near the stem of each plant. Water will be dripping continuously into the soil around the plant. When the water is finished in the jerry can, it should be refilled.
Harvesting: Citrus stays on the tree from 6 – 9 months depending on the variety and climatic conditions. Maturity of citrus is indicated by color changes in the rind.
However, in the low land tropics, fruit may remain green at maturity. Rind color changes from dark green to light green. Though the lack of colouring decreases the attractiveness of the fruit, flavour is un affected.
As long as an orange fruit hangs on the tree, the brix continues to rise. But if the fruit remains too long on the tree, it becomes over ripe, unpleasantly sweet and is not acceptable for processing or export.
Citrus fruits do not contain starch and therefore must not be picked in the hard green state as it will remain in the same state.
Citrus fruits are harvested by clipping or pulling fruits from the tree. Harvested fruits should be protected from sun and rain. Damaged fruits should be discarded or used immediately.
Avoid picking citrus shortly after rains, wait until the fruit is dry. This is to prevent mould.
Yield- 7-15 tons/ha. It is affected by variety, climate and soils, management and age of the tree.
Citrus pests and diseases:
Pests:
1 Leaf miner Very serious pest of citrus right from the nursery to the field. The adult is a moth it deposits tiny eggs on the lower surface of the leaf. Upon hatching, the larva enters the leaf where it remains until mature; it then comes out of the leaf and forms a pupation cell by folding over a part of the leaf margin. The larvae some times mine in the green bark of the young growth but injury is mainly caused when they mine in the leaf between the upper and lower surfaces. The serpentine mine produced is characteristic and conspicuous. As a result of the leaf mining, the leaves curl some what as they do from the effects of attack by aphids.
Control: The pest is resistant to most available insecticides. Bulldock alternated with Folimat are the only pesticides that can control leaf miners effectively.
Citrus psyllid- common but minor pest of mature citrus. Serious pest in the cooler areas of the tropics. The adult is an aphid – like insect about 2 mm long, with transparent wings almost twice the length of the body.
Damage: the leaves are conspicuously pitted, the pits opening of the lower leaf surface. In severe attacks comer, young leaf blades are cupped or otherwise distorted and yellow in color. Leaves with many picks tend to curl upwards-citrus psyllid causes a serious problem by transmitting citrus greening disease.
Control-Spray young trees with dimethoiate as a full- cover spray taking particular care to wet the flush leaves. Treatment of mature trees is not usually economical unless the greening disease is present.
2. Aphids –these are shinny black or brown insects and may be winged or wingless.
The main injuries caused by aphids on citrus are:
- severe curling and deformation of young leaves and stunted growth of leaves and twigs. There are clusters of black and brown and brown aphids on flush foliage. Some branches and twigs show die back symptoms.
impairement of leaf functioning from the presence of sooty mould fungus, which grows in the copious quantities of honey dew excreted by the aphids.
Control- Sprays should only be applied in periods of flush growth and before populations build up( before much curling of leaves). Spray using Dimethoate. Some contact pesticides like Dursban may be used during heavy infestation.
3. Orange dog. Adult is a butterfly, dark brown with numerous pale yellow markings. The caterpillar has 5 instars; all stages are harmful. The first 3 are dark brown with white markings and resemble bird droppings; the fourth and fifth instars are pale green caterpillars with black, brown and grey markings. If the caterpillars are disturbed they shoot out a pink Y-shaped organ from just behind the head.
Control
- hand collection of caterpillars is often effective on young trees.
- Where infestation is high, spray using Dimethoate.
Other pests include:
• Citrus wooly white fly
• Scales
• Thrips
• Mites
CITRUS DISEASES
Damping off
It is a worldwide problem, which can affect recently germinated seedlings of any citrus cultivars.
Symptoms
The typical damping off symptom results from fungal penetrations just above the soil level. However damping off fungi can also cause a seed rot or a pre emergence rot, resulting in sparse stands of seedlings. It is caused mostly by rhizoctonia solani. Also phytophthora spp and pythium spp
Control
- Avoid planting in infected fields.
- Plant in soil less media.
- Fumigating soil with methyl bromide
- Hot water treatment of seed.
- Avoid over watering seedbeds
- Contaminated soil, tools or irrigation water should not be used in or near seedbeds.
However one method for controlling damping off in the nursery, and has been tried with success involves the following.
A media for planting seed in the bed is constituted using manure and sand at the ratio of 1:2 respectively. The sand ratio is twice the ratio of manure .The aim is to increase aeration to improve on drainage in the bed.
Poor drainage enhances damping off.
The media is set to make a bed, and a 2-inch depth of sand is spread on the surface of the bed.
Seed is planted in the sand zone and when it germinates, it plugs its roots into the sand manure mixture. But the stem will remain in the sand zone. Damping off attacks and kills seedlings at the stem zone. Sand on the surface will help to drain away water from the stem zone and thus creating unfavorable conditions for damping off at the stem zone.
Greening Disease
Greening is just one of the names given to a disease of citrus and is the name by which
the disease is best known. The name has been given unto it on the basis of one of the fruit symptoms i.e the failure of affected oranges to completely change color , leaving patches of green peel. A rather better name for it , but no longer widely accepted, is yellow branch, which clearly illustrates a major symptom, that of sectorial infection.
Most typically the disease is seen as one or more branches of the tree (sectorial infection) looking, from a distance, somewhat yellow and having a spiky appearance.
A more typical leaf symptom, but apparently seasonal, is a leaf mottling, a blend of light and dark green patches on the leaf which are not bounded by the veins. Leaves on affected branches usually abscise readily so that severely affected trees can easily be seen through because of the reduced foliage.
Flowering and flushing occur out of season so that at times only affected branches are in flower. The fruits on affected branches, if any set at all, is small and deformed.
Even the maturing fruit colors are different. Healthy oranges start to change color from the free end whist,while greening affected fruits start changing from the stalk end. The green patches do not change color even after picking.
With time a number of affected branches on the tree increases until the whole tree is affected. The affected tree will be totally unproductive! The Asian strain is able to kill trees.
If affected bud wood is used for grafting, or the disease affects the rootstock or young budded plant, then the young tree is systematically affected. In such instances the sectorial infection seen in older trees is not seen and the whole young tree will be yellow and spiky.
The symptoms may take up to two years to fully develop but are generally much faster in higher areas.
The overall nutritional status of the tree also contributes to the severity of symptoms. Many of the symptoms of greening are induced zinc deficiency symptoms. If zinc is sprayed onto the foliage some of the symptoms will temporarily disappear but this is in no way a control.
Causual Organism-The mycoplasma –like bodies transmitted by psyllids.
Control
• Preventing introduction.
• Prevent its spread
• Reduce existing infection.
Preventing Introduction
Greening can be introduced into an orchard with infected plant material or through the spread of psyllids from neighbouring infected orchards. The preventive action is based on good nursery management and selection of healthy plants. Bud wood should not be obtained from trees showing signs of infection.
To produce healthy nursery material therefore requires that only healthy bud wood be used and that the seedlings and plants be protected at all times.
The usually recommended pesticide for psyllid control is Dimethoate (Rogor, Perfekthion,Roxion), to be applied at the rate of 4ml/litre. Great care must be takenwith dimethoate in nurseries as the foliage of rough lemon in highly sensitive to it.
Because of abslue necessity of keeping the nursery stock free of these insects, monthly applications are recommended.
Preventing spread
One of the crucial aspects of greening control is the prevention of spread by vectors. Many pesticides can be used for psyllid control in the orchard, and so far Dimethoate stands as number one preference. In citrus more than any other crop, the principle of integrated pest control is accepted and practiced. Broadly IPM means that the pest complex is managed in such a way that all suitable and locally applicable control tactics are utilized. In most instancesthis means that pest populations should be reduced to sub-economic levels, not necessarily to be eradicated. In the case of psyllids, however, almost total control has to be the aim
Reducing existing infection
It must be stressed that only in very exceptional circumstances is there a recommendation to uproot and this almost always refers to very young trees which are systematically affected. In such cases any inputs are wasted as the trees are impossible to treat and will never fruit, the tree serving solely as an infection source.
In such instances the tree should be removed and replaced by a healthy tree, and protected from psyllids.
The soil is completely unaffected by greening and replanting presents no problem.
In orchards with a very low level of pysillid activity and where only isolated branches are infected, the branches can be removed with a saw, the cut being made as close to the trunk as possible. With very severely affected trees a method used successfully elsewhere has been the complete pruning of the tops of trees.
Sooty mould
It is a black removable fungal growth, mostly commonly of capnodium citri, which appears on the surface of leaves, stems, and fruit after trees become infected with honeydew excreting insects.
The mould deposits are always heavier on the upper side of the leaf than on the lower surface of leaves.
The mould deposits may delay fruit coloring and can be difficult to remove in the packinghouse from cultivars with a rough rind.
Honeydew excreting insects include aphids, soft scales mealy bugs and white flies.
When sooty mould is serious enough to potentially affect fruit yield or quality, the insects responsible for honeydew should be controlled.
Application of Dithane to loosen the mould deposits thereby facilitating their removal by wind and rain. However addition of Dithane and insecticide to kill honey-dew producers would be more desirable.
Melanose and stem end rot
Caused by diaporhe citri (Ascomycete)
Grape fruit is the most susceptible.
Symptoms
Numerous small raised, small to black pustules are scattered over the leaves.
These start as sunken chlorotic specks on young leaves, shoots and fruits.
Severe infection may result in leaf defoliation, chlorosis and defoliation. Badly diseased shoots may die back.
Diseased fruits show a surface russetting, which may occur in patterns running down the fruit and cracked crusted areas.
Stem infection may start in the field as a brown discoloration of the penduncle, which may result in a premature fruit drop.
The pathogen remains dormant in the calyx until the fruit ripens. Then it invades the fruit causing brown discoloration of the peel with a progressive internal rot.
Transmission
Dead twigs bearing the pathogen provide the major source of the inoculum to initiate epidemics. Conidia are water borne and dispersed during wet weather to infect young susceptible leaves shoots and fruits. Mature tissues are infected through wounds.
The disease is prevelant on mature trees because of the greater amount of inoculum on dead twigs.
Control
Removal of dead branches and twigs and debris beneath the trees.
Application of fungicide spraya especially to new flushes and petal fall will give adequate control.
Copper fungicides have been used to control the post harvest fruit rot stage.
Citrus scab
Cause : Elsinoe fawettii
Common hosts: Lemon, grape fruit, sour orange, manderine,tangerine. Sweet orange, and lime are resistant to this disease.
Symptoms.
Initial infection produces small semi transluscent spots which become raised and edvelop a creamy yellow colour. They become suberised , warty and cracked to reproduce the characteristic scab.
Minute dark fruiting bodies of the fungus can be seen embedded in the scab.
Only young shoots are susceptible but infection can occur on leaves , twigs and fruits causing distorted groqth with shedding of diseased leaves and fruit if the disease is severe. Scabby fruits cannot be marketed and damage to nursery stock is another important aspect of the disease.
Transmission
Spots are mostly water borne and require water or high humidity for their production and germination. The disease is consequently most prevalent in wet areas . Flushes of growth during warm wet weather are most susceptible. The main source of inoculum for initial attack is scab pn trees and a crop debris beneath
Control
Chemical control in orchards where it is prevalent and where susceptible spp are grown and apply captafol and Thiophamate.
Crop sanitation involving pruning out badly diseased plant parts and destroying crop debris.
Phytosanitary measures are particularly important at the nursery.
Canker
Caused by strains of the bacterium xanthomonas campestris .It is leaf spotting and rind- blemishing disease. When conditions are favorable for infection, it also causes defoliation and fruit drop. The internal quality of the fruits that remain on the tree until maturity is not affected, but they have reduced marketability as fresh fruits.
Grapefruit, lime and trifoliate are highly susceptible. Sour orange lemons and sweet orange are moderately susceptible. Mandarins are moderately resistant.
Symptoms
Lesions start as pinpoint spots, initially circular but later may develop irregularity. They are often aggregated at the leaf margin or leaf tips or in a restricted area of the leaf. All lesions on the leaf tend to be about the same size. On the fruit the lesions can vary in size because the rind is susceptible for a long time and further infection circles can occur. A characteristic symptom of the disease on leaves is the yellow halo that surrounds lesions. However other spots such as those caused by some fungal diseases and abiotic factors can also be surrounded by a yellow tissue. The distinguishing factor is that the yellow halo tends to disappear as canker lesions age. A more reliable diagnostic symptom of canker is the water soaked margin that develops around the necrotic tissue.
Control
- Planting resistant varieties e.g. Valencia and mandarin
- Apply strict quarantine, inspection of nurseries and orchards
- Cultural practices e.g manuring, pruning are essential