CITRUS AGRONOMYBYDennis Owusu Boateng (Technical officer)BUNSO COCOA COLLEGECSSVD CONTROL UNIT (COCOBOD)
Introduction• Botanical name for sweet orange is citrus senensis• Citrus, common name for several related evergreen trees and shrubs of the Rue family, and generally for the fruits they produce, including the citron, grapefruit, lemon, lime, orange, shaddock, tangerine, and bergamot (A pear shaped orange).• The plants are characterized by wing like appendages on the leaf stalks, white or purplish flowers, and fruit with a spongy and a juicy pulp divided in sections.• The leaves, flowers, and rind of the fruit abound in volatile oil and emit a sharp fragrance. Many citrus plants have thorny branches.
Distribution♦ Citrus are subtropical, with growing temperatures ideally at 24-27 ºC and intolerant of frost.♦ Native to Southeast Asia♦ Site selection, always an important consideration, is particularly critical in regions subject to cold winter temperatures.
Botany♦ That is the biological description of a plant.♦ The citrus tree consists of Shoot, Leaves, Flowers, Fruits and Roots.
Flowers♦ Reproductive organ of most seed-bearing plants. Flowers carry out the multiple roles of sexual reproduction, seed development, and fruit production.♦ The flowers are solitary or in small corymbs, each flower 2–4 cm diameter, with five (rarely four) white petals and numerous stamens; they are often very strongly scented.
Leafs♦ A leaf is an extension of a plants branch.♦A plant leaf helps in transpiration and guttation, both of which remove excess water from the plant, and respiration, the process by which a plant obtains oxygen and energy.♦ Leaves also may store food and water and provide structural support.♦ These plants are large shrubs, reaching 5–15 m tall, with spiny shoots and alternately arranged evergreen leaves with an entire margin.
Fruits♦ The fruit is a hesperidium, a specialised berry, globose to elongated, 4–30 cm long and 4–20 cm diameter, with a leathery rind surrounding segments filled with pulp vesicles.♦ Citrus fruits are notable for their fragrance, partly due to flavonoids and limonoids contained in the rind, and most are juice-laden.♦ The juice contains a high quantity of citric acid giving them their characteristic sharp flavour.
Varieties• Citrus consists of many species, some of which are not even edible. Some varieties of the edible ones have however been recommended to the public.• These include: – Sweet Orange – Grapefruit – Tangerine – Tangelo – Lemon – Lime
Propagation♦ Citrus is almost entirely propagated by budding. Planting unbudded seedlings may not be true-to-type, and often takes a long time to fruit.♦ They are also susceptible to disease attack.♦ Obtain buding by selecting a required bud and uniting this with a suitable rootstock.♦ Most trees are propagated in the nursery by T- budding.
♦ ROOTSTOCKS♦ Traits most important in rootstock selection include: – compatibility with scion – adaptation to soil and climatic conditions – ease and uniformity in propagation – influence on precocity, consistent cropping and yield, and fruit quality – influence on winter hardiness – suckering tendency – sensitivity to disease, pests, and replant problems.
Selecting a site for Nursery♦ The site should be level.♦ It should be safe from erosion, floods or heavy winds—that is, you will need windbreaks.♦ It must be fenced to protect it against damage from animals.♦ It needs a permanent water supply.♦ It must be near a road for transport of inputs and plants and ready access by clients.♦ The soil used must be free of soil borne diseases and pests, such as nematodes.♦ Local construction materials, such as wood, bricks, poles and banana stems, should be available.♦ Labour to manage the nursery must be available.♦ Well-rotted farmyard manure, sand or decomposed sawdust, compost or fertile topsoil for soil mixture must be available.
Site selection and Preparation• Sandy loam is best for the cultivation of Citrus.• The most important factor in the choice of site is free from high drainage of water.• Soils with an underlying hardpan of about 1m or less to the top mock soils, or slain sites are not suitable. Apart from these, citrus can be grown on a wide range of soil types.• Citrus is not shade loving, therefore clear the site of all tree stumps and plough before setting in the plants.
♦ Citrus can develop a large number of nutrient deficiencies, including N, P, K, Mg, Ca, Mn, Zn, Cu, Fe, B and Mo.♦ The level of N fertility has more influence on the growth, yield, and quality of citrus than any other single plant nutrient.♦ Adequate supplies of N are necessary to optimize growth and development of newly planted citrus trees.♦ Compost, animal manure and green manure can be worked into the soil to a depth of 1 m, to serve as pre plant fertilizer.
Orchard Establishment• Plant the orchard when the rainy season has fully set in.• Clear the land of any bush.• Complete planting holes, well in advance, before purchasing the budding.• Always ensure that the trees are planted no deeper than they had grown in the nursery.• Citrus has a planting distance of 3m x 3m
How to Plant♦ Prepare the planting hole – Dig a hole only as large as necessary to accommodate the root system. – Trees should be planted with their top major roots even with the soil line. – Prune any damaged roots back beyond the damaged area. – If container-grown trees have a tap root curled in the bottom of a container, cut this root off at the point where it begins to curl.
– Separate and trim the roots of container trees that may be root-bound. – Save the soil from the hole to use as backfill.• Position the tree – Place the tree in the hole at the same depth it was growing previously.♦ Filling the hole – Remove any debris from the dug-up soil. Back-fill with the same soil that was removed from the hole. Never back-fill with an amended soil mix of a lighter texture. Such a practice will create drainage problems and cause tree roots to suffocate during periods of excessive moisture.
♦ Water – After the tree is planted, water well. Check the original soil line one last time. – If the tree does settle, now is the time to move it back to the correct position with the soil level .♦ Stake if needed – Unless the tree bends over, it will not need support from staking. – If stakes are needed, place them on opposite sides of the tree, perpendicular to the direction of the prevailing wind.
Maintenance Operations1. Training and Pruning (Nursery) – The tree must produce a canopy of sufficient size before economic fruit production begins.♦ What to do during training and pruning. – Remove shoots (i.e. suckers) originating from the rootstock and low on the scion (i.e. water sprouts) – Remove branches that cross from one side of the tree to the other.♦ This will produce an abundance of vegetative growth that will, in turn, assist in producing new vegetative growth.
2. Irrigation(Nursery) – Young trees seldom need watering in the fall. For young trees, it is best to water twice per week, from March through June is to maintain optimum moisture in the upper soil layer where most of the roots are, especially during the crucial period of leaf expansion, bloom, fruit set and fruit enlargement –(January/February to June, usually). – Soil type will affect how well the soil holds the water.
♦ Good irrigation help – Proper growth and fruiting it is essential that trees receive water in ‘on time’. – To ensure adequate moisture the soil should be thoroughly wet before wilting occurs. – To avoid overwatering, excess water must drain away. – Alternate wetting and drying allows oxygen necessary for root growth to enter the soil.
3. Fertilizing (Nursing) – Mineral nutrients are classified as macronutrients and micronutrients. – The term "macronutrients" refers to those elements that plants require in large amounts (N, P, K, Ca, Mg, S). – The term "micronutrients" (or “trace elements”) applies to plant nutrients that are essential to plants but are needed only in small amounts (Fe, Zn, Mn, B, Cu, Mo, Ni, Cl). – Visual deficiency symptoms of N, P, K, Ca, Mg, Fe, Zn, Mn, B, Cu, and Mo can usually be recognized by distinctive symptoms with lack of nutrients.
♦ Types of fertilizer to use: Generally, only two types of fertilizers are required: – A balanced fertilizer (8-8-8, 13-13-13), and – A nitrogen fertilizer such as ammonium nitrate (33-0-0), or ammonium sulfate (21-0-0). Urea (46- 0-0) is a good source of nitrogen. The nitrogen fertilizer stimulates vegetative growth later in the year. The numbers 8-8-8 or 13-13-13 represent the percentage of nitrogen-phosphorus-potassium the fertilizer has.
4. Weeds Removal (Nursing) – Hand pick weed and unwanted plants from the nursery. – These weeds compete with the citrus for nutrients which may end up killing the seedlings.5. Pest Control (Nursing) 5. Young Plants at nursery are affected by a lot of stem borers, Leaf Chewers and leaf miners. 6. Recommended insecticide such as Green Light Spinosad can be sprayed
Orchard ManagementThe activities for citrus management include:♦ Pruning♦ Irrigation♦ Weed Control♦ Fertilizer Application♦ Pests, Parasites and Diseases , and their control
♦ Pruning (Matured Stage) – For a matured tree, we do want is called maintenance pruning. This is a situation were we do selective pruning; If heavy pruning of large wood is required, pruning half of the trees in the first year, and the second half next year will reduce the impact of crop loss.♦ Annual pruning of diseased and pest attacked branches is also done in the matured stage.♦ To avoid damage to the fruit, mature fruit trees should not be pruned immediately before harvest, or when mature fruit are on the tree in early spring.
♦ Irrigation (Matured Stage) – Citrus trees will wilt and yield will be depressed unless moisture is provided during the long dry season of November to February. – Therefore, make sure that the trees are watered during this period. – Ten liters of water (stream water, well water, tap water, etc.) delivered at the base of the trees twice a week is sufficient for good development and yield.
♦ Weeding (Matured Stage) – Absolute weed control under a hot, humid climate may be very expensive, nevertheless there is need for some level of weed control. – Slash the orchard at least 3 times in the year with a hoe, the last being at the on-set of the dry season. – In addition, ring weed individual trees constantly. Ring-apply Diuron or Paracol to check weed growth.
♦ Fertilizer Application (Matured Stage) – 2–4 years of age: • 500 grams of compound fertilizer N.P.K (15:15:15) per trees, twice in the year. – 5–10 years of age: • 2 kg (15:15:15) + 600 grams per tree, twice in the year. – 10 years and above: • 3 kg (15:15:15) + 800 grams per tree, twice in the year.♦ These applications should be done in June and September to obtain maximum benefit.
Pests of citrus♦ Citrus Rust Mites♦ Main Species – Citrus bud mites – Citrus red mites♦ Symptoms of Damage – causing external blemishing and, in extreme infestations, smaller fruits, pre-mature falling and even shedding of leaves. disfigure the surface of the fruit and the foliage mainly in the winter and during droughts
♦ Biological Control – Mass rearing and release of parasitic fungi (Hirsutella thompsonii and Triplosporium floridana) help to eradicate rust mites and the Texas citrus mite.♦ Chemical Control – To control Citrus Bud Mites spray the tree with Bug Buster or Trounce.
Citrus Scale Insects♦ Main Species – Snow scale, Purple scale, Red scale and glover scale.♦ Symptoms of damage – Affect woody portions of the tree, suck sap from the branches, twigs, leaves and fruit, induce shedding of fruit and foliage, produces green spots on the fruit, infests young trees.
♦ Biological control – Maintaining populations of the Vedalia lady beetle in nurseries and groves is a fairly effective means of controlling this scale, Parasitic wasps (Aphytis spp.) are able to control Citrus snow scale, purple scale and Florida red scale.♦ Chemical Control – To effectively control scale insects and limit damage, Horticultural Oil should be sprayed on the tree. The Horticultural oil serves to suffocate the scale and eggs. In the spring if the crawling nymphs are present, spray the trees with Bug Buster to prevent the new nymphs from further infecting the tree.
Citrus Aphids (Plant lice)♦ Main Species – Brown Citrus aphid, Toxoptera citricidus – Citrus Whitefly – Orange dog caterpillar♦ Symptoms of Damage – Aphids (plant lice) cause leaves to curl, crinkled with puckered marks, yellowing and the twisting of the leaves, which gives the appearance of deformed leaves. As the severity of the aphid infestation increases, leaf drop and twig and branch die back can be seen.
♦ Chemical control – Use a specific aphicide or horticultural spray oil. Always check the label before spraying, as not all oils are registered for use in citrus. Oils can also cause phytotoxic damage if not used correctly.♦ Biological control – Predators, parasites, and fungal diseases attack aphids and occur naturally in the orchard. The honeydew produced by the aphids provides a good food source for many natural enemies.
Fruit Flies♦ Main Species – Queensland fruit fly – Mediterranean fruit fly – Papaya fruit fly♦ Symptoms of damage – Infested fruit usually falls from the tree as a result of the activity of the larvae in it. The style of damage by fruit fly larvae tunnelling in fruit varies with the type and maturity of the fruit, the number of the larvae in it, and the
– weather . Frequently citrus fruits, although stung, do not develop larvae, but the stung fruit sometimes fall.♦ Control – Traps • Traps are used to monitor male fruit fly populations in orchards. The lures in the traps attract only the male fruit fly, which are then killed. • The traps, commonly called Dak-pots, contain a mixture of the male attractant or parapheremone and an insecticide. • The traps are normally hung in trees. The traps are effective and convenient but do not control it.
– Baits • Bait mixture is prepared using an attractant (protein source) and an insecticide. Flies are attracted to the protein which they require for egg maturation. • As they feed they are killed by the insecticide. Bait spray components include the insecticides maldison or chlorpyrifos and yeast autolysate or hydrolysate as the attractant.– Chemical spray • Registrations also exist for the chemicals dimethoate and fenthion for cover sprays for Qld fruit fly control in citrus.
Diseases of Citrus♦ Citrus Canker♦ Causative organism-Bacterium♦ Symptoms – formation of scabs (similar to halo lesions) in the leaves, branches and fruits causing yellow halo-like lesions or scabs on the fruit, leaves and twigs of citrus trees. Severe infections can cause leaf loss, blemished fruit, fruit drop and die back.
♦ Control – This contagious disease is treated by applying antibiotic formulation. – Management options include replacing susceptible citrus cultivars with resistant cultivars, applying preventive sprays of copper- based bactericides, and destroying infected trees and all surrounding trees within an appropriate radius.
♦ Citrus Sooty Mold♦ Causative organism-fungus♦ Symptoms – Characterized by black coloration of the affected citrus tree leaves. The black color is nothing but the spores of sooty mold. – The mold forms on the leaves as a result of honeydew secretions from insects such as whiteflies, aphids and mealybugs.♦ Control – Using formulations of neem oil, which is an organic broad spectrum insecticide, fungicide and miticide such as orthene, malathion, or diazinon
♦ Citrus Greasy Spot♦ Causative organism-fungus♦ Symptoms – After infection takes place, the fungal spores germinate within the leaf tissues, causing the lower leaf portion to develop slightly raised blisters. – Telltale signs include yellowish-brownish blister spots on leaves, often on the underside of the leaf. As the disease develops, the spots develop into oily looking blisters.
♦ Control – Spraying copper fungicide two times in summer help in controlling this fungal disease. Spraying petroleum oil at the rate of 5-10 gallons per acre is generally applied.
♦ Citrus Melanose♦ Causative Organism-fungus♦ Symptoms – Darkly colored, raised, corky pustules appear after leaf infection. The pustules may be surrounded by yellowed leaf tissue or yellow halos. The yellow color may later turn green. Severely infected shoot apices may become distorted or die back. – Fully expanded, mature leaves resist infection. On fruits infection appears as darkly colored and usually raised pustules of various sizes
♦ Control – Cultural control by periodically pruning away dead branches. This will reduce pathogen survival, increase air circulation to dry out the canopy, and allow for more effective fungicide penetration and coverage of the foliage. – Sprays of copper fungicides to young fruits and leaves may be necessary for disease management. Where the disease tends to be severe, frequent fungicide applications may be required
♦ Citrus Root Rot Disease♦ Causative organism-Fungus♦ Symptoms – Notable signs include developing brown patches in bark, oozing of plant sap, leaf turning yellow, premature leaf fall and browning of citrus fruits. – Over time, as the disease advances the bark dries, cracks and dies. The infected area is then left as a dark sunken canker. The disease can also cause browning and decaying on the fruit and yellowing and die-back on the foliage.
♦ Control – Cultural control such as trimming down all the affected twigs and spray appropriate fungicide in correct dose. To control the brown rot it is important to remove all leaves and damaged fruit when it falls to the ground; prune of all lower branches off the tree so that the tree branches are more than 2 feet from the ground; – Chemical control can be done by spraying the tree with a fungicide such as Agri-Fos and Captan
Parasite♦ Mistletoes – Mistletoes are plant parasites that live on other plants to obtain food and water.♦ Carrier organism- Birds, squirrels and porcupines♦ Symptoms – The symptoms are difficult to note at the early stage because the leaves mimic that of citrus. it can really be seen when the parasitic plant bears yellow or red flowers at the adult stage.
♦ Control – Cultural control is so far the only method that is effective; chemicals are not effective, as it is impossible to apply them safely and efficiently. This is done with a standard pruner – The recommended time is when the Mistletoe is in flower as this makes it easier to spot them in the cocoa canopy.
Harvest♦ Citrus must be clipped, not pulled from the tree, to prevent plugging the peel. It is best to harvest citrus on a clear, sunny day with low humidity. The fruit should be harvested as soon as the dew has evaporated. On a cloudy day, the fruit should be harvested in the afternoon. Fruit should not be harvested at all on a rainy day.
Storage♦ Only fruit which have not been damaged in harvest are used for storage. Citrus may be stored for periods of up to 1-2 months at low temperatures (0-4.4 °C).♦ Plastic crates or boxes are used for storing fruit. Boxes should be stacked inside the storage room in a way that maintains good ventilation. Fruit will store longer if kept cold. Sunlight should not be able to penetrate inside the storage room. Any rotting fruit should be removed.
Uses♦ Food Value – Their juice is used as an ingredient in a variety of dishes; it can commonly used in salad dressings and squeezed over cooked meat or vegetables. – A variety of flavours can be derived from different parts and – Marmalade, a condiment derived from cooked orange and lemon to produce a jam- – Lemon or lime is commonly used as a garnish for water, soft drinks, or cocktails. – The colourful outer skin of some citrus fruits, known as zest, is used as a flavouring in cooking;
♦ Medical Value – Citrus juice also has medical uses; lemon juice is used to relieve the pain of bee stings. – Oranges were historically used for their high content of vitamin C, which prevents scurvy. – After consumption, the peel is sometimes used as a facial cleanser. – A peel of lemons or orange is commonly used as a means to moisten medical cannabis when stored with it. – Before the development of fermentation-based processes, lemons were the primary commercial source of citric acid. – Orange juice may help prevent calcium oxalate stone formation