Citrus

CITRUS {Citrus sinensis L.}

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.

Biophysical limits
Altitude: 0-2 000 m,
Mean annual temperature: 5-40 deg. C,
Mean annual rainfall: 900-2500 mm .Trees are intolerant of water logging.
Soil type: Trees will grow in almost any soil type if well aerated. The optimum conditions for citrus orchards are fertile, light to medium, well-drained, deep, loose loams; soils with a high water table should be avoided.  Trees are intolerant of water logging. The species is sensitive to excess salts; pH range of 5-8 is preferred
C. sinensis starts flowering and bearing fruit after 2 - 3 years

ORCHARD SANITATION AND PRUNING
Seedlings should never be planted out deeper than they were in the nursery.
Trees are planted at a spacing of 3-5 m squares, but a spacing of up to 1.3 m can be used, depending on the climate. Irrigation is necessary if rainfall is not adequate.
If soil lacks nutrients, fertilizer containing nitrogen, phosphorus potash, magnesium, calcium, zinc, manganese and iron should be added. Pruning is practised to encourage branching and keep the tree low for easy harvesting of the fruit. Any growth below where budding took place should be removed.
 A windbreak should be provided to protect trees. Trees aged 3-4 years produce 2.5-5 t/ha of fruit and 8-12 year old trees produce 20-40 t/ha of fruit. Single trees may live up to 100 years, but the economic life of an orchard seldom exceeds 30 years.
Maintaining good sanitation in the orchard is very important in citrus health management. Twigs and fallen leaves should be collected from under and around the trees, and either buried or burned.
Pruning is also important, because like good sanitation it helps eliminate the source of pathogens and insects. In winter, growers should prune twigs which show signs of die-back. They should also remove any twigs which show symptoms of canker, scab (Elsinoë fawcetti), anthracnose (Collectotrichum acutatum) or stem-end rot (Diaporthe citri and Diplodia natalensis).

Many scale insects and mites may survive in the canopy over the winter in a dormant state. The trees should be sprayed at least once during the cold months. This should kill any over-wintering insects such as scale insects, rust mites and red spiders mites.

Covering the soil surface with a grass or straw mulch is an effective way of reducing levels of disease, as well as improving the soil. Covering the soil within 30 cm of the trunk with a layer of
straw helps prevent infection of the roots with fungus diseases caused by Phytophthora.

Trees aged 3-4 years produce 2.5-5 t/ha of fruit and 8-12 year old trees produce 20-40 t/ha of fruit. Single trees may live up to 100 years, but the economic life of an orchard seldom exceeds 30 years.


DISEASES AND PESTS OF CITRUS TREES

Diseases
1.    Alternaria Brown Spots
Alternaria Brown Spot affects tangerines more than any other type of citrus fruit. Symptoms:-
-    dark brown, pitted areas on the fruit and the fruit may drop prematurely. 
-     leaves of tangerine trees  may also develop similar spots and sometimes the leaves fall off.
     Control:- Applying a copper spray controls this fungal disease e.g. CupproCaffaro
2.    Greasy Spot
Fallen decomposing leaves that surround the bottom of the tree cause greasy spot.
Symptoms:-
-    trees infected by greasy spot exhibit oily looking spots below the surface of the leave, or black specks may appear on the rind of the fruit.
-    If the tree is severely infested, leaves begin to drop prematurely and the tree becomes susceptible to other diseases.
Control:-
-    Prevention is the best medicine for greasy spot. Keep dead leaves clear from the tree, especially before rains begin.
-    Also, one can do copper sprays to control it.

3.    Foot Rot
It is a fungal disease of the soil that affects trees exposed to wet conditions and poorly drained soil. The fungus attacks the trunk and possible the roots of the tree.
 Symptoms :-
-     Fruit rot, discoloration of the fruit rind, rancid fruit, and leaf drop. When infected with fruit rot, the trees do not grow vigorously or produce as much fruit.
Control/prevention:-
-    Addressing the conditions that foster foot rot by pruning lower branches, keeping mulch away from the base of the trees and providing good drainage is the best prevention.
-     Scraping foot rot and painting the affected area with copper paint helps trees already infected with the fungus.
4.    Black Spot
It is a fungal problem showing as circular, dark, sunken spots on the fruit.  Spray the young fruit and foliage with a copper spray in early spring (September – November), and then repeat again in 6 weeks time. 
5.    Collar Rot
It is a fungal disease which affects the bark on the trunk of the tree.
Symptoms: -
-    The bark begins to rot or flake just above ground level and this band of soft decay gradually encircles the trunk.  If not detected before it fully encircles the trunk, it is too late and the tree will die.
-    This problem can be caused by injury from mowing, weeding etc, or by incorrect mulching, where the mulch is too close to the trunk, allowing harmful fungi and bacteria to invade the bark.  The first signs may be loss of vigour and possibly small fruit which turn black and drop off the tree.  If collar rot is discovered, use a knife to gently scrape away soft, diseased bark, down to clean wood.
Control:-
-    Mix copper spray to a paste and paint generously on to the affected section.
-    Make sure any mulching material, plants or weeds are kept well clear of the trunk so that there is good air circulation.
-    Remove any weak or congested branches from the tree, and all fruit and blossoms.
-    Preventing collar rot is easier than trying to cure it so make sure the lower section of the trunk is kept clear of all mulch or plant material.  
6.    Septoria Spot
It is a fungal disease and generally a sign of high humidity in the area around the tree.  It presents as brownish marks about the size of a finger print on the fruit, and slightly sunken.  The quality of the fruit is not usually affected but discontinue overhead watering and improve the air circulation in the canopy by thinning the central branches.
Pests
1.    Leaf miners: - are insects which feed on the leaves, particularly the young leaves. By defoliating trees, they reduce their productivity, so that there is a decline in yields. Leaf miners reproduce fast, and may produce three or four generations in each growing.
They are controlled by spraying the trees with Dimethaote, another alternative is to apply (Confidor) to the trunks of the trees

2.    Scale insects and mealybugs: - these species attack the leaves, twigs and fruits. They cause injury by sucking sap from the tree and excreting large amounts of honeydew. This honeydew serves as a medium for the growth of sooty mold fungus. Sooty mold deposits on the fruit require extra scrubbing on the packing line for removal. Continual sucking of the sap by scale insects and mealybugs from foliage and twigs lowers the vitality of the tree. Heavy infestations can result in severe defoliation. Growers should watch for a build-up of mealybugs and scale, and apply a timely spray of the insecticide Malathion combined with Dimethoate.

Overwintering insects should be eliminated by pruning infested branches, and spraying a mixture of insecticide and oil. Carrying out these measures after harvesting the fruit is indispensable to prevent scale and mealybug problems the following season

3.    Mites: - attack the leaves and fruits. Rust mites in particular cause the atrophy of leaves and buds, and turn fruit a rusty brown color. Mites are controlled by spraying an acaricide once a week throughout the growing season. At least two different kinds of acaricides should be applied in rotation, to prevent the mites from becoming tolerant of the chemical. Spraying over the winter and pruning are the same as for scale and mealybugs.

4.    Fruit fly: - Fruit flies (Bactrocera dorsalis) attack fruit when it is ripe or maturing. The adults lay their eggs in the fruit. The eggs hatch into yellowish white larvae which burrow into the fruit. Infected fruit have little market value, and often drop before they are ripe.

Other challenges in Citrus production
Defoliation can be caused by several factors.  Salt winds in coastal areas.  Unfortunately citrus cannot tolerate salt winds and the only solution to this is to provide complete protection;
Manure which is too fresh - Only composted manure should be used on all plants; 
Poor drainage -  Citrus cannot tolerate poor drainage and the plant should be moved to a raised bed containing good soil and aged manure and watered well. All flowers and fruit should be removed and no fruit permitted to form for a period of 12 months. Also, insufficient water in hot weather may cause leaf fall (defoliation)
Iron Deficiency is indicated by the new leaves showing light green to yellowish colour during the warmer months.  Apply iron chelates according to the directions and composted cow manure.  This problem usually occurs on alkaline soils.  
Failure to Fruit in the first few years is normal as citrus are usually not productive until the main branches have formed.  Young citrus trees should have all fruit removed for at least the first 2 years.  When large, healthy plants fail to set fruit it can be caused by excessive nitrogen.  Apply 1 handful of superphosphate per square metre of area below the tree and just beyond the outer branches and water in thoroughly. 
Fruit Drop is most often caused by dry conditions and insufficient watering of plants.
Magnesium Deficiency is indicated by a green ‘V’ shape through the centre of mature leaves, with yellowed outer edges.  Leaves fall prematurely. 

Manganese Deficiency may cause yellowing or light coloured leaves between the midrib and the veins of young leaves.   Add rotted poultry manure to make the soil more alkaline and increase the plant’s access to manganese. 
Stunted Plants with lots of flowers may indicate stress caused by soil problems, strong winds or root bound container plants.   Remove all flowers, water and feed well.  Container plants will need to be repotted
Zinc Deficiency is fairly common in citrus and presents as yellow areas between the midrib and the main lateral veins of leaves.  In severe cases the new leaves will be very small and narrow on short stems, giving a crowded appearance.  Some twigs may die back and some may have multiple buds and dense shoots giving a stunted, bushy appearance.  Fruit may be pale, elongated and small.  Feed the tree with a balanced citrus fertiliser, watering before and after fertiliser application, or apply zinc sulphate.  Can also be treated by applying composted cow manure. 

Boron Deficiency is indicated by fruit with dry, brown flesh.  Sometimes, if the soil is very acidic, this may limit the boron available to the plant and adding rotted poultry manure to make the soil more alkaline increases the plant’s access to boron.