Mandarin oranges

Description

Mandarin is the group name given to several classes of small oranges that includes mandarins, satsumas, clementines and tangerines which all belong to the species Citrus reticulata and the satsuma mandarin Citrus unshiu. Generally, mandarin trees are small and spiny with long, slender branches. They can have an erect or drooping growth habit depending on variety. The leaves of the trees are lanceolate and may be hairless or toothed with narrowly winged petioles. The trees produce flowers singly or in clusters and globose fruits with a bright orange to red-orange peel and segmented orange flesh. Mature mandarin trees can reach 7–8 m 23–26 ft in height and can be very long lived if they do not succumb to disease. Mandarins originate from Southeast Asia.


Uses

Mandarin oranges are commonly eaten fresh or may be processed for canned segments. They can be pressed or squeezed to produce juice which is used in many beverages. Mandarin essential oil is used as a flavoring in alcoholic drinks.


Propagation


Requirements
Mandarin oranges are subtropical plants and the trees grow best in regions with a pronounced change in season. They will grow best at temperatures between 12.8 and 37.8°C (55–100°F) during the growing season and 1.7 to 10°C (35–50°F) during dormancy. Mature mandarin orange trees can survive short periods of freezing, whereas young trees will be killed. Fruit will also be damaged by freezing conditions. The trees will tolerate drought conditions but perform poorly in water-logged soil. Trees will grow best when planted in a well-draining sandy loam with a pH between 6.0 and 7.5. Soil must be deep enough to permit adequate root development. Mandarin orange trees require full sun and should be protected from wind.

Mandarin oranges propagation
Mandarin orange seedlings are usually produced by grafting or budding to an appropriate rootstock as seeds will not produce fruit true to type. Grafting is the process by which a scion from plant is joined to the rootstock of another to produce a new tree. Budding is a special type of grafting where the scion that is joined to the rootstock consists of a single bud. Budding is commonly used in citrus propagation as it is the easier of the two processes and works very well. Common rootstocks for grafting and budding of citrus trees include sour orange and rough lemon.

Budding
Budding should be carried out when seedling stems have reached roughly the diameter of a pencil (6–9 mm/0.25–0.36 in) and at a time when the bark of the rootstock tree is slipping (this is the term used to describe a period of active growth when the bark can be easily peeled from the plant). Twigs (budwood) should be collected from the previous growth flush or the current flush so long as the twig has begun to harden. The twigs should have well developed buds and should be as close as possible to the diameter of the rootstock onto which it will be joined. It is extremely important to only collect budwood from disease-free trees. The use of diseased budwood can cause the spread of many serious citrus diseases which can kill trees. The budwood to be used for propagation should be trimmed to create budsticks which are 20–25 cm (8–10 in) by removing any unwanted wood and leaves. These budsticks can be stored for 2–3 months under the correct conditions but it is best to use them as soon as possible after cutting.

The simplest way to join the budwood the the rootstock is by T-budding. The area to be joined should be pruned to remove any thorns or twigs and the cut made approximately 15 cm (6 in) from the ground. Using a sharp knife, a 2.5–3.8 cm (1–1.5 in) vertical cut should be made in the stem of the rootstock, through the bark. A horizontal cut should be made at either the top or the bottom of the vertical cut to produce a “T-shape” The horizontal cut should be made a slightly upward-pointing angle and should reach through the bark. Remove a bud from a budstick by slicing a thin, shield-shaped piece of bark and wood from the stem, beginning about 1.25 cm (0.5 in) above the bud. This piece should measure 1.9–2.5 cm(0.75–1.0 in) in length. Immedietely insert the piece of bud into the cut on the rootstock by sliding it under the opened bark so that the cut surface lies flat against the wood of the rootstock plant. Finish the join by wrapping the bud with budding tape.

After the union has formed and the tape is removed, the bud is forced to grow by cutting the rootstock stem 2.5–3.9 cm (1.0–1.5 in) above the join about 2/3 of the way through the stem on the same side as the join. The top of the seedling should then be pushed over towards the ground. This process, known as “lopping” allows all of the nutrients to be diverted to the bud Once the bud begins to grow and reaches several inches in length, the lop can be removed completely from the seedling.

Planting seedlings
Mandarin orange trees can be purchased as seedlings which have already been grafted and only require planting in the garden or orchard. The best time to plant citrus trees is in Spring after all danger of frost has passed in your area. Standard sized trees should be spaced 3.7–7.6 m (12–25 ft) apart in an area that receives full sunlight, but is protected from strong winds which can damage the trees. Planting against a south facing wall will help protect the tree in cooler climates.

General care
Newly planted trees require proper irrigation to ensure they become established. During the first year, water should be applied at the base of the trunk so that the root ball is kept moist to allow the roots to establish in the soil. Newly planted trees should be provided with water every 3–7 days. The soil should be moist, but not wet. Trees planted in sandy soils will require water more frequently. Young trees will also require a light application of fertilizer every month in the first year


References

CABI Crop Protection Compendium. (2010). Citrus reticulata (mandarin) datasheet. Available at: http://www.cabi.org/cpc/datasheet/13463. [Accessed 10 February 15]. Paid subscription required.

Fadamiro, H., Nesbitt, M. & Wall, C. (2007). Crop profile for satsuma mandarin in Alabama. Alabama A & M & Auburn University Extension. Available at: http://www.aces.edu/anr/ipm/old/crop_.... [Accessed 10 February 15]. Free to access.

Fake, C. (2010). Growing Citrus in the Sierra Nevada foothills. University of California Cooperative Extension. Available at: http://ucanr.org/sites/placernevadasm.... [Accessed 10 February 15]. Free to access.

Lazaneao, V. (2008). Citrus for the home garden. University of California Cooperative Extension. Available at: http://www.mastergardenerssandiego.or.... [Accessed 10 February 15]. Free to access.

Timmer, L. W., Garnsey, S. M. & Graham, J. H. (2000). Compendium of Citrus Diseases. American Phytopathological Society Press. Available at: http://www.apsnet.org/apsstore/shopap.... Available for purchase from APS Press.


Common Pests and Diseases

Huanglongbing (Citrus greening, Yellow dragon disease)
Candidatus Liberibacter asiativus
Candidatus Liberibacter africanus
Candidatus Liberibacter americanus

Symptoms
Yellowing of one limb or one area of canopy; yellowing of leaf veins; blotchy mottling on leaf blades; twig and limb dieback; fruits dropping prematurely; small upwardly pointing leaves; small, misshapen fruit; fruit very bitter
Cause
Bacteria
Comments
Asiatic form of the bacteria found in Florida; transmitted by Asian citrus psyllid
Management
Regulating planting material is necessary for the control of the disease; in areas where disease is endemic, control strategies should focus on populations of the insect vectors, reducing inoculum and planting trees in conditions unfavourable to the spread of the pathogen; heat treatment of budwood can treat the disease

Citrus canker
Xanthomonas axonopodis

Symptoms
Raised lesions on leaves, often at leaf margin or tip; lesions may also be present on twigs and fruits; young lesions are usually surrounded by yellow halo; depressed brown craters formed from collapse of lesions
Cause
Bacterium
Comments
Can cause serious economic losses to grapefruit crop; bacteria survive in lesions; the main method of spread is via wind driven rain; bacteria may enter through pruning wounds
Management
If the disease is introduced to an area, all infected trees should be removed and destroyed; in areas where disease is endemic, windbreaks can help to reduce disease severity; cultural control of the disease should focus on controlling leaf miner populations, utilizing wind breaks and applications of copper sprays

Stubborn disease
Spiroplasma citri

Symptoms
Stunted trees; leaves shorter and broader, cupped and upright; may be chlorotic or have a mottled appearance; stunted, malformed fruits and low yield
Cause
Bacterium
Comments
Transmitted by leafhoppers; can cause serious losses in hot, dry conditions
Management
Plant only material from disease-free budwood; if disease is endemic to the are then nursery trees should be grown in an enclosure to protect the trees from vectors; if a young orcahrd becomes infected, it should be removed and replanted with healthy material

Blast
Pseudomonas syringae

Symptoms
Water-soaked or black lesions on leaf petioles;which rapidly expand along the leif midrib; cankers on twigs and branches; twigs may be girdles and die; leaves turning black and dying; black lesions may be present on fruit
Cause
Bacterium
Comments
Symptoms most severe on south facing side of tree exposed to winds
Management
In areas where disease is severe, copper fungicides should be applied in Fall and WInter prior to the first rains

Anthracnose
Colletotrichum gloeosporioides

Symptoms
Dieback of twigs; premature leaf drop; dark staining on fruit; leaves and twigs covered in dark spores
Cause
Fungus
Comments
Disease common during wet Springs or long periods of wet weather late in season
Management
If disease is damaging then appropriate fungicides should be applied to whole tree

Black root rot
Thielaviopsis basicola

Symptoms
Small brown-black lesions on roots which may coalesce and turn entire root black; root cortex may slough off to reveal the vascular tissue below; leaves of plant may be chlorotic
Cause
Fungus
Comments
Serious disease of glasshouse grown citrus trees; pathogen usually drops to non-damaging levels after tree is transplanted to the field
Management
Keep glasshouses well lit and warm during winter to encourage vigorous root growth; use good quality potting soil which provides goo aeration

Armillaria root rot (Mushroom root rot)
Armillaria mellea

Symptoms
Trees may wilt suddenly and collapse or decline slowly; leaves become chlorotic and drop from tree; if large parts of root are destroyed then whole canopy is affected; trunk may have area of rotting bark at the base; lesions on the trunk resemble Phytophthora gummosis; clusters of mushrooms may be present at the bottom of the tree and fan shaped mycelial mats are often present between the bark and the wood
Cause
Fungus
Comments
Healthy trees are usually infected by infected pieces of wood or tree stumps which have been left in the ground after an orchard is cleared
Management
Disease is difficult to control once it becomes established in an orchard; affected trees showing signs of decline should be removed along with as much of the roots system as possible; area where infected tree was should not be replanted with health citrus for a period of at least one year; fumigating soil can help to reduce soil inoculum but is not always completely effective

Citrus leaf miner
Phyllocnistis citrella

Symptoms
Thin, winding trails on leaves; heavy infestation can result in curled and distorted leaves; adult leafminer is a tiny moth which lays its eggs in the leaf; larvae hatch and feed on leaf interior
Cause
Insect
Comments
Leaf miners attack flushes of young growth and are unable to enter leaves once they harden
Management
Insecticide application are rarely warranted in mature orchards as yields are unaffected; young trees should be treated with appropriate insecticides to prevent retarded growth; cultural control methods include removal of water sprouts from trees and refraining from pruning live branches more than once a year to encourage uniform growth flushes which are short in duration

Aphids (Black citrus aphid)
Toxoptera aurantii

Symptoms
Leaves curling; leaves and twigs covered in sticky substance which may be growing sooty mold; trees may show symptoms of tristeza (see entry); insects are small and soft bodied and are black in color
Cause
Insect
Comments
Aphids transmit tristeza virus on citrus
Management
Aphid numbers tend to naturally decline as leaves harden off but can be a problem on young trees or varieties which continually produce flushes of new growth; pesticides are not generally recommended due to resistance and trees can withstand a high degree of leaf curling

Thrips
Scirtothrips citri

Symptoms
Insect feeds under sepals of young fruit and causes a ring of scarred tissue as the rind expands; adult thrips are orange-yellow in color
Cause
Insect - Citrus thrips
Comments
Insects overwinter on trees as eggs and can undergo multiple generations per year
Management
Insecticide application is rarely required as healthy trees can withstand heavy feeding damage; insecticides can actually promote thrips populations by stimulating reproduction

Soft scales (Black scale, Brown soft scale , Citricolla scale)
Saissetia oleae
Coccus hesperidum
Coccus pseudomagnoliarum

Symptoms
Leaves covered in sticky substance and may have growth of sooty mold; reduced tree vigor; leaves and/or fruit dropping from plants; presence of black, brown or gray flattened scales on leaves, twigs and/or branches
Cause
Insects
Comments
Insects can produce several overlapping generations per year
Management
Organically acceptable methods of control include the application of horticultural oils and preservation of natural enemies

Brown rot
Phytophthora spp.

Symptoms
Water-soaked lesions on fruit close to maturation; leather tan to dark brown lesions on fruit; lesions with a pungent smell; leaves, twigs and flowers may be turning brown
Cause
Oomycete
Comments
Disease emergence favored by cool, wet conditions
Management
Cultural control methods should focus on reducing leaf wetness e.g. mowing around trees to prevent grasses growing too long, proper irrigation management, pruning branches hanging low to the ground etc.; if fruit become infected, harvest should be delayed to allow all infected fruit to drop to the ground and minimizing contamination in the harvest; applications of copper fungicides to foliage can help protect the trees

Phytophthora gummosis
Phytophthora spp.

Symptoms
Sap oozing from cracks in bark; bark cracking, drying and falling off; lesions girdling trunk; severely infected trees have pale green leaves with yellow veins
Cause
Oomycete
Comments
Disease can develop rapidly in moist, cool conditions; spread by water splash
Management
Only plant disease-free nursery stock; plant trees in well-draining soil and avoid injuries to bark on trunk; trunk wraps can provide protection from freezing

Tristeza disease
Citrus tristeza virus (CTV)

Symptoms
Light green foliage; poor new growth; leaves may be dropping from tree; young trees blooming early; severely infected trees are stunted and bushy in appearance with chlorotic leaves and brittle twigs; some strains of the virus cause elongated pits in the trunk and branches which give the wood a rope-like appearance
Cause
Virus
Comments
Disease spread from infected grafting material or by aphids
Management
Quarantine procedures are used to control tristeza and prevent the pathogen from entering areas which are currently free of the disease