Grapefruit, Citrus × paradisi, is a large evergreen tree in the family Rutaceae grown for its edible fruit. The tree has a spreading canopy with a rounded top and densely packed dark green, long and thin leaves with winged petioles. The tree produces white flowers singly or in clusters and large spherical fruits, usually in clusters. The fruits are 8–12 cm (3–5 in) in diameter with yellow-orange skin and segmented white, red or pink flesh. Grapefruit trees can reach heights between 10 and 15 m (33–50 ft) and live for many years if well maintained. Grapefruit likely originated in the West Indies as a hybrid of pomelo and sweet orange or mandarin.
Grapefruit can be eaten fresh and is commonly eaten as a breakfast fruit. It can also be used to make juice or processed for canning of segments or pulp. Grapefruit essential oils are used in perfumery.
Grapefruit is a tropical to subtropical plant and the trees grow best in areas with hot daytime temperatures and warm to hot night time temperatures. Grapefruit is less hardy than orange but continual exposure to cooler temperatures will induce a certain degree of cold tolerance. Mature trees can tolerate short exposures to temperatures below freezing but ice will begin to form in fruit after 2-3 hours at -3°C (27°F). Young trees may be killed by these temperatures. 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. Grapefruit trees will grow best when planted in full sun.
Grapefruit 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 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 lengthe, the lop can be removed completely from the seedling.
Grapefruit 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. Trees should be planted at or higher than the level of the nursery pot. Once the tree is positioned in the planting hole, backfill the soil by about half and water to allow the soil to settle around the lower roots before filling in the hole. The newly planted tree should be watered every few days.
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. Grapefruit trees will need protected from cold temperatures to prevent damage. Soil can be mounded up around the trunk during the winter and removed in the Spring. Young trees can also be protected from frosts by covering them with tarps or blankets as required.
Dieback of twigs; premature leaf drop; dark staining on fruit; leaves and twigs covered in dark spores
Disease common during wet Springs or long periods of wet weather late in season
If disease is damaging then appropriate fungicides should be applied to whole tree
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
Symptoms most severe on south facing side of tree exposed to winds
In areas where disease is severe, copper fungicides should be applied in Fall and WInter prior to the first rains
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
Disease emergence favored by cool, wet conditions
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
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
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
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
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
Leaf miners attack flushes of young growth and are unable to enter leaves once they harden
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
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
Origins and spread
Huanglongbing, or citrus greening, was first reported from Southern China in 1919 by American botanist Otto August Reinking who described a “yellow shoot” disease of citrus while evaluating diseases of economic plants in Southern China. A subsequent field survey conducted between 1941 and 1955 on citrus plants in the provinces of Guangdong, Fujian and Jiangx by Chinese plant pathologist Lin Kongxiang (Kung Hsiang) determined that that the disease likely originated in Chaozhou county in Guangdong as early as the 1870s. Lin adopted the name the local farmers had given to the disease of “huang long bing”, which translates to “yellow dragon disease”, a reference to the yellow coloration of new shoots on the infected trees. By 1936 Huanglongbing was considered a serious disease of citrus in China and it subsequently spread across Southeast Asia reaching Indonesia in 1948 and Taiwan in 1950 before spreading to the Philippines, Thailand and Malaysia in the 1950s, 60s and 70s respectively. The disease has been known by various names in different countries - “greening” in South Africa, “mottle leaf” in the Philippines, “dieback” in India and “vein phloem degeneration” in Indonesia - but in 1995 the disease was officially named Huanglongbing by the International Organization of Citrus Virologists (IOCV) and this name is now widely used to describe the disease in Africa, America and Asia.
Biology and ecology
The organism that causes Huanglongbing is a Gram-negative bacterium that is limited to the plant phloem - the plant system responsible for the delivery of sugars from the leaves to the growing parts of the plant. The bacteria involved have so far not been isolated and cultured but the disease is believed to be caused by bacteria belonging to the genus Candidatus Liberibacter. It is believed that there are at least two different forms of the disease, an African heat-sensitive form, L. africanus which survives in cool areas with temperatures below 30-32 C, and an Asian heat-tolerant form which occurs in areas where temperatures greatly exceed 30C. A third species, L. asiaticus, found in .A third species, L. americanus was detected in citrus trees in Sao Paulo, Brazil but there is presently little information on its climatic requirements. As this species is found in the same areas as L. asiaticus it seems likely that is has similar requirements.
Huanglongbing can be transmitted by citrus psyllids or by grafting. The Asian citrus psyllid, Diaphorina citri is responsible for the spread of the disease in Asia and Oceania, Brazil and North America whereas the African citrus psyllid, Trioza erytreae is the main vector in Africa and Madagascar. Both psyllid species are present the Indian Ocean islands of Reunion and Mauritius,
Citrus psyllids are tiny (3-4 mm) sap-sucking insects that excrete a sticky, sugary substance called honeydew. Both the Asian and African citrus psyllids are mottled brown in color but the Asian citrus psyllid possesses a brown head and the African species has a black head. Adult citrus psyllids will jump and/or fly for a short distance when they are disturbed. They are usually found on the undersides of leaves, often in high numbers. When a psyllid feeds on an infected plant, it acquires the disease after 15 to 30 minutes and feeding and is able to transmit the disease to new hosts after a period of 21 days. In order to transmit the disease successfully, the psyllids need only feed on a new host for a period of 15 minutes in order for successful transmission to occur. It is hypothesized that the bacterium multiplies within the body of the psyllid prior to transmission but this theory requires validation through experiments.
Although the primary method of spread of the Huanglongbing bacterium is via the movement of citrus psyllids, the disease can also be transmitted through grafting practices. The ability of Huanglongbing to be transmitted by grafting was first demonstrated by Lin Kongxiang through experimental work which was published in 1956. The disease in not transmitted at high rates through grafting as as not all buds on infected trees contain the bacterium.
(i) Cultural control
Once a tree becomes infected with HLB, it cannot be cured. Control is therefore reliant on preventing the disease occurring in the first place and this is achieved through strict quarantining to prevent the introduction of citrus psyllids to areas which are currently free of the pest. Areas which are subject to quarantine have restrictions placed on the movement of citrus plants, fruit, equipment and items made from citrus.
Infected trees should be removed as quickly as possible from plantations and destroyed. Identification of infected trees should be achieved through several surveys to ensure that infected trees which are not yet showing symptoms are identified. In Florida, the recommendation is to scout groves at least 4 times a year for disease symptoms.
(ii) Control of citrus psyllids
Citrus psyllid populations can be controlled through the application of chemical sprays. Insecticides have proved very effective at controlling T. eryreae in South Africa where systemic insecticides are applied to the tree at the base of the trunk. In areas of the USA, Citrus health management areas (CHMAs) have been created to encourage neighbouring growers to work together to prevent the disease. Control strategies which have been implemented by the program include scouting, mapping and large-scale spraying to control citrus psyllids.
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
Disease can develop rapidly in moist, cool conditions; spread by water splash
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
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
Insects can produce several overlapping generations per year
Organically acceptable methods of control include the application of horticultural oils and preservation of natural enemies
Stunted trees; leaves shorter and broader, cupped and upright; may be chlorotic or have a mottled appearance; stunted, malformed fruits and low yield
Transmitted by leafhoppers; can cause serious losses in hot, dry conditions
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
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
Insects overwinter on trees as eggs and can undergo multiple generations per year
Insecticide application is rarely required as healthy trees can withstand heavy feeding damage; insecticides can actually promote thrips populations by stimulating reproduction
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
Disease spread from infected grafting material or by aphids
Quarantine procedures are used to control tristeza and prevent the pathogen from entering areas which are currently free of the disease
CABI Crop Protection Compendium. (2008). Citrus x paradisi datasheet. Available at: http://www.cabi.org/cpc/datasheet/13461. [Accessed 12 December 14]. Paid subscription required.
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.
Williamson, J. G. (1997). The Grapefruit. University of Florida Cooperative Extension Service. Available at: http://university.uog.edu/cals/people.... [Accessed 15 December 14]. Free to access.