Lemon, Citrus limon
, is a small evergreen tree in the family Rutaceae grown for its edible fruit which, among other things, are used in a variety of foods and drinks. The tree has a spreading, upright growth habit, few large branches and stiff thorns. The tree possesses large, oblong or oval, light green leaves and produces purple-white flowers in clusters. The lemon fruit is an ellipsoid berry surrounded by a green rind, which ripens to yellow, protecting soft yellow segmented pulp. Lemon trees can reach 3–6 m (10–20 ft) in height and can live for many years, reaching full fruit bearing capacity in approximately 40 years. Lemon may also be referred to as bush lemon or Persian apple and likely originated from the eastern Himalaya of India.
Seeds inside fruit
Fruit sliced open to reveal flesh
Cluster of lemon fruits
Lemon fruit ripening on tree
Due to their bitter taste, lemon fruit is not usually consumed fresh. It is used widely to make juices such as lemonade, as garnishes in cooking and as a flavoring in cooking and baking.
Lemon is a subtropical plant and the trees grow best in regions with a pronounced change in season. They will grow best at temperatures between 26–28°C (79–82°F) and are very sensitive to cold. Trees and fruit will be damaged or killed by freezing conditions without protection.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. Lemon trees will grow best when positioned in full sunlight.
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.
Lemon 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. Lemon 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.
CABI Crop Protection Compendium. (2008). Citrus limon (lemon) datasheet. Available at: http://www.cabi.org/cpc/datasheet/13450. [Accessed 10 February 15]. Paid subscription required
Lacey, K., Ramsey, H. & Hoffman, H. (2009). Growing healthy citrus. Department of Agriculture and Food. Available at: http://archive.agric.wa.gov.au/objtwr/imported_assets/content/hort/fn/cp/citrus_propagation.pdf. [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/shopapspress/Pages/42481.aspx. Available for purchase from APS Press
Common Pests and Diseases
Category : Fungal
Dark spores on spots
Anthracnose symptoms on lemon leaf
Dieback of twigs; premature leaf drop; dark staining on fruit; leaves and twigs covered in dark spores.
If disease is damaging then appropriate fungicides should be applied to whole tree.
Armillaria root rot (Mushroom root rot)
Mycelial mats on wood
Armillaria spp. often produce abundant quantities of white spores
Mushrooms of Armillaria
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.
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.
Black root rot
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.
Keep glasshouses well lit and warm during winter to encourage vigorous root growth; use good quality potting soil which provides good aeration.
Discolored wood on lemon tree infected with mal secco
Lemon tree infected with mal secco
Lemon tree infected with mal secco
Chlorosis of leaf veins; wilting leaves which drop from plant; the midrib of fallen leaves may have a red discoloration; infected bark may turn silver-grey in color.
Spread of the disease into new areas can be prevented through the use of clean planting material; if trees become infected, diseased shoots and branches should be pruned out as soon as possible; avoif over head irrigation; trees can be protected from the disease by spraying with copper fungicides.
Category : Bacterial
Bacterial canker (Blast)
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.
In areas where disease is severe, copper fungicides should be applied in Fall and WInter prior to the first rains.
Symptoms on fruit
Symptoms on leaves
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.
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.
Huanglongbing (Citrus greening, Yellow dragon disease)
Candidatus Liberibacter asiativus
Candidatus Liberibacter africanus
Candidatus Liberibacter americanus
citrus greening (Candidatus Liberibacter asiaticus) infected leaves
Symptoms of citrus greening (Candidatus Liberibacter asiaticus)
citrus greening (Candidatus Liberibacter asiaticus) 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.
(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.
Category : Viral
Citrus tristeza virus (CTV)
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.
Quarantine procedures are used to control tristeza and prevent the pathogen from entering areas which are currently free of the disease.
Category : Insects
Aphids (Black citrus aphid, Brown citrus aphid)
Brown citrus aphids on leaves
Brown citrus aphid (Toxoptera citricida)
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.
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.
Citrus leaf miner
Tunneling caused by leaf miner
Citrus leafminer (Phyllocnistis citrella) damaged leaves
Leaf curling distortion from citrus leafminer injury
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.
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.
Soft scales (Black scale, Brown soft scale , Citricolla scale)
Citricola scale (Coccus pseudomagnoliarum) on leaves
Brown soft scale (Coccus hesperidum)
Black scale (Saissetia oleae)
Brown soft scale (Coccus hesperidum) adults on citrus stem
Citricola scale (Coccus pseudomagnoliarum)
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.
Organically acceptable methods of control include the application of horticultural oils and preservation of natural enemies.
South African citrus thrips (Scirtothrips aurantii) damaged lime
California citrus thrips (Scirtothrips citri)
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.
Insecticide application is rarely required as healthy trees can withstand heavy feeding damage; insecticides can actually promote thrips populations by stimulating reproduction.