Olea europaea (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Dye / tannin|
|Forage / feed|
- Protologue: Sp. pl. 1: 8 (1753).
- Family: Oleaceae
- Chromosome number: 2n = 46
- Olive (En).
- Olivier (Fr).
- Oliveira (Po).
- Mzeituni, mzaituni (Sw).
Origin and geographic distribution
Olive is a characteristic fruit tree of the Mediterranean Basin. The wild Mediterranean olive or oleaster (Olea europaea subsp. europaea var. sylvestris (Mill.) Lehr) is a typical component of the Mediterranean shrub vegetation and the most likely progenitor of the cultivated olive (Olea europaea subsp. europaea var. europaea). First domestication is associated with early civilizations in the eastern Mediterranean or Middle East. Archaeological evidence of olive cultivation dates back to the 4th millennium BC. The Phoenicians and Greeks in particular contributed to the expansion of olive cultivation around the Mediterranean Sea during the first millennium BC. In the Roman empire of the 2nd century AD, olive oil became one of the most economically important commodities. Eastwards, olive cultivation spread up to north-western India and the Caucasus. Olive cultivation was introduced to the New World (Peru, Chile, Argentina, Mexico and United States (California)) in the 16–18th centuries by the Spanish, to Australia and South Africa by Italian and Greek immigrants and to Japan and China from France in the 19th century. Nevertheless, about 97% of the world’s 850 million olive trees are still grown in the Mediterranean region. In tropical Africa a small olive industry producing table olives is developing in Namibia.
The wild African olive (Olea europaea subsp. cuspidata (Will. ex G.Don) Cif.) occurs in Central, East and southern Africa and in the Indian Ocean Islands. It is also found in Arabia and from south-western Asia to China.
The main product of the olive tree is the edible oil extracted from the mesocarp (pulp) of the fruit and commonly used as a cooking and salad oil and in the preservation of various foods. It is much appreciated for its specific flavour and supposedly beneficial effects on health due to the high concentration of mono-unsaturated fatty acids and polyphenolic anti-oxidants. Lower grade olive oil is used in the manufacture of soap, cosmetics and lubricants. In perfumery the oil is a good, although sticky, carrier oil for essential oils. Traditionally, olive oil also has various pharmaceutical applications and has served as lamp oil, as well as for treating wool.
Fruits are processed into whole green and black table olives, often mixed with various condiments. They are sometimes pitted, and then stuffed with sweet pepper or anchovy. They are sliced, minced or made into paste such as ‘tapenade’ in the south of France. They are eaten as an appetizer or used in cooking. The presscake is not a very suitable livestock feed, but can be used as fuel or fertilizer. The leaves provide cattle feed and in Tanzania they are used in brewing beer. The wood is valuable, hard and fairly durable, but it is rarely available in large sizes. It is used for turnery and furniture, and is much appreciated for handicrafts; in larger sizes, it is also used for flooring and railway sleepers. The Maasai people of East Africa use it to make clubs and for poles for houses. It makes excellent fuelwood and charcoal.
The leaves have been used for a long time to clean wounds. Olive leaves are applied to lower blood pressure and to help improve the function of the circulatory system. They are also taken as a mild diuretic and may be used to treat conditions such as cystitis. Having some ability to lower blood sugar levels, the leaves have been taken to treat diabetes. The oil is traditionally taken with lemon juice in teaspoonful doses to treat gallstones.
Olive trees are planted for ornamental purposes, as firebreaks and to control soil erosion.
Production and international trade
Average world production of olive oil during the period 2002–2005 was 2.5 million t/year, almost all from the Mediterranean region. The biennial bearing habit of the olive tree and variable weather conditions cause considerable fluctuations in annual world production (2.1–2.9 million t). The total area planted with olive trees is estimated at 8.1 million ha in 25 countries. The principal olive oil producing countries are Spain (32%), Italy (23%), Greece (14%), Turkey (8%), Tunisia (5%), Syria (5%), Morocco (3%), Egypt (2%), Portugal (2%) and Algeria (1%), which together account for 95% of the world supply. About 600,000 t per year reach the international vegetable oil market; the European Union and United States are the main importers of olive oil. Olive oil commands better prices than other table oils.
The 1.1 million t of table olives produced annually represent about 8% of total olive fruit yields. Spain is the largest producer of table olives (25%) followed by the United States (14%), Turkey, Morocco, Syria, Greece and Italy (6–9% each). In the Mediterranean Basin, table olives are sold in great variety by specialized sellers.
Mature olive fruits weigh 2–12 g. They consist of mesocarp 70–90%, endocarp (stone) 9–29% and seed 1–3%. Per 100 g fresh edible portion, the mesocarp contains: water 60–70 g, crude protein 1–2 g, fat 15–30 g, carbohydrate 3–6 g, cellulose 1–4 g, phenolic compounds 1–3 g, ash and other substances 1–3 g. The fatty acid composition of the oil is: palmitic acid 7.5–20%, palmitoleic acid 0.3–3.5%, stearic acid 0.5–5%, oleic acid 55–83%, linoleic acid 3.5–20%, linolenic acid 0–1.5%, arachidic acid 0.1–0.6% and traces of gadoleic acid, behenic acid and lignoceric acids. The anti-oxidant effect of the phenolic compounds (50–400 ppm) and the high oleic content combine to give an oil of exceptional stability even during deep frying. Olive oil is classified into two main quality classes: cold-pressed or virgin oil and refined olive oil. Virgin olive oil is one of the few vegetable oils that is traded and consumed without any refinement and contains its full complement of secondary compounds. Mainly oleuropein but also other phenolic compounds are responsible for the intense bitterness of olive fruits, as well as for fruit blackening and inhibition of micro-organisms during processing. The bitterness in table olives is largely removed in the early stages of processing.
The heartwood is yellowish brown to reddish brown, with dark streaks, demarcated from the pale yellow sapwood. The wood is heavy and hard, and oily to the touch. The grain is straight or slightly wavy, the texture fine and even. The density at 12% moisture is more than 1150 kg/m3. It dries moderately slowly with high shrinkage and considerable distortion. Shrinkage from green to 12% moisture content is about 4.5% radial and 6.5% tangential. The wood is difficult to work because of its hardness and tends to blunt cutting edges rapidly. With care it may be turned and planed. It produces a nice finish. Its natural durability is high, but it is moderately susceptible to termites and borers, The heartwood and water extracts from it are fluorescent.
- Evergreen tree up to 20 m tall or densely branched shrub up to 5 m tall; root system extensive with main roots thickened by fasciation; bole often fluted or crooked, up to 100(–200) cm in diameter, at the base with protuberances (spheroblasts) with additional lateral roots; bark rough, longitudinally fissured, grey to dark brown; crown with spreading branches, young branches 4-angular, whitish, thorny, with numerous lenticels.
- Leaves opposite, simple and entire, without stipules; petiole up to 1.5 cm long; blade elliptical to lanceolate, 3–9 cm × 0.5–3 cm, cuneate at base, acute at apex, leathery, dark grey-green and glabrous above, densely silvery scaly beneath, pinnately veined.
- Inflorescence an axillary panicle, 3–8 cm long, many-flowered.
- Flowers bisexual, regular, 4-merous, fragrant; pedicel short; calyx cup-shaped with broadly triangular lobes, persisting in fruit; corolla c. 2.5 mm long, white, with short tube and 4 elliptical lobes; stamens 2, filaments short, anthers large; ovary superior, 2-celled, style short, stigma 2-lobed.
- Fruit a globose to ellipsoid drupe 0.5–4(–6) cm × 0.5–2.5 cm, bright green, turning purple-black, brown-green or ivory-white at maturity, mesocarp rich in oil; endocarp stony, usually containing 1 seed.
- Seed ellipsoid, 9–11 mm long with straight embryo and copious endosperm.
- Seedling with epigeal germination.
Other botanical information
Olea comprises 33 species, most of them occurring in eastern and southern Africa and in tropical Asia. In the complex species Olea europaea, 6 subspecies, one of which with 2 varieties, are recognized based on morphological characters and geographic distribution:
- subsp. europaea var. europaea, the cultivated olive.
- subsp. europaea var. sylvestris (Mill.) Lehr., the wild olive or oleaster of the Mediterranean basin.
- subsp. cuspidata (Wall. ex G.Don) Cif. (synonyms: Olea africana Mill., Olea chrysophylla Lam., Olea europaea subsp. africana (Mill.) P.S.Green), the most widespread wild olive in tropical Africa.
- subsp. laperrinei (Batt. & Trab.) Cif., a wild olive endemic to the Saharan mountains.
- subsp. maroccana (Greut. & Burdet) P.Vargas, a wild olive occurring in the Atlas mountains in Morocco.
- subsp. cerasiformis G. Kunkel & Sunding, the wild olive of Madeira.
- subsp. guanchica P.Vargas, J.Hess, F.Muñoz Garmendia & J.Kadereit, the wild olive of the Canary Islands.
The wild types are generally distinguishable from the cultivated olive by their much smaller fruits (5–12 mm long) with thin layer of oil-bearing mesocarp and often more dense, twiggy and spiny habit. Although recent molecular evidence based on chloroplast and mitochondrial DNA polymorphisms has confirmed considerable genetic diversity between the taxa, all have remained largely interfertile and the wild types should provide a valuable gene-pool for the improvement of the cultivated olive, e.g. for disease and pest resistance and adaptation to new environments.
More than 2000 cultivars are known, which according to their use, can be distinguished into three groups:
- Cultivars for oil extraction, e.g. ‘Picual’, ‘Arbequina’ and ‘Blanqueta’ in Spain; ‘Frantoio’ and ‘Leccino’ in Italy and ‘Koroneiki’ in Greece.
- Cultivars for fruit consumption, e.g. ‘Gordal Sevillana’ and ‘Manzanilla de Sevilla’ in Spain, ‘Conservolea’, ‘Kalamata’ and ‘Chaldiki’ in Greece, ‘Picholine du Languedoc’ in France, ‘Manzanillo’ and ‘Mission’ in the United States and ‘Oliva di Spagna’ and ‘Oliva di Cerignola’ in Italy.
- Dual-purpose cultivars (for oil extraction and fruit consumption), e.g. ‘Hojiblanca’, ‘Manzanilla Cacereña’ and ‘Aloreña’ in Spain, ‘Tanche’ in France, ‘Picholine marocaine’ in Morocco, ‘Dan’ in Syria and ‘Arauco’ in Argentina.
Growth and development
Practically all olive trees in the world are grown from clonal cultivars. Seeds germinate within 25–50 days after sowing, but seed viability of cultivated olives is generally low. Olive seedlings have a distinct juvenile phase lasting 4–9 years and characterized by strong vegetative growth and profuse branching. Plants raised from cuttings have a more adult growth habit with monopodial branching and may start flowering within 3–7 years after field planting. The life of leaves is 2–3 years. Flowering occurs annually in spring on branch segments formed during the previous season, with 50–80% of the leaf axils developing inflorescences. Wind pollination and cross-fertilization are the rule due to self-incompatibility. Even under optimum conditions of pollination and initial fruit set, generally only 1–5% of the flowers will develop into mature fruits due to severe early (up to 50%) and late physiological fruit abscission, water stress, diseases and pests. In a year of profuse flowering, such low fruit set still represents a large crop. Olive is a strongly biennial bearer, because a heavy fruit load in one year inhibits adequate shoot extension necessary for the following year’s bearing wood and vice versa. Olive fruit development takes 6.5–7 months from anthesis to harvesting, the last 20–40 days being essential for oil formation in the mesocarp.
The commercial life span of an olive tree is about 50 years, but individual trees can become very old (hundreds of years). Very often, old trees are hollow, usually because during its history, fungus diseased wood has been cut away repeatedly. Such old, gnarled trees are often also twisted and slanting, giving the tree a peculiar appearance: abundant, fresh, lively, young, green sprouts on an old, grey, twisted, gnarled and slanting, hollow cylinder.
The olive tree is well adapted to the seasonal and relatively dry climate of the Mediterranean region. Worldwide cultivation is concentrated between 30–45° latitudes in the northern and southern hemispheres, from sea-level to 900 m altitude on south-facing slopes (higher than 1200 m in Argentina). Frost in spring can damage young shoots and flowers, and the ripening fruits in late autumn. Olive trees are fairly frost-hardy during winter, tolerating –8°C to –12°C. For flower initiation, most olive cultivars require a vernalization period of 6–11 weeks below 9°C which ends 40–60 days before anthesis. Optimum temperatures for shoot growth and flowering are 18–22°C. Temperatures above 30°C in spring can damage flowers, but the tree can withstand much higher temperatures in summer. The xerophytic physiology of olive trees makes them highly tolerant of long periods of water stress, but for economic yields, low and irregular rainfall (less than 300 mm) should be supplemented by irrigation during critical growth stages to 500–800 mm per year.
Soils should be light textured (less than 20% clay), well drained and have a depth of at least 1.5 m. Olives can do well on very poor soils, except when these are waterlogged, saline or too alkaline (higher than pH 8.5).
In tropical Africa wild olive occurs in montane woodland, rainforest and wooded grassland at 1000–3150 m altitude. They are often found on rocky hillsides, in forest margins and along dry riverbeds, and may occasionally form almost pure stands.
Propagation and planting
The main method of propagation of olive is based on rooting of semi-hardwood cuttings prepared from one-year-old branches (10–12 cm long with 4–5 nodes and two pairs of leaves). Propagation by seed is possible but gives rather variable seedlings because of cross fertilization. Seed is mostly used for breeding purposes. In-vitro micro-propagation of olive explants has not yet passed the experimental stage, partly because of large variation in rates of success between different cultivars. Somatic embryogenesis is very difficult to achieve from adult tissues and cannot be used for propagation purposes. Traditional methods of clonal propagation are: large hardwood cuttings, grafting on seedlings or mature trees, grafting on wild olive trees and rooting of fragments of protuberances with a shoot attached. Protuberances can also be used for in situ rejuvenation of very old and decaying olive trees.
Plants from rooted cuttings are raised in beds or polythene bags in nurseries for 1.5–2 years prior to planting in the field in spring. They are planted in large holes (40 cm × 40 cm × 60 cm) which are later refilled with topsoil, compost and fertilizers, especially P and K. Plant densities traditionally vary from 40–60 trees/ha in very dry areas to 300–400 trees/ha under optimum soil conditions and water availability (more than 600 mm) and using cultivars with more compact and erect growth habit. Field experiments with high density olive orchards (up to 2000 trees/ha planted in hedges) are in progress in Spain and France. The majority of olive orchards in the Mediterranean region have traditional densities of 100–250 trees/ha. Planting along contour lines or in terraces is necessary in sloping terrain to prevent soil erosion. Leguminous and cereal crops have been planted as intercrops in olive groves.
The olive tree requires pruning to shape it into the desired main frame and crown, to maintain a proper balance between vegetative growth and fruit production and so reduce biennial bearing and to rejuvenate senescent trees. There is a long tradition of manual pruning methods and some are region specific. Mechanized maintenance pruning is done in modern olive orchards, but requires adaptation of tree shape and careful management to prevent excessive branch damage and subsequent disease problems.
Regular fertilizer application is needed for sustained fruit production, but type and rate vary with local climate, soil condition and agronomic practice. Foliar analysis provides information on the nutrient status of olive trees. Nutrients removed by 3 t of fruit amount to about 19 kg N, 9 kg P2O5 and 25 kg K2O. A general fertilizer recommendation would be: annual applications of 0.8 kg N (in 2–3 split applications), 0.3 kg P2O5 and 0.9 kg K2O per tree at medium planting density (150 trees/ha). This corresponds to 120 kg N, 45 kg P2O5 and 135 kg K2O per ha. Occasional correction of calcium, magnesium and boron deficiencies may also be needed. Triennial application of organic manure or compost (50 kg/tree) is recommended to improve soil texture and fertility. This can also be done before planting.
Only 15% percent of areas planted with olive trees worldwide are actually irrigated but this is steadily increasing. Surface, sprinkler and drip irrigation are some of the methods applied to supplement deficient rainfall in intensive olive cultivation. Correctly timed and dosed irrigation is required to produce economic responses in yield and fruit quality. Irrigation combined with ground cover positively influence olive production and soil conservation.
Diseases and pests
Leaf spot or peacock spot caused by Spilocaea oleagina (Cycloconium oleaginum) is the most common disease in olive cultivation. Methods of control include preventive copper-based fungicide sprays and host resistance. Copper sprays also have a tonic effect of promoting longer leaf retention. Other diseases are sooty mould caused by secondary infection of Alternaria, Capnodium and Cladosporium spp. following black scale infestation, Verticillium wilt caused by Verticillium dahliae and bacterial canker or olive knot caused by Pseudomonas syringae pv. savastanoi.
There are numerous pests, which generally cause much more economic harm to olive cultivation than diseases. The most damaging insect pests are the olive fly (Bactrocera oleae) and olive moth or kernel borer (Prays oleae, synonym: Prays oleellus) on fruits, black scale (Saissetia oleae) on branches, jasmin moth (Margaronia unionalis) on young shoots, bark beetles (Hylesinus oleiperda and Phloeotribus scarabaeoides) on branches and trunk, psyllids (Euphyllura olivina) sucking on flowers, mites (Aceria oleae) on leaves and fruits, and thrips (Liothrips oleae) on flowers and young leaves. Insect control in olive cultivation is increasingly based on systems of integrated pest management including monitoring, pheromone trapping, promoting or releasing natural enemies, Bacillus thuringiensis -based insecticides and cultural measures such as pruning and irrigation.
Olive fruits intended for oil are harvested at full maturity in late autumn or early winter, either mechanically or with the use of rakes, beating poles and collecting nets. Table olives are harvested by hand; mature green fruits in early autumn and black olives in late autumn. Manual fruit picking (capacity about 80 kg/person per day) accounts for 50–60% of field production costs. Machines developed to reduce harvesting costs include trunk and branch shakers in combination with inverted umbrellas or rolling canvas frames to catch the fruits. Self-propelled overhead harvesting machines in olive orchards planted in hedge rows and the application of chemicals (e.g. ethephon) to promote fruit abscission shortly before harvesting are still in the testing stage.
World average yield in 2005 was 2.0 t of olive fruits per ha. Fruit yield per ha varies from 1–3 t in traditional olive groves to 4–10 t under irrigation and optimum agronomic practices (e.g. in Italy at 280 trees per ha). In well-managed plantings under rainfed conditions, fruit yield is 2–5 t/ha. There is always considerable year-to-year variation in productivity. About 5–6 kg of fruits are needed to produce 1 kg oil, giving a world average of 350–400 kg/ha in 2005.
Handling after harvest
Oil extraction should start within 1–3 days after fruit harvesting to avoid a change in flavour and increase in free fatty acid content. The fruits are washed, crushed and mashed into a uniform paste, from which the oil is cold-extracted by mechanical pressing or centrifuging. The ‘margine’, or mixture of water and oil, is allowed to settle and the oil is separated by decantation, centrifugation and filtration. Oil prepared exclusively by this process, i.e. by physical means only and without any heating, is called virgin olive oil. In the European Union, virgin olive oil is graded into 4 classes based on many characteristics of which the most important ones are free fatty acid content and organoleptic test score: extra virgin oil, virgin oil, standard and ‘lampante’ virgin oil. ‘Lampante’ virgin oil and oil obtained by heating or solvent extraction are either used industrially or have to be refined by neutralization, bleaching and deodorization to produce refined olive oil. The cold-extracted cake or pomace may undergo further solvent extraction to produce an industrial grade ‘olive-pomace’ oil.
Preservation of table olives starts with soaking fruits in an alkaline solution to reduce the bitterness before pickling in brine (Spanish-style and Californian-style). The Greek-style preservation of fully ripe, black olives involves pickling in brine without alkaline pre-treatment.
The numerous traditional olive cultivars (estimated at 2000) are gradually disappearing because of abandonment of marginal groves and urbanization or replacement by modern cultivars. Programmes to collect and preserve this valuable olive germplasm are in progress with the support of the International Olive Oil Council (COI) and the European Union. In addition to the Olive World Collection in Cordoba (Spain) with 310 accessions, there are 73 collections of olive germplasm in 23 countries and a project of a second world collection at Marrakech (Morocco).
Wild olive is widespread in tropical Africa and locally common, and not under threat of genetic erosion.
Olive improvement has a long tradition of clonal selection. Breeding programmes based on inter-cultivar crosses followed by selection within segregating seedling populations are of fairly recent date. The long juvenile phase of olive seedlings has been an impediment to breeding, but forcing methods and existing genetic variation in length of juvenile phase have contributed to shorter breeding cycles. Main criteria of selection in the olive are fruit yield, regular production, cold tolerance, early first bearing, compact growth, oil content of the mesocarp, quality of the oil and resistance to diseases and pests. Quality of olive oil is determined by standard physical and chemical analyses and sensory assessment of taste and flavour. Host resistance to Spilocaea oleagina has been reported in Israel and to Pseudomonas syringae pv. savastanoi in Portugal. Progress is also being made with the application of molecular biology in the olive, including molecular markers for cultivar identification, the construction of a linkage genome map and marker assisted selection. There are no crossing barriers for introgression of desired characters from the oleaster and other wild subspecies of Olea europaea.
Increasing interest in the olive as a source of high quality and healthy vegetable oil may have a positive effect on world production, notwithstanding its high production costs in relation to other vegetable oils. Olive also contributes considerably to environmental protection (soils, flora and fauna) in dry and hilly areas. There may be opportunities for olive cultivation in Central, East and southern Africa, in particular where wild olive trees already occur.
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Correct citation of this article
van der Vossen, H.A.M., Mashungwa, G.N. & Mmolotsi, R.M., 2007. Olea europaea L. [Internet] Record from PROTA4U. van der Vossen, H.A.M. & Mkamilo, G.S. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>.
Accessed 28 March 2021.
- See the Prota4U database.