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Citrullus lanatus (Thunb.) Matsum. & Nakai

Cat. Sem. Spor. Hort. Bot. Univ. Imp. Tokyo 30: No 854 (1916).
Chromosome number
2n = 22
Momordica lanata Thunb. (1794), Citrullus vulgaris Schrad. ex Eckl. & Zeyh. (1836), Colocynthis citrullus (L.) Kuntze (1891).
Vernacular names
– Egusi melon, egusi watermelon, West African watermelon (En). Pastèque égousi, melon à pistache (Fr).
– Watermelon, dessert watermelon (En). Pastèque (Fr). Melancia (Po). Mtikiti, mtikiti maji (Sw).
– Cooking melon, fodder melon, preserving melon, citron (En). Pastèque à cuire, pastèque fourragère, citre, méréville (Fr).
Origin and geographic distribution
Citrullus lanatus originates from the western Kalahari region of Namibia and Botswana, where it can still be found in the wild in a diversity of forms together with other Citrullus species. In this region there are two major types, one with small fruits that are generally bitter and mainly used for their seeds, called ‘tsama melon’. This is the probable ancestor of egusi melon. The other type has fruits that are mainly used as a source of water during periods of drought or as cooking melons, and may well represent the ancestral form of the watermelon, fodder melon and cooking melon. Following first domestication of Citrullus lanatus in southern Africa in prehistoric times, its cultivation became widespread in Mediterranean Africa, the Middle East and West Asia more than 3000 years ago. Introduction into India must also have occurred in ancient times and here a strong secondary centre of genetic diversity developed. Citrullus lanatus reached China around the 10th century and Japan in the 16th century. It was introduced to the Americas in early post -Columbian times.
Egusi melon, grown for its seed, has probably been domesticated in the southern Sahel zone or in regions surrounding the Kalahari desert. It is especially important in West Africa. People in Namibia and Botswana still harvest most of their seeds from the wild, but some landraces have been selected specifically for their oil-rich seed. Cultivars grown in western Sudan are probably of the same type as the egusi melon of West Africa. Seed melons are also important in China.
Watermelon is now widespread in all tropical, subtropical and warm temperate (hot summers) regions of the world, including Africa. Cooking melons are mainly found in the northern and eastern parts of the Kalahari desert. Fodder melon is mainly grown in the United States and South Africa.
Citrullus lanatus comprises overlapping groups of cultivars that yield seed or edible fruits. Fruits of wild or semi-wild plants are used in the Kalahari region as a source of drinking water. The same use is reported from Sudan. Other primitive forms are used as forage.
Most important in Africa are cultivars of which the only edible portion are the seeds. The fruit pulp of these cultivars is too bitter for human consumption. In West Africa they are called ‘egusi’, derived from the Yoruba language; in Wolof language (Senegal) they are called ‘beref’. In the Kalahari region, the seeds are considered a delicacy. After roasting, they are ground into a coarse, whitish meal, which is nutritious and pleasantly nutty-tasting. In West Africa the seeds are made into pulp and added as thickener to soups. They are also fermented to produce a sweetener locally called ‘ogiri’ or they are roasted, pounded, wrapped in leaves and then boiled to produce another sweetener called ‘igbãlo’. The pulp of roasted and salted seeds is eaten in Sudan and Egypt, where it is called ‘tasali’. In the far northern parts of Sudan seeds of some types are eaten whole, including the seedcoat, after being roasted; these are called ‘gorom’. A highly prized vegetable oil is extracted from the seed. This oil is used for cooking and for cosmetic purposes and is of interest to the pharmaceutical industry. The residue from oil extraction is made into balls that are fried to produce a local snack called ‘robo’ in Nigeria, or is used as cattle feed. The oil is used in making soups and in Namibia it has traditionally been used for making soap. The seeds can be roasted to make a substitute for coffee.
Many cultivars are grown as a vegetable crop for fresh consumption of the refreshingly juicy and sweet flesh of the mature fruit. In most parts of the world the watermelon is the most important type of Citrullus lanatus.
In several African countries, local non-sweet, non-bitter cultivars are used as cooking melon, e.g. in Kenya, Namibia, Botswana and Zimbabwe. Related types in Sudan are sometimes referred to as ‘citron’ or ‘citronnel’. Young fruits from which the seeds have been removed are cooked until they are soft. In Zimbabwe the cooked melons are mixed with cooked beans or cowpeas, and powdered seeds of bottle gourd are added. To preserve the fruit flesh, the seeds and rind are removed and slices are dried in the sun. A stiff porridge is made from mature fruits mixed with maize or pearl millet flour. The leaves are occasionally used as a cooked vegetable. In the United States the rind of some cultivars is made into pickle or a sweet preserve. In the south of France, the preserving melon or citron is popular for jams. In the extensive farming systems of semi-arid regions, leaves and fruits of fodder melons are a source of forage and water for livestock. The fruits are used as a drastic purgative in Senegal; they are diuretic and used to treat diarrhoea and gonorrhoea in Nigeria. Tar is extracted from the seeds and used for the treatment of scabies and for skin tanning.
Production and international trade
West and Central Africa are the largest producers of egusi seed, but statistics are scarce. FAO statistics include melon seed, but they also refer to seed of Cucumeropsis mannii Naudin and several Cucurbita species. World production of melon seed in 2002 was reported as 576,000 t from 608,000 ha. Production in Nigeria amounted to 347,000 t from 361,000 ha, Cameroon produced 57,000 t, Sudan 46,000 t, DR Congo 40,000 t, Central African Republic 23,000 t and Chad 20,000 t. Outside Africa, China is important with a production of 25,000 t. An estimated 5000–7000 t is traded from Nigeria to other West African countries. Sudan (where production for seed is important especially in Kordofan) exports about 27,000 t, mainly to Arab countries; however, quantities fluctuate strongly from year to year. Unrecorded trade of pulp and oil from the seed to African communities in North America and Europe occurs. For the Kalahari region international trade has recently started, but is still on a small scale.
Annual world production of watermelon increased from 30 million t from 2.1 million ha in 1992 to 81 million t from 3.2 million ha in 2002. China alone produced 57 million t from 1.8 million ha in 2002. Other major producers of watermelon include Turkey with 3.9 million t, North Africa (Algeria, Egypt, Libya, Morocco and Tunisia) 2,9 million t, Commonwealth of Independent States 2.8 million t, Iran 1.9 million t, United States 1.8 million t, Mediterranean Europe (Greece, Italy, Spain) 1.8 million t, Mexico 871,000 t and Brazil 620,000 t. Countries in tropical Africa with sizeable watermelon production are Senegal with 224,000 t, Sudan 143,000 t, Cameroon and Somalia each 28,000 t and Mauritania 11,000 t. Watermelon is mainly produced for local and urban markets, each country having its preferences for size and type. Production for export markets has developed in the Mediterranean region, Mexico, Taiwan, Malaysia and Thailand, with mainly smaller-fruited F1 hybrid cultivars, including seedless types.
No information is available on the production and trade of cooking melon.
The composition of dried egusi seed without shell per 100 g is: water 5.1 g, energy 2340 kJ (557 kcal), protein 28.3 g, fat 47.4 g, carbohydrate 15.3 g, Ca 54 mg, P 755 mg, Fe 7.3 mg, thiamin 0.19 mg, riboflavin 0.15 mg, niacin 3.55 mg, folate 58 μg. The seed is an excellent source of energy and contains no hydrocyanic acid, making it suitable as livestock feed. The seed oil consists of glycosides of linoleic, oleic, palmitic and stearic acids. The fruit flesh contains bitter cucurbitacins.
The composition of watermelon per 100 g edible portion (50–70% of the mature fruit) is: water 91.5 g, energy 134 kJ (32 kcal), protein 0.6 g, fat 0.4 g, carbohydrate 7.2 g, Ca 8 mg, P 9 mg, Fe 0.17 mg, thiamine 0.08 mg, riboflavin 0.02 mg, niacin 0.2 mg, folate 2 mg, ascorbic acid 9.6 mg (USDA, 2002). The total soluble solids (TSS) content of the fruit flesh, mainly sucrose and fructose and measured by a refractometer (°Brix, equivalent to% TSS), is an important property for quality; 8% is marginal, 10% acceptable and 12% or higher excellent. Watermelon is a rich natural source of lycopene, a carotenoid of great interest because of its antioxidant capacity and potential health benefits.
Adulterations and substitutes
Seeds of Citrullus lanatus may be replaced by several other cucurbit seeds which are generally referred to as egusi seeds; the most important of these is the real egusi, Cucumeropsis mannii.
Monoecious, annual, scandent or trailing herb up to 4(–10) m long, climbing by (1–)2(–4)-fid tendrils; roots shallow, with a taproot and many lateral roots; stem ridged, rather softly long-hairy. Leaves simple, alternate; stipules absent; petiole 2–14 cm long, long-hairy; blade oblong-ovate in outline, 4–25 cm × 3–19 cm, deeply palmately 3–5(–7)-lobed, lobes usually more or less pinnately sinuate-lobulate, margin shallowly sinuate-toothed, long-hairy on the veins, becoming scabrid-punctate. Flowers solitary in leaf axils, unisexual, 2–3.5 cm in diameter, regular, 5-merous, yellow; pedicel 1.5–4 cm long; calyx campanulate; petals united below; male flowers with 3 free stamens; female flowers with inferior, 1-celled, hairy ovary, stigma 3-lobed. Fruit a berry, usually globose to oblong or ellipsoid, sometimes ovoid, 5–70 cm long and weighing 0.1–30 kg (0.1–2.5 kg in egusi melon, 1.5–30 kg in watermelon), white to green, grey or yellow, uniform or mottled or striped, flesh white to pale green, yellow or red, many-seeded. Seeds obovate to elliptical, flattened, 0.5–1.5 cm × 0.5–1 cm, smooth, yellow to brown or black, rarely white. Seedling with epigeal germination; cotyledons leafy, rounded to oblong.
Other botanical information
Citrullus belongs to the tribe Benincaseae of the subfamily Cucurbitoideae. It comprises 4 species, 2 of which are endemic to Namibia. All species can be hybridized with relative ease, giving fertile F1 hybrids, but F2 progeny of crosses with Citrullus lanatus as one of the parents showed a high degree of sterility.
Citrullus lanatus is sometimes divided into 3 subspecies:
– subsp. lanatus: including wild plants, the ‘tsama melon’ and Citroides Group, comprising fodder melon and citron melon;
– subsp. mucosospermus Fursa: egusi types from West Africa;
– subsp. vulgaris (Schrad.) Fursa: comprising cultivated watermelon types classified in Dessert Group, also including cultivars grouped together under Cordophanus Group, a variable cultivar-group in the Sahara, Sahel and East Africa.
However, there is a complete overlap between the various types of this highly polymorphic species. In Africa, seed melons are by far the most important group. In Nigeria, there are two major seed types, which can be differentiated by the presence or absence of a seed edge. The two types are referred to as ‘bara’ (with prominent thick seed edge with black or white colour) and ‘erewe’ (without pronounced seed edge). The two seed types can be regarded as different cultivar-groups.
Hundreds of open-pollinated and F1 hybrid cultivars are available to watermelon growers in the world. ‘Charleston Gray’ with oblong to cylindrical fruits of 6–15 kg, pale green rind with small veins and pink-red flesh, and ‘Sugar Baby’ with globular fruits of 3–8 kg, dark green rind and red flesh are still the most important cultivars in tropical Africa. New F1 cultivars with improved agronomic characteristics and disease resistance, e.g. ‘Logone’, ‘Sugar Dragon’ and the seedless ‘Sunshine’, are now being introduced to the African market gardeners.
Growth and development
Seed will remain viable for at least 8 years when stored dry at temperatures below 18°C. Emergence of the seedling takes 5–7 days. Cotyledons unfold after 10–12 days and the first true leaf appears one week later. In egusi melon lateral branches are produced on the main stem from node 4–6 and the first male flower is formed on node 8–11 at 35–50 days after sowing, the first female flowers on node 15–25 at 45–60 days after sowing. In watermelon both the first male and female flowers are formed somewhat later. The first female flowers often have poorly developed ovaries and fail to set fruit. Flowering peaks 50–80 days after germination. Flowers open shortly after sunrise and remain open only one day. Pollination occurs in the morning and is done by insects, predominantly bees. Ample deposition of pollen on all three stigmas is necessary for regular fruit development. Within 24 hours after pollination the pedicel starts to elongate and bends downward with the swelling ovary. The fruits of watermelon are mature 30–50 days after pollination. At maturity the green fruit stalk turns brown.
In the wild Citrullus lanatus prefers deep sands and occurs especially in dry watercourses and the sandy flats surrounding them. It is also found naturally in disturbed areas or as a weed in cultivated land. It is daylength neutral. Egusi watermelon is cultivated in tropical lowlands up to 1000 m altitude, watermelon up to 2000 m. Both perform better in the savanna region than in the wet forest zone. West African seed types require an average annual rainfall of at least 700–1000 mm and a daytime temperature of 28–35ºC. In the Kalahari region, seed melons usually only get 400–650 mm of rain. Excessive rainfall and high humidity give excessive vegetative growth and promote disease infection, mainly leaf and fruit rot, with consequent low yields. Although irrigated dry season cropping gives higher yields, local farmers prefer to plant seed melons during the rainy season, due to lack of irrigation facilities. Seed yield in the dry savanna zone is 2–3 times higher than in the rainforest zone.
Well-drained sandy loam soil with pH 6–7 is adequate; at lower pH values soilborne diseases (Fusarium) may become a serious problem. A waterlogged soil predisposes the crop to attacks of anthracnose disease and fruit rot. A moderately rich soil is required for early and close vegetative cover, which is suitable for control of weeds and erosion.
A wide crop rotation (cucurbits once in 4–6 years) is essential to avoid damage by soilborne diseases and pests. Supplementary irrigation is required before ploughing in case of prolonged drought. Two or three weedings are needed before the stems grow into a thick vegetative cover over the soil surface, attained in 6–8 weeks after sowing. The movement in the crop should then be reduced to prevent plant damage. Citrullus lanatus responds well to fertilizers, especially to organic matter. The amount required depends on the nutrient status of the soil. In general application at a rate of 20–30 t/ha organic manure, 50–60 kg N, 10–15 kg P and 20–30 kg K per ha is suitable for good performance. In commercial production of watermelon mulching with polythene sheets (black, transparent or silver-painted) or straw is common practice to conserve moisture, raise or lower soil temperatures, suppress weeds and prevent direct contact of the fruits with the soil. Stems are trimmed to prevent excessively dense vegetative growth and usually only 2 fruits per plant are left to mature, or up to 6 in small -fruited cultivars.
Propagation and planting
There is no seed dormancy, but germination is retarded under high temperature regimes. Germination can be accelerated by pre-soaking in water for 24 hours after scarifying the seed at one end, especially for cultivars which have a hard seed coat. Seed germinates best at temperatures of 17°C at night and 32°C at daytime and also at a constant temperature of 22°C. It will not germinate at temperatures below 15°C. Light has an inhibitory effect.
After soil preparation seed melon is sown directly on ridges or in flat plots. Both sole cropping and mixed cropping with maize, pearl millet, sorghum or cowpeas occur. Per hill 3–4 seeds are sown at a depth of 3–4 cm; 3–4 weeks after sowing, at the 2–4 leaf stage, seedlings are thinned to 1–2 per hill. A spacing of 2 m × 2 m or 1 m × 4 m with a population of about 2500 plants/ha is suitable in sole cropping, but a spacing of 2 m × 1 m is also used. The minimum seed requirement for sole cropping is 0.8–1.5 kg/ha; in mixed cropping farmers use 1.0–2.0 kg/ha.
Watermelon is seeded directly in a similar way or transplanted after raising seedlings in pots of 9 cm diameter. Seedlings are transplanted to the field when they have 3–4 true leaves, some 5 weeks after sowing. Planting distances are 0.9–1.2 m × 1.2–1.8 m giving a density of 5000–9000 plants/ha. Seed rates per ha are 1–2 kg for direct-seeded and 0.3–0.5 kg for transplanted watermelon. Seedless (triploid) cultivars are practically male sterile and require a diploid cultivar planted every third row for adequate pollination and fruit set. Fruits of the diploid pollinator cultivar should be clearly distinct for easy separation of the more valuable seedless fruits.
Diseases and pests
There are a number of important diseases, although Citrullus lanatus is less susceptible than cucumber and melon. Fusarium wilt (Fusarium oxysporum f.sp. niveum) can be prevented by wide crop rotation (preferably once in 6 years), ensuring good drainage and using tolerant/resistant cultivars. Anthracnose (Glomerella cingulata var. orbiculare, synonym: Colletotrichum lagenarium) can be controlled by dithiocarbamates and organic fungicides, but cultivars resistant to some of the 5 known races are available. Gummy stem blight (Didymella bryoniae, synonym: Mycosphaerella citrullina) can also be controlled with fungicides, and sources of resistance have been identified in wild Citrullus accessions. Powdery mildew (Sphaerotheca fuliginea) may occur, but in hot and humid climates downy mildew (Pseudoperonospora cubensis) is more important. Bacterial rind necrosis (Erwinia carnegieana) may be serious, but differences in susceptibility exist amongst accessions. Bacterial fruit blotch (Acidovorax avenae subsp. citrulli) is a relatively new disease, reported in China and the United States since 1989, which requires disease-free seeds and seedlings, crop rotation and preventive copper sprays to avoid serious outbreaks. Major virus diseases are watermelon mosaic virus (WMV-2), papaya ringspot virus (PRSV-W) and zucchini yellow mosaic virus (ZYMV), which are all transmitted by aphids and cucumber beetles. Some egusi melon lines are resistant to WMV-2. Other diseases include damping-off and root rot (Pythium spp.), Alternaria leaf blight (Alternaria cucumerina), Cercospora leaf spot (Cercospora citrullina), Phytophthora root, crown and fruit rot (Phytophthora capsici), southern blight (Sclerotium rolfsii) and squash lead curl virus (SqLCV) transmitted by silverleaf whitefly (Bemisia argentifolia).
Root-knot nematodes (Meloidogyne spp.), particularly serious on sandy soils, can be prevented by crop rotation, destruction of susceptible weed hosts, solar sterilization of the soil (instead of expensive and environmentally hazardous fumigation) and grafting on resistant rootstocks.
Common insect pests are thrips (Thrips spp.), mites (Tetranychus spp.), aphids (Aphis gossypii), fruit fly (Dacus ciliatus), cucumber beetles (Diabrotica spp.), red pumpkin beetle (Aulacophora spp.), loopers (e.g. Spodoptera exigua, Trichoplusia ni), Epilachna beetles and leaf miner (Liriomyza spp.). There are many types of insecticides to control the various insect pests, but indiscriminate spraying usually aggravates the situation by destroying useful parasites. Polythene mulch, especially when coated with reflective aluminium paint, repels thrips and aphids. In southern Africa, the most serious pest is the cucurbit bug Coridius viduatus. However, in many cases farmers are not worried about this pest since their larvae form part of the local cuisine.
In egusi melon fruit maturity is best determined by the withering of the fruit stalk and ancillary tendril. Fruits are cracked using strong short wooden poles. They are then heaped and covered with dry grass to ferment. Fermentation takes about 14 days. The seeds are then scooped out from the pulp and washed. They are spread out in the open to dry under the sun, and turned over several times to ensure an even drying. Depending on temperatures and cloud cover, the seeds are dry in 5–7 days.
For most watermelon cultivars under tropical conditions the first fruits are ready for harvesting 65–90 days after transplanting to the field. Indications of maturity are a muffled sound when tapping the fruit, the pale spot where the fruit rests on the ground turning yellow, the fruit skin increasing in lustre and losing hairs, and the tendrils directly opposite the fruit stalk turning yellow and shrivelling. Watermelon fruits show non-climacteric respiratory behaviour and therefore do not ripen further after harvest. The fruit is cut with about 5 cm of stalk. Fruits harvested in the afternoon are less turgid and less likely to crack during handling and transport.
In West Africa egusi seed yields vary from 225 kg/ha in Senegal to 1100 kg/ha in Nigeria. In Namibia the seed yield ranges from 550 kg/ha to over 3000 kg/ha, depending on the cultivar used. In China an average seed yield of 1500 kg/ha has been reported.
Worldwide the yield of watermelon averages about 25 t/ha, varying from 5–60 t/ha, depending on cultivar and cultural practices. Seed yields of watermelon are 150–400 kg/ha for most cultivars. Experimental yields of cooking melon in Namibia are very high and have exceeded 100 t/ha.
Handling after harvest
In Nigeria the harvested seeds are bulked and packed in jute bags or bath cloth (20–25 kg) for proper aeration and further drying. Plant parts and sand or stones are removed from the seed lots. The seeds are marketed whole (with the seed coat) or as kernels (without seed coat). A portion of the seeds (in Nigeria 10–20%) is made into pulp for sale in the market. ‘Ogiri’ is made by alkaline fermentation with Bacillus and Alcaligenes species. The best temperature for fermentation is 30–35°C.
Watermelon fruits are rather fragile and susceptible to breakage and bruising and should be handled and shipped with care. They can be stored for 2–3 weeks at 10–15°C and 85% relative humidity. In Africa fruits are often greatly damaged as a result of poor handling and transportation.
Genetic resources
In Africa most farmers plant and carefully maintain their own local cultivars. Germplasm collections of Citrullus lanatus (mainly watermelon) are maintained at universities, horticultural institutes and genebanks in the major producing countries. There is a need to complement existing collections with additional germplasm of Citrullus lanatus, related Citrullus species and Acanthosicyos naudinianus (Sond.) C.Jeffrey from the primary (central and southern Africa) and secondary (India, China) centres of genetic diversity. Egusi watermelon germplasm has been collected and maintained ex situ, e.g. by the Horticultural Research Institute (NHR) and Universities in Nigeria and the Plant Genetic Resources Unit of the Agricultural Research Corporation in Sudan. The diversity of watermelon found in Namibia is represented by seed collections held at Namibia’s National Plant Genetic Resources Centre in Windhoek. Improved cultivars have not yet spread sufficiently well among the farmers to pose serious danger to local genetic resources.
Citrullus lanatus is self-compatible but outcrossing is more common. The potential for selfing allows breeders to create a uniform crop in a relatively short time. Breeding programmes in Nigeria and Sudan aim at egusi cultivars with profuse branching, early soil coverage, high number of fruits per plant, many large seeds per fruit, seeds with white colour for export quality and disease tolerance or resistance. In Namibia and Botswana, additional attention is given to the oil content and oil quality of the seeds. Three standard cultivars and nine other promising lines of egusi melon are available at the National Horticultural Research Institute in Nigeria. These are vegetatively very similar but differ in fruit colour, seed type and yield.
Emphasis in breeding watermelon has shifted from developing open-pollinated to F1 hybrid cultivars. Heterosis effect on yield is not always very large, but other advantages of F1 hybrids are larger uniformity of plants and fruits, easier manipulation of fruit shape and seed size and effective accumulation of dominant genes for disease resistance in one genotype. For the increasingly popular seedless cultivars F1 hybrids are the only option. Production of F1 hybrids in watermelon requires hand-pollination and the seedless hybrids are even more difficult and expensive to produce (only 40–100 seeds per fruit of the tetraploid female line against 200–800 for diploid parents). Seed production of watermelon hybrids is therefore generally carried out in countries with low costs of labour and in arid regions to facilitate the production of high-quality and disease-free seed. The recent discovery of cytoplasmic male sterility in China without linkage to negative plant characteristics offers the opportunity to produce legitimate hybrid seeds by bee-pollination.
Major watermelon breeding programmes are presently carried out by government institutes and private seed companies in Japan, China, Taiwan, India and the United States. The most popular F1 hybrid cultivars grown worldwide originate from Taiwan, but more recently seed companies in other countries have started breeding specifically for tropical lowland conditions. Main breeding objectives include compact plant types, earliness, small and round fruits, fruits with a thin but strong rind, high sugar content, finely grained flesh with small seeds and no hollow heart, disease and pest resistance (especially Fusarium wilt, anthracnose and viruses), heat tolerance and improved seed production. There are hundreds of open-pollinated and F1 hybrid cultivars.
Citrullus lanatus seeds are increasingly used for their oil in semi-arid regions and also the use of the oil in the cosmetic and pharmaceutical industry is increasing. There are also prospects for use of the seeds in the improvement of infant nutrition in view of their high protein and fat content. The introduction of new egusi melon cultivars into traditional cropping systems combined with appropriate cultural practices will improve the suppression of weeds and soil erosion and will reduce the production costs. The large diversity of germplasm offers scope for the development of new cultivars. Future research priorities should emphasize high yields through development of vigorous cultivars with early soil coverage and good disease and pest resistance. Further research is needed on the development of improved agronomic practices and labour-saving equipment for seed extraction and shelling. Appropriate storage and oil extraction devices need to be developed whereby oxidation of the oil is minimized.
The demand for watermelon, especially the smaller, ovoid to globular and seedless fruit types, is rapidly increasing in many countries. Sources of resistance to the most important diseases and pests have been identified. Molecular marker-assisted selection and genetic transformation will further increase breeding efficiency in watermelon and may realize effective resistance to diseases and pests, unattainable by conventional breeding. The longer-term prospects of reducing dependence on pesticides when producing watermelon fruits are therefore good. The costs of hybrid seeds are likely to come down once effective male sterility has been incorporated into the female lines of F1 hybrid cultivars.
Major references
• Fursa, T.B., 1981. Intraspecific classification of watermelon under cultivation. Kulturpflanze 29: 297–300.
• Kihara, H., 1951. Triploid watermelon. Proceedings of the American Society for Horticultural Science 58: 217–230.
• Maggs-Kolling, G.L., Madsen, S. & Christiansen, J.L., 2000. A phenetic analysis of morphological varieties in Citrullus lanatus in Namibia. Genetic Resources and Crop Evolution 47(4): 385–393.
• Maynard, D.N. (Editor), 2001. Watermelons: characteristics, production and marketing. American Society for Horticultural Science (ASHS) Press. Horticulture Crop Production Series. Alexandria, VA, United States. 227 pp.
• Mohr, H.C., 1986. Watermelon breeding. In: Bassett, M.J. (Editor): Breeding vegetable crops. Avi Publishing Company, Westport, Connecticut, United States. pp. 37–66.
• Oyolu, C., 1977. A quantitative and qualitative study of seed types in egusi (Colocynthis citrullus L.). Tropical Science 19(1): 55–62.
• Paje, M.M. & van der Vossen, H.A.M., 1993. Citrullus lanatus (Thunberg) Matsum. & Nakai. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 144–148.
• Rhodes, B. & Zhang, X.P., 1999. Hybrid seed production in watermelon. In: Basra, A.S. (Editor). Hybrid seed production in vegetables: rationale and methods in selected crops. Food Products Press, New York. pp. 69–88.
• Robinson, R.W. & Decker-Walters, D.S., 1997. Cucurbits. CAB International, Wallingford, United Kingdom. 226 pp.
• Schippers, R.R., 2002. African indigenous vegetables, an overview of the cultivated species 2002. Revised edition on CD-ROM. National Resources International Limited, Aylesford, United Kingdom.
Other references
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• Babu, P.G., Reddy, D.J., Jadhav, D.R., Chiranjeevi, C. & Khan, M.A.M., 2002. Comparative efficacy of selected insecticides against pests of watermelon. Pesticide Research Journal 14(1): 57–62.
• Burkill, H.M., 1985. The useful plants of West Tropical Africa. 2nd Edition. Volume 1, Families A–D. Royal Botanic Gardens, Kew, United Kingdom. 960 pp.
• FAO, 2003. FAOSTAT Agriculture Data. [Internet] http://apps.fao.org/page/collections?subset=agriculture. Accessed 2003.
• Fehner, T., 1993. Watermelon, Citrullus lanatus (Thunb.) Matsum. & Nakai. In: Kallo. G. & Bergh, B.O. (Editors). Genetic improvement of vegetable crops. Oxford, Pergamon Press, New York. pp. 295–314.
• Graves-Gillaspie Jr, A. & Jarret, R.L., 1997. Watermelon mosaic virus resistant watermelon breeding lines wm-1, wm-2, wm-3, wm-4. HortScience 32: 1136.
• Huang, H.X., Zhang, X.Q., Wei, Z.C., Li, Q.H. & Li, X., 1998. Inheritance of male-sterility and dwarfism in watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai). Scientia Horticulturae 74(3): 175–181.
• Hulugalle, N.R., Ezumah, H.C. & Leyman, T., 1994. Changes in surface soil properties of a no-tilled tropical Alfisol due to intercropping maize, cassava and ‘egusi’ melon. Field Crops Research 36: 191–200.
• Ikeorgu, J.E.G., 1991. Effects of maize and cassava on the performance of intercropped egusi melon (Citrullus lanatus (L.) Thunb.) and okra (Abelmoschus esculentus (L.) Moench) in Nigeria. Scientia Horticulturae 48: 261–268.
• Iwuoha, C.I. & Eke, O.S., 1996. Nigerian indigenous fermented foods: their traditional process operation, inherent problems, improvements and current status. Food Research International 29: 527–540.
• Kolo, M.G.M., 1995. Effects of intercropping melon (Citrullus lanatus) with yam (Dioscorea rotundata) under different staking methods on weed control and crop production. Tropical Agriculture 72: 102–104.
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• Levi, A., Thomas, C.E., Joobur, T., Zhang, X. & Davis, A., 2002. A genetic linkage map for watermelon derived from testcross populations: (Citrullus lanatus var. citroides × C. lanatus var. lanatus) × C. colocynthis. Theoretical and Applied Genetics 105(4): 555–563.
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Sources of illustration
• Stevels, J.M.C., 1990. Légumes traditionnels du Cameroun: une étude agrobotanique. Wageningen Agricultural University Papers No 90-1. Wageningen University, Wageningen, Netherlands. 262 pp.
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H.A.M. van der Vossen
Steenuil 18, 1606 CA Venhuizen, Netherlands
O.A. Denton
National Horticultural Research Institute, P.M.B. 5432, Idi-Ishin, Ibadan, Nigeria
I.M. El Tahir
Plant Genetic Resources Unit, Agricultural Research Corporation, P.O. Box 126, Wad Medani, Sudan

G.J.H. Grubben
Prins Hendriklaan 24, 1401 AT Bussum, Netherlands
O.A. Denton
National Horticultural Research Institute, P.M.B. 5432, Idi-Ishin, Ibadan, Nigeria
Associate Editors
C.-M. Messiaen
Bat. B 3, Résidence La Guirlande, 75, rue de Fontcarrade, 34070 Montpellier, France
R.R. Schippers
De Boeier 7, 3742 GD Baarn, Netherlands
General editors
R.H.M.J. Lemmens
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
L.P.A. Oyen
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
Iskak Syamsudin
PROSEA Network Office, P.O. Box 332, Bogor 16122, Indonesia
Photo Editor
E. Boer
PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article:
van der Vossen, H.A.M. & Denton, O.A. & El Tahir, I.M., 2004. Citrullus lanatus (Thunb.) Matsum. & Nakai In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Légumes. [CD-Rom]. PROTA, Wageningen, Netherlands.
Distribution Map wild and planted

1, part of stem with female flower; 2, part of stem with male flower; 3, egusi melon fruit; 4, seeds of egusi melon; 5, watermelon fruit (rind and flesh partly removed).
Redrawn and adapted by Iskak Syamsudin

flowering and fruiting plant, ssp. mucosospermus.

fruit and halved fruit of the cooking melon

young leaves are eaten as vegetable

fruits on the market

ssp. mucosospermus Fursa cv. Group Egusi melon; diversity in fruit shapes

different seed shapes of Egusi melon

Seeds used as egusi. Cucumeropsis mannii, is at top right. Seeds at bottom right and top left both belong to Citrullus lanatus. Seeds at bottom left belong to a variety of Lagenaria siceraria from Côte d’Ivoire.