Watermelon (Citrullus lanatus) is the most cultivated cucurbitaceous vegetable crop in the world. It is popularly known as Tarbuj, Kalinda, and Matera in India. The fruits of watermelon are fleshy, juicy, and sweet. Mostly eaten fresh, provide a delicious and refreshing dessert in hot weather. The major nutritional components of the fruit are carbohydrates (6.4 g/100g), vitamin A (590 IU), and lycopene (4,100 ug/100g). Pink-fleshed watermelon contains a significantly higher quantity of ß-carotene than yellow-flesh varieties. It is rich in iron content among all members of cucurbitaceous crops. Watermelon seeds are rich in fat (about 45% edible oil) and protein (30-40%). The seeds are powdered and baked like bread in India. The seed kernels are also used in various sweets and other delicacies. The unripe fruits are also cooked as a vegetable in some parts of India. The juice is an alternative drink to drinking water in desert areas. During 2012-13 India produced 1,789.20 thousand metric tonnes of watermelon from an area of 80.59 thousand ha. Watermelon is widely cultivated in Uttar Pradesh, Himachal Pradesh, Rajasthan, Odisha, Gujarat, Punjab, Haryana, Asom, West Bengal, Karnataka, Telangana, Andhra Pradesh, Maharashtra, and Tamil Nadu.
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Climate and soil
Watermelon simply desires heat- the hotter the better in fact, and can be grown in sub-tropical as well as hot arid regions. It is a warm-season crop and cannot withstand cold climates, especially frost. Watermelon requires dry weather and plenty of sunshine. Continuous rain or cloudy weather will not only stunt the plant growth but also reduce the flowering and fruit set. Short days and low-temperature conditions favor the expression of female flowers. If watermelons mature in the rainy season, the sugar content will be greatly reduced. High humidity adversely affects the yield and quality of fruits and increases the incidence of diseases. Seeds germinate well and the crop thrives best when the temperature is 25-30°C. Fruits mature best at 30°C. Cool-weather below 16°C adversely affects the growth and development of crops. Watermelon can be grown on a variety of soil types. Sandy loam to loam soils with good drainage are preferred for the cultivation of watermelon. Heavier soils with adequate drainage or silt soil could also be used. When sandy soils are used, they should be supplemented with humus or compost. Poorly drained soils should be avoided. The crop can withstand acid conditions better than other cucurbits. The optimum pH range is 6.5 to 7.0. Watermelon should not be grown on the same soil year after year because of Fusarium and nematode problems.
Some important varieties of watermelon are described here.
Arka Aiswarya: It is an F, hybrid having a crop duration of 95-100 days. Fruits are oval, green with light green broken stripes, red flesh, crispy, delicious, juicy, and 13-14% TSS weighing 7.5 kg. It is good in keeping and transport qualities.
Arka Akash: High yielding F, hybrid. Fruits are oblong, red-fleshed, juicy weighing 6.5 kg with TSS of 12-13%, and very good with taste. Yields 65-70 tonnes/ ha with a crop duration of 90-95 days.
Arka Jyoti: A mid-season F, a hybrid developed by crossing IIHR-20 Crimson Sweet. It produces round fruits weighing 6-8 kg with light green rind and dark green stripes. Flesh color is crimson having excellent texture, excellent flavor, and sweetness (11-13% TSS). Yields 50-60 tonnes/ha.
Arka Madhura: It is a triploid and seedless variety. Suitable for year-round production under protected conditions. Fruits are sweet (13-14% TSS) and juicy. Gives 50-60 tonnes/ha fruit yield. Shelf-life is long with good transport quality.
Arka Manik: Developed by crossing IIHR-21 x Crimson Sweet followed by modified backcross method. Fruits are round to oval weighing 6 kg with light green rind and dark green stripes. The flesh is deep crimson with a granular texture, pleasant aroma, and very sweet (12-15% TSS). Seeds are small with dark brown specks on the testa. It stands well in transport and storage. It possesses triple resistance against powdery mildew, downy mildew, and anthracnose. Produces 60 tonnes/ha in crop duration of 100-115 days.
Arka Muthu: High yielding variety with the unique character of the dwarf vine (1.2 m), shorter internodal length, and early maturing type (75-80 days). It has round to oval fruits with dark green stripes and deep red flesh. The average fruit weight is 2.5-3 kg with 12-14% TSS. Fruit yield is 55-60 tonnes/ha.
Asahi Yamato: It is a mid-season Japanese introduction. Fruits are medium-sized weighing 6-8 kg with rind light green, deep pink, and crisp flesh having 11-13% TSS. Seeds are small and brown. Fruits ripen about 90-95 days after sowing.
Durgapura Kesar: It is a late variety. Fruits are slightly ovicular in shape with light green rind. Flesh color is saffron, juicy, moderate in sweetness (10-11% TSS), and seeds large. The average fruit weight is 4-5 kg and the average fruit yield is 35 tonnes/ha.
Durgapura Lal: Leaves are non-lobed which helps to distinguish the variety easily (marker gene). Fruits are round, dark green with dark lining, thin and hard rind weighing 4-5 kg. The flesh is crisp, dark red with 10-11% TSS. First picking starts at 105-110 days after sowing and produces 35-45 tonnes/ha fruit yield. It is moderately resistant to blight, powdery mildew, and bud necrosis under field conditions.
Durgapura Meetha: It is a late-maturing variety (125 days). Fruits are round with light green thick rind, red-fleshed with 10-11% TSS. First picking starts in 110-120 days after sowing. The average fruit weight is 6-8 kg with a yield potential of 40-50 tonnes/ha. It possesses good keeping and transport quality.
Kashi Pitamber: Early in maturity, small-fruited (2.50-3.40 kg), TSS 10.5-11% produces yellow color fruits with red flesh. The yellow color of fruits appears right from the ovary stage.
Pusa Bedana: It is a seedless triploid variety developed by crossing Tetra-2 (Tetraploid) x Pusa Rasal (Diploid). Fruits are dark green weighing 5-6 kg, somewhat triangular in shape with tough rind, red-fleshed, seedless with 12-13% TSS. It takes 115-120 days for the first fruit harvest.
Sugar Baby: It was introduced in the USA. Fruits are slightly small in size weighing 3-5 kg, round having bluish-black rind and deep pink flesh with 11 to 13% TSS and the small brown seed. Fruits are ready for harvesting in 85-90 days after sowing.
Thar Manak: It is developed through selection from the local landraces found in the arid regions. It is drought-hardy and suitable for cultivation in arid regions during the rainy season. Early and first fruit harvesting can be done in 75-80 days after sowing.
Watermelon is raised through seeds. The seed rate required is 3.0-3.5 kg/ha, however, for sowing hybrids about one-third seed rate is sufficient. In north India, sowing is done in both summer (mid-February mid-March) and a rainy season (June-July), whereas, in Central and South India, the best time of seed sowing is winter (November-January). In the northern hills, the crop is sown in summer (April-May). In North and West Indian conditions watermelon is also grown during winter (December-January) in river-bed cultivation system to fetch high prices from off-season crops.
Watermelon can be direct seeded to the field or grown as transplants in polybags/ pro-trays and then transplanted to the field. Prior to sowing, the seeds must be treated with Captan or Thiram @2 g/ kg of seed. Seeds should be soaked in water overnight and kept in moist gunny bags for 2-3 days in a warm place to initiate germination. The soil surface should be smooth, loose, friable, and free from clods at the time of sowing. Generally, 2-3 pre-germinated seeds/ hill on both sides of the raised bed (ridge) or furrow and 3-4 pre-germinated seeds/ hill in river-bed cultivation are sown at 2-2.5 cm depth. After sowing, the seeds should be covered with a thin layer of well rotten fine farmyard manure (FYM). At the sowing the soil must have sufficient moisture for better germination hence, the channels should be irrigated two days before seed sowing.
The important methods for sowing watermelon are given here.
Shallow pit or flatbed method:Shallow pits of 45 x 45 x 45 cm size are dug and left open for 21 days before sowing for partial soil solarization. These pits are filled with a mixture of topsoil, well-decomposed FYM or compost, and recommended dose of fertilizers.
Deep pit or trench method: Deep pit method is commonly practiced for raising watermelon in riverbeds. In this method, the circular pits of 60-75 cm diameter and 1.5 m depth are dug at the recommended distance, or about 60 cm wide trenches are made at a distance of about 2.5-3 m across the slope up to the depth of the clay layer. These pits or trenches are filled with a mixture of topsoil, well rotten FYM, and recommended NPK mixture. The pre-germinated seeds are sown in trenches at a distance of 90-120 cm apart.
Sowing of seeds on ridges: In this method, 50-60 cm wide channels are prepared manually or mechanically maintaining 2.5-3 m distance between two channels, depending upon cultivars. Seed is sown on both ridges of the channel at a spacing of about 60-90 cm. Generally, two to three sprouted seeds are sown per hill in spring-summer and adequate moisture is maintained at the time of emergence. The vines are allowed to spread in-between space of channels.
Watermelon is grown best in soils that have high organic matter content. The doses of fertilizers depend on the nutritional status of the soil, climate, and variety to be grown. Apply well-decomposed FYM @ 15-20 tonnes/ha in the furrows or pits made for sowing and mix thoroughly. This is supplemented by a half dose of N (40-50 kg/ha) and full dose each of P₂0, (60-80 kg/ha) and K₂O (50-60 kg/ha). The remaining nitrogen is divided into two equal parts and applied at the time of vine initiation and 10-15 days later. Two foliar sprays of water-soluble fertilizer 19:19:19 (NPK) @5-6 g/litre water should be done at 25-30 days after sowing and again at the time of flower initiation. Foliar sprays of boron 25 ppm along with urea 1% at 2-3 times beginning from 2-4 true leaf stage to flower initiation increase number of female flowers and yield. The complete doses of fertilizers should be applied before the fruit set. Excessive application of nitrogen should be avoided as it results in more male flowers, which affects the fruit yield adversely. While applying fertilizers, care must be taken not to cause damage to roots and there should be sufficient moisture in the soil. Irrigation should be supplied shortly after applying fertilizers to move them towards roots.
Thinning of plants should be done 10-15 days after sowing, retaining only 2 healthy seedlings per hill. Weeds pose a serious problem during the early period of crop growth and compete with crops for nutrients, moisture, sunlight, and space and also act as reservoirs of many insect pests and diseases. Good weed control is essential for high-quality watermelon. The sugar content of the fruits is lowered where weeds shade and compete with the crop. Two or three weedings are required to control weeds before the vines cover the ground area. First wedding should be done at 30 days after sowing and subsequent weeding is done at an interval of 30 days followed by hoeing and earthing-up. Deep hoeing should be avoided as it may destroy many of the fine roots near the soil surface. Application of herbicides like Fluchloralin or Trifluralin (0.75-1.5 kg/ ha) as pre-plant soil incorporation at two weeks before sowing and Butachlor (1 kg/ha) as post-emergence after first weeding proved better in controlling the weeds in watermelon. Mulching is also found effective for controlling weeds. Mulching the bed surface with rice straw or sugarcane leaves not only retains the soil moisture, prevents nutrient leaching, improves soil aeration, but also controls the weeds and provides support for tendrils. The weeds can also be controlled effectively by the use of biodegradable plastic mulch. Mulching with silver color ultra-violet reflective plastic mulch reflects thrips and aphids hence reduces the incidence of viral diseases.
Canopy management: Generally watermelon does not require pruning. However, it is better to restrict excessive vegetative growth by pinching apical shoots and allowing 2-4 side shoots to promote a high female: male flower ratio. Remove the first fruit on the primary vine as early as possible, because this fruit is too near the crown that cannot develop well and will affect the later fruit setting. Thinning of excess fruits should also be done by retaining only 2 fruits/vine, which improves fruit size and yield.
The soil must have sufficient moisture at the time of sowing for better seed germination. Light irrigation should be given at the time of seedling emergence. Soil moisture should be adequate throughout the early growing season to produce good plant growth. Frequent irrigation promotes excessive vegetative growth and delayed flowering which needs to be avoided. In light-textured soils, watermelon requires a minimum of 380 mm water during growth. Less irrigation is required for watermelon than for other melons. The crop should be irrigated when the irrigation water/ cumulative pan evaporation (IW/CPE) ratio is one. It is essential to maintain optimum moisture in the field at critical stages, viz. vine development, pre-flowering, flowering, and fruit development period to get a high yield. When the female flower appears, it is advocated to withhold water supply to improve the fruit set. When fruits start to develop, irrigation should be applied to produce good size fruits. During ripening, excess soil moisture reduces the sugar content and adversely affects the development of flavor. Therefore, irrigation should be completely stopped during fruit ripening. Crop sown in riverbeds does not need any irrigation unless the soil is too dry.
Irrigation in furrows needs more quantity of irrigation water, which also frequently wets the vines or vegetative parts and promotes the incidence of diseases. Among the irrigation systems, drip irrigation is the most efficient method. The single lateral lines (12-16 mm size) with in-line or on-line drippers of 4 liter/hr discharge capacity can successfully be used.
Watermelon is a highly cross-pollinated crop being monoecious in sex expression with a very high male to female sex ratio. Honey bees are the chief pollinating agent, which ensures good fruit set and development. Bee pollination is most effective in the early morning hours hence, no spray should be scheduled in the morning hours particularly in the flowering span, which may deter the bees. If the number of bees is insufficient to ensure adequate pollination, one or two beehives per hectare under normal conditions should be kept in the field when 5-10% of plants have open flowers. Beehives should be placed in clusters around the periphery of the field, with additional hives inside the larger fields. Very high and low temperatures can also affect pollen viability, resulting in poor pollination.
Watermelon being monoecious in sex form produces staminate (male) and pistillate (female) flowers separately on the same plant. In this sex form, fruit set and yield depend on the number of female flowers as the proportion of male flowers is very high than the female flowers. Therefore, a high female to male ratio is desirable to increase fruit set and finally the fruit yield. The yield of watermelon can be appreciably enhanced by suppressing the appearance of male flowers with the application of plant growth substances. Foliar spray of tri-iodobenzoic acid (TIBA) at 25-50 ppm, molybdenum at 3 ppm, and calcium at 20 ppm at 2- and 4-leaf stages of growth proved to be effective in inducing more female flowers and increasing fruit set and fruit yield. Similarly, 2-3, foliar application of boron (25 ppm) at the beginning from 2-4 true leaf stage to flower initiation increases the number of female flowers and yield. Adding urea as an adjuvant at 1% concentration to the spray solution improves the absorption of boron by leaves. A favorable status of boron in watermelon vine causes (i) proper growth of pollen tube after fertilization leading to enhanced fertilization of eggs in the ovary which ultimately increases the fruit set in each vine; and (ii) substantial growth or enlargement of the fruit since every fertilized egg in the ovary releases growth hormones responsible for fruit enlargement leading to a significant increase in fruit weight.
The excessive vine growth due to a high dose of nitrogen application coupled with high temperature and high soil moisture conditions also promote male flowers on the vines, resulting in poor fruit set and low yield. The best way to control vine growth within reasonable limits is by adjusting nitrogenous fertilizer doses and frequency of irrigation.
It is a kind of vegetable forcing where watermelon is sown in December-January under plastic low tunnels to get an early crop. The area under riverbed cultivation is very limited and cannot be extended further in such conditions, watermelon can be grown very early in the spring or summer. Plastic low tunnels provide the best way for off-season cultivation of cucurbits during winter by modifying the micro-climate around the plants. Low tunnels installed over the rows of directly sown or transplanted watermelon, enhances plant growth by warming the temperature around the plants during winter. These tunnels are quite cost-effective for the growers in northern parts of the country, where the night temperature during winter falls below 8°C for a period of 30-40 days. Low tunnels offer the advantage of crop advancement from 30-40 days over their normal season of cultivation and ultimately high prices of the produce.
Seedlings are raised in the greenhouse in plastic pro-trays having 3.75 cm cell size in soil-less media in December-January. Nursery can also be raised even in polythene bags under very simple and low-cost protected structures. About 32-35 days old seedlings at 4 true leaf stages are transplanted with a ball of earth in the open field in January and covered with plastic low tunnels. When the seedlings are transplanted at the end of February there is no need to cover with plastic low tunnels. Transplanting should be done during evening hours followed by irrigation. Direct sowing of seed under plastic low tunnels can also be done beginning from mid-December to mid-January.
Transplanting or sowing is done in rows at a recommended distance on the drip system of irrigation. The flexible galvanized iron hoops are fixed manually at a distance of 2.0-2.5 m. The width of the two ends of the hoop is kept 1 m with a height of 60-70 cm above the level of ground for covering the plastic on the rows or beds for making low tunnels. Transparent, 30-micron biodegradable plastic is generally used for making low tunnels, which reflects infra-red radiation to keep the temperature of the low tunnels higher than outside. The 3-4 cm size vents are made on the eastern side of the tunnels just below the top at a distance of 2.5-3.0 m when temperature increases within the tunnels during the peak daytime. These vents also facilitate the pollination of crops by visits of bees. Plastic is completely removed from the plants in February-March depending upon the prevailing night temperature in the area.
Diseases and pests
A large number of diseases and pests affect the watermelon adversely at different stages of the crop, which are described here.
Powdery mildew (Erysiphe cichoracearum): Tiny white to dirty grey spots on the foliage, leaves, and green stem appears. Later these spots become powdery and enlarge into patches. Under severe infection, the fruits may also be covered with powdery mass. The humid weather is favorable for the spread of this disease.
Downy mildew (Pseudoperonospora cubensis): Angular yellow-colored spots often restricted by the veins appear on the upper surface of leaves giving purplish growth on the lower surface. The affected leaves die quickly.
Anthracnose (Colletorichum lagenarium): Reddish-brown spots are formed on the affected leaves and become angular or round when many spots collapse. It results in shriveling of leaves which later die.
Fruit rot (Pythium aphanidermatumand P. butleri): Water-soaked lesions girdle the stem, later extending upwards and downwards. The rotting of affected tissues occurs and even grown-up plants collapse. The fungus causes the rotting of fruits, which have direct contact with soil.
Fusarium wilt (Fusarium oxysporum f. sp. niveum): The fungus is seed-borne as well as a persistent soil inhabitant. Seedling injury is high at 20-30°C temperature. Wilt development is also favored by a temperature of about 27°C. No infection occurs at temperatures below 15°C and above 35°C. The plants are prone to attack at all stages of growth. Germinating seeds may also rot in the soil. The affected plants turn yellow, show wilting, and later the whole plant dies. It causes damping-off disease of seedlings. Small leaves lose their green color and wilt.
Watermelon mosaic virus: The young leaves develop small greenish-yellow areas and they become more translucent than those in the remaining parts of the leaf. Yellow mottling is seen on leaves and fruits. Leaf distortion and stunting of infected plants occur. The virus is transmitted through sap, seed, and aphids.
Bud necrosis: It is a serious disease of watermelon and caused by Tospovirus. The infected plants have small internodes, erect shoots and finally, the apical portion of the infected vines die. It is transmitted through thrips.
Red pumpkin beetle (Aulacophora foveicollis): Both grub and adult attack the crop at the seedling stage and make holes in the cotyledonary leaves. When the attack is severe, the crop is totally destroyed.
Aphid (Aphis gossypii or Myzus persicae): Both nymphs and adults of this tiny insect suck cell sap from the tender leaves, reducing plant vigour. As a result, the leaves curl up and ultimately wilt. The aphids excrete honeydew on which black sooty mould develops, which hampers photosynthetic activity. Besides, these aphids act as vector for the transmission of many viruses. The attack is severe during March-April.
Fruit fly (Bactrocera cucurbitae): The damage is caused by maggots of the fly. The adult fly punctures tender fruits and lays eggs below the fruit skin (epidermal layer). The maggots feed inside the fruits and make them unfit for consumption. The hot and humid weather is most suitable for its attack.
Mite (Polyphagotarsonemus latus):Both nymphs and adults suck the sap from young foliage and growing tips. Downward curling and crinkling of leaves giving an inverted boat-shaped appearance, stunted growth, and elongation of the petiole are the characteristic symptoms.
Harvesting and post-harvest management
Watermelon is non-climacteric fruit it must thereby be harvested at the full ripe stage. The fruits are ready for harvesting in 80-110 days after sowing depending upon variety and season. Uniform development of red color from center to rind is a must, as in some of the cultivars, the white heart is seen at the central portion resulting in poor quality fruits. Similarly, when harvesting is delayed hollow heart and fibrous flesh signify the maturity of fruits. The maturity is judged by taking into consideration several factors Le. (i) dull sound when the fruit is thumbed, in contrast to metallic sound, (ii) withering of tendril at the fruit axil, (iii) ground spot (where the fruit touches the ground) turning yellow, (iv) the rind of the ripe melon emits a crisp, cracking noise, and (n) fruit weight is decreased with the advancement of ripening. The other important criteria followed in judging the maturity of watermelon fruits are: (1) when the fruits are tapped in the center with knuckle and if it sounds like taping forehead then it is under-ripe, (ii) if it sounds like your chest, it is just ripe, and (iii) if it sounds like your stomach, it is overripe. Sometimes a single method is not a conclusive indication that the fruit is ready for harvest therefore, it requires experience.
The ripe fruits should be separated from the vines with the help of a knife. The average fruit yield of watermelon is 30-40 tonnes/ ha but it is greatly influenced by variety, season, and crop management practices. The harvested fruits should be handled gently to avoid bruising. When loading, the fruits should not be stacked so high that their weight bruises the bottom fruit. Fruits should not be left long in the sun to avoid sun-scald. Watermelon is graded according to its size for the local market. The distinction among grades is based predominantly on external appearances. However, watermelon should be symmetrical and uniform in appearance. The surface should be waxy and bright in appearance devoid of scars, sunburn, transit abrasions, or other surface defects. Watermelon can be stored for 1-3 weeks at 2.2-4.4°C and 80% relative humidity. The extended holding at this temperature induces chilling injury. Many watermelons are still shipped without pre-cooling or refrigeration during transit. These fruits must be utilized for prompt market sales as quality declines rapidly under these conditions.
A delicious ready-to-serve beverage can be prepared from watermelon by using an edible color portion by blending and adjusting TSS, acidity, and addition of ginger extract and heat processing. The processed juice can be stored either in refrigerated conditions (4-5°C) or in ambient conditions. The shelf-life of the beverage is 6 months at ambient conditions and 12 months at refrigerated conditions. The left-out white rind of watermelon, which is normally discarded after eating the fruit, can be converted into value-added products such as candy. The watermelon rind candy can be used in confectionery, toffees, bread, cakes, custards, and sweetmeats.