Watermelon

Table watermelon (Citrullus lanatus)

The root is taprooted, strongly branched, penetrates the soil to a depth of 1–5 m, spreads sideways up to 7 m, and is distinguished by a high absorbing capacity. Formation of the root system is completed at the flowering phase.

The stem is prostrate, creeping, long-vined up to 2–5 m, with 5–10 branches (up to 50), pubescent with stiff hairs, round-pentagonal.

The leaves are dissected into pinnately incised lobes, grey-green, stiffly pubescent, long-petioled.

The flowers are pentamerous, yellow, unisexual, sometimes bisexual. The plant is monoecious. Pollination is cross-pollinated by insects (entomophilous). There can be up to 150 male flowers on one plant; female flowers are about 5 times fewer. Flowering of male flowers begins earlier than that of female flowers.

The fruit is a multi-seeded false berry (pepo) on a long peduncle, spherical, oval or oblong, coloured white-greenish or dark green, usually with a marbled pattern. The fruit rind is leathery, fragile, from 0.5 to 2 cm thick. The flesh is of various consistency, carmine-red, pink, sometimes white or yellow, sweet or slightly sweet. Sugar content in the flesh is 5.7–13%. Fruit weight varies from 2 to 20 kg.

Seeds are flat, ovate, 0.5–2 cm long, up to 10 mm wide, up to 3 mm thick, with a hilum along the edge and a hard seed coat, having white, cream, yellow, grey, red, brown and black colour, sometimes with a spotted pattern of various shapes. The mass of 1000 seeds is 60–150 g.

Fodder watermelon (Citrullus colocynthis)

Fodder watermelon, or colocynth, Citrullus colocynthis, is a perennial that is rarely cultivated for medicinal use of the dry pulp of unripe but full-sized fruits. Although colocynth fruits have bitter white flesh, the non-bitter seeds are eaten and used to make oil in Africa. This species grows wild in northern Africa and south-western Asia and exists as an escape from cultivation in other areas, including Australia and southern Europe.

The root system of fodder watermelon is more powerful than that of table watermelon and penetrates the soil to 2–5 m. With sufficient moisture in the upper soil layer, adventitious roots can form from the nodes of the vines.

The stem is pentagonal, prostrate, vines up to 2–5 m long, strongly branched, pubescent with stiff hairs.

The leaves are deeply dissected, have larger shortened lobes, and are stiffly pubescent.

The flowers are large, unisexual, with a pale yellow or yellow corolla. Male flowers are located on long pedicels, female flowers on shortened ones.

The fruits are spherical to oval-oblong, green or light green with dark stripes in a marbled pattern and a thick rind. The flesh is pale green, dense, unsweet, with a sugar content of 1.2–2.6%. Fruit weight is from 10–15 to 25–30 kg.

Seeds are without a hilum. The mass of 1000 seeds is 100–200 g.

Egusi watermelon (Citrullus mucosospermus)

Egusi watermelon (Citrullus mucosospermus (Fursa) Fursa) is the species closest to dessert watermelon. Egusi watermelons native to western Africa are usually spherical with bitterish flesh and a soft mucilaginous seed coat. They are cultivated for consumption of their seeds, which vary in size and shape.

Genetic diversity

Fruit shape and skin pattern of watermelon (Citrullus lanatus):

golden rind, spotted rind and interrupted stripes (first column); grey rind, narrow stripes, medium-width stripes and wide stripes (second column); light solid, medium solid and dark solid rind (third column).

The lack of diversity in general contrasts with the large morphological variation observed in flesh colour, seed size and colour, and fruit skin pattern (discussed in detail in the section above) (fig.). Watermelon, which appeared in Africa at least 4000 years ago, was domesticated as a source of nutrients (seeds and flesh) and water in semi-arid and arid climates (Paris, 2015). From Africa, dessert watermelon was brought to the Mediterranean in the 2nd century CE, reaching Italy and France in the 5th and 6th centuries. As it spread, this crop was constantly improved by farmers.

Having gained popularity in the Mediterranean, dessert watermelon spread to Europe and then to China (a secondary centre of diversity) via the Silk Road (McCreight et al., 2013). The tertiary centre of diversity is Central Asia. India was considered a secondary centre of watermelon spread because of the presence of tinda there. However, since then tinda (formerly Citrullus) has been classified as Praecitrullus. Watermelon probably appeared in India during the Islamic conquests in the medieval period.

The fruit rind of lemon melon (Citrullus amarus, formerly Citrullus lanatus var. citroides) is used to make pickles or preserves, and the fruits are also fed to livestock. Citron has hard, white or pale green flesh, tasteless or bitterish. Large seeds have various colours, including light green (for example, ‘Colorado Preserving Melon’). In commercial production, more than 100 fruits per plant and up to 200 seeds per fruit yield a seed yield of 500–700 kg/ha.

Citron grows wild in Africa and as an escape elsewhere. It is a weed in watermelon-growing areas in North America, causing serious problems in cotton and sorghum fields in Texas, where three different citron species exist. Citron easily crosses with watermelon, so seed production fields must be isolated and monitored.

Temperature requirements

Watermelon is a heat-loving and heat-tolerant plant.

In moist soil, seeds begin to germinate at a temperature of 15–17°C; seedlings appear after 8–10 days. Seedlings die at a temperature of -1°C.

The optimal temperature for growth of stems and leaves is 20–22°C, for fertilisation 18–25°C, and for fruit development 25–30°C. At temperatures below 15°C, photosynthesis and growth processes slow down.

The sum of active temperatures above 10°C is 2200–2500°C.

Moisture requirements

Watermelon is a very drought-resistant plant due to its powerful developed root system, but it is responsive to irrigation.

Light requirements

Watermelon is a light-loving short-day plant and poorly tolerates shading.

Soil requirements

Sandy and sandy loam chernozem soils are optimally suitable.

Loamy and clay soils are poorly suitable, as they retain moisture firmly and warm up poorly.

Optimal soil acidity pH is 6.5–7.5.

Soil properties also affect fruit quality and sugar content in fruits.

Germination, emergence and seedling growth

Cucurbit seeds are non-endospermic, and germination is epigeal. If seeds remain in fruits too long, they germinate in place. Collected seeds can be sown the next day, since watermelon seeds do not have a dormant period. Germination of seeds takes from 2 days to 2 weeks and depends on temperature, water activity in the environment and seed age. The optimal germination temperature for triploid watermelon seeds is 29–32°C. Maximum germination of watermelon seeds was noted at temperatures from 18 to 22°C (Milotay et al., 1991; Singh et al., 2001) and at an optimal temperature of 25°C (Jennings and Saltveit, 1994).

Seeds do not germinate at temperatures below 15°C, and under prolonged low-temperature conditions, despite available soil moisture, germination decreases due to the effect of pathogenic microorganisms on germinating seeds (Singh et al., 2001). Germination of watermelon seeds improved at 15°C after priming with an inorganic osmoticum (Sachs, 1977).

Germination positively correlated with water activity at a certain temperature. Treatment of seeds with fungicides and bactericides is useful and improves germination and emergence. Emergence was high in seeds grown in sterilised soils.

Tetraploid watermelon has poor germination and low seedling viability mainly due to the thick seed coat and adherence of the seed coat to the emerged cotyledons. Seed treatment, such as nicking and pre-soaking in distilled water or H₂O₂, increased germination of tetraploid seeds but showed genotypic differences (Muhammad et al., 2006). When planting seeds positioned with the radicle upwards, adherence of the seed coat to the cotyledons decreased. Sowing watermelon seeds soaked in water increased the germination percentage (Hall et al., 1989).

Photosynthesis

Treatment with 5-aminolevulinic acid (ALA) (50–200 mg/L) increased the net photosynthetic rate, stomatal conductance and transpiration rate of watermelon seedling leaves (Kang et al., 2006) and significantly increased maximum fluorescence (Fm), variable fluorescence (Fv), maximum photochemical efficiency of PS II (Fv/Fm), potential photochemical efficiency of PS II (Fv/F0) and the ability of the PS II reaction centre to capture antenna pigment energy (1/F0-1/Fm) in dark-adapted leaves of watermelon seedlings. Measurements of light response curves showed that the photochemical efficiency of PS II (Fv»/Fm»), actual photochemical efficiency of PS II (OPS II), photochemical quenching (qP) and photochemistry (P) of light-adapted leaves decreased with increasing actinic light intensity. Non-photochemical quenching (NPQ), electron transport rate (ETR), photochemistry rate (PCR), antenna heat dissipation (D) and non-photochemistry in the PS II reaction centre (E) increased as actinic light intensity increased.

At an actinic light intensity of about 40 gmol/m²/s on the light response curves of chlorophyll fluorescence parameters, including Fv»/Fm», OPS II, qP, NPQ, P, D and E, an inflection was found, which may be related to the light compensation point of photosynthesis. In addition, treatment with exogenous ALA significantly increased Fv»/Fm», OPS II, P, PCR and ETR. When actinic light intensity exceeded about 1500 μmol/m²/s, NPQ was higher in ALA-treated leaves than in the control, which indicates that photoinhibition caused by ALA treatment was useful for energy dissipation to protect leaf photosynthesis. Analysis of energy distribution in the PS II reaction centre showed that ALA treatment led to a decrease in D and an increase in E and P, which indicates that ALA enhances the collection of energy by antenna pigments in the PS II reaction centre, where part of the energy is dissipated non-photochemically, which is important for maintaining a higher level of PS II photochemistry (Kang and Wang, 2008).

The protective mechanism against photodamage during photosynthesis in watermelon was induced by ALA treatment by reducing minimum fluorescence (Fo) and increasing Fm, which led to an increase in Fv/Fm in dark-adapted leaves and an increase in the actual photochemical efficiency of photosystem (phiPS II), ETR, qP and NPQ in light-adapted leaves. Thus, photon energy irradiated on watermelon leaves was more easily collected by PS II, converted into bioelectric energy in the reaction centre and transferred along the photosynthetic electron chains in ALA-treated plants (Wang et al., 2010).

ALA treatment increased the activity of antioxidant enzymes such as SOD and POD, but had practically no effect on the activity of CAT and APX in watermelon seedling leaves, which indicates that ALA treatment enhanced reactive oxygen species metabolism by increasing SOD and POD activity in watermelon plants, thereby maintaining cellular photosynthetic capacity. 30 days after ALA treatment, dry weight, starch content and soluble sugar content in treated watermelon plants increased by 42–54%, 62.2–207.0% and 32.0–87.1%, respectively. Thus, ALA treatment can increase the activity of antioxidant enzymes in watermelon seedlings under low temperature and low light, and also stimulate photosynthesis and photosynthate accumulation, as well as plant growth (Kang et al., 2006).

Vegetative growth

Watermelon is a trailing vine with large, hairy, pinnately lobed leaves and white or yellow flowers. Watermelon is grown for its fruits — berries known under the botanical name PEPO. The fruit has a smooth, hard rind, usually green with dark green stripes or yellow spots, and juicy, sweet inner flesh, usually dark red or pink, but sometimes orange, yellow or white, with a large number of seeds. The stems are thin, hairy, angular, grooved, with branched tendrils at each node. The stems are strongly branched and reach 30 feet in length in standard cultivars, and are shorter and less branched in dwarf cultivars. The roots are extensive but shallow, with a taproot and many lateral roots.

Inoculation with the arbuscular mycorrhizal fungus Glomus versiforme can activate the activity of defence enzymes (root phenylalanine ammonia-lyase [PAL], catalase [CAT], peroxidase [POD], 0-1,3-glucanase and chitinase) of non-grafted and grafted watermelon seedlings, allow the roots of seedlings to respond quickly to adverse conditions and improve the ability of watermelon seedlings to resist continuous cropping obstacles (Chen et al., 2013).

Generative phase

Watermelon is a warm-season crop. It requires a long growing season. Flowering and fruit development are favoured by high light intensity and high temperature.

Watermelon is annual. Flowering begins approximately 8 weeks after sowing. Flowers are born in the order staminate (male), perfect (hermaphroditic) or pistillate (female). There are andromonoecious (staminate and perfect) types, observed mainly in older cultivars or cultivars collected from the wild. Pistillate flowers have an inferior ovary, and the size and shape of the ovary correlate with the final size and shape of the fruit. In many cultivars, pistillate or perfect flowers appear at every seventh node, and staminate flowers at intermediate nodes. The flower ratio in typical watermelon cultivars is 7:1 staminate/pistillate, but it varies from 4:1 to 15:1.

The watermelon fruit has a round or cylindrical shape, length up to 60 cm and rind thickness of 1–4 cm. The edible part of the fruit is the endocarp (placenta). Fruits usually weigh 3–16 kg; exceptions include fruits weighing up to 120 kg and smaller ones in the range of 0.9–3.6 kg. The fruit rind varies from thin to thick and from fragile to hard. Seeds continue to mature as the fruit ripens, and the rind lightens.

Harvest begins from July to August. Harvest indices are a combination of the following indicators, which may vary depending on location, cultivar and plant growth (Crop profile, VA2003):

• Branches or tendrils on the vines nearest the fruits wilt and change colour from green to brown.
• The ground spot on the belly of the fruit changes from white to light yellow. A typical response of watermelon to biotic and abiotic stresses.
• The sound of tapping changes from a metallic ring in the unripe state to a soft hollow sound when ripe.
• Green stripes (in striped cultivars) gradually break up where they intersect on the watermelon peduncle.

Seeds are easier to extract from the fruit if the fruit is kept in storage (in shade or in a seed processing room) for several days after harvest. The optimal storage temperature is 16°C.

Nutrient deficiency stress

Productivity is optimal on well-drained, irrigated soils with balanced nutrition (kg/ha 135–200 N, 170 P₂O₅, 135–170 K₂O). Nitrogen deficiency in watermelon is usually observed in Arizona soils. If nitrogen deficiency is detected after fruit set, it cannot be corrected. Soil analysis before planting is more valuable because nitrogen deficiency affects growth, vigour and yield of watermelon. Nitrate levels in petioles are usually high at an early growth stage and gradually decrease by the beginning of fruiting. The nitrate concentration in the petiole should be maintained above 4000 ppm NO₃-N throughout the season. If the level decreases to 2000 ppm NO₃-N, visual symptoms appear, such as pale green foliage or reduced vine growth (Anonymous, 2015).

Early yield positively correlated with the application of phosphorus (P) fertilisers, and maximum yield was achieved with a small amount of P fertiliser (Hoschmuth et al., 1993). K⁺ is necessary for flowering, fruit development and improving their quality. K deficiency and the associated blossom-end rot due to calcium deficiency were often observed in watermelons grown on soils where cultivars with long fruiting and/or less irrigation had previously been grown, as, for example, in Florida. Higher levels of N and K increased the need for banded P uptake, but P uptake did not affect yield (Hochmuth and Hanlon, 2015).

The watermelon genotype ‘YS’ was more resistant to K⁺ deficiency and demonstrated less inhibited root growth than the genotype ‘8424’, and transcriptome analysis results showed that suppression of defence and stress responses may save energy for better root growth in YS, which may facilitate K⁺ uptake and increase K use efficiency and resistance to K⁺ deficiency (Fan et al., 2013).

Copper is vital for the metabolism of higher plants and physiological and biochemical processes, especially electron transport during photosynthesis (a component of plastocyanin) and accumulation of carbohydrates and proteins. On very acidic fine sandy soil Leon in Gainesville, Florida, copper (Cu) deficiency led to reduced growth, with correctly shaped leaves curled upwards, and marginal burn of young, unfolding leaves and death occurred at growth points. Fruiting was absent in plants with severe Cu deficiency. Increased yield due to Cu fertiliser increased the Cu level in leaf tissues (Locascio et al., 1964). Organic nitrogen fertilizer made Cu and zinc (Zn) available to growing watermelon (Fiskell et al., 1964).

Manganese (Mn) deficiency begins before the beginning of watermelon flowering and is characterised by yellowing of leaves with interveinal chlorosis with white spots after fruit set and over time spreads from lower leaves to upper ones. Soils with high pH and a level of easily reducible manganese of less than 100 mg Mn/kg (extracted with 2% hydroquinone + 1 M ammonium acetate) usually have Mn deficiency. Low-molecular-weight organic compounds, green manure and biowaste added to soil with high Mn content increase Mn phytotoxicity.

Watermelon is sensitive to molybdenum (Mo) deficiency. Mo deficiency is characterised by stunted plants with whitish-purple interveinal chlorosis and marginal burn of leaves in severe cases. Older leaves are affected first.

Salinity stress

Watermelons grow best on sandy loam soils resistant to slight acidity (pH 6–7). Inhibition of shoot and root development is the main response to salinity stress. Root elongation is strongly inhibited by high NaCl concentrations in the growth medium compared with lateral root formation (Bernstein and Kafkafi, 2002).

The rooting rate of transplanted seedlings significantly decreased with increasing salinity of the nutrient solution, and significantly higher values of relative growth rate (RGR), net assimilation rate (NAR) and leaf area ratio (LAR) were recorded in root-pruned and grafted seedlings (Balliu et al., 2014).

Drought stress

Agriculture mainly depends on irrigation. Drought is the most destructive abiotic stress, leading to economic yield losses. Under normal growth conditions, roots absorb water and nutrients from the soil to maintain plant vital activity and a balanced system. Under water stress, the entire plant system changes, which leads to various structural, molecular, cellular, metabolic, functional and phenotypic changes, such as strengthening of the cell wall and reduction or elongation of roots to adapt to changing environmental conditions (Atkinson and Urwin, 2012; Ghosh and Xu, 2014).

In the early stages of drought, root development of wild watermelon (Citrullus lanatus sp.) intensified, which indicates activation of a drought avoidance mechanism for absorbing water from deep soil layers, while in the later stages of drought, resistance to physical desiccation intensified by regulating the root proteome (Yoshimura et al., 2008). Wild watermelon demonstrated active root growth, and the domesticated cultivar showed slowed root growth under drought stress (Kajikawa et al., 2010). Under water deficit conditions, wild watermelon showed reduced leaf transpiration and a simultaneous increase in leaf temperature (Akashi et al., 2011).

Under drought conditions in the presence of strong light, wild watermelon accumulates high concentrations of citrulline in its leaves (Kawasaki et al., 2000), glutamate, arginine and DRIP-1, a drought-induced peptide (DRIP) (Yokota et al., 2002). Comparison of the proteome of stressed plants with that of non-stressed plants revealed 23 stress-induced proteins and 6 stress-repressed proteins, and confirmed that 15 of the 23 upregulated proteins were heat shock proteins (HSP), and 10 of the 15 HSPs were small HSPs (sHSP). The defence response of wild watermelon includes several functional proteins associated with cell protection and metabolism to provide protection against water deficit stress (Akahsi et al., 2011), and these traits can be used to breed new cultivars of cultivated watermelon with drought tolerance.

Temperature stress

The optimal temperature for watermelon growth ranges from 20 to 32°C (Bates and Robinson, 1995). Low-temperature stresses (< 5°C) are most common in temperate regions such as North America and Europe, and high-temperature stresses (>45°C) are extreme in tropical and subtropical regions of the world such as China, India and Africa, as well as in southern and central regions of temperate countries such as Europe and America, leading to famine due to complete crop failure.

Plant height, relative chlorophyll content, dry weight of leaves and stems of watermelon linearly decreased with decreasing daytime temperature from 25 to 15°C (Inthichack et al., 2014). High temperature usually affects reproductive growth of plants rather than vegetative growth, due to pollen sterility, which ultimately reduces yield (Bita and Gerats, 2013).

Ultraviolet B radiation

The flux of photochemically active ultraviolet B photons (UV-B) (wavelength, λ < 315 nm) into the troposphere is limited by the amount of stratospheric O₃ (Cicerone, 1987). The decrease in the amount of stratospheric ozone (O₃) and, as a consequence, the possible increase in UV-B is a critical problem. The effects of UV-B, CO₂ and O₃ on plants have been studied in growth chambers, greenhouses and field conditions. The response of plant biomass to enhanced UV-B can be negative (adverse effect), positive (stimulating effect) or neutral (tolerance). There are inconsistencies between results obtained under controlled conditions and field observation results. Based on measurements of biomass accumulation, watermelon proved to be very sensitive to increased UV-B radiation (Krupa and Kickert, 1989).

Heavy metal stress

Heavy metal (HM) toxicity is one of the main abiotic stresses leading to dangerous consequences for plants. A common consequence of heavy metal toxicity is excessive accumulation of ROS and methylglyoxal (MG), which can cause lipid peroxidation, protein oxidation, enzyme inactivation, DNA damage and/or interaction with other vital components of plant cells. Higher plants have developed a complex antioxidant defence system and a glyoxalase system for removing ROS and MG. In addition, MG entering the cell can be absorbed by amino acids, organic acids and GSH or specific metal-binding ligands.

As a central molecule of both the antioxidant defence system and the glyoxalase system, GSH participates in both direct and indirect control of ROS and MG and their reaction products in plant cells, thereby protecting the plant from HM-induced oxidative damage. Recent molecular studies of plants have shown that GSH itself and its metabolising enzymes — in particular, glutathione S-transferase, glutathione peroxidase, dehydroascorbate reductase, glutathione reductase and glyoxalases I and II — act additively and coordinately for effective protection against ROS- and MG-induced damage, in addition to detoxification, complexation, chelation and compartmentation of HMs.

A copper level in soil of 50 mg/kg was favourable for the growth of watermelon plants, while a level above 100 mg/kg proved toxic. The phytoremediation role of watermelon on copper-contaminated soil is low (Vijayarengan and Jose, 2014). It has been shown that the Mt2 and PCS genes act as powerful scavengers of hydroxyl radicals in watermelon (Citrullus lanatus) under HM exposure (Akashi et al., 2004). Several studies have shown that some rootstocks are capable of limiting the uptake and/or translocation of heavy metals (for example, Cd, Ni, Cr) and micronutrients (for example, Cu, B and Mn) to the shoot, thereby mitigating stress caused by their excessive external concentration (Savvas et al., 2010).

Pollution stress

Ozone is formed in the air during daylight from chemicals emitted by vehicle exhaust and during the combustion of fossil fuels (oil and coal). Ozone or the ozone layer in the stratosphere (mainly 90% O₃) serves as protection against biologically harmful solar ultraviolet radiation, initiates key chemical reactions in the stratosphere and converts solar radiation into heat and mechanical energy of atmospheric winds. The remaining 10% of O₃, located in the troposphere, is harmful to humans, animals and vegetation (Krupa and Kickert, 1989; Cicerone, 1987).

Damage from pollution was observed primarily on pollinator plants. Ozone is the most phytotoxic air pollutant, damaging the leaves of melons and watermelons grown in field conditions in several Midwestern and Eastern US states and Europe. Damage to watermelon leaves consists of premature chlorosis (yellowing) of old leaves. Subsequently, brown or black spots with a white coating appear on the leaves. Watermelons are generally more susceptible to ozone damage than other cucurbit crops. Damage is more common during fruit ripening or when plants are under stress. Damage first appears on crown leaves and then spreads outward.

The autumn watermelon crop (cultivar Sugar Baby) in open chambers in Indiana, treated with filtered charcoal or unfiltered air, showed a significant decrease in marketable yield by weight and number (21%) compared with the crop grown in clean air (Synder et al., 1991). In two studies using open-top chambers in commercial fields in Spain, the content of soluble solids in watermelons decreased by 4–8% due to the seasonal ozone level in the environment (Gimeno et al., 1999).

Symptoms caused by air pollution are very similar to symptoms caused by biotic or nutrient stress. Therefore, when checking for damage caused by ozone air pollution, leaf tissues should be checked for Mn toxicity; N, P, Mg, B and Fe deficiency; moisture stress; damage by insect mites and thrips; pesticide toxicity and viral infection. If pollution is caused by sulphur dioxide, leaf tissues should be checked for N, K, Mn, Mg and Ca deficiency, moisture stress, high temperature, pesticide toxicity and other organisms causing leaf diseases (Lacasse and Treshow, 1976; Simon et al., 1988).

The key to preventing air pollution damage is planting tolerant cultivars to limit plant stress. Seedless watermelon cultivars are generally more resistant to air pollution than seeded cultivars, and icebox-type cultivars are the most susceptible. Fungicides, thiophanate-methyl (Topsin), provide some protection against ozone damage. Watermelon cultivars were classified as resistant (Charleston Gray, Jubilee, Picnic and Supersweet), intermediate (Chilean Black, Mirage, Tender Sweet and Orange Flesh) and susceptible (Crimson Sweet, Madera, Sugar Baby, Moran, Petite Sweet and Blue Belle) to environmental pollution based on assessments conducted at the Purdue Southwest Agricultural Research Center, Vincennes, Indiana, in 1984–1985.

Fusarium wilt

A soilborne disease caused by Fusarium oxysporum f. sp. niveum (FON) is a major production problem worldwide and in the USA. There are three races (0, 1 and 2); race 1 is the most common throughout the USA and worldwide, and race 2 is aggressive and present in Delaware, Florida, Indiana, Oklahoma, Maryland and Texas in the USA. The disease is severe at a temperature of 25–27°C.

Wilt can occur in young seedlings and decay in the soil, where the hypocotyl is surrounded by watery and soft rot, causing stunting of plants. Later, wilt occurs in more mature plants, leading to their death. Infected plants were noticeably wilted on one side, the remaining shoots were healthy, observed to be wilted, dried out and had brown leaves with discoloured brown vascular tissues.

Watermelon cultivars Afternoon Delight, Crimson Sweet, Indiana, Calhoun Gray, Smokylee and Summit were highly resistant, and Sweet Princess, Jubilee, Charleston 76, Klondike R7 and Summerfield were weakly resistant to FON (Elmstrom and Hopkins, 1981). Long crop rotations with grass crops, delayed final thinning and resistant cultivars are best for controlling FON. Combined use of green manure, organic mulch from hairy vetch (Vicia villosa Roth) and partial resistance of cultivars can provide adequate FON suppression (Zhou and Everts, 2006). This combination also reduced the severity of powdery mildew (caused by Podosphaera xanthii) and Plectosporium blight (caused by Plectosporium tabacinum) on pumpkin, a vegetable of the Cucurbitaceae family (Everts, 2002).

The self-regulation ability and return to normal state in the resistant watermelon cultivar ‘Kelunsheng’ was higher than in the susceptible ‘Zaohua’ in terms of malondialdehyde (MDA) content, relative electrical conductivity, superoxide dismutase (SOD) and CAT activity when infected with FON at the seedling stage. The ability of the resistant cultivar to inhibit chlorophyll destruction and maintain a higher carotenoid content was significantly stronger than that of the susceptible cultivar. The resistant cultivar maintained higher dehydrogenase activity, relatively higher vitamin C content and relatively lower soluble sugar content with high soluble protein content than the susceptible cultivar (Wang et al., 2002).

A procedure combining inoculation by dipping roots in water and the method of culturing spores at the seedling stage and further testing resistant material in the open field was recommended for obtaining accurate and consistent results over a relatively short period and accelerated the process of breeding watermelons resistant to FON (Zhang et al., 1991). The disease incidence of FON in highly resistant cultivars Tianniu and Zaochunhongyu was 19.67% and 18.45%, respectively, and the disease incidence of highly susceptible cultivars Tianshi and Heimeiren was 88.22% and 93.33%, respectively, and clearly showed the physiological race that infected watermelon, provided a basis for breeding FON-resistant watermelon cultivars and selecting production cultivars in Heilongjiang (An et al., 2009).

Genetic populations segregating for race 2 FON are currently being developed. Three single nucleotide polymorphisms (SNPs) and a major quantitative trait locus (QTL) associated with resistance to race 1 were identified (Amnon, 2014), as well as QTL markers for fruit, seed and flower traits, and rind thickness (Cecilia, 2012).

Anthracnose

Two races of the anthracnose fungus (Colletotrichum orbiculare) are common on cucumber crops. Warm, humid weather favours infection and spread of the disease. Most watermelon cultivars are resistant to race 1, and none possesses resistance to race 2. The race 2 fungus infects the aboveground parts of watermelon. The severity of infection leads to numerous lesions on leaves, and defoliation of the vine leads to yield loss and reduced fruit quality.

The best time to study resistance to anthracnose is 7–10 days after inoculation with 1 x 10⁶ CFU/ml at the 2nd leaf stage (Zhao-Ming et al., 2009). An antifungal protein isolated from bacterial strain MET0908 acted on the cell wall of Colletotrichum lagenarium, isolated from anthracnose-infected fruits, leaves and stems of watermelon, and exhibited β-1,3-glucanase activity (Kim and Chung, 2004).

The activity of POD and polyphenol oxidase (PPO) and chlorophyll content were higher in anthracnose-resistant cultivars (cv. Xinongbahao and Hongxiaoyu) compared with susceptible cultivars (Linglongwang), and the peak value of soluble protein content was observed earlier in resistant cultivars compared with susceptible ones when seedlings were inoculated with a spore solution of Citrullus orbiculare 1 x 10⁶ CFU/ml (Ting et al., 2009).

Downy mildew

Pseudoperonospora cubensis is a fungus that causes the appearance of small, round, irregular, chlorotic spots on the upper surface of leaves, later becoming brown, necrotic and affected with purple or grey downy growth on the underside of lesions. Lesions are not limited by veins (Lebeda and Cohen, 2011). Infection occurs in winter, and infected leaves curl upwards at the edges. Higher temperatures and relative air humidity prevailing in winter under closed covers usually favour the development of downy mildew on watermelons. The disease is common in warm humid climates of temperate regions such as America, Europe, Japan, Australia and South Africa, and in tropical and semi-arid regions such as the Middle East.

Reduced photosynthetic activity in the early stages of plant development, stunted plants, sunscald, reduced fruit quality and yield due to premature defoliation are some of the symptoms associated with growth and development due to this disease (Colucci and Holmes, 2010; Savory et al., 2011).

Gummy stem blight (black rot)

Didymella bryoniae (Phoma cucurbitacearum) causes crown rot, extensive defoliation and fruit rot of watermelon in the south-eastern USA. The fungus is transmitted by air, seed and soil. Infection is enhanced by wet plant surfaces and high relative humidity. Crop rotation, various irrigation methods and solarisation proved effective in controlling the spread of the disease. Plant stand, vine length and number of fruits increased in plots with added cabbage and solarisation, while the area affected by the disease decreased compared with annual watermelon-only crops (Keinath, 1996).

Among the USDA collections, the most resistant PIs were identified as 279461, 254744, 482379, 244019, 526233, 482276, 164248, 482284, 296332, 490383, 271771 and 379243, and the most susceptible as 226445, 534597, 525084, 223764, 169286 and 183398 (Song et al., 2004).

Powdery mildew

Symptoms of watermelon infection by powdery mildew Podosphaera xanthii include round, tan to dark brown spots that begin at the leaf edges and spread over the entire leaf, formation of spots, stem cankers, extensive defoliation and damage to cotyledons, hypocotyls and fruits (Antonia, 2008; Maynard and Hopkins, 1999). Exposed vines and fruits may drop and shrivel. The following cultivars were recognised as the most resistant: PI 632755, PI 386015, PI 346082, PI 525082, PI 432337, PI 386024, PI 269365 and PI 189225 (Antonia, 2008).

Phytophthora rot

Phytophthora blight and rot are caused by Phytophthora capsici Leon and Phytophthora drechsleri Tuck and were first recorded in Colorado and California, then in the northern and south-eastern US states and China. Phytophthora capsici causes seedling blight, leaf spotting, leaf wilt, root and crown rot, stem damage and fruit rot.

Crown rot — the initial symptom on the root under field conditions — causes complete destruction and death of the entire plant in a short period. The affected vine tissue becomes brown, appears water-soaked and often collapses. Dark, water-soaked lesions form on infected fruits, usually covered with white mould. Two cultivars resistant to Phytophthora capsici (IT185446 and IT187904), identified during germplasm characterisation, can be used as rootstock and a source of resistance in breeding Phytophthora-resistant watermelon cultivars (Kim et al., 2013).

Bacterial fruit blotch

This disease is caused by Acidovorax avenae subsp. citrulli and can destroy 100% of marketable watermelons if infection develops early (Lewis et al., 2015). Symptoms include dark angular spots on seedlings, mature leaves and fruits. In severe infection, small dark green spots appear on the upper surface of fruits, which lead to fruit cracking and fruit rot; although the disease is limited to the rind, it causes serious economic damage. Resistance to the disease has not yet been identified.

Viral diseases

Among the various cucurbit viruses in the southern USA, the most common are WMV-1 and WMV-2, watermelon vine decline (WVD), potato ringspot virus (PRSV-W), zucchini yellow mosaic virus (ZYMV), tobacco ringspot virus (TRSV), squash leaf curl virus (SLCV) and squash mosaic virus (SqMV) (Ali, 2011).

PRSV-W causes mosaic, blisters, wrinkling and chlorosis of infected watermelon leaves. PRSV-W and WMV cause mottling, leaf deformation and vine stunting in watermelons. ZYMV causes yellowing and mosaic. WMV-2 infects various cucurbits (muskmelon, pumpkin), leading to physiological and metabolic changes in the synthesis of chlorophyll and carotenoids, availability of cellular nutrients (phosphorus, nitrogen) in leaves and reduced photosynthetic activity (Muqit et al., 2007).

WMV-1 caused changes in leaf chlorophyll (14.4–54.9%) and carotenoids (30.7–70.4%), reducing yield and fruit quality (Sandhu et al., 2007). The virus also caused disintegration of infected leaf tissues, hypertrophy, tissue compaction and a decrease in the number of chloroplasts in tissues depending on virulence. The virus also caused a decrease in vine viability and length, and reduced the number of branches per vine. WVD virus is transmitted by whiteflies, which led to serious economic losses in Florida due to yellowing and browning of the vine and death of the entire vine (Baker et al., 2008).

Viruses are transmitted by aphids and whiteflies. Measures to control insect vectors, the use of silver plastic mulch and insect/virus-resistant cultivars will be useful. High resistance to mosaic virus was observed in watermelon collections PI 595203 (ZYMV-CH and WMV) and PI 482261 (ZYMV-FL) and was controlled by a single recessive gene (Xu et al., 2004).

Resistance to PRSV-W was recorded in cultivars PI 244017, PI 244019, PI 485583 (Guner et al., 2008) and PI 595203 (Strange et al., 2002). It was controlled by a single recessive gene prv (Guner et al., 2008). Resistance to ZYMV was controlled by a single recessive gene zym-FL in Citrullus lanatus and zym-FL-2 (Wehner, 2012) and zym-CH (Xu et al. 2004).

Materials and methods

The experiment was conducted at the headquarters of the National Horticultural Research Institute (NIHORT), Ibadan, Nigeria (3°54"E, 7°30"N, 213 m above sea level) from October 2009 to June 2010 using supplementary irrigation during periods of low rainfall. Procedures included three watermelon sowing densities: 1.5 x 0.45 m; 1.5 x 0.90 m; 1.5 x 1.50 m. Grain amaranth as an intercrop was transplanted at intervals of 0.75 x 0.75 m. The control plot remained empty without crops, and the control plot — only with grain amaranth, forming five treatments in each of five replicates in a randomized complete block design (RCB). Weeding was carried out manually at 3-week intervals, starting from the 3rd week after planting (WAP) of watermelon. A natural weed population was used, that is, no additional weeding was applied. The size of the net plot was six data rows, 15 m long, at a distance of 0.75 m from each other (i.e., 15 m x 4.5 m), plus a border row on each side to minimize interference from weed control agents. The treatments, design, and agronomic methods used were based on experimental results of the National Horticultural Research Institute (NIHORT) from 1985 to 1999 (NIHORT 1985-1999) and on results obtained by David (1997). Data on maximum and minimum soil temperature were collected by installing two maximum-minimum dial thermometers per plot at a depth of 0.10 m for 12 weeks, starting from the 3rd week after transplanting (WAT) of amaranth. Readings were taken at 3 weeks after planting amaranth, 50% anthesis of amaranth, and harvest of amaranth. Instrument readings at each plot were averaged for use in data analysis. Soil moisture content at the 0.15 m surface was determined gravimetrically at 50% amaranth fruit set. Four soil samples were taken randomly from each plot and combined in a sealed plastic bag for drying as soon as possible. The soil was dried at 120 °C for 3 days, and soil moisture content was expressed as the percentage of moisture lost on drying relative to the dry weight of the soil. Photosynthetic photon flux density (PPFD) was measured using an AccuPAR PAR 80 model within 1 h after solar noon in clear weather at 50% amaranth anthesis between the center of amaranth and watermelon rows. Percentage transmittance was determined based on readings below and above the canopy after adjusting for differences between sensors in readings of unobstructed sunlight. Weed samples in four quadrats of 0.50 m² per plot were taken at 3 WAT of amaranth, 50% amaranth anthesis, and amaranth harvest. Weeds in the quadrats were cut at ground level and dried in an oven at 80 °C to constant weight to determine total weed biomass. All data were subjected to analysis of variance using SAS software procedures. Interaction effects are indicated only if they were significant at P = 0.05.

Results

On average over two consecutive sowings, grain yield of amaranth and fruit yield of watermelon were highest at a watermelon plant spacing of 1.5 x 0.90 m. These values are within the range reported by David (1997).

The use of watermelon as a cover crop consistently reduced weed density and biomass compared with the control in crops.

The lowest average light transmittance of living mulch was 0.29% and 0.22% in the first and second sowings, respectively. All mulch densities transmitted less PPFD compared with the control. To activate phytochrome-mediated germination, a light transmittance level of less than 0.1% is required, and for the very low fluence response an even lower level is required (Kronenberg et al., 1986). This conclusion is confirmed by previous studies, according to which living mulch reduces light penetration into the soil by a quarter or one and a half times compared with bare natural soil infested with weeds (Teasdale et al., 1993).

Maximum soil temperature was reduced by amaranth and melon in each sowing, and there was a tendency toward a significant increase in minimum soil temperature under living mulch. Britow (1988) first noticed this under mulch for modifying the temperature of the soil environment, but the implications for adaptation to climate change needed clarification (Challinor et al., 2007). The practical significance is that living mulch can regulate the ambient and soil temperature by lowering the temperature in a hotter environment and raising it in a colder environment and during cool periods of the day. Melon may have a greater effect on weed germination by affecting the daily amplitude between maximum and minimum temperatures. Germination of seeds of some weeds requires a daily temperature change of about 10 °C (Taylorson, 1987), but it is unknown whether the species (predominantly elephant grass) in this experiment have a temperature amplitude requirement.

Soil moisture content was significantly higher under cover crop use compared with bare soil.

Cultivar types

Watermelons reach sizes of 159 kg, but most cultivars produce fruits weighing 1–15 kg. Fruits can be micro (< 1.4 kg), mini (1.4–3.7 kg), icebox (3.8–6.1 kg), small, sometimes called pee-wee (6.2–8.3 kg), medium (8.4–11.1 kg), large (11.2–14.5 kg), and giant (> 14.5 kg). The cultivar 'Sugar Baby', developed by M. Hardin in 1956, produces small dark green spherical fruits. It is popular among home gardeners and widely grown in Asia. In addition, there are cultivars that ripen in production zones with a short season.