Watermelons: commercial production

R. Grattidge, revised by N. Meurant, T. Grundy T. Boland & Eric Coleman, Department of Primary Industries and Fisheries.

Watermelons (Citrullis vulgaris) are a member of the cucurbit family, which also includes rockmelons, honeydew melons, cucumbers, pumpkins, squash, zucchini and other gourds. Common in fruit platters or as a refreshing desert at a picnic, watermelons are available in a wide range of sizes and shapes.

Dark red fleshed, black seeded varieties are the most popular on the market place. Yellow fleshed (champagne) melons and small mini melons also find a ready market. "Seedless" melons, although more difficult to grow, have become popular in the market.

Climate and soils

Watermelons require long warm growing periods. The plants are frost susceptible and low temperatures can cause growth abnormalities, poor setting and hollow fruit. Sunburn can be a problem for the fruit, particularly in the "ground spot" area if the fruit are placed upside down after harvest. Light green and grey-green melons are less likely to sunburn than the darker green or striped varieties.

Watermelons can be grown in all districts in QLD during the warm months the main harvest times are set out below.

Major harvest periods in Queensland (shaded)

Foliage and fruit disease can be a major problem in high rainfall areas.

Soils

Provided the soil is well prepared, watermelons can be grown on a wide range of soil types. The main requirement is good drainage. Rigid irrigation practices must be adopted when growing on heavy soils otherwise plant losses from root rot may be high . Mud sticking to the harvested fruit can also make them unsightly and reduce market acceptance.

Establishment and maintenance

A cover crop is recommended before planting watermelons. If root-knot nematodes are a problem, a cereal crop such as forage sorghum should be used instead of legumes. All cover crop residues should be completely broken down before planting the watermelon crop. Ripping may be required to improve drainage on shallow or compacted soils.

Laser levelling is recommended before planting. As watermelons are often used as a quick rotational crop, the cropping history of the ground they are to be planted on should be researched to ensure no carry over herbicides have been used. In sugar growing districts, it is a wise precaution to notify surrounding cane farmers of the crop's presence, to avoid damage from herbicide spray drift.

Plastic mulch in conjunction with trickle irrigation and containerised seedlings is widely used in watermelon production. The plastic mulch used can vary from 0.9 to 1.2 m in width and is used to cover the plant row. Soil in the rows is usually raised about 10 cm in height and the trickle tape laid down the centre of the row or slightly to the side. The end is buried to about 5 cm deep. Both the bedding up and laying of the trickle tape can be carried out during or prior to the laying of the plastic.

Varieties

There is a large range of varieties available for planting. They vary in shape, size and internal and external colour. The most sought after standard types are those with dark striped skin with rich red flesh and very black seeds. Mini melons are popular on the market and are sold in cartons. Seedless melons have become an important crop.

Some of the main varieties grown include:

Large Types	Mini Melons	Yellow Champagne	Seedless
Red Tiger Bengal Tiger Phantom Pharoah Red Dragon Genghis Hercules	Gemini Minilee (O.P.) Baby lee Baby Tiger Sugar Baby	Yellow Doll Orange Dragon Champagne	Honeyheart (yellow) Raven (red) Dragon Heart (red) Triple Heart (red) Banquet (red) Golden acre (yellow) Seedless 1600

Planting

Field planting is carried out by direct seed sowing in the field or by using containerised seedlings. As most commercial varieties are hybrids, the latter technique is preferred because of the expense of hybrid seed. Where plastic mulch is used, direct seeding machines on waterwheel planters are used, punching the seed or seedlings directly through the plastic mulch. For direct sowing approximately 1.5 to 2 kg of seed is required per hectare. Pregerminated seed may be used to replant any which do not germinate.

Row spacings vary from 1.5 to 3.0 m and plant spacings range from 0.75 to 1.5 m. sSeedless melons need a standard variety in close proximity to provide pollen for fertilisation to occur. Without pollinators fruit will not set,no more than two rows of the seedless variety should be planted to every one row of a standard variety for ease of identification at harvesting. Always ensure the seedless type is readily distinguishable from the standard variety. Seeds take 4 to 14 days to germinate, soil temperature should be at least 11 ° C at time of planting.

Nutrition and irrigation

A soil analysis is recommended. In the absence of a complete soil analysis,a complete fertiliser should be applied by spreading and incorporating (sandy soils) or drilling (red soils). NPK mixes at 700-1200 kg/ha of 5:6:4 or 300-500 kg/ha of 14:14:12 and nitrogen side-dressings can be used. Late applications or excessive nitrogen use in cold weather should be avoided as hollow melons may result. Excessive nitrogen application also favours the development of male flowers.

Where trickle irrigation is used, side dressings are applied through the trickle system. Depending on the results of soil or sap analysis, side dressings of calcium nitrate alternated with potassium nitrate can be applied every two weeks commencing at early running. Both these fertilisers are applied at between 20 and 40 kg per hectare. Magnesium sulphate used at the rate of 20 kg per hectare should be applied after every 2 to 3 applications of the calcium or potassium nitrate. Plant tissue analysis can be used to gain an indication of the nutrient status of the crop during the season.

Nutritional disorders

Molybdenum deficiency is common in areas with a low pH. Symptoms include stunting of plant growth and mottling, yellowing and rolling of leaves. This deficiency can be remedied by a foliar spray of sodium molybdate at the four leaf stage, repeated if necessary at the runner stage of growth. Zinc, copper and boron deficiencies may also occur, but are most likely in leached, strongly acid, sandy soils or where large quantities of liming materials are present. Hence, zinc and boron deficiencies are also common on the alkaline soils of the Lockyer and Fassifern Valleys.

Manganese toxicity is a problem associated with acid soils (pH 5.8 or less). Seedlings suffering from Manganese toxicity exhibit stunted growth and yellowish crinkled leaves. Older plants can have an unhealthy pale green cast with pinhole size brown lesions occurring in clusters between the leaf veins of affected plants. These symptoms are often confused with infectious diseases such as gummy stem blight. Manganese toxicity can be avoided by maintaining soil pH between 6.0 and 6.5.

Irrigation

Water can be applied to the crop through overhead, furrow, or trickle irrigation. Trickle irrigation allows the most efficient use of available water and is compatible with fertigation (application of soluble fertilisers in the irrigation water)

The amount of water used to grow a crop will vary depending on the time of year and soil type but, the following figures are a useful guide-

Over head sprinkler systems	4 to 5 ML / hectare
Furrow / Flood	4 to 5 ML / hectare
Trickle	3 ML / hectare

Overwatering should be avoided when the plants are young but quantities should increase as the plant grows. Highest water requirements are during fruit set and fruit fill.

Water quality is important, particularly if using overhead irrigation. EC (electrical conductivity) readings of much greater than 1300 to 1400 m s/cm could cause leaf burn and reduction in yield. With furrow or trickle systems, irrigation water with E.C readings of up to 2000 m s/cm can be used with care. Above this figure, losses from a disorder known as blossom end rot may occur. Avoid overhead irrigation between 7 - 11 am during flowering and fruit set, as this may deter bees from pollinating flowers.

Pollination

Watermelons produce separate male and female flowers. Male flowers are produced initially, followed by production of both sexes usually at a ratio of 1 female to 7 males.

Watermelons, pumpkins, cucumbers, and zucchinis can be planted side by side. Cross-pollination can occur between cultivars of the same crop (eg watermelons and watermelons) but not between different crops (eg watermelons and pumpkins).

Bees are necessary for pollination. Bees need to be present and active in the crop. If bees are not plentiful, then import at least 2 hives/ha, spreading them around the field perimeter and check to ensure that male flowers are producing pollen. Flowers are most receptive to pollination during the morning hours when bee activity is usually the highest. Bee activity is related to climate and is lower in cooler weather. Spraying and irrigation should be coordinated to occur when bees are least active. If possible, use agrochemicals with minimal bee toxicity.

Figure 1. Typical male (a) and female (b) cucurbit flowers.

Pests and diseases

Weeds

Plastic mulch provides excellent weed control over the plant row. The inter-row space can be kept clean by using a registered knockdown herbicide. Where crops are grown without plastic mulch, grass control is obtained by applying registered post emergence herbicides (It is important to read the chemical label, melons are very susceptible to herbicide damage especially when the plants are small). Mechanical methods will give good control and are most effective if the vines are "tucked in" before cultivation. Cultivation between rows should be shallow to avoid injury to the root system.

Insects

A number of insect species including caterpillars, mites and thrips can cause damage to the plants but are readily controlled using registered insecticides. Aphids can be very damaging as they are the vector for mosaic viruses. Sliver leaf whitefly is a new pest in north Queensland and has the potential to cause extensive damage to crops. It is very difficult to control and produces copious quantities of honeydew resulting in the presence of sooty mould. It is the vector of gemini viruses.

Vertebrates

Mice can cause major problems in melon crops prior to emergence, they will dig out and eat large quantities of seed. The crops may have to be replanted several times, resulting in delayed harvests. Pre-germinating seed or planting container grown seedlings will reduce loss.

Crows can be a devastating and annoying pest. Just before harvest they can reduce the saleability of melons by punching holes through the skin with their sharp beaks. Often not content with eating from just one melon, the crow will hop from melon to melon, in this way populations of crows can wipe out an entire crop in just a few days.

Crows can be deterred with the use of a sound device such as a gas gun, or bird alarm.

Diseases

Diseases can cause extensive damage to crops. Some of the more important of these are listed below.

Disease	Symptom	Management
Bacterial fruit blotch	Oval water-soaked areas on fruit	*Registered seed treatment, crop hygiene, *registered copper-based sprays
Fusarium wilt	Yellowing, wilting, stunting	Rotation and resistant varieties
Gummy stem blight	Brown spots on leaves; reddish ooze from runners; black sunken spots on fruit	*Registered fungicides and crop hygiene
Mosaic viruses	Mosaic pattern on leaf, leaf and fruit distortion	Deter aphids, crop hygiene
Powdery mildew	Greyish patches on older leaves	*Registered fungicides or resistant strains
Damping off	Rotting and death of seedlings	Good soil drainage
Root-knot nematodes	Wilting and stunted growth	Soil treatment
Sudden death	Rapid wilting Rotting roots	Improve drainage control irrigation crop rotation.

*The infopest CD-Rom provides an up to date database of registered pesticides of all vegetable crops including pumpkins(see information section)

Fruit disorders

Blossom-end rot is caused by calcium deficiency in the flower(blossom) end of the fruit. It is worse in hot, dry, windy conditions where moisture stress is more likely to occur. Symptoms include young fruit drop and brown rotting lesions at the blossom end of older fruit. Good water management and ensuring sufficient soil calcium availability will usually address the problem. Soil or irrigation water salinity may also promote blossom end rot.

Internal cracking is caused by cool temperatures during early fruit-filling period. Other influences are stop-start growth, excess nitrogen, low boron levels, or heavy infrequent watering at fruit fill. Affected melons tend to be flattened in shape and feel lighter than usual.

Spongy end occurs in melons, which have been poorly pollinated. These melons may turn yellow and drop off the vine early in their development or partly develop with the stylar end soft and spongy. This area is also slightly pointed. Internally, there is very poor seed development at the spongy end.

Sunburn can be a major problem in dark or darker striped melons. It is rarely seen in light coloured melons. Sunburning can also occur in melons, which have been stacked in the field with their underside facing upward.

Harvesting and marketing

Harvesting usually begins 3-4 months after planting. Maturity is sometimes difficult to determine. Useful maturity indicators are listed below, however it is still advisable to cut open a few fruit to check maturity before harvesting commences. The watermelon stem should be cut rather than pulled from the vine to avoid damage to the stem end. Do not stack fruit on their ends, as this is where the rind is thinnest.

Maturity indicators include:

• A dull hollow sound when the fruit is tapped with the knuckles
• The change from white to cream or pale yellow of the skin area where the melon has been resting on the soil
• Shrivelling of tendrils on nodes to which melons are attached.
• Slight ribbing on surface of fruit can indicate maturity in some varieties.
• The Brix test is the most objective way of testing maturity. It assesses the total soluble solids (soluble solids is related to sugar content and is an indicator of sweetness) of the melon flesh. The test is becoming more popular with many retailers insisting on specific brix levels particularly in seedless lines.

Melons should be packed in clearly marked cardboard bins. Inspect all containers to ensure no sharp objects, which may damage the fruit, are present. Ensure minimum handling of melons, as extra handling is expensive and may harm the fruit. Watermelons do not store well as they are susceptible to chilling injury, and are subject to decay at higher temperatures. Under the ideal conditions of 7 ° C and a relative humidity of 80 to 90 per cent melons can be stored for up to two weeks.

Further information

The DPI&F produces a handbook for Growing pumpkins, grammas and watermelons in Queensland which is currently undergoing revision. The third edition is due for release in 2001. For further general information and specialist referral services Queensland residents can contact the DPI&F Call Centre on 132523 for the cost of a local call. The Call Centre is open 8am to 6pm Monday to Friday (excluding public holidays); E-mail callweb@dpi.qld.gov.au.

Current national information on agricultural chemicals registered on pumpkins is available on INFOPEST CD-ROM. Write to DPI&F GPO Box 46, Brisbane Qld 4001, or email infopest@dpi.qld.gov.au, or phone DPI&F call centre 132523.

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Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.

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Last updated February 2001