Pests and diseases of watermelon

Weeds

Plastic mulch provides excellent weed control over the plant row. You can keep the inter-row space clean by using a registered knockdown herbicide. Where crops are grown without plastic mulch, you can control grass by applying registered post-emergence herbicides. It is important to read the chemical label, as 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 prevent injury to the root system.

Insects

A number of insect species, including beetles, caterpillars, mites and thrips, can damage the plants but are readily controlled using registered insecticides. Aphids can be very damaging as they are the vector for mosaic viruses. Sliverleaf whitefly has the potential to extensively damage 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 because they dig out and eat large quantities of seed. If this occurs, you may need to replant the crops several times, resulting in delayed harvests. You can reduce losses by pre-germinating seed or planting container-grown seedlings.

Crows can be a devastating and annoying pest. Just before harvest, they can make melons unsaleable by punching holes through the skin with their sharp beaks. Often they damage many melons, and populations of crows can wipe out an entire crop in just a few days.

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

Diseases

Diseases can extensively damage crops. Some of the more important of these are listed below.

Table 1. A list of major diseases, their symptoms and management techniques
Disease Symptoms 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 Aphid deterrent, crop hygiene
Powdery mildew Greyish patches on older leaves *Registered fungicides or resistant strains
Damping off Rotting and death of seedlings Good soil drainage; decomposition of organic matter before planting
Root-knot nematodes Wilting and stunted growth Avoidance of legume crops; soil treatment
Sudden death Rapid wilting, rotting roots Improved drainage; controlled irrigation crop rotation

*The Infopest DVD provides an up-to-date database of registered pesticides of all vegetable crops, including pumpkins(see Further information below).

Fruit disorders

Blossom-end rot

This 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 dropping off and brown rotting lesions appearing at the blossom end of older fruit. Salinity of soil or irrigation water may also promote blossom end rot. Good water management and sufficient soil calcium availability will usually address the problem.

Internal cracking

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

Spongy end

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

Sunburn

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

Further information

Last updated 20 December 2011