Spoilage by diseases can cause serious losses in post-harvest of watermelon. The most common postharvest diseases are anthracnose and stem-end rot. Postharvest treatments are required to control them.

Anthracnose is caused by Colletotrichum orbiculare , which can penetrate the cuticle and epidermis of leaves, stems and fruits. It is characterised by dark green circular or elongated lesions which affects the fruit surface. The lesions gradually enlarge, turn brown and become sunken. The disease can be a latent infection from the field and becomes apparent at postharvest especially under warm and humid condition. The disease may also infect fruits through cross contamination with infected fruits and the use of dirty containers. Anthracnose can be controlled by implementing measures at preharvest including the use of mancozeb at 8-10 g/4.5 L or maneb at 9-18 g/4.5 L.

Stem end rot is caused by Lasiodiplodia theobromae , which causes development of brown, soft, water-soaked areas. The pathogen usually enters the cut surface of the stem or through open wounds or bruises. It can be controlled by allowing a few centimetres of the stem attached to the fruit.

In summary, the important factors in determining watermelon quality during harvesting and handling are:

Watermelons are either loaded as bulk shipments or packed into bins and cartons and loaded onto transport trucks. Bulk shipments are loaded adjacent to the watermelon-growing field. Bulk loading requires protection of melons from vibration and load shifting injury during transit. This is accomplished by placing a layer of straw in the floor of the trailer and between the melons and the front and sidewalls of the trailer. Sufficient straw is needed at the rear between the melons and rear doors to avoid damage caused by sudden stops. During bulk loading operations, watermelons are not physically protected by packaging. Therefore, workers must not sit, stand or walk over melons while loading because this will cause compression damage. A layer of foam will help protect the fruits from injury.

Plate 1: Placing a layer of foam between melons will protect them from injury

Shipping cartons hold three, four or five melons, depending on size; fibreboard bins hold on average 50 melons and are used for bulk shipments. The immediate benefit of containers is less bruising because of less handling for individual melons. Inexperienced packers cause internal melon bruising during bin and carton packing. Dropping melons into bins produces impact shock and standing or sitting on loaded melons inside bins causes compression injury. All handlers should learn that the absence of rind injury does not justify rough handling because bruising occurs first to the internal flesh.

Bulk shipments of watermelons must be padded by sufficient straw to protect them from damage during transit.

In addition, trailers should be adequately vented. An enclosed trailer should have two vent doors on the front and two on the rear to allow air passage within the melon load. Otherwise, overheating of melons can result in increased decay and quality loss.

Minimise damage to bin- and carton-packed melons during shipment by doing the following:

Temperature management is important for optimum watermelon quality. Precooling the fruits before storage to achieve maximum storage period is recommended. Precooling can be done by room cooling or forced air cooling to bring the pulp temperature as close to optimum as quickly as possible.

The optimum storage temperature for melons is 10-15°C, with a relative humidity of 85-90%. Watermelons stored in these conditions should keep for 2 to 3 weeks.

At temperatures below 10°C, chilling injury can develop, causing decreased redness and juice leakage of the internal flesh, and surface pitting. Chilling injury can also increase fruit susceptibility to decay and rots especially after being returned to warm ambient temperature following cold storage.

Once melons are sliced for sectional display, they should be wrapped with film and stored at 2-4°C to avoid souring and surface dryness.

Watermelons are normally shipped in open or closed trucks and trailers without refrigeration. The fruits should not be shipped in closed trucks if not refrigerated. Melons should not be stored with other fruits (e.g. bananas) and vegetables (e.g. tomatoes) that emit ethylene . Ethylene is a colourless gas regarded as the natural aging or ripening hormone. When exposed to ethylene , watermelons break down internally, and the flesh takes on a water-soaked appearance. This leads to flesh softening and flavour loss.