Commodities / Watermelons

Flavor | Texture | Color | Size | Shape | Defects and Disease | Shelf Life | Nutritive Value
MW Vegetable Production Guide | Postharvest Information (from UC Davis) | USDA Grade Standards

The quality of a watermelon is determined primarily by genetics. The inbred parents of a hybrid provide a relatively stable "set" of characteristics for a variety which establishes the potential for expression of the components of quality. The actual perceived or measured quality is influenced by the conditions under which the watermelon is grown and handled.

Flavor top

Sweetness is the component of watermelon flavor most commonly linked with quality. Soluble solids above 10% as measured by a refractometer (preferably above 12%) indicate a high quality watermelon. According to United States Standards for Grades of Watermelons, optional internal quality requirements may be specified in connection with the grade. For "very good internal quality", random samples of internal edible portions must contain not less than 10% soluble solids. For, "good internal quality" the soluble solids must be at least 8%. The genetic differences in watermelon sweetness are reported annually in the Midwest Vegetable Variety Trial Report.

Mineral nutrition must be optimal to establish a healthy canopy of leaves, which is essential to sugar development. Inadequate mineral nutrition will result in poor growth and low sugar levels. High soil moisture during ripening can reduce the concentration of sugars in the fruit. Many growers reduce irrigation this phase of fruit development. The combination of a healthy canopy of leaves, sunlight and warm temperature is essential to sugar development. Cool temperatures and cloudy conditions result in poor sugar development. Recommendations for fertilizing watermelons are available in the Midwest Vegetable Production Guide for Commercial Growers or through your local Cooperative Extension Service.

Fungal diseases such as alternaria, anthracnose, gummy stem blight and fusarium wilt can reduce leaf area enough to interfere with sugar development in the fruit. Competition from weeds for light, water and nutrients can reduce vine growth and lower sugar content of the fruit.

The timing of harvest is important in regard to sugar content. Immature watermelons will likely not have the sweetness characteristic of a mature watermelon for the type. An overripe watermelon has flesh that is mealy, may have slight cavities around seeds and may have an insipid taste. In the field watermelons are harvested when the tendril nearest the fruit is wilting and the ground spot is yellow.

Cooling and cool storage slow the loss of sugars in watermelon fruit. Watermelons should be stored at 50˚F to 60˚F, or for short periods, 45˚F to 50˚F. Temperatures below 45˚F cause chilling injury resulting in loss of flavor in addition to reduced shelf life.

Texture top

Watermelon flesh of high quality has a crisp texture. Although not specifically mentioned in USDA grade standards, it is an important component of overall quality. This characteristic is measured subjectively. Watermelon varieties are likely to differ in texture due to different genetic backgrounds, although these differences are not regularly reported in published reports.

An overripe watermelon has flesh that is mealy, may have cavities around seeds and may have an insipid taste. Exposure to ethylene in storage/transit will increase the mealiness of the flesh, in addition to causing separation of the rind and softening of the entire fruit.

Color top

Rind and flesh color are important characteristics that are genetically controlled. Among seeded watermelons rind color patterns are referred to as Allsweet, Jubilee, Crimson Sweet, and black. Red is the most common flesh color, but orange and yellow are also available. Intensity and shade of flesh and rind color can vary among varieties. Characteristics of numerous varieties are reported in the annually published Midwest Vegetable Variety Report.

Whiteheart is a hard white streak of flesh through the heart of the watermelon. If this streak exceeds 0.25 in. (6 mm) in diameter it is classified as damage in the United States Standards for Grades; if it exceeds 0.5 in. (13 mm) it is classified as serious damage. Whiteheart is associated with excessive moisture and nitrogen during maturation.

Storage at temperatures below 50˚F reduces red color in the flesh. Temperatures above 70˚F intensify the red flesh color.

Size top

Potential size of watermelon fruit is determined by genetics. Watermelon varieties traditionally come in two size categories: standard and icebox (small). Within these categories, the market determines desired size. Average fruit size of numerous varieties are reported in the annually published Midwest Vegetable Variety Trial Report.

Plant population influences fruit size, with higher populations producing smaller fruit, on average. Changes in spacing between plants within a row generally have a greater impact on fruit size than changes in between-row spacing. Plant population recommendations may be found in the Midwest Vegetable Production Guide, or through your local Cooperative Extension Service.

Drought stress, especially during fruit development, will limit fruit growth and may predispose the fruit to blossom-end rot.

Diseases, insects and weeds can reduce fruit size. Control recommendations for commercial growers may be found in the Midwest Vegetable Production Guide, or through your local Cooperative Extension Service.

Shape top

Watermelon shape is governed by genetics. Standard watermelon shapes are oblong, oval and round. Within a category, a variety may be more or less blocky or elongated. The market determines the desired shape. Fruit that is asymmetrical is of lower quality. Occasionally melons of any variety may be misshapen because they lie on uneven ground or were injured while small in size. Shapes of varieties are reported in the annually published Midwest Vegetable Variety Trial Report.

Poor pollination of watermelon, especially elongated types, may result in "bottlenecks", or constricted growth at the stem end of the fruit. Poor pollination can be due either to a low population of bees or poor bee activity due to weather. Research has shown that a minimum of 1,000 grains of pollen are required to be distributed over the three lobes of the stigma of the female flower in order to produce a uniformly shaped fruit. One hive for 1 to 5 acres is recommended depending on the strength of the hive. A strong hive has 50,000 bees. Many hives will not have that many bees early in the spring.

Defects and Disease top

Anthracnose

Anthracnose (Colletotrichum orbiculare) is a fungal pathogen that can cause round sunken spots on the fruit. Anthracnose is the only disease mentioned by name in the USDA grade standards for watermelon. Some watermelon varieties have a partial genetic resistance to anthracnose, but it is not enough to prevent the problem.

High humidity and periods of leaf wetness promote anthracnose development. Applying overhead irrigation in the early morning so leaves can dry, or using drip irrigation may reduce infection and disease pressure. Temperatures between 70˚F and 80˚F are best for development of anthracnose.

Anthracnose on the fruit is prevented by controlling the disease from the seedling stage in the greenhouse and throughout the growing season. Recommendations for managing this disease in commercial fields may be found in the Midwest Vegetable Production Guide, or through your local Cooperative Extension Service.

Fruit with anthracnose is a source of infection for other fruit. Watermelons with anthracnose lesions should not be harvested.

Sunscald

Sunscald or sunburn first appears as a gray or white area on the upper surface of the fruit where it is exposed to the sun. Fruit with dark rinds are more susceptible to sunscald than those with light colored rinds like Charleston Grey.

The amount of sunscald is related directly to fertility management and other conditions like field tilth and drainage because of their influence on foliage cover. Proper fertility and soil management promotes adequate vine growth and coverage of fruit. Hot sunny days will increase the incidence of sunscald if foliage cover is poor.

The amount of sunscald is also related directly to management of diseases that reduce foliage cover, such as anthracnose, alternaria, gummy stem blight and downy mildew. Recommendations for managing these diseases in commercial fields may be found in the Midwest Vegetable Production Guide or through your local Cooperative Extension Service.

Hollow Heart

Hollow heart is an internal crack in the flesh of the melon. The cause is not known. The incidence is higher in seedless varieties and is higher in crown-set fruit that would be the first harvest of the season.

Shelf Life top

Watermelons may last 2 to 3 weeks under proper storage conditions.

Watermelons should be harvested when mature, preferably when soluble solids are 10% or more. Bruising during harvest or transport will injure watermelon fruit, promoting decay and reduced shelf life. Fruit with anthracnose lesions or evidence of bacterial fruit blotch should be left in the field.

Watermelons should be stored at 50˚F to 60˚F, but may be stored at 45˚F to 50˚F for short periods. This is a compromise between cooler temperatures that retard decay but lead to chilling injury, and warmer temperatures which prevent chilling injury. Chilling injury symptoms may include surface pitting, discoloration, water-soaked spots, development of necrotic spots, shorter storage life and poor flavor.

Nutritive Value top

Serving Size: 2 cups diced pieces, 1/8 medium melon (280g)
Amount Per Serving

% Daily Value*

Calories 80 Calories from Fat 0
Total Fat 0g 0%
___Saturated Fat 0g 0%
___Cholesterol 0mg 0%
Sodium 10mg 0%
Total Carbohydrate 27g 9%
___Dietary Fiber 2g 8%
___Sugars 25g  
Protein 1g  
Vitamin A 20%
Vitamin C 25%
Calcium 2%
Iron 4%
*Percent Daily Values are based on a 2,000 calorie diet.
(Source: Produce Marketing Association's Labeling Facts, February 1994.)
Watermelon, Raw diced, 1 cup
Weight of Household Measure % Water Food Energy
K cal
 Protein Fat Saturated Fatty Acid Mono - unsaturated Fatty Acid Poly - unsaturated Fatty Acid
160g 92 50 1g  1g 0.1g 0.1g 0.3g
Cholesterol Carbohydrate Calcium Phosphorus Iron Potassium Sodium Vitamin A (IU)
 0mg 11g 13mg 14mg 0.3mg 186mg 3mg 590
Vitamin A (RE) Thiamin Riboflavin Niacin Ascorbic Acid      
59 0.13mg 0.03mg 0.8mg 15mg
(Source: USDA. Nutritive Value of Foods (HG-72), Release 3.2. 1990.)


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Content authors: J. Neibauer and E. Maynard, 2002. Links updated January 2012.