Gordon Johnson, Extension Vegetable & Fruit Specialist;gcjohn@udel.edu
Watermelon harvest will be in “full gear” soon across Delmarva. The following is a review of common fruit disorders that can occur in watermelon.
Misshapen Fruits
Poor pollination due to low bee activity, may result in “bottlenecks”, or constricted growth at the stem end of the fruit, especially in seeded/elongated watermelons. 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 to produce a uniformly shaped fruit. In seedless watermelon, poor pollination may lead to undesirable “triangular” fruits.
Sunscald
Sunscald occurs when fruits are exposed to direct sunlight, especially on extremely hot days. Under these conditions, rind surfaces can reach temperatures exceeding 120ºF killing cells and resulting in sunburn spots. Fruits with little or no foliar cover are at most risk. Sunscald or sunburn first appears as a gray or white area on the exposed upper surface of the fruit. Fruit with dark rinds are more susceptible to sunscald than those with light colored rinds. Sunscald severity is related directly to fertility regime and foliage cover. Proper fertility and soil management promotes adequate vine growth and coverage of fruit. Sunscald severity is also associated with diseases that reduce foliage cover, such as anthracnose, or gummy stem blight.
Hollow Heart
Hollow heart is an internal split or void in the flesh of the watermelon. While the cause of hollow heart has not be definitively determined, evidence strongly points to inadequate pollination. Hollow heart is generally more severe in seedless fruit and in the crown set. Varieties vary considerably in their susceptibility to hollow heart. Dense fleshed varieties, mini, and personal type watermelons have lower hollow heart incidence. Factors that would influence pollination such as cold weather during fruit set or delayed male flower production on pollenizers will increase hollow heart potential in susceptible varieties.
Water Soaking
This disorder occurs where excess water accumulates at the bottom of the fruit resulting in a water soaked appearance of internal flesh. Water accumulates during cloudy weather when transpiration from vines is low. Water soaking sometimes appears in fruits where foliage has deteriorated since excess water cannot be transpired.
Splitting
Splitting during handling occurs in fruits under excessive water pressure. Excess irrigation or rainfall are the usual causes.
Irregular Ripening
Irregular ripening can be a problem in some years and varieties. Watermelons are classified as non-climacteric since they do not continue to ripen significantly after harvest. However, recent research has shown that watermelon fruit produce a burst of ethylene at the white fruit stage and factors that reduce ethylene will slow ripening. Watermelon fruit development and ripening are also dependent on the accumulation of sugars. Loss of foliage or stem tissue due to diseases such as gummy stem blight or insect or mite feeding can reduce the amount of sugars available to the fruit. Different varieties, low potassium nutrition, or variability in vine health will lead to variability in fruit ripening.
Internal Rind Necrosis
Internal rind necrosis is indicated by the presence of a corky, red-brown layer of tissue that occurs on the inside of the rind of affected fruit but that does not extend into the fruit flesh. The disease occurs sporadically and is thought to be caused by bacteria (Erwinia) that are naturally present on fruit. Drought stress has been implicated in this disorder.