Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
The first harvest of watermelons has started in the region. Irregular ripening is a common problem that occurs in some watermelon fields each year. This is where varieties planted at the same time do not ripen evenly in a field. Fruits that look mature on the outside are not fully ripe inside, often with significant amounts of white flesh.
Watermelons are classified as non-climacteric, that is, they do not continue to ripen significantly after harvest. Other fruits, particularly those that soften, such as peaches, release ethylene gas during the ripening process and will continue to ripen after harvest. It was once thought that ethylene was not involved in watermelon ripening, however, in 2009, USDA researchers found that watermelons released a burst of ethylene at the white fruit stage. Watermelon fruit development and ripening also is dependent on the accumulation of sugars. Sugars are produced by photosynthesis in the foliage of the watermelon plant and are translocated to the fruit.
So, what is cause of irregular ripening? One possible explanation is deteriorating vine health. Loss of foliage or stem tissue due to diseases such as gummy stem blight or insect or mite feeding on leaves and stems can reduce the amount of sugars available to translocate into the fruit. In a field, variability in vine health therefore would lead to variability in fruit ripening. Certain viruses can also affect watermelon ripening.
The burst of ethylene that researchers found could also be an issue. In plants where ethylene production is compromised, this could lead to later ripening or incomplete ripening.
Potassium may also be an issue. Potassium is important in fruit ripening and low or variable potassium levels may lead to irregular ripening. In fields with pre-plant potassium applications only, heavy irrigation could leach potassium out of the root zone creating lower than normal levels in the soil and potential deficiencies leading to irregular ripening.
Hot weather (temperatures in the 90s) can also lead to fruit disorders. In general, watermelons tolerate high temperatures; however, some varieties are less tolerant of extended hot weather, leading to irregular ripening. Long season varieties often take longer to ripen, even when outwardly they appear to be mature.