Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
There are several fruit disorders in watermelons being found across the region currently.
One of the most common is sunscald or sunburn on fruits. This occurs when fruits are exposed to direct sunlight, especially on extremely hot days. Rind surfaces can reach temperatures exceeding 140º F. This kills rind cells and results in sun burnt spots where the cells have died. Fruits with little or no vine cover are at most risk. Also at greater risk are watermelons with dark colored rinds.
Irregular ripening has been a problem in some fields this year. Varieties planted at the same time are not ripening 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 seen this year? 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.
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. Watermelons do tolerate high temperatures but some varieties are less tolerant of extended hot weather, leading to irregular ripening.
Another disorder that can be found is water soaking in fruits. This occurs where excess water accumulates at the bottom of the fruit, leaving a water soaked appearance in the flesh when cut open. Water accumulates during cloudy weather when transpiration from fruits is low. Water soaking in fruits is also found in fields where foliage has deteriorated. In this situation, water is still being translocated in the xylem but there is limited transpiration through the leaves. Watermelon fruits are still transpiring, but due to the nature of the fruit (thick rind, waxy surface); transpiration is lower than in leaf tissue, leading to water buildup in the fruit. A related disorder is watermelon splitting during handling. In fruits with excess water, the high turgor pressure makes the fruit susceptible to splitting as it is handled (i.e. harvested into busses or trucks, grading, and placing in bins). Even small drops can lead to these splits.