Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
There are a number of fruit disorders in watermelons that are being found in Delmarva fields at this time. 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 140 ºF. This kills rind cells and results in the 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.
Another disorder that is being found is water soaking or water belly 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. This year we have also seen water soaking in fruits 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.
Another water 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 will lead to these splits. This year as growers were irrigating heavily due to the high heat, the potential for excess water in fruits was much higher, especially in certain varieties.
Irregular ripening has been a problem in some fields this year. The main cause in early fields was the variable growth during cold May weather with some stunted plants taking more time to grow out. In addition, many fields required considerable replanting also delaying maturity. Another causes of irregular ripening 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. This may also be the case where severe storms have damaged fields in some areas. 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.
In our variety trial, we are seeing higher than normal levels of hollow heart. Hollow heart is an internal split or void in the flesh of the watermelon between the 3 carpels or fruit sections. Research at the University of Delaware strongly points to inadequate pollination as a major factor in hollow heart. Hollow heart is generally more severe in the crown set. Varieties vary considerably in their susceptibility to hollow heart. Dense fleshed (crisp) 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.
We have also seen higher incidence of separation in the locules of watermelon fruit in our variety trials. This is seen as half-moon shaped gaps in the seed cavity areas.
In addition, we are seeing higher incidence of white and yellow tissue in fruits. Areas of the fruit flesh have hardened and may be white or yellow in color. This tissue will be chewy when eaten. This disorder often is associated with vascular traces that have become hardened. This is very similar to white tissue in tomato fruits. In tomatoes, low fruit potassium has been associated with white tissue. This may also be the case in watermelons; however, research is needed to confirm this. Yellow tissue also forms in hollow heart gaps and locular gaps.
Severe hollow heart in a watermelon fruit caused by limited pollen during pollination.
Hollow heart, half-moon locular separation and yellow tissue.