Hollowheart of watermelons is a physiological fruit disorder. Flesh separates inside of the fruit, typically forming three gaps (Figure 1 and 2). In severe cases, hollowheart could cause watermelon load rejection.
Watermelon fruit that has hollowheart tends to be triangular shaped. Poor pollination is the primary reason causing hollowheart. Scientists were able to prove that seedless watermelons are more likely to develop hollowheart when the pollenizer plants (diploid watermelons) are located further away from the seedless plants. The study found hollowheart incidence starts to increase when the distance between the seedless plant and the pollenizer plant is more than 6 feet.
Figure 1. A minor symptom of hollowheart.
Figure 2. A severe case of hollowheart watermelon. Note the fruit is trianggular shaped.
Cold weather and the lack of bee movement during pollination period causes poor pollination and increases the chance of hollowheart. Some growers use mixed pollenizer plants with different flowering peaks to ensure availability of pollen matching the blooming period of seedless plants. Bumblebees, in addition to honeybees, are sometimes used; bumblebees are relatively more active at relatively low temperatures and low light intensity conditions.
In addition to poor pollination, water and fertility management are also believed to affect the incidence of hollowheart. The assumption is that hollowheart is caused when the fruit inner cells can not keep pace with the expansion rate of the rind. This situation is more likely to occur on ‘forced’ plants, for example, plants with excessive fertilizers.
Incidence of hollowheart occurs more frequently in crown-set fruit compared to lateral-set fruit. Part of the reason is that crown-fruit have larger intercellular spaces with fewer and larger cells. Another reason is that crown-fruit set earlier in the season when environmental factors such as low temperatures are more likely to affect pollination.
Although hollowheart symptoms are observed on both seedless and seeded watermelons, seedless watermelons tend to have more hollowheart than the seeded watermelons. Under the same environmental conditions, yellow and orange-fleshed watermelon varieties tend to have the most severe hollowheart symptoms. Among typical seedless watermelon varieties, a negative correlation existed between watermelon flesh firmness and the ratings of hollowheart. Firmer fleshed-watermelons tend to have lower ratings of hollowheart.