picture of the week
September 13, 2021
Hallowheart of Watermelons
Wenjing Guan, Clinical Engagement Assistant Professor-SWPAC, Horticulture and Landscape Architecture, Purdue University
Hollowheart of watermelons is a physiological fruit disorder. Flesh separate inside of the fruit, typically forming three gaps (Figure 1 and 2). In severe cases, hollowheart could cause watermelon load rejection.
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Figure 1 
Figure 2 Watermelon fruit that has hollowheart tends to be triangular shaped. Poor pollination is the primary reason causing hollowheart. Scientists were able to approve 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 hollow heart incidence started to increase when the distance between the seedless plant and the pollenizer plant is more than 6 feet.
Cold weather and the lack of bee movement during pollination period also cause poor pollination. Some growers use mixed pollenizer plants with different flowering peaks to ensure availability of pollens matching the blooming period of seedless plants. Bumblebees, in addition to honeybees, are used. This is partly because 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 hollow heart. The assumption is that hollow heart is caused when the fruit inner cells can not keep pace with the expansion rate of the rind, and this situation is more likely to occur on ‘forced’ plant, for example, planting 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. It could also because crown-fruit set earlier in the season when environmental factors such as low temperatures are more likely to affect pollination.
Although hollowheart symptom was 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 have lower hollowheart ratings.
