PPDL Picture of the Week
September 4, 2018
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.
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.