Conditions Influencing Hollow Heart Disorder In Triploid Watermelon

Two studies were conducted in triploid watermelons in 2010 to gather preliminary information on conditions affecting hollow heart disorder.  In the first study, test beds were planted to Liberty triploid watermelon, a variety known to have had significant hollow heart Delaware production fields in the past.  Plants were transplanted at a distance of 0.9 m apart with no diploid pollenizers.  These test beds were separated by beds that were planted in normal fashion with seedless varieties at 0.9 m apart and a diploid pollenizer plant placed between every third and fourth plant in the bed thus creating varying distances from a pollen source in test beds.  In the second study, seedless varieties were transplanted at increasing distances from a pollen source.  At maturity, watermelon fruits from test plots were split and hollow heart incidence, length and diameter of hollow heart, melon diameter and length, distance from seedless mother plant crown, distance from nearest pollenizer crown, distance from nearest pollenizer plant, and node of attachment  were recorded.  Seedless fruit in the adjacent beds were also split to record hollow heart incidence only.  There was no hollow heart in the beds with normal pollenizer spacing.  There was a large increase in hollow heart frequency as triploid fruits approached and exceded 1.5 m distance from a pollenizer crown.   At 1.5 m the hollow heart incidence was 56%; at 2.4 m hollow heart incidence was 74%.  This suggests that there is a critical distance at which pollinating insects do not transfer enough pollen to allow for normal fruit development, also suggesting a relationship between pollination and hollow heart.  There was a significant logarithmic relationship between hollow heart incidence and distance from the nearest pollenizer plant (r² = 0.83). There was a significant linear relationship between hollow heart incidence with distance from the nearest pollenizer vine (r² = 0.75).  There were no differences in hollow heart by weight class; however, highest hollow heart frequency was found with a length to width ratio of 1.26 suggesting that longer watermelons may be more susceptible to hollow heart.  Fruits on the 10^th node on seedless watermelon plants had the highest frequency of hollow heart indicating that the early fruit set was affected the most.  2011 trials will focus on the effects of cytokinin and auxin applications on hollow heart where pollen is limited.