Factors Affecting Fruit Set in Triploid Watermelons from Delaware On-farm Surveys

On-farm surveys were conducted in Delaware watermelon fields in 2010 and 2011 to assess factors affecting fruit set. Twenty watermelon fields representing 14 cooperating growers were surveyed from the third week in June through the second week in August in 2011; Fourteen watermelon fields with 21 plantings representing 12 cooperating growers were surveyed from the third week in May through the last week in June in 2011.  Data was collected weekly on male flower counts, fruit set, stem number and length, vigor, and health for triploids and pollenizers.  In 2011, plant crown and leaf temperatures were also recorded.  In 2010, crown set did not differ significantly by pollenizer or triploid; however, there was a significant trend toward reduced early set with reduced pollenizer or triploid vigor.  Fruit numbers varied significantly with pollenizer but not triploid.  Mean fruit numbers ranged between 2.0 and 2.7 per plant.  There as a linear reduction in fruit numbers (R² = 0.25) with reduced triploid vigor. Reductions in pollenizer vigor, male flower counts, or pollenizer vine length also reduced fruit numbers; however, adding these factors only explained an additional 2% of the variability. There were no differences in mean fruit numbers between bumble bee and honey bee pollinators. In 2011, crown set occurred earlier when leaf temperatures were below 23.8 °C.  Mean fruit number varied significantly with pollenizer and triploid. Several special pollenizer varieties produced male flowers earlier than standard diploid pollenizers and the pollenizer ‘Accomplice’ had high male flower counts throughout  the survey.  There was a significant linear reduction in the number of fruit carried with an increase in plant temperature measured over the crown of triploids from 2.9 at 18.3 °C to 2.0 in temperatures above 32.2 °C. Fruit numbers varied between triploid cultivars in relation to crown temperature.