Grafted Seedless Watermelon Production with Different Planting Densities

Grafting watermelon with hybrid squash rootstocks can provide resistance to certain soil-borne diseases and improve plant vigor, thus leading to enhanced fruit yield. Given the yield improvement of grafted plants, the potential of reducing grafted plant population to increase cost effectiveness was explored. This study was designed to assess rootstock effects on yield and fruit quality of seedless watermelon grown with different in-row planting densities. The field trial was carried out in Fall 2015 in Citra, FL., with seedless watermelon (Citrullus lanatus) ‘Melody’ grafted onto interspecific squash (Cucurbita maxima x C. moschata) hybrid rootstock ‘Super Shintosa’. Grafted and non-grafted ‘Melody’ were planted using in-row spacings of 0.76, 1.07, 1.37, and 1.68 m, with a constant between-row spacing of 2.44 m, resulting in populations ranging from 2444 to 5378 plants/ha. The experiment was arranged in a split-plot design with four replications, with planting densities as the whole plot treatments and grafting as the subplot factor. Marketable and unmarketable fruit weight and number were recorded for two harvests. At the first harvest, six fully ripe fruit from grafted and non-grafted plants in the 0.76 and 1.68 m treatments were sampled and stored overnight at 12°C before fruit quality assessment. Instrumental measurements included flesh firmness, total soluble solids, titratable acidity, pH, flesh color, and lycopene content. Overall acceptability, flavor, and firmness were evaluated by consumer sensory analysis using a 1-9 hedonic scale, while sweetness, firmness, and juiciness were scored using a 1-5 “just about right” scale. At the second harvest, all the fruit were cut for evaluation of hollow heart. After the final harvest, plant roots were scored for root-knot nematode galling using a 0-10 rating scale. Grafting and in-row spacing showed significant effects on fruit yield. Despite higher root-knot nematode galling ratings in grafted plants, across all in-row spacings, grafting significantly improved marketable fruit number and average fruit weight on both per-plant and per-acre bases. With respect to in-row spacing, both marketable fruit number and weight per plant were highest at 1.68 m spacing while they were highest on a per-acre basis at 1.07 m spacing. At the first harvest, fruit quality analysis showed some differences in lycopene (non-grafted ‘Melody’ watermelon at 0.76 m spacing had the highest value) and lightness of fresh color (non-grafted ‘Melody’ watermelon at 1.68 m spacing had the lowest value). Interestingly, grafting significantly reduced hollow heart incidence at the second harvest.