Grafting watermelon is a common practice in many parts of the world, including China, Korea, Japan, Spain, Italy, and Israel, but has yet to be adopted widely in the United States (Kubota et al., 2008; Sakata et al., 2007). Reported benefits of grafting include resistance to diseases caused by soilborne pathogens, abiotic stress tolerance, and enhanced yield and fruit quality (Davis et al., 2008b; Lee and Oda, 2003; Louws et al., 2010). Modern vegetable grafting was first introduced in watermelon production in 1920, when Japanese growers grafted watermelon to squash rootstocks to provide resistance to fusarium wilt (Fusarium oxysporum f. sp. niveum) and other soilborne pests (Lee and Oda, 2003; Tateishi, 1927). In Turkey, higher yields were reported in grafted watermelon compared with nongrafted watermelon, although grafted watermelon exhibited a decrease in fruit quality (Turhan et al., 2012). In Japan and Korea, watermelon production has undergone an intensification on limited arable acreage, and many growers use permanent high tunnel structures or greenhouses for production. Permanent structures restrict the ability for crop rotation, which led to the adoption of grafting to address diseases incited by soilborne pathogens (Kubota et al., 2008; Lee and Oda, 2003; Sakata et al., 2007).
Vegetable grafting became an important pest management option following the removal of methyl bromide as a soil fumigant (Zagheni, 2003). The loss of methyl bromide and the success of grafting in Asian countries have caused an increased interest in vegetable grafting in western regions including Europe, North Africa, the Middle East, and Central and South America (Kubota et al., 2008; Miguel et al., 2004; Moreno et al., 2016). A study in Egypt reported only 67% survival of nongrafted watermelon that were planted in a fusarium wilt-infested field, whereas grafted plants had 83% to 100% survival and a corresponding increase in yield (Mohamed et al., 2012). In Spain and South America, economic analyses support the use of grafting where growers observe high levels of fusarium wilt in fields (Miguel et al., 2004; Moreno et al., 2016).
Despite reported benefits of grafting, U.S. watermelon growers are slow to adopt the practice because of the associated increases in labor costs, seed costs, greenhouse space, and management time required to oversee the grafting process (Kubota et al., 2008). If grafted seedlings are purchased, growers can expect to pay $0.75–$1.00 per seedling compared with $0.28 per nongrafted seedling, so grafting must provide substantial benefits to offset the expense (Taylor et al., 2008). Research in Oklahoma demonstrates that if a grower expects high yields (50,000 kg·ha−1), grafted watermelon fruit would have to sell for $0.22/kg to break even compared with a price of $0.15/kg for nongrafted watermelon fruit (Taylor et al., 2008). Furthermore, researchers in Oklahoma emphasize the importance of the history of the field being considered for watermelon production. If a field is historically free of fusarium wilt and disease is not anticipated to become a problem, a grower would benefit by avoiding the increased cost associated with grafted seedlings (Taylor et al., 2008). In Washington, researchers developed cost estimate analyses that favor the implementation of grafting for growers in the Pacific northwestern United States. The results were based on a series of assumptions, particularly an increase in yield of grafted plants and the effectiveness of grafting as an alternative to fumigation for disease control (Galinato et al., 2016). However, the authors note that growers should review the assumptions to check whether the expenses and yields in the models match those of the grower.
Several studies report no difference or even reduced yields in grafted watermelon as compared with nongrafted watermelon, particularly in the absence of disease pressure (Bertucci et al., 2017; Kokalis-Burelle et al., 2016). Under disease-free conditions in Florida, grafted watermelon yielded nearly 50% less total fruit weight than nongrafted watermelon (Kokalis-Burelle et al., 2016). Yields from seeded watermelon scions grafted to bottle gourd rootstocks were increased and yields were decreased when grafting to ISH rootstocks (Yetisir et al., 2003). This disparity emphasizes the need for further investigation of yield response to grafting watermelon in the southeastern United States.
It is important that grafted plants are tested and evaluated in the environment where they will be grown. Rootstock performance is dependent on edaphic factors and climatic conditions, which will define how each rootstock performs in a given environment (Kumar et al., 2017). Thus, one of the major difficulties of grafting is finding the appropriate rootstock–scion combination. The objectives of this study were to graft both standard and mini watermelons (cultivars Exclamation and Extazy, respectively) on 20 commercially available rootstocks to determine the effect of grafting on early season growth, time to maturity, fruit yield, and fruit quality of standard and mini watermelons in the southeastern United States.
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Colla, G., Rouphael, Y., Mirabelli, C. & Cardarelli, M. 2011 Nitrogen-use efficiency traits of mini-watermelon in response to grafting and nitrogen-fertilization doses J. Plant Nutr. Soil Sci. 174 6 933 941
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Miller, G., Khalilian, A., Adelberg, J.W., Farahani, H.J., Hassell, R.L. & Wells, C.E. 2013 Grafted watermelon root length density and distribution under different soil moisture treatments HortScience 48 1021 1026
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