Grafted watermelon transplants are an important part of worldwide watermelon production because they confer resistance to soilborne diseases such as fusarium wilt (Fusarium oxysporum f. sp. niveum) and Monosporascus root rot (Monosporascus cannonballus) (Beltran et al., 2008; Guan et al., 2012; Louws et al., 2010). Grafting can also control root-knot nematode (Meloidogyne spp.) using wild watermelon (Citrullus lanatus var. citroides) or Cucumis metuliferus as a rootstock (Sigüenza et al., 2005; Thies et al., 2010). In addition to disease resistance, grafting watermelon onto vigorous rootstocks provides various other benefits, including increased yield and fruit quality (Ozlem et al., 2007), increased resistance to abiotic stresses (Savvas et al., 2010), decreased planting densities (Cushman and Huan, 2008), and increased nutritional components (Davis and Perkins-Veazie, 2005). Currently, the United States is one of the few countries worldwide that does not use grafted cucurbit transplants in commercial production. A key reason for this is that soil fumigants have been available at modest costs, whereas there is high cost associated with grafted transplants (Edelstein, 2004). However, the loss of the widely used and inexpensive soil fumigant methyl bromide as a result of the Montreal Protocol and the Clean Air Act (USDA NASS, 2013) has made the use of grafted watermelon transplants a more attractive option for producers looking for a solution to soilborne diseases and pests. Of the 105,000 acres of seedless watermelon planted in the United States in 2012 (U.S. Department of Agriculture, National Agricultural Statistics Service, 2012) 5% (5250 acres) was reported to be affected by fusarium wilt (Dean Liere, Syngenta Corporation, personal communication). As arable land for rotation decreases, the percentage of fusarium-infested soils will continue to rise. Currently, no control options are available other than grafted transplants; however, their increased cost remains the major impediment to adoption in U.S. production (Davis et al., 2008).
The two most common commercially used grafting methods (over 90%) are the hole-insertion and the one-cotyledon method (Hassell et al., 2008). These methods require at least one cotyledon to remain intact to ensure graft success (Bisognin et al., 2005; Hassell et al., 2008), and both require manual meristem removal with a blade during grafting. This method often removes the meristem only partially, and meristem regeneration occurs. The extent of meristem regeneration varies widely and is dependent on the method used, timing of grafting, and the experience of the individual doing the grafting.
Rootstock meristem regrowth also contributes greatly to the cost associated with grafted watermelon transplant production (Choi et al., 2002; Memmott and Hassell, 2010) because it decreases graft success and requires additional labor to control. If the regrowth is not removed manually during production, it will outcompete the watermelon plants (the scion) for light, space, and nutrients, preventing effective healing. In the field, unremoved regrowth can also affect yields by competing with the scion. Even if the regrowth is removed at the transplant stage, additional labor is required to scout and remove regrowth in the field. The labor required for manual regrowth control makes grafted transplants economically impractical for commercial production in the United States. Because grafted transplant production is not currently taking place in the United States, it is unclear what the specific production costs would be. Regardless, a labor-free method of eliminating meristematic regrowth should significantly reduce the overall cost of grafted watermelon transplant production and might help to increase the adoption of grafted cucurbit transplants in the United States.
Chemical inhibition of the meristem region of rootstocks would not only decrease the labor required for grafted transplant maintenance, but would address the variance in regrowth based on grafter skill, timing, and method. However, acceptable removal of the meristem without damaging the rootstock cotyledons has been a challenge. Preserving the quality of the cotyledons is essential, because cucurbit seedlings rely heavily on at least one cotyledon to supply energy for growth and establishment (Bisognin et al., 2005). A chemical treatment would be acceptable for use only if it could destroy the meristem without damaging the cotyledons, which would provide the energy required for rootstock health and graft healing.
Previous studies (Choi et al., 2002) have examined silver nitrate and hydrogen peroxide applications to the meristem region of cucurbit rootstocks, but the application resulted in unacceptably low rootstock survival. Other preliminary research of chemicals including maleic hydrazide, oryzalin, sulfuric acid, and fatty alcohols indicated that only the fatty alcohol successfully destroys the rootstock meristem without damaging the rootstock cotyledons (Hassell, unpublished data). This promising method of regrowth control (US Patent 8,629,330) involves applying a dilute C6, C8, C10, C12 fatty alcohol solution to the meristem area of a rootstock seedling, where it destroys only the rapidly dividing meristem tissue (Steffens et al., 1967) to prevent regrowth while the rootstock seedling remains viable for grafting. Commercially available fatty alcohol products are used on tobacco to control axillary meristem growth (suckers) after topping. The compound acts by disrupting the cell’s plasma membrane, causing plasmolysis of the cells and desiccation of the tissue (Wheeler et al., 1991). Although the mode of action on tobacco is understood, the specifics of fatty alcohol applications on cucurbit rootstock tissues have not been characterized. The first objective of this experiment was to determine the optimal application rate of commercially available fatty alcohol compounds that would control rootstock regrowth without damaging the rootstock cotyledons and, thus, their potential for grafting. The second objective was to determine whether the fatty alcohol treatment affected the graft success of the rootstocks.
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