Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is an important specialty crop in Florida, a leading watermelon producer in the United States, with an average production value exceeding $80 million each year (USDA, 2017). Seedless cultivars are commonly grown by Florida growers, in response to the increasing market demand for seedless watermelon in the United States (Elwakil et al., 2017; Ferreira and Perez, 2016). The tetraploids used in developing triploid watermelons usually have very limited resistance to fusarium wilt and this may have resulted in most of common seedless watermelon cultivars being susceptible to fusarium wilt (Bruton et al., 2007).
Fusarium wilt of watermelon, caused by FON, is a reemerging pathogen that can cause 100% yield losses in extreme cases (Bruton, 1998). Among the first described fusarium wilt diseases, fusarium wilt of watermelon is still economically important as it occurs worldwide and the pathogen continues to evolve into new and more aggressive races, for which most commercial cultivars lack or have limited resistance (Egel and Martyn, 2013). The phaseout of the broad-spectrum soil fumigant methyl bromide has made it more difficult to manage fusarium wilt (King et al., 2008), thus, requiring producers to use more integrated management strategies including host resistance, biological and chemical controls, crop rotation, and grafting (Everts and Himmelstein, 2015).
Grafting has been widely used in solanaceous and cucurbitaceous crops as a novel integrated disease management strategy, especially when the availability of resistant cultivars is limited. By using selected rootstocks, grafting can efficiently control the soil-borne diseases caused by a wide range of pathogens including nematodes (e.g., root-knot, Meloidogyne), fungi (e.g., Verticillium, Fusarium, Pyrenochaeta, and Monosporascus), oomycetes (e.g., Phytophthora), bacteria (e.g., Ralstonia), and several soil-borne viral pathogens (Louws et al., 2010; Thies et al., 2010). Because many commercial watermelon cultivars are susceptible to FON race 2 (Miguel et al., 2004) and race 3 (Egel and Martyn, 2013), interspecific and intergeneric grafting, and the use of interspecific hybrid rootstocks are commonly practiced (Keinath and Hassell, 2014; Louws et al., 2010). Grafting can provide other benefits (e.g., improved fruit yield and lycopene content) besides disease management to watermelon producers, but these benefits can vary depending on the plant material and production systems implemented (Kyriacou et al., 2017; Rouphael et al., 2010).
The vigorous root system from the rootstock can also help improve growth and fruit yield of grafted plants regardless of infections from soil-borne pathogens (Lee et al., 2010). Several studies have confirmed the positive impact of specific rootstocks on plant growth and fruit quality (Alan et al., 2007; Chouka and Jebari, 1997; Kyriacou et al., 2016; Yetisir and Sari, 2003). The use of ‘Shintoza’ (C. maxima × C. moschata) rootstock increased fruit size and yield stability of grafted plants without affecting fruit quality (Miguel et al., 2004). The interest in watermelon grafting as an effective tool for fusarium wilt control has been identified among growers in Florida; however, to date limited research-based information is available regarding the use of grafted plants in fusarium wilt management in the Florida watermelon production systems.
Depending on grafting skill, available space, and healing environment, different grafting techniques, including tongue approach, hole insertion, and one-cotyledon grafting, are commonly used for commercial production of grafted watermelon transplants (Davis et al., 2008). In addition, root excision with regeneration of adventitious roots has been used in cucurbit grafting especially when the grafting process is mechanized (Guan and Zhao, 2015). It has also been suggested that a primary nursery can conduct grafting and remove the root system of the grafted plants after healing, while a secondary nursery receiving the shipped grafted plants with root excision can re-root the plants and distribute the re-rooted grafted plants locally (Sabatino, 2013). Root excision could conserve rootstock hypocotyl carbohydrate to improve the healing process (Memmott, 2010). However, it is unclear whether re-rooted, grafted watermelon seedlings will differ from the grafted plants without root excision in terms of their effectiveness in suppressing FON.
It was hypothesized that seedless watermelon plants grafted with selected C. moschata and C. maxima × C. moschata hybrid squash rootstocks could be highly resistant to FON infection and that root excision and regeneration would not affect the performance of grafted plants. Specifically, the objectives of this study were to 1) assess the growth and yield performance of grafted and nongrafted seedless watermelon plants when inoculated with FON race 2 and 2) determine the effect of root excision and regeneration on grafted plant performance under FON race 2 inoculation.
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