The southern RKN (Meloidogyne incognita) is a serious constraint to U.S. watermelon production and can significantly reduce watermelon yields in the southern United States (Davis, 2007; Sumner and Johnson, 1973; Thies, 1996). Pre-plant fumigation of soil beds with methyl bromide has been the primary method for controlling root-knot nematodes and soilborne diseases in watermelon (Thies et al., 2010). However, in accordance with the Montreal Protocol and the U.S. Clean Air Act, methyl bromide was phased out effective 1 Jan. 2005 and use of methyl bromide for pre-plant fumigation of soil has been restricted to that allowed through Critical Use Exemptions (CUE) granted to growers in specific states or geographic regions by the U.S. Environmental Protection Agency (USEPA) (USEPA, 2011). The loss of methyl bromide for pre-plant soil fumigation was estimated to result in annual yield losses of 15% to 20% for watermelon in Georgia and Florida (Lynch and Carpenter, 1999) and the total economic impact of the methyl bromide ban for U.S. watermelon production and consumption was calculated to be –$9,111,000 annually (Carpenter et al., 2000). Additionally, nematicides used for controlling RKN in watermelon and other vegetable crops have been lost from the U.S. market because of human health risks and groundwater contamination (Chitwood, 2003). Thus, there is increased interest in the development of alternative technologies such as use of resistant rootstocks for grafting to manage RKN in watermelon and other vegetable crops.
Grafting vegetables onto resistant rootstocks to prevent damage by soilborne diseases or pests has been practiced for many years in eastern Asia (Cohen et al., 2007). Grafting has proven to be a quick alternative to long-term breeding programs aimed at incorporating resistance to soilborne disease, especially fusarium wilt, into elite vegetable cultivars (Miguel et al., 2004). Rootstocks also proved to be useful in enhancing tolerance to abiotic stresses and in improving fruit yield and quality (Cohen et al., 2007; Core, 2005; Edelstein and Ben-Hur, 2006).
Grafting of watermelon is an emerging production practice in U.S. agriculture, which is being adopted largely for control of soilborne diseases such as fusarium wilt and vine decline of melon caused by Monosporascus cannonballus. The extensive use of methyl bromide for pre-plant soil fumigation (before 2005) and the availability of large acreages of land, which accommodated multiyear crop rotations, curtailed the buildup of soilborne disease pathogens in watermelon production fields in the United States in past decades. However, availability of agricultural land is becoming more limited in the United States and methyl bromide for pre-plant soil fumigation is only available through limited CUE allocated to growers by the USEPA (USEPA, 2011). Thus, grafting is becoming a feasible alternative for disease control in U.S. watermelon production. Although the cost of grafted seedless watermelon transplants has been estimated at $0.75 per plant compared with $0.28 per non-grafted plant in the United States (Taylor et al., 2008), the use of newly improved grafting equipment can reduce the labor requirements and costs of grafting. Current commercial grafting methods require maintenance of at least one rootstock cotyledon during the graft healing period to ensure high survival of grafted plants (Lee, 1994; Lee and Oda, 2003). Regrowth of the rootstock from meristematic tissue adjacent to one of the cotyledons has been a major drawback to cost-effective use of grafted plants in U.S. agriculture (Edelstein, 2004). However, Hassell et al. (2008) recently developed the cotyledon devoid method, which involves the removal of meristematic tissue that prevents rootstock shoot regeneration. If further testing proves this new grafting method to be successful, grafting costs could be significantly reduced as a result of complete elimination of rootstock regrowth (Memmott and Hassell, 2010).
Squash hybrid (Cucurbita maxima × C. moschata) and bottle gourd (Lagenaria siceraria) rootstocks are among the most commonly used rootstocks for grafting watermelon and other cucurbits because these species are not susceptible to fusarium wilt caused by Fusarium oxysporum f. sp. niveum and they have vigorous roots systems (Edelstein and Ben-Hur, 2006; Miguel et al., 2004). However, the squash hybrid rootstocks and bottle gourd rootstocks are extremely susceptible to RKNs (Thies et al., 2010), and thus these rootstocks are not suitable for grafting when fields or high tunnels are infested with RKNs. RKNs have been identified as the primary pathogen of squash hybrid and bottle gourd rootstocks worldwide (Xingping Xie, personal communication).
Germplasm with resistance to RKN should be useful for development of resistant rootstocks to manage RKN in grafted watermelon. The C. lanatus var. citroides PI collection at the USDA, ARS Plant Genetic Resources and Conservation Unit in Griffin, GA, represents wide genetic diversity and is considered a valuable germplasm source for genes conferring resistance to diseases or pests of watermelon (Jarret et al., 1997; Levi et al., 2001, 2010). In previous studies we have identified U.S. PIs of C. lanatus var. citroides that contain resistance to RKN (Thies and Levi, 2003, 2007). At the U.S. Vegetable Laboratory, USDA, ARS, we have selected and developed several improved lines derived from these wild watermelon PIs that could be useful as rootstocks for grafted watermelon (Thies et al., 2008, 2010). The objective of these studies was to evaluate the performance of these improved C. lanatus var. citroides rootstock lines vs. commercial rootstocks of bottle gourd and hybrid squash for managing RKN with and without methyl bromide treatments.
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