Two improved germplasm lines of wild watermelon (Citrullus lanatus var. citroides) designated USVL246-FR2 and USVL252-FR2 were released in 2012 by the Agricultural Research Service of the U.S. Department of Agriculture (Wechter et al., 2012). These lines are each highly uniform for growth characteristics, fruit size, and fruit shape. In both greenhouse and field tests, these lines also exhibit high levels of tolerance to the phytopathogen Fusarium oxysporum f. sp. niveum race 2 (Fon-R2) when compared with the watermelon cultivars Charleston Gray, Allsweet, Calhoun Gray, and Sugar Baby (all C. lanatus var. lanatus). In addition, they have shown higher vigor and equal or higher tolerance levels to Fon-R2 than PI 296341-FR, an improved C. lanatus var. citroides germplasm line derived from U.S. PI 296341, which has been reported previously as resistant to Fon-R2 (Martyn and Netzer, 1991).
Fusarium wilt [caused by F. oxysporum Schlechtend.:Fr. f. sp. niveum (E.F. Sm.) W.C. Snyder & H.N. Hans (Fon)] is one of the most serious soilborne diseases of watermelon (Martyn, 1985, 1987; Netzer, 1976). There are four pathogenic races, 0, 1, 2, and 3 (Martyn and Netzer, 1991; Netzer, 1976; Netzer and Martyn, 1989; Zhou et al., 2010). Various commercial cultivars have been introgressed with resistance to Fon race 0 and 1, but not to race 2 or 3. Fon-R2 has been reported throughout the United States (Bruton and Damicone, 1999; Bruton et al., 1988, 2008; Egel et al., 2005; Keinath and DuBose, 2009; Martyn, 1985; Martyn and Bruton, 1989; Zhou and Everts, 2001, 2003). Fusarium wilt can reduce yield of susceptible watermelon by 40% (Keinath et al., 2010).
USVL246-FR2 and USVL252-FR2 were developed at the U.S. Vegetable Laboratory, Charleston, SC. The PIs from which the released lines were developed, PI 482246 and PI 482252, were initially reported as resistant to Fon-R2 based on standard Fusarium assay definitions (Wechter et al., 2012). The term tolerance is now being used instead of resistance to characterize these two PIs and the lines described here. This decision is based on subsequent studies (data not shown) showing that after root or seed inoculation, the plants grow normally through fruit harvest without showing symptoms of wilt; however, the pathogen can be isolated from parts of the plant not directly associated with the site of inoculation.
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