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Phytophthora fruit rot, caused by Phytophthora capsici, is prevalent in most watermelon-producing regions of southeastern United States and is known to cause preand post-harvest yield losses. A non-wound inoculation technique was developed to evaluate detached mature fruit belonging to U.S. watermelon PIs for resistance to fruit rot caused by P. capsici. Mature fruit were harvested and placed on wire shelves in a walk-in humid chamber [greater than 95% relative humidity (RH), temperature 26 ± 2 8C] and inoculated with a 7-mm agar plug from an actively growing colony of P. capsici. Twenty-four PIs that exhibited resistance in a preliminary evaluation of 205 PIs belonging to the watermelon core collection in 2009 were grown in the field and greenhouse in 2010 and 2011 and evaluated in the walk-in humid chamber. Fruit rot development was rapid on fruit of susceptible controls ‘Black Diamond’, ‘Sugar Baby’, and PI 536464. Several accessions including PI 560020, PI 306782, PI 186489, and PI 595203 (all Citrullus lanatus var. lanatus) were highly resistant to fruit rot. One C. colocynthis (PI 388770) and a C. lanatus var. citroides PI (PI 189225) also showed fruit rot resistance. Fruit from PIs that were resistant also had significantly lower amounts of P. capsici DNA/gram of fruit tissue compared with the susceptible commercial cultivars Sugar Baby and Black Diamond. The sources of resistance to Phytophthora fruit rot identified in this study may prove useful in watermelon breeding programs aimed at enhancing disease resistance. Phytophthora capsici has been documented as a pathogen on a wide variety of vegetable crops in the families Solanaceae (tomato, pepper, eggplant), Cucurbitaceae (cucumber, watermelon, squash, pumpkin, melon), Fabaceae (lima bean) (Babadoost and Zitter, 2009; Erwin and Riberio, 1996; Gubler and Davis, 1996; Hausbeck and Lamour, 2004), frasier fir (Quesada-Ocampo et al., 2009), and plants belonging to 23 other families (Granke et al., 2012b). The pathogen has also been reported to have caused severe losses in cucurbit crop production since the 1940s resulting in cessation of production in severely infested fields (Babadoost, 2004; Babadoost and Zitter, 2009; Hausbeck and Lamour, 2004; Kreutzer et al., 1940). Phytophthora fruit rot of watermelons caused by P. capsici was first reported in 1940 (Wiant and Tucker, 1940) and is prevalent in many watermelon-growing regions of the United States (Gevens et al., 2008; Hausbeck et al., 2012; Kousik et al., 2011a; McGrath, 1996). The disease is particularly severe in the southeastern United States where 50% of the watermelons are grown (Florida, Georgia, Alabama, South Carolina, North Carolina, and Virginia) and optimal conditions for development of Phytophthora fruit rot are prevalent. Between 2003 and 2008 many watermelon growers in Georgia, South Carolina, and North Carolina did not harvest their crop as a result of severe fruit rot. In some instances fruits rotted after shipping, resulting in rejection of entire loads and loss of revenue (Jester and Holmes, 2003; Kousik et al., 2011a). Such instances led the watermelon growers’ consortium, National Watermelon Association, to list Phytophthora fruit rot as one of their most important research priorities (Morrissey, 2006). The current recommended strategy for managing P. capsici includes a combination of several control methods, cultural practices that ensure well-drained soils, crop rotation, soil solarization, reducing splash dispersed soil, and application of fungicides (Babadoost, 2004; Granke et al., 2012b; Hausbeck and Lamour, 2004; Kousik et al., 2011a; McGrath, 1996). Several commercial fungicides that are effective in managing Phytophthora fruit rot of watermelon have been identified (Kousik et al., 2011a, 2011b). However, the prevalence of P. capsici isolates insensitive to fungicides such as cyazofamid and mefenoxam also has been well documented (Granke et al., 2012a, 2012b; Hausbeck and Lamour, 2004; Kousik and Keinath, 2008; Jackson et al., 2012). Furthermore, the application of fungicides is not very effective when disease pressure is high (Granke et al., 2012b; Kousik et al., 2011a; McGrath, 1994). Therefore, alternative strategies such as host resistance for managing Phytophthora fruit rot are needed. Host plant resistance can be considered the cornerstone of an integrated disease management program, and watermelon cultivars with resistance to P. capsici would be extremely useful in management of Phytophthora fruit rot. Host resistance to P. capsici in peppers was first reported by Kimble and Grogan (1960) and additional sources of resistance were subsequently reported by Barksdale et al. (1984) and Peter et al. (1984). Recently, commercial bell pepper hybrids with resistance to P. capsici have become available (Candole et al., 2010; Foster and Hausbeck, 2010). However, there are few reports on resistance of cucurbit crops to P. capsici. Resistance to crown rot caused by P. capsici was reported in a Korean pumpkin cultivar (Lee et al., 2001) and squash (Cucurbita pepo) accessions (Padley et al., 2008). Gevens et al. (2006) identified cucumber varieties that limit development of P. capsici; however, none of the varieties tested had complete fruit rot resistance. Resistance in cucumber was related to the developmental stage of the fruit, increasing with increasing age and size of the fruit (Ando et al., 2009; Gevens et al., 2006). Ando et al. (2009) also noted that there have been no studies to identify sources of resistance to P. capsici in watermelon or melon. A majority of the evaluations to identify sources of resistance to P. capsici in pepper, tomato, and some cucurbits has been performed on seedlings. Over four months are usually needed to grow watermelon fruit for evaluations and because of larger fruit, more resources are required to complete the evaluations. Furthermore, resistance at the seedling stage is generally not correlated with resistance in the fruit (Gevens et al., 2006). In pepper, it was reported that resistance is controlled by different genes in the leaves and the roots (Sy et al., 2005; Walker and Bosland, 1999). In cucumbers, the plants are generally resistant to P. capsici; however, the fruit are very susceptible and it is possible that more than one mechanism of resistance may be involved (Gevens et al., 2006). We evaluated seedlings of over 1800 U.S. watermelon PIs for resistance to crown rot (Kousik et al., unpublished data) caused by P. capsici Received for publication 26 July 2012. Accepted for publication 1 Oct. 2012. We acknowledge the technical assistance of Danny Cook, Richard Carrington, Kim Alford, Stephanie Berry, Mira Patel, and Paul Cossman in conducting many of these experiments. Critical review of the manuscript by Drs. Merle Shepard and Craig Webster is also appreciated. This research was supported in part by funding from the Cucurbit Corp Germplasm Committee (CGC) of the USDA. Mention of a trademark name or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture, Agricultural Research Service (USDA, ARS), nor does it imply exclusion of other products that may also be suitable. To whom reprint requests should be addressed; e-mail shaker.kousik@ars.usda.gov. 1682 HORTSCIENCE VOL. 47(12) DECEMBER 2012 and 205 PIs for resistance to fruit rot (Kousik, 2011) and there appears to be no clear correlation between resistance in fruit and resistance in seedling to the same pathogen. Because fruit rot is a major problem in watermelon, identifying sources of resistance that can be useful for developing resistant cultivars is important. In 2009, we conducted a preliminary unreplicated evaluation of 205 PIs in the core collection of U.S. watermelon PI available with the Plant Genetic Resources and Conservation unit (PGRCU), Griffin, GA () and reported the results as an abstract (Kousik, 2011). The present studies were conducted to identify and confirm resistance in watermelon PI to Phytophthora fruit rot. In addition, we quantified the development of P. capsici in inoculated fruit tissue using real-time quantitative polymerase chain reaction to further confirm resistance. Materials and Methods Seeds of U.S. watermelon PIs. Seeds of U.S. watermelon PIs belonging to the core collection were obtained from PGRCU, Griffin, GA. The details of the various PIs evaluated in this study are available on the germplasm resources information network (GRIN) web site (). Seeds of commercial watermelon cultivars Black Diamond, Sugar Baby, Mickey Lee, and Charleston Gray used as controls were purchased from Willhite seeds (Willhite Seeds, Pooleville, TX). Phytophthora capsici isolate. Phytophthora capsici isolate RCZ-11, which was highly aggressive on watermelon fruit and caused severe rot, was used as the inoculum. The isolate was collected in 2003 from zucchini (Cucurbita pepo) plants in South Carolina and belongs to mating type A2. The isolate was kindly provided by Dr. A.P. Keinath, Clemson University. The isolate was routinely maintained on V8 juice agar amended with antibiotics (PARP) as described by others (Keinath, 2007; Quesada-Ocampo et al., 2009). The isolate also was routinely inoculated and then re-isolated from watermelon fruit to maintain its virulence. Evaluation of watermelon core collection for resistance. The first experiment was conducted in the Summer of 2009 to identify potential sources of resistance among the watermelon PIs. Five plants each of 205 PIs from the watermelon core collection were seeded in 50-cell Jiffy trays (Jiffy Products of America, Norwalk, OH) filled with Metro Mix (Sun Gro Horticulture, Bellevue, WA) and allowed to germinate and grow in a greenhouse for 4 weeks. Four-week-old plants were transplanted onto 96-cm wide raised beds covered with white plastic mulch. Beds were spaced 6.4 m apart. Each PI plot was a single row of five plants spaced 46 cm apart with 2.7-m spacin

Resistance to Phytophthora Fruit Rot of Watermelon Caused by Phytophthora capsici in U.S. Plant Introductions