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Summary Years of selection for desirable fruit quality traits in dessert watermelon (Citrullus lanatus) has resulted in a narrow genetic base in modern cultivars. Development of novel genomic and genetic resources offers great potential to expand genetic diversity and improve important traits in watermelon. Here, we report a high‐quality genome sequence of watermelon cultivar ‘Charleston Gray’, a principal American dessert watermelon, to complement the existing reference genome from ‘97103’, an East Asian cultivar. Comparative analyses between genomes of ‘Charleston Gray’ and ‘97103’ revealed genomic variants that may underlie phenotypic differences between the two cultivars. We then genotyped 1365 watermelon plant introduction (PI) lines maintained at the U.S. National Plant Germplasm System using genotyping‐by‐sequencing (GBS). These PI lines were collected throughout the world and belong to three Citrullus species, C. lanatus, C. mucosospermus and C. amarus. Approximately 25 000 high‐quality single nucleotide polymorphisms (SNPs) were derived from the GBS data using the ‘Charleston Gray’ genome as the reference. Population genomic analyses using these SNPs discovered a close relationship between C. lanatus and C. mucosospermus and identified four major groups in these two species correlated to their geographic locations. Citrullus amarus was found to have a distinct genetic makeup compared to C. lanatus and C. mucosospermus. The SNPs also enabled identification of genomic regions associated with important fruit quality and disease resistance traits through genome‐wide association studies. The high‐quality ‘Charleston Gray’ genome and the genotyping data of this large collection of watermelon accessions provide valuable resources for facilitating watermelon research, breeding and improvement.

The oxidative burst is a critical early event in plant-pathogen interactions that leads to a localized, programmed cell death (PCD) called the hypersensitive response (HR). The HR and associated PCD retard infection by biotrophic pathogens, but can, in fact, enhance infection by necrotrophic pathogens like Botrytis cinerea. In addition to signaling the induction of the HR, reactive oxygen species (ROS) produced during the oxidative burst are antimicrobial. We hypothesize that pathogens such as B. cinerea survive the antimicrobial effects of ROS, at least partially by secreting the antioxidant mannitol during infection. This is supported by the previous observation that overexpression of the catabolic enzyme mannitol dehydrogenase (MTD) can decrease a plants susceptibility to mannitol-secreting pathogens like B. cinerea. To extend the above hypothesis, and test the general utility of this approach in an important horticultural crop, we overexpressed celery MTD in tomato (Solanum lycopersicum cv. "Moneymaker"). In these studies, we observed a significant increase (up to 90%) in resistance to B. cinerea in transgenic tomatoes expressing high amounts of MTD.

Anthracnose, caused by the fungal pathogen Colletotrichum orbiculare (Berk. & Mont.) Arx syn. lagenaria, is one of the most important diseases of watermelon in the United States and worldwide. The study was conducted to identify C. orbiculare race 1 resistance quantitative trait loci (QTL) in a 'Charleston Gray', resistant parent, and 'New Hampshire Midget', susceptible parent, biparental mapping population. The mapping population consisted of 228 F 2 and the validation population consisted of 60 individuals each in BC 1 P 1 and BC 1 P 2 . The disease severity was rated using a disease index comprising a rating scale of 0 to 100%. IciMapping was used to draw the linkage map and R/qtl non-parametric method ('model = np') was used to identity QTL. We identi ed a major disease resistance QTL, Qar1-8, on chromosome 8. The signi cant SNP marker S8_5149002, part of a putative coiled-coil (CC)-nucleotide-binding site (NBS)leucine-rich repeat (LRR) (CC-NBS-LRR or CNL; ClCG08G002410), had a LOD of 14.06. The signi cant marker was validated on mapping populations using R package functions 'chisq.test ', 'wilcox.test', 'kruskal.test', and 'dunn.test'. The signi cant marker S8_5149002 was also tested for its ability to differentiate race 1 anthracnose resistance on 61 watermelon germplasm including 41 plant introduction (PI) lines. Hence, the diagnostic SNP marker S8_5149002 could be used for marker assisted selection (MAS) for race 1 anthracnose resistance in watermelon breeding programs.

Reactive oxygen species (ROS) made by plants in response to pathogen infection not only initiate local and systemic defenses, they are also antimicrobial. A number of fungi are hypothesized to secrete the antioxidant mannitol to protect against this antimicrobial ROS during infection. This hypothesis is supported by reports that overexpression of the mannitol catabolic enzyme mannitol dehydrogenase (MTD) in plants increases resistance to mannitol-secreting pathogens like Botrytis cinerea and Alternaria alternata. To extend this hypothesis and test the general utility of this approach, we overexpressed celery MTD in a tomato breeding line (NC1 Grape) currently used in our program. Although we reported earlier that MTD overexpression provides resistance to Botrytis gray mold in a greenhouse tomato, this is the first report of overexpression in an elite breeding variety providing heritable, whole-plant resistance to A. solani (tomato early blight). In this study, progeny from a high-MTD-expressing line had infection rates <65% those of nontransformed plants, and transformants outgrew infection by 7 days post-inoculation. Finally, our results suggest that screening for higher innate MTD expression in plants, rather than screening solely for the presence of the Mtd gene, might be a more effective way to identify parental lines for use in conventional breeding of early blight resistance. Accepted for publication 7 August 2015. Published 14 August 2015.