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Phytophthora capsici Leonian, the causal agent of foliar blight, root rot, fruit rot and crown rot syndromes in squash (Cucurbita moschata), is a devastating pathogen worldwide. Resistance to Phytophthora crown rot in University of Florida breeding line #394-1-27-12 (C. moschata) is conferred by three independent dominant genes (R1R2R3). Availability of DNA markers linked to R1R2R3 genes would allow efficient breeding for Phytophthora crown rot resistance through marker-assisted selection (MAS). The goal of the current study was to identify quantitative trait loci (QTLs) associated with resistance to Phytophthora crown rot in an F 2 population (n = 168) derived from a cross between #394-1-27-12 (R) and Butter Bush (S) using QTL-seq bulk segregant analysis. Whole-genome resequencing of the resistant (n = 20) and susceptible (n = 20) bulk segregants revealed ~900,000 single nucleotide polymorphisms distributed across C. moschata genome. Three QTLs significantly (P < 0.05) associated with resistance to Phytophthora crown rot were detected on chromosome 4 (QtlPC-C04), 11 (QtlPC-C11) and 14 (QtlPC-C14). Several markers linked to these QTLs are potential targets for MAS against Phytophthora crown rot in C. moschata. The present study reports the first QTLs associated with Phytophthora crown rot resistance in C. moschata. Disease epidemics caused by the oomycete Phytophthora capsici Leonian are a major challenge for squash (Cucurbita pepo L., C. moschata Duchesne, and C. maxima Duchesne) growers worldwide 1. The pathogen causes foliar blight, root rot, fruit rot and crown rot syndromes, and is particularly severe under flooding conditions, often resulting in total crop loss 2. Current strategies for managing P. capsici in commercial squash production rely heavily on chemical fungicides, however, existence of fungicide-resistant P. capsici isolates in major squash growing regions has rendered many chemicals ineffective for the control of the pathogen 3-5. Cultural management practices such as crop rotation and soil-water management focus on inoculum reduction or avoidance, but are not solely effective, particularly under heavy disease pressure 6. Host resistance is the best strategy for managing this disease, but no commercial cultivars resistant to the pathogen are currently available 7 to support the U.S. squash industry currently valued at 230 million dollars annually 8. Extensive efforts have led to identification of sources of resistance to Phytophthora crown rot in unimproved germplasm of Cucurbita. Padley et al. 9 identified sixteen plant introductions (PIs) of C. pepo that showed moderate to high resistance to Phytophthora crown rot. Among these, PIs 181761 and 615132 were the most resistant (disease severity (DS) ≤ 1.3 out of 5) 9. In C. moschata, Chavez and Kabelka 10 identified five PIs (176531, 458740, 442266, 442262 and 634693) that exhibited high resistance (DS ≤ 1 out of 5) to Phytophthora crown rot. Kabelka et al. 11 identified a source of resistance in C. lundeliana that was successfully introgressed int...

Egusi watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai subsp. mucosospermus var. egusi (C. Jeffrey) Mansf.] is known for its distinctive fleshy-pericarp seed phenotype and high seed oil percentage (SOP). The seed is part of the daily diet in West Africa where it is used in soups and stews or processed for cooking oil. Genetic mapping studies have revealed that most of the variation in SOP between egusi and normal, non-egusi seed is explained by the egusi (eg) locus, which is also associated with the unique seed phenotype. However, variation in SOP is also observed within egusi and normal seed types although the basis of this variation remains to be elucidated. A high correlation between kernel percentage (KP) and SOP has been observed in watermelon and other crops, and recent data also suggest an association between seed size and SOP in watermelon. The aim of this study was to elucidate the relationship among SOP, KP, and seed size traits in watermelon and to identify quantitative trait loci (QTL) associated with the latter traits to facilitate marker-assisted selection (MAS) for traits correlated with SOP. KP showed a significant (α = 0.05) positive correlation with SOP in both egusi and normal seed types, whereas seed size traits showed significant negative correlations with SOP. QTL associated with KP and seed size traits in normal seed were colocalized with a previously mapped locus for SOP on linkage group (LG) 2, but in egusi seed, a QTL explaining 33% of phenotypic variation in KP was localized on LG 7. The results of this study show that SOP in watermelon is correlated with KP and seed size, but KP is associated with different loci in normal and egusi seed phenotypes.

a b s t r a c t Management of fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon) (E.F. Sm.) W.C. Snyder & H.N. Han. in watermelon [Citrullus lanatus (Thunb.) Matsum.& Nakai] is largely dependent on cultivation of resistant cultivars. Application of marker-assisted selection (MAS) in conventional breeding programs can accelerate the release of new watermelon cultivars resistant to fusarium wilt. Towards developing tools for MAS, physical (1024 SNPs) and genetic (389 SNPs) maps were developed in the current study for an F 2 population (n = 89; Calhoun Gray x Sugar Baby) segregating for resistance against Fon race 1 using the genotyping by sequencing platform. A modified tray-dip method was established for high-throughput phenotyping of the segregating F 3 population. A major quantitative trait locus (QTL) accounting for up to 38.4% of the phenotypic variation in the F 3 population was identified on chromosome 1 on both the physical and genetic maps in a region previously associated with Fon race 1 resistance. This resistance locus was consistently detected over five different time points and three different phenotypic screens, showing the reliability of the screening method in discriminating susceptible and resistant genotypes. Eight resistance genes were found within the confidence interval of the identified QTL. SNPs close to this QTL may be exploited in MAS for fusarium wilt resistance in breeding programs. This study confirms the resistance locus on chromosome 1 and demonstrates the use of a physical map for QTL detection in watermelon. The SNPs reported here will be useful for future genetic studies in watermelon.

Phytophthora crown rot, caused by Phytophthora capsici Leonian, is a devastating disease in commercial squash (Cucurbita pepo L.) production across the United States. Current management practices rely heavily on the use of chemical fungicides, but existence of fungicide-resistant pathogen populations has rendered many chemicals ineffective. Host resistance is the best strategy for managing this disease; however, no commercial cultivars resistant to the pathogen are currently available. Resistance to Phytophthora crown rot in PI 181761 (C. pepo) is an important genetic resource for squash breeders worldwide; however, the underlying genetic basis of resistance in PI 186761 that would allow designing of sound breeding strategies is currently unknown. The goal of the current study was to determine the inheritance of resistance in breeding line #186761-36P, a resistant selection of PI 181761, using phenotypic data from F1, F2, and backcross populations derived from a cross between #181761-36P and a susceptible acorn-type cultivar, Table Queen. The results indicated that resistance in #181761-36P is controlled by three dominant genes (R4, R5, and R6). Introgression of these genes into susceptible cultivar groups of C. pepo will provide an important tool in the integrated management of Phytophthora crown rot.

Fusarium wilt of watermelon (Citrullus lanatus), caused by Fusarium oxysporum f. sp. niveum (FON), is a devastating soil-borne disease limiting watermelon production across the world. Although many watermelon cultivars have been bred for resistance to FON races 0 and 1, the only released cultivars that are resistant to FON 2 are nonharvested pollenizers. The lack of FON 2–resistant edible cultivars is thought to be associated with linkage drag and/or preferential inheritance patterns observed when crossing the resistant, wild source (Citrullus amarus), with edible watermelon. A potential way to overcome these obstacles is to use a resistant C. lanatus as the source of resistance and to develop molecular markers to increase the efficiency of selection. Here we describe the identification of a quantitative trait locus (QTL) associated with FON 2 resistance in watermelon. The genotyping by sequencing (GBS) platform was used to generate single nucleotide polymorphisms (SNPs) in an F2 population (n = 178) developed from a cross between UGA147 (resistant) and ‘Charleston Gray’ (susceptible). Five hundred and one SNPs were placed on the watermelon physical map and used in the mapping of QTL. F3 lines were phenotyped for resistance to FON 2 in the greenhouse. An intermediate QTL associated with resistance to FON 2 was identified on chromosome 11 (Qfon11). This QTL is a potential target for marker-assisted selection (MAS) for FON 2 resistance in watermelon.

Summer squash (Cucurbita pepo L.) is a major vegetable crop produced in Georgia and Florida during the fall season. This production is vulnerable to whitefly (Bemisisia tabaci Genn.)-transmitted viruses that lead to severe yield losses. Over the past several years, whitefly populations have increased during the fall, thus leading to an increase in whitefly-transmitted viruses such as Cucurbit leaf crumple virus (CuLCrV) and Cucurbit yellow stunting disorder virus (CYSDV). Whitefly management for summer squash relies on the use of insecticides and can be costly without providing adequate management of the viruses. Deployment of host resistance to whiteflies and their transmitted viruses (CuLCrV and CYSDV) is the best strategy for mitigating yield loss of summer squash; however, no resistant cultivars are commercially available. In the current study, resistance or tolerance to whiteflies, CuLCrV, and CYSDV was determined for squash germplasm from the U.S. Department of Agriculture (USDA) Germplasm Resources Information Network (GRIN), university breeding programs, and commercial companies in Georgia and Florida across 2 years. In both locations and years, visual virus symptom severity scores were collected and a quantitative polymerase chain reaction (qPCR) was used to determine the CuLCrV viral load and CYSDV presence in Georgia. Whitefly-induced feeding damage was evaluated by directly assessing the intensity of silverleaf symptoms and visual counts of whitefly adults on the foliage in the field or in photographs. Virus symptom severity was lower in C. moschata Duchesne ex Poir. genotypes, namely, PI 550689, PI 550692, PI 550694, PI 653064, and Squash Betternut 900, than in other evaluated genotypes. Two C. pepo accessions were common between both locations for viral severity (PI 442294) or viral severity and viral load (PI 171625). Lower CuLCrV loads were identified in C. ecuadorensis Cutler & Whitaker (PI 540895), and C. okeechobeensis (Small) L.H.Bailey (PI 540900) than other evaluated genotypes. Four genotypes tested negative for CYSDV during both years: C. pepo (PI 507882), C. moschata (PI 483345), C. ecuadorensis (PI 390455), and C. okeechobeensis (PI 540900); they are potential sources of resistance. Six C. moschata accessions (PI 211999, PI 550690, PI 550692, PI 550694, PI 634982, and PI 653064) showed high tolerance to silverleaf disorder and had the lowest adult whitefly counts. Collectively, the accessions identified in the current study are potential sources of resistance or tolerance to whitefly and whitefly-transmitted viruses (CuLCrV and CYSDV).

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