Genetic linkage map of melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.) and localization of a major QTL for powdery mildew resistance

Powdery mildew caused by Podosphaera xanthii has become a major problem in melon since it occurs all year round irrespective of the growing system. The TGR-1551 melon genotype was found to be resistant to several melon diseases, among them powdery mildew. However, the corresponding resistance genes have been never mapped. We constructed an integrated genetic linkage map using an F2 population derived from a cross between the multi-resistant genotype TGR-1551 and the susceptible Spanish cultivar ‘Bola de Oro’. The map spans 1,284.9 cM, with an average distance of 3.6 cM among markers, and consists of 354 loci (188 AFLP, 39 RAPD, 111 SSR, 14 SCAR/CAPS/dCAPS, and two phenotypic traits) distributed in 14 linkage groups. QTL analysis identified one major QTL (Pm-R) on LG V for resistance to races 1, 2, and 5 of powdery mildew. The PM4-CAPS marker is closely linked to the Pm-R QTL at a genetic distance of 1.9 cM, and the PM3-CAPS marker is located within the support interval of this QTL. These codominant markers, together with the map information reported here, could be used for melon breeding, and particularly for genotyping selection of resistance to powdery mildew in this vegetable crop species.     

Role of leaf glandular trichomes of melon plants in deterrence of Aphis gossypii Glover

External characteristics of the leaf epidermis and their effects on behaviour of Aphis gossypii Glover were evaluated in two Cucumis melo L. genotypes, ‘Bola de Oro’ (aphid susceptible) and TGR‐1551 (aphid resistant) in order to explore their role in the early rejection of TGR‐1551 by this aphid. No differential effects of epicuticular waxes on aphid behaviour were observed. The type, distribution and number of trichomes on melon leaves were also studied. Pubescence in melon, measured as the number of non‐glandular trichomes per cm², was not sufficient to prevent aphid settling. However, there was a high density of type I glandular trichomes on leaves of the aphid‐resistant genotype. According to microscopic observations and stain testing, these trichomes store and secrete phenols and flavonoids. Free‐choice tests were conducted to determine the effect of these glandular trichomes on A. gossypii preference, revealing that aphids reject leaf disks of TGR‐1551 from the onset of the experiment. Additional experiments after removal of leaf type I glandular trichome exudates showed that A. gossypii preferred washed TGR‐1551 leaf disks over unwashed disks, while this effect was not observed in experiments using washed and unwashed ‘Bola de Oro’ leaf disks. These results suggest that a high density of glandular trichomes and chemicals secreted by them deter A. gossypii and disturb aphid settling on TGR‐1551.     

<Emphasis Type="Italic">FaRCg1</Emphasis>: a quantitative trait locus conferring resistance to Colletotrichum crown rot caused by <Emphasis Type="Italic">Colletotrichum gloeosporioides</Emphasis> in octoploid strawberry

Colletotrichum crown rot (CCR) is an important disease of strawberry (Fragaria?×ananassa) throughout the Southeastern US and in subtropical climates around the world, where hot and humid conditions facilitate rapid disease development. Yet no resistance loci have been described to date, as genetic studies have been historically difficult in allo-octoploid (2n?=?8x?=?56) strawberry. In the present study, we investigate the genetic architecture of resistance to CCR. Four population sets from the University of Florida were inoculated in four different seasons from 2013–2014 to 2016–2017. Two large, multiparental discovery population sets were used for QTL discovery, and two validation sets of cultivars and advanced selections representing the parent pool of the breeding program were also assessed. Subgenome-specific single-nucleotide polymorphism (SNP) markers were mapped, and FlexQTL? software was utilized to perform a Bayesian, pedigree-based QTL analysis. A quantitative trait locus on linkage group 6B, which we name FaRCg1, accounts for most of the genetic variation for resistance in the discovery sets (26.8–29.8% in 2013–2014 and 17% in 2015–2016). High-throughput marker assays were developed for the most significant SNPs which correlated with the mode of the QTL region. The discovery and characterization of the FaRCg1 locus and the molecular tools developed from it will be utilized to achieve increased genetic gains for resistance.     

Simple sequence repeat markers linked to QTL for resistance to Watermelon mosaic virus in melon

A population of recombinant inbred lines (RIL) derived from a cross between the Watermelon mosaic virus (WMV) resistant genotype TGR-1551 and the susceptible Spanish cultivar ‘Bola de Oro’ has been evaluated for WMV resistance in spring, fall and growth chamber conditions. The quantitative trait loci (QTL) analyses detected one major QTL (wmv) on linkage group (LG) XI close to the microsatellite marker CMN04_35. This QTL controls the resistance to WMV in the three environmental conditions evaluated. Other minor QTLs affecting the severity of viral symptoms were identified, but they were not detected in all the assayed environments. The screening of the marker CMN04_35 in an F₂ progeny, derived from the same cross, confirmed the effect of this QTL on the expression of WMV resistance also in early generations, which evidences the usefulness of this marker for a marker assisted selection program.     

<Emphasis Type="Italic">qNCLB7.02</Emphasis>, a novel QTL for resistance to northern corn leaf blight in maize

Northern corn leaf blight (NCLB), which is caused by the hemibiotrophic fungal pathogen Setosphaeria turcica, is a devastating foliar disease that results in considerable maize yield losses. In the present study, quantitative trait locus (QTL) analysis was conducted across two environments using an ultra-high-density bin map constructed using recombinant inbred lines (RILs) derived from a cross between Ye478 and Qi319. A total of 11 QTLs, located on chromosomes 1, 4, 5, 6, 7, 8, 9, and 10, were detected that confer resistance to physiological race 0 of NCLB. Each QTL could explain 3.53–15.29% of the total phenotypic variation in disease resistance after artificial inoculation in two environments. Among these QTL, qNCLB7.02, which is located on chromosome 7, had the largest effect, accounting for 10.11 and 15.29% of the phenotypic variation in resistance in two field trials and BLUP. The common confidence interval (CI) for qNCLB7.02 was 1.4 Mb, according to the B73 RefGen_v3 sequence. The resistance effect of qNCLB7.02 was validated in 2016 by using chromosome segment substitution lines (CSSLs) derived from Qi319 as the donor in the genetic background of Ye478. The type 6 CSSL, which harbors introgressed qNCLB7.02, was found to be significantly associated with resistance to NCLB by linked marker bnlg1808 and exhibited greater resistance than the other CSSLs that did not carry this QTL (P?=?0.0008). The combination of linkage mapping in RILs and validation in CSSLs is a powerful approach for the dissection of QTL for disease resistance in maize.     

Construction of a genome-anchored,high-density genetic map for melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.) and identification of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">melonis</Emphasis> race 1 resistance QTL

Key message

Four QTLs and an epistatic interaction were associated with disease severity in response to inoculation with Fusarium oxysporum f. sp. melonis race 1 in a recombinant inbred line population of melon.

Abstract

The USDA Cucumis melo inbred line, MR-1, harbors a wealth of alleles associated with resistance to several major diseases of melon, including powdery mildew, downy mildew, Alternaria leaf blight, and Fusarium wilt. MR-1 was crossed to an Israeli cultivar, Ananas Yok’neam, which is susceptible to all of these diseases, to generate a recombinant inbred line (RIL) population of 172 lines. In this study, the RIL population was genotyped to construct an ultra-dense genetic linkage map with 5663 binned SNPs anchored to the C. melo genome and exhibits the overall high quality of the assembly. The utility of the densely genotyped population was demonstrated through QTL mapping of a well-studied trait, resistance to Fusarium wilt caused by Fusarium oxysporum f. sp. melonis (Fom) race 1. A major QTL co-located with the previously validated resistance gene Fom-2. In addition, three minor QTLs and an epistatic interaction contributing to Fom race 1 resistance were identified. The MR-1 × AY RIL population provides a valuable resource for future QTL mapping studies and marker-assisted selection of disease resistance in melon.

     

Combination of Acylglucose QTL reveals additive and epistatic genetic interactions and impacts insect oviposition and virus infection

Acylsugars are important insect defense compounds produced at high levels by glandular trichomes of the wild tomato, Solanum pennellii. Marker-assisted selection was used to select for plants containing the three AGQTL named AG3QTL, AG4QTL, and AG11QTL from self-pollinated populations derived from an interspecific backcross population of CU071026 x (CU071026 x S. pennellii LA716). High acylglucose-accumulating lines were selected from these populations that possess these three AGQTL and the fewest number of extraneous S. pennellii LA716 introgressions. Incorporation of these three acylglucose QTL in the presence of the five standard S. pennellii introgressions of CU071026 altered acylsugar level and sugar moiety, demonstrating epistatic interactions between the acylglucose QTL on both of these traits. Comparison of the lines generated from the two breeding techniques indicated the three acylglucose QTL are essential but not necessarily sufficient for the production of elevated levels of acylglucose acylsugars. Fine-mapping of AG3QTL, AG4QTL, and AG11QTL resulted in less than 1 Mbp intervals for the locations of AG4QTL and AG11QTL; proposals of the causal genes underlying these acylglucose QTL are discussed. Characterization of the fatty acid profile of lines selected out of the interspecific backcross populations revealed an increase in the proportion of acylsugar n-C10 fatty acid acyl chains, possibly governed by one or more of the three acylglucose QTL. Characterization of the acylsugar profile of acylglucose lines selected from the interspecific backcross populations also demonstrated interactions among the acylglucose QTL to further modulate the diversity of acylsugars accumulated. Evaluation of an acylglucose line and controls against the tomato insect pest Frankliniella occidentalis demonstrated that levels of resistance differed among these lines and that the acylsugars accumulated by the acylglucose line were effective at reducing both F. occidentalis oviposition and incidence of Tomato spotted wilt orthotospovirus. However, of some of the acylglucose lines and hybrids tested against Spodoptera exigua did not indicate differences for larval weight gain and survival.     

Flower palate structure of the aquatic bladderworts <Emphasis Type="Italic">Utricularia bremii</Emphasis> Heer and <Emphasis Type="Italic">U. minor</Emphasis> L. from section <Emphasis Type="Italic">Utricularia</Emphasis> (Lentibulariaceae)

There is an enormous diversity in the structure of the flower palate of the carnivorous rootless genus Utricularia. This study aims to examine the structure of the palates in Utricularia bremii Heer and U. minor L of the Utricularia sect. Utricularia, which have a glandular palate type. In both species, the palate has only one type of glandular trichomes. Because of the occurrence of cell wall ingrowths in its glandular cells, any exudation may be transported via eccrinous secretion. It was proposed that the palate trichomes of the examined species act as scent glands and that the palate may play a role as an unguentarium. Both U. bremii and U. minor are of an open flower type. Thus, U. bremii and U. minor flowers can be penetrated by small, weak insects, which then easily have access to their generative structure. Small Hymenoptera (member of families Mymaridae and Braconidae) were observed as flower visitors of the male-sterile species Utricularia bremii.     

Round fruit shape in WI7239 cucumber is controlled by two interacting quantitative trait loci with one putatively encoding a tomato <Emphasis Type="Italic">SUN</Emphasis> homolog

Key message

QTL analysis revealed two interacting loci, FS1.2 and FS2.1, underlying round fruit shape in WI7239 cucumber; CsSUN , a homolog of tomato fruit shape gene SUN , was a candidate for FS1.2.

Abstract

Fruit size is an important quality and yield trait in cucumber, but its genetic basis remains poorly understood. Here we reported QTL mapping results on fruit size with segregating populations derived from the cross between WI7238 (long fruit) and WI7239 (round fruit) inbred cucumber lines. Phenotypic data of fruit length and diameter were collected at anthesis, immature and mature fruit stages in four environments. Ten major-effect QTL were detected for six traits; synthesis of information from these QTL supported two genes, FS1.2 and FS2.1, underlying fruit size variation in the examined populations. Under the two-gene model, deviation from expected segregation ratio in fruit length and diameter among segregating populations was observed, which could be explained mainly by the interactions between FS1.2 and FS2.1, and segregation distortion in the FS2.1 region. Genome-wide candidate gene search identified CsSUN, a homolog of the tomato fruit shape gene SUN, as the candidate for FS1.2. The round-fruited WI7239 had a 161-bp deletion in the first exon of CsSUN, and its expression in WI7239 was significantly lower than that in WI7238. A marker derived from this deletion was mapped at the peak location of FS1.2 in QTL analysis. Comparative analysis suggested the melon gene CmSUN-14, a homolog of CsSUN as a candidate of the fl2/fd2/fw2 QTL in melon. This study revealed the unique genetic architecture of round fruit shape in WI7239 cucumber. It also highlights the power of QTL analysis for traits with a simple genetic basis but their expression is complicated by other factors.

     

Low validation rate of quantitative trait loci for Gibberella ear rot resistance in European maize

Key message

Six quantitative trait loci (QTL) for Gibberella ear rot resistance in maize were tested in two different genetic backgrounds; three QTL displayed an effect in few near isogenic line pairs.

Abstract

Few quantitative trait loci (QTL) mapping studies for Gibberella ear rot (GER) have been conducted, but no QTL have been verified so far. QTL validation is prudent before their implementation into marker-assisted selection (MAS) programs. Our objectives were to (1) validate six QTL for GER resistance, (2) evaluate the QTL across two genetic backgrounds, (3) investigate the genetic background outside the targeted introgressions. Pairs of near isogenic lines (NILs) segregating for a single QTL (Qger1, Qger2, Qger10, Qger13, Qger16, or Qger21) were developed by recurrent backcross until generation BC₃S₂. Donor parents (DP) carrying QTL were backcrossed to a susceptible (UH009) and a moderately resistant (UH007) recurrent parent. MAS was performed using five SNP markers covering a region of 40 cM around each QTL. All NILs were genotyped with the MaizeSNP50 assay and phenotyped for GER severity and deoxynivalenol and zearalenone content. Traits were significantly (P < 0.001) intercorrelated. Out of 34 NIL pairs with the UH009 genetic background, three pairs showed significant differences in at least one trait for three QTL (Qger1, Qger2, Qger13). Out of 25 NIL pairs with the UH007 genetic background, five pairs showed significant differences in at least one trait for two QTL (Qger2, Qger21). However, Qger16, Qger10 and Qger13 were most likely false positives. The genetic background possibly affected NIL pairs comparisons due to linkage drag and/or epistasis with residual loci from the DP in non-target regions. In conclusion, validation rates were disappointingly low, which further indicates that GER resistance is controlled by many low-effect QTL.

     

Flower palate ultrastructure of the carnivorous plant <Emphasis Type="Italic">Genlisea hispidula</Emphasis> Stapf with remarks on the structure and function of the palate in the subgenus <Emphasis Type="Italic">Genlisea</Emphasis> (Lentibulariaceae)

In the genus Genlisea as well as in its sister genus Utricularia, the palate probably plays a key role in providing the colour, mechanical and olfactory stimuli to attract insect pollinators and to guide them to the generative structures and the nectary spur. However, information about the micro-morphology of the palate of Genlisea is scarce. This study aims to examine the structure of the palate in Genlisea hispidula in detail as well as the palate from other five species from the subgenus Genlisea. In particular, its aim is to ascertain whether these palates function as an area for the osmophores in the flower or whether they produce nectar. We showed that the palate in all of the species that were examined was the glandular type and that it had capitate, glandular trichomes, which had a similar general architecture across the species that were examined. No nectar secretion was observed on the palates. The ultrastructure of the palate trichomes showed that the palate glandular trichomes most probably function as scent glands that produce an olfactory stimulus for flower pollinators.     

Inheritance and gene mapping of spotted to non-spotted trait gene <Emphasis Type="Italic">CmSp-1</Emphasis> in melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L<Emphasis Type="Italic">.</Emphasis> var. <Emphasis Type="Italic">chinensis</Emphasis> Pangalo)

Melon (Cucumis melo L.) is one of the most popular and highly nutritious vegetable species within Cucurbitaceae. Because appearance is used as an important indicator of quality, the spotted to non-spotted trait associated with this product somewhat influences the buying habits of consumers. We tested a six-generation family to determine the inheritance and genetic basis of this trait. Genetic groups F₁, F₂, BC₁P₁, and BC₁P₂ were from a cross between “IM16559” (non-spotted) and “IM16553” (spotted). Our genetic analysis showed that the spotted to non-spotted trait was controlled by a single dominant gene that we named CmSp-1. Whole-genome resequencing-bulked segregant analysis (WG-BSA) demonstrated that this gene was located on the end of chromosome 2, in the intersections of 22,160,000 to 22,180,000 bp and 22,260,000 to 26,180,000 bp, an interval distance of 3.94 Mb. Insertion-deletion (InDel) markers designed based on WG-BSA data were used to map this gene. Using 13 InDel markers, we produced a genetic map indicating that CmSp-1 was tightly linked to markers I734-2 and I757, with genetic distances of 1.8 and 0.4 cM and an interval distance of 280.872 kb. The closest marker was I757. Testing of 107 different melon genotypes presented an accuracy of 84.11% in predicting the phenotype. By being able to locate CmSp-1 in melon, we can now use the findings to identify potential targets for further marker-assisted breeding and cloning projects.     

Comparative studies of leaf surface chemical biosynthesis in different tobacco cultivars

To understand the differences in the biosynthesis of leaf surface chemicals and their influence on aphid preference for different tobacco cultivars (Nicotiana tabacum), we analyzed the secretory characteristics of glandular trichomes of four commercial cultivars, K326 (flue-cured), Beinhart 1000-1 (cigar), Basma YNOTBS1 (oriental), and Dabai 1 (burley), and their parental species, Nicotiana sylvestris and Nicotiana tomentosiformis. Trichome-type observation showed that K326 and N. sylvestris have three kinds of glandular trichomes (non-glandular, long stalked glandular, and short stalked glandular trichomes), whereas Beinhart 1000-1, Basma YNOTBS1, Dabai 1, and N. tomentosiformis had two kinds of glandular trichomes (long and short stalked glandular trichomes). The gas chromatography–mass spectrometry profiles of leaf exudates indicated that N. tomentosiformis synthesized only labdanoids; N. sylvestris, K326 and Dabai 1 synthesized only cembranoids; and Beinhart 1000-1 and Basma YNOTBS1 synthesized cembranoids and labdanoids. Gene expression pattern analysis revealed that the labdanoid synthesis-related genes NtABS and NtCPS2 were expressed in N. tomentosiformis, Beinhart 1000-1, and Basma YNOTBS1, whereas the cembranoid synthesis-related genes NtCYC and NtCYP71D16 were expressed in N. sylvestris and all four commercial cultivars. Evolutionary analysis indicated that NtCYC and NtCYP71D16 might be phylogenetically originated from N. sylvestris, whereas NtABS and NtCPS2 expressed in Basma YNOTBS1 and Beinhart 1000-1 might be derived from N. tomentosiformis. In addition, aphid attraction (number of aphids) was significantly and positively correlated with the total glandular secretion (r² =?0.9425, P?≤?0.05), and it was significantly and positively correlated with amount of CBT-diol (r²?=?0.9224; P?≤?0.05). These results provide new insights into the biosynthesis of diterpenoids and biotic stress resistance in tobacco.     

Assessment of genetic diversity of <Emphasis Type="Italic">Saltol</Emphasis> QTL among the rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) genotypes

Eight Saltol quantitative trait locus (QTL) linked simple sequence repeat (SSR) markers of rice (Oryza sativa L.) were used to study the polymorphism of this QTL in 142 diverse rice genotypes that comprised salt tolerant as well as sensitive genotypes. The SSR profiles of the eight markers generated 99 alleles including 20rare alleles and 16 null alleles. RM8094 showed the highest number (13) of alleles followed by RM3412 (12), RM562 (11), RM493 (9) and RM1287 (8) while as, RM10764 and RM10745 showed the lowest number (6) of alleles. Based on the highest number of alleles and PIC value (0.991), we identified RM8094 as suitable marker for discerning salt tolerant genotypes from the sensitive ones. Based upon the haplotype analysis using FL478 as a reference (salt tolerant genotypes containing Saltol QTL), we short listed 68 rice genotypes that may have at least one allele of FL478 haplotype. Further study may confirm that some of these genotypes might have Saltol QTL and can be used as alternative donors in salt tolerant rice breeding programmes.     

High-resolution mapping of genes involved in plant stage-specific partial resistance of barley to leaf rust

Partial resistance quantitative trait loci (QTLs) Rphq11 and rphq16 against Puccinia hordei isolate 1.2.1 were previously mapped in seedlings of the mapping populations Steptoe/Morex and Oregon Wolfe Barleys, respectively. In this study, QTL mapping was performed at adult plant stage for the two mapping populations challenged with the same rust isolate. The results suggest that Rphq11 and rphq16 are effective only at seedling stage, and not at adult plant stage. The cloning of several genes responsible for partial resistance of barley to P. hordei will allow elucidation of the molecular basis of this type of plant defence. A map-based cloning approach requires to fine-map the QTL in a narrow genetic window. In this study, Rphq11 and rphq16 were fine-mapped using an approach aiming at speeding up the development of plant material and simplifying its evaluation. The plant materials for fine-mapping were identified from early plant materials developed to produce QTL-NILs. The material was first selected to carry the targeted QTL in heterozygous condition and susceptibility alleles at other resistance QTLs in homozygous condition. This strategy took four to five generations to obtain fixed QTL recombinants (i.e., homozygous resistant at the Rphq11 or rphq16 QTL alleles, homozygous susceptible at the non-targeted QTL alleles). In less than 2 years, Rphq11 was fine-mapped into a 0.2-cM genetic interval and a 1.4-cM genetic interval for rphq16. The strongest candidate gene for Rphq11 is a phospholipid hydroperoxide glutathione peroxidase. Thus far, no candidate gene was identified for rphq16.     

Dissection of two quantitative trait loci with pleiotropic effects on plant height and spike length linked in coupling phase on the short arm of chromosome 2D of common wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Key message

Two QTL with pleiotropic effects on plant height and spike length linked in coupling phase on chromosome 2DS were dissected, and diagnostic marker for each QTL was developed.

Abstract

Plant height (PHT) is a crucial trait related to plant architecture and yield potential, and dissection of its underlying genetic basis would help to improve the efficiency of designed breeding in wheat. Here, two quantitative trait loci (QTL) linked in coupling phase on the short arm of chromosome 2D with pleiotropic effects on PHT and spike length, QPht/Sl.cau-2D.1 and QPht/Sl.cau-2D.2, were separated and characterized. QPht/Sl.cau-2D.1 is a novel QTL located between SNP makers BS00022234_51 and BobWhite_rep_c63957_1472. QPht/Sl.cau-2D.2 is mapped between two SSR markers, SSR-2062 and Xgwm484, which are located on the same genomic interval as Rht8. Moreover, the diagnostic marker tightly linked with each QTL was developed for the haplotype analysis using diverse panels of wheat accessions. The frequency of the height-reduced allele of QPht/Sl.cau-2D.1 is much lower than that of QPht/Sl.cau-2D.2, suggesting that this novel QTL may be an attractive target for genetic improvement. Consistent with a previous study of Rht8, a significant difference in cell length was observed between the NILs of QPht/Sl.cau-2D.2. By contrast, there was no difference in cell length between NILs of QPht/Sl.cau-2D.1, indicating that the underlying molecular mechanism for these two QTL may be different. Collectively, these data provide a new example of QTL dissection, and the developed diagnostic markers will be useful in marker-assisted pyramiding of QPht/Sl.cau-2D.1 and/or QPht/Sl.cau-2D.2 with the other genes in wheat breeding.

     

QTL-seq and marker development for resistance to <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">niveum</Emphasis> race 1 in cultivated watermelon

Fusarium wilt, caused by the fungus Fusarium oxysporum f. sp. niveum (Fon), is one of the predominant diseases of watermelon. Resistance to Fon race 1 is conferred by a single major quantitative trait locus (QTL), Fo-1.1, but resolution of this region has been poor due to low marker density. In this study, a combination of whole genome resequencing of bulked segregants (QTL-seq analysis) followed by QTL mapping with kompetitive allele specific PCR (KASP) markers developed across Fo-1.1 successfully increased the resolution from 2.03 to 1.56 Mb and 315 kb, respectively. The linkage of the KASP markers to Fon race 1 resistance across a wide range of watermelon germplasm was validated in a set of elite watermelon cultivars. The linked markers described here provide a breeder-friendly toolkit immediately available for high-throughput genotyping in large-scale breeding programs for fine mapping and incorporation of Fon race 1 resistance in watermelon.