Mapping and validation of Fusarium wilt race 2 resistance QTL from Citrullus amarus line USVL246-FR2.

KEY MESSAGE: Fon race 2 resistant QTLs were identified on chromosomes 8 and 9. Families homozygous for resistance alleles at a haplotype of three KASP markers had 42% lower disease severity than those with susceptible alleles in an independent, interspecific validation population confirming their utility for introgression of Fusarium wilt resistance. Fusarium oxysporum f. sp. niveum (Fon) race 2 causes Fusarium wilt in watermelon and threatens watermelon production worldwide. Chemical management options are not effective, and no resistant edible watermelon cultivars have been released. Implementation of marker-assisted selection to develop resistant cultivars requires identifying sources of resistance and the underlying quantitative trait loci (QTL), developing molecular markers associated with the QTL, and validating marker-phenotype associations with an independent population. An intraspecific Citrullus amarus recombinant inbred line population from a cross of resistant USVL246-FR2 and susceptible USVL114 was used for mapping Fon race 2 resistance QTL. KASP markers were developed (N = 51) for the major QTL on chromosome 9 and minor QTL on chromosomes 1, 6, and 8. An interspecific F2:3 population was developed from resistance donor USVL246-FR2 (C. amarus) and a susceptible cultivar 'Sugar Baby' (Citrullus lanatus) to validate the utility of the markers for introgression of resistance from the wild crop relative into cultivated watermelon. Only 16 KASP markers segregated in the interspecific C. amarus/lanatus validation population. Four markers showed significant differences in the separation of genotypes based on family-mean disease severity, but together explained only 16% of the phenotypic variance. Genotypes that inherited homozygous resistant parental alleles at three KASP markers had 42% lower family-mean disease severity than homozygous susceptible genotypes. Thus, haplotype analysis was more effective at predicting the mean disease severity of families than single markers. The haplotype identified in this study will be valuable for developing Fon race 2 resistant watermelon cultivars.

Genome-Wide Association Mapping and Genomic Prediction of Fusarium Wilt Race 2 Resistance in the USDA Citrullus amarus Collection.

Fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon) race 2 is a serious disease in watermelon and can reduce yields by 80%. Genome-wide association studies (GWAS) are a valuable tool in dissecting the genetic basis of traits. Citrullus amarus accessions (n = 120) from the USDA germplasm collection were genotyped with whole-genome resequencing, resulting in 2,126,759 single nucleotide polymorphic (SNP) markers that were utilized for GWAS. Three models were used for GWAS with the R package GAPIT. Mixed linear model (MLM) analysis did not identify any significant marker associations. FarmCPU identified four quantitative trait nucleotides (QTN) on three different chromosomes (i.e., chromosomes 1, 5, and 9), and Bayesian-information and linkage-disequilibrium iteratively nested keyway (BLINK) identified one QTN on chromosome 10 as significantly associated with Fon race 2 resistance. FarmCPU identified four QTN that explained 60% of Fon race 2 resistance, and the single QTN from BLINK explained 27%. Relevant candidate genes were found within the linkage disequilibrium (LD) blocks of these significant SNPs, including genes encoding aquaporins, expansins, 2S albumins, and glutathione S-transferases which have been shown to be involved in imparting resistance to Fusarium spp. Genomic predictions (GP) for Fon race 2 resistance using all 2,126,759 SNPs resulted in a mean prediction accuracy of 0.08 with five-fold cross-validation employing genomic best linear unbiased prediction (gBLUP) or ridge-regression best linear unbiased prediction (rrBLUP). Mean prediction accuracy with gBLUP leave-one-out cross-validation was 0.48. Thus, along with identifying genomic regions associated with Fon race 2 resistance among the accessions, this study observed prediction accuracies that were strongly influenced by population size.

Widespread Resistance to Tebuconazole and Cross-Resistance to Other DMI Fungicides in Stagonosporopsis citrulli Isolated from Watermelon in South Carolina.

Tebuconazole, a demethylation-inhibitor (DMI) fungicide, is widely used on watermelon and muskmelon because it is inexpensive and has been effective against Stagonosporopsis citrulli, the primary causal agent of gummy stem blight in the southeastern United States. Most isolates (94% of 251) collected from watermelon in South Carolina in 2019 and 2021 were moderately resistant to tebuconazole at 3.0 mg/liter in vitro. Ninety isolates were identified as S. citrulli, and no isolates of S. caricae were found in this study. On watermelon and muskmelon seedlings treated with the field rate of tebuconazole, sensitive, moderately resistant, and highly resistant isolates were controlled 99, 74, and 45%, respectively. In vitro, tebuconazole-sensitive isolates were moderately resistant to tetraconazole and flutriafol but sensitive to difenoconazole and prothioconazole, while highly resistant isolates were highly resistant to tetraconazole and flutriafol and moderately resistant to difenoconazole and prothioconazole. On watermelon seedlings treated with field rates of five DMI fungicides in the greenhouse, severity of gummy stem blight did not differ significantly from the nontreated control when seedlings were inoculated with a highly resistant isolate, while severity was lower with all DMIs on seedlings inoculated with a sensitive isolate, although severity was greater with tetraconazole than with the other four DMIs. In the field, tetraconazole rotated with mancozeb did not reduce severity of gummy stem blight caused by a tebuconazole-sensitive isolate when compared to the nontreated control, while the other four DMIs did. With a highly resistant isolate, all DMIs rotated with mancozeb reduced severity of gummy stem blight compared to the nontreated control, but severity with tetraconazole and tebuconazole was greater than with mancozeb alone, and severity with flutriafol, difenoconazole, prothioconazole, and difenoconazole plus cyprodinil did not differ from mancozeb applied alone. Results from in vitro, greenhouse, and field experiments with the five DMI fungicides were highly correlated with each other. Thus, determining relative colony diameters with a discriminatory dose of 3 mg/liter of tebuconazole is an effective way to identify isolates of S. citrulli highly resistant to tebuconazole.

First Report of Bacterial Leaf Spot on Spinach (Spinacia oleracea) Caused by Pseudomonas sp. in South Carolina.

A large grower of Brassica leafy greens and spinach in South Carolina observed a severe outbreak of leaf spot on 150 hectares of spinach (Spinacia oleracea) in Orangeburg County, SC in 2013. The entire field was lost due to the outbreak. Symptoms appeared on 8-week old plants as tan to white necrotic spots with black centers, water-soaking and no discernable chlorotic borders. Lesions varied from 2 mm to 1 cm in diameter and often coalesced to cover >50% of the leaves. Symptomatic spinach plants cv. Vancouver were collected in 2013 from the field. Bacterial streaming was evident from the border of necrotic lesions under magnification. Lesion border regions were excised, surface-disinfested with 0.5% NaOCl, macerated in sterilized distilled water and streaked onto nutrient agar (NA) and Pseudomonas Agar F (PAF). Bacterial growth was observed on NA and PAF; several single colonies were selected and re-streaked onto PAF. Colonies fluoresced blue under UV light after 48 h at 28oC. Two of the strains were subjected to 16S rRNA sequencing (GenBank accessions OM983506 and OM983507) and Fatty Acid Methyl Ester (FAME) analysis (MIDI LABS, Newark, DE). FAME results had a best similarity index (0.788) to Pseudomonas cichorii/viridiflava. The 16S sequences were queried to Pseudomonas type-strains in GenBank resulting in best matches: P. ovata (99.23% identity with 99% coverage) and P. maditerranea (99.04% identity with 100% coverage). Additionally, sequences had 97.33% identity with 100% coverage as a P. cichorii type strain, and only 96.86% identity with 97% coverage as a P. viridiflava type strain. These two strains were tested for pathogenicity on the spinach cv. Vancouver. Bacteria were grown on PAF for 48 h, and a bacterial suspension was prepared with sterile distilled water with the addition of 0.001% Latron (Plant Health Technologies, Boise, ID) and adjusted to an optical density of 0.4 at OD600. Six-week-old plants (eight plants) were sprayed with the bacterial suspension to runoff, placed at 100% relative humidity for 72 h, and then put in a growth chamber at 25oC with a 12 h diurnal light cycle for 10 days. Eight plants of 'Vancouver' were sprayed with water and 0.001% Latron as controls. Both strains were pathogenic on 'Vancouver' and caused symptoms similar to those observed in the field. Symptoms were not observed on negative controls. The same bacterial colonies were recovered from the lesions on inoculated plants, fulfilling Koch's postulates. Comparative rep-PCR analysis using the BOXA1R primer (Versalovic et al. 1994) showed both strains had identical DNA-banding profiles. All identification methods used indicate that this is a different Pseudomonas species from the one reported on spinach in California by Koike et al (2002). The top producers of spinach in SC stopped large-scale production in 2014 as a result of this pathogen. In 2020, due to inability of processors to obtain sufficient quantities of spinach, SC growers again planted the crop. Growers experienced yield losses due to similar symptoms on the crop. BOX-PCR of isolated strains of bacteria from these plants showed a DNA banding pattern similar to the 2013 strains.

Mapping Resistance to Alternaria cucumerina in Cucumis melo.

Infection with Alternaria cucumerina causes Alternaria leaf blight (ALB), a disease characterized by lesion formation on leaves, leading to substantial yield and quality losses in Cucumis melo (melon). Although fungicides are effective against ALB, reduction in the frequency of application would be economically and environmentally beneficial. Resistant melon lines have been identified but the genetic basis of this resistance has not been determined. A saturated melon genetic map was constructed with markers developed through genotyping by sequencing of a recombinant inbred line population (F6 to F10; n = 82) derived from single-seed descent of a F2 population from a cross between the ALB-resistant parent MR-1 and the ALB-susceptible parent Ananas Yokneum. The population was evaluated for A. cucumerina resistance with an augmented block greenhouse study using inoculation with the wounded-leaf method. Multiple quantitative trait loci (QTL) mapping identified two QTL that explained 33.9% of variation in lesion area. Several candidate genes within range of these QTL were identified using the C. melo v3.5 genome. Markers linked to these QTL will be used to accelerate efforts to breed melon cultivars resistant to ALB.

Comparative high-density microarray analysis of gene expression during growth of Lactobacillus helveticus in milk versus rich culture medium.

Lactobacillus helveticus CNRZ32 is used by the dairy industry to modulate cheese flavor. The compilation of a draft genome sequence for this strain allowed us to identify and completely sequence 168 genes potentially important for the growth of this organism in milk or for cheese flavor development. The primary aim of this study was to investigate the expression of these genes during growth in milk and MRS medium by using microarrays. Oligonucleotide probes against each of the completely sequenced genes were compiled on maskless photolithography-based DNA microarrays. Additionally, the entire draft genome sequence was used to produce tiled microarrays in which noninterrupted sequence contigs were covered by consecutive 24-mer probes and associated mismatch probe sets. Total RNA isolated from cells grown in skim milk or in MRS to mid-log phase was used as a template to synthesize cDNA, followed by Cy3 labeling and hybridization. An analysis of data from annotated gene probes identified 42 genes that were upregulated during the growth of CNRZ32 in milk (P < 0.05), and 25 of these genes showed upregulation after applying Bonferroni's adjustment. The tiled microarrays identified numerous additional genes that were upregulated in milk versus MRS. Collectively, array data showed the growth of CNRZ32 in milk-induced genes encoding cell-envelope proteinases, oligopeptide transporters, and endopeptidases as well as enzymes for lactose and cysteine pathways, de novo synthesis, and/or salvage pathways for purines and pyrimidines and other functions. Genes for a hypothetical phosphoserine utilization pathway were also differentially expressed. Preliminary experiments indicate that cheese-derived, phosphoserine-containing peptides increase growth rates of CNRZ32 in a chemically defined medium. These results suggest that phosphoserine is used as an energy source during the growth of L. helveticus CNRZ32.