Fusarium Oxysporum F. Sp. Niveum Research Articles

Inactivated pathogenic mycelia as a biocontrol agent against Fusarium wilt and its effects on continuously cropped watermelon

ABSTRACT Fusarium wilt is a widespread disease causing serious production losses in watermelon (Citrullus lanatus L.). Accordingly, control agents to manage this disease should be developed to ensure the sustainability of watermelon cultivation. In this study, inactivated Fusarium oxysporum f. sp. niveum (FON) mycelia were used as a decoy inoculation agent, and the effects of this treatment on Fusarium wilt and its possible underlying mechanisms were investigated. Inoculation with inactivated FON mycelia significantly decreased rhizosphere FON number and Fusarium wilt disease severity, and this might be closely related to the observed improved abundances and community structures of native beneficial rhizosphere microorganisms. In addition, inactivated FON mycelia inoculation significantly enhanced soil enzyme activity levels and plant systemic resistance, which promoted resistance to Fusarium wilt and thus watermelon growth. These results demonstrated that inactivated FON mycelia could be used as a novel biocontrol agent to alleviate Fusarium wilt and increase yields in watermelon.

Microbial assembly and association network in watermelon rhizosphere after soil fumigation for Fusarium wilt control

Soil fumigation is an effective method to control soil-borne diseases like Fusarium wilt, however the processes and mechanisms driving microbial community reestablishment and pathogen suppression in the rhizosphere after fumigation remain poorly understood. In this study, we examined the dynamics of the rhizosphere microbiome and microbial network associations across different watermelon development stages and plant statuses (i.e. healthy and dead) after soil fumigation and organic fertilizer (OF) application in plastic shelters with Fusarium oxysporum f. sp. niveum (FON) heavily infected. Our results showed that fumigation treatments significantly reduced Fusarium wilt disease incidence and pathogen abundance, meanwhile, decreased soil microbial metabolic activity, fungal biomass and diversity. Bacterial community recovered from fumigation suppression in a short period, while fungal suppression was longer lasting, resulting in decreased fungi to actinomycetes (F/A) and fungi to bacteria (F/B) ratios in PLFA profiles. We further found some bacterial families, such as Actinospicaceae within Actinobacteria, Haliangiaceae, Rhizobiaceae and uncultured Rhodospirillales within Proteobacteria, Sporolactobacillaceae and Limnochordaceae within Firmicutes were greatly enriched after fumigation and might potentially contribute to pathogen suppression. The fumigation treatments significantly reduced microbial network complexity and the percentage of fungal nodes in comparison to un-fumigated control treatment. In contrast, a more complex microbial network was observed in the rhizosphere soil of healthy plants than that in the soil surrounding dead plant roots within fumigation treatments. Furthermore, healthy plant rhizosphere significantly enriched potential beneficial and nitrogen cycle-related bacterial phyla like Gemmatimonadetes, Verrucomicrobia, and Nitrospirae. More interestingly, Fusarium were markedly enriched in the rhizosphere soil of healthy plants and mainly represented by non-FON Fusarium like F. verticillioides and F. solani, implying a potential niche competition between FON and nonpathogenic Fusarium species in the rhizosphere of healthy watermelon. Taken together, our results provide vital information on the reconstruction of microbial communities and potential interactions between plant and its beneficial consortium after fumigation, which is instructive to develop more systematic strategies through targeting both beneficial and pathogen-similar taxa to improve disease control and soil suppression.

Marker Development for Differentiation of Fusarium Oxysporum f. sp. Niveum Race 3 from Races 1 and 2.

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (FON), is pathogenic only to watermelon and has become one of the main limiting factors in watermelon production internationally. Detection methods for this pathogen are limited, with few published molecular assays available to differentiate FON from other formae speciales of F. oxysporum. FON has four known races that vary in virulence but are difficult and costly to differentiate using traditional inoculation methods and only race 2 can be differentiated molecularly. In this study, genomic and chromosomal comparisons facilitated the development of a conventional polymerase chain reaction (PCR) assay that could differentiate race 3 from races 1 and 2, and by using two other published PCR markers in unison with the new marker, the three races could be differentiated. The new PCR marker, FNR3-F/FNR3-R, amplified a 511 bp region on the “pathogenicity chromosome” of the FON genome that is absent in race 3. FNR3-F/FNR3-R detected genomic DNA down to 2.0 pg/µL. This marker, along with two previously published FON markers, was successfully applied to test over 160 pathogenic FON isolates from Florida, Georgia, and South Carolina. Together, these three FON primer sets worked well for differentiating races 1, 2, and 3 of FON. For each marker, a greater proportion (60 to 90%) of molecular results agreed with the traditional bioassay method of race differentiation compared to those that did not. The new PCR marker should be useful to differentiate FON races and improve Fusarium wilt research.

Dazomet application suppressed watermelon wilt by the altered soil microbial community

Fusarium wilt disease causes severe decline of watermelon yield and quality. Researches have been reported that soil fumigation with dazomet can help control crop disease. Firstly, we discovered that the dazomet application suppressed watermelon wilt in field experiment compared to the control group. While the importance of microbial community in regulating plant health has been rising up, we therefore focused on examining the soil microbial diversity at six different sampling times after dazomet application by using Illumina MiSeq platform. Remarkably, our research results showed that some beneficial microbial genera have been altered, and these beneficial microbial genera have dominated the entire community, such as Nitrolancea, Pseudomonas and Penicillium after dazomet application. Instead, the relative abundance of Fusarium genus and the pathogen FON (Fusarium oxysporum f. sp. niveum, FON) had the decreased. As there was a significant accumulation of AP (available soil phosphorus) after dazomet application, we noticed that the beneficial microbes as Bacillus, Nitrolancea, Paenibacillus and Penicillium have significant positive correlation with AP but negatively related to morbidity. Together, these results demonstrate that the altered soil microbial community structure by dazomet application is critical to suppress watermelon Fusarium wilt. Thus, our results will drive investigations aimed to deploy interaction of microbiota contribute and plant immunity.

Soil inoculation of Trichoderma asperellum M45a regulates rhizosphere microbes and triggers watermelon resistance to Fusarium wilt

Fusarium wilt (FW) caused by Fusarium oxysporum f. sp. niveum (FON) is a soil-borne disease that seriously limits watermelon production. In the present study, Trichoderma asperellum (T. asperellum) M45a was shown to be an effective biocontrol agent against FW. In a pot experiment, the application of 105 cfu/g of T. asperellum M45a granules had an improved control effect on FW during the blooming period (up to 67.44%) in soils subjected to five years of continuous cropping with watermelon, while the average length of watermelon vines was also significantly improved (P < 0.05). Additionally, the acid phosphatase (ACP), cellulase (CL), catalase (CAT), and sucrase (SC) activities in the M45a-inoculation group were significantly higher than those in the control (CK) group, and transformation of the soil nutrients (total N, NO3-N, and available P) was significantly increased. Moreover, T. asperellum M45a inoculation reduced fungal diversity, increased bacterial diversity and especially enhanced the relative abundance of plant growth-promoting rhizobacteria (PGPR), such as Trichoderma, Sphingomonas, Pseudomonas, Actinomadura, and Rhodanobacter. Through functional prediction, the relative abundance of ectomycorrhiza, endophytes, animal pathotrophs, and saprotrophs in the fungal community was determined to be significantly lower than that observed in the M45a-treated soil. Correlation analysis revealed that Sphingomonas, Pseudomonas, and Trichoderma had the most differences in terms of microorganism abundance, and these differences were positively correlated with ACP, CL, CAT, and SC. These findings provide guidance for the use of fungicides to achieve microecological control of FW in continuously cropped watermelon plots.

Effect of anti-fungal compound phytosphingosine in wheat root exudates on the rhizosphere soil microbial community of watermelon

The relationship between wheat root exudates and watermelon rhizosphere microbial communities in wheat/watermelon cropping system is well documented now, however, less is known about the activity of allelochemicals in wheat root exudates against watermelon Fusarium oxysporum f. sp. niveum (FON), and how it may influence belowground plant-microbe interactions for disease control. TLC board and UPLC-Q-TOF-MS techniques were used to isolate and analyze the composition of wheat root exudates, and responses of watermelon rhizosphere community against phytosphingosine were analyzed by high-throughput sequencing and quantitative PCR (qPCR). Total of 15 inhibitory compounds were identified in wheat root exudates and most of them were long chain alcohols. Due to highest mass spectral fragment ion matching with already known compounds, we selected phytosphingosine as a model component to study the influence of allelochemical compounds of wheat root exudates on FON and overall rhizosphere microbial community composition of watermelon. Treatment with phytosphingosine increased the diversity of total fungi and bacteria, and altered the compositions of their communities particularly at lower concentration. The abundances and relative abundances of certain potentially plant beneficial bacteria and pathogen-antagonistic fungi were stimulated at lower concentration, while potentially pathogenic fungi including FON were inhibited at different degree. Phytosphingosine inhibited the FON directly, regulated the microbial community structure in watermelon rhizosphere and formed an environment not conducive to pathogen invasion, eventually alleviating the incidence of watermelon Fusarium wilt in wheat/watermelon companion cropping system.

Biocontrol potential of Bacillus subtilis IBFCBF-4 against Fusarium wilt of watermelon

Fusarium wilt of watermelon caused by Fusarium oxysporum f. sp. niveum (FON) is a destructive soil-borne disease. The objective of this study was to screen antagonistic bacteria with potential applications for controlling this disease. Strain IBFCBF-4 with obvious antifungal activity against FON and seven other phytopathogens was successfully isolated from a rhizosphere soil. Based on the morphological, physiological, biochemical characteristics and 16S rDNA sequence analysis, IBFCBF-4 was identified as Bacillus subtilis. Under greenhouse condition, IBFCBF-4 could significantly reduce the incidence of Fusarium wilt of watermelon by 51.1% compared to the control. The studies on the antagonistic mechanism showed that IBFCBF-4 could cause abnormal swelling and increased branching to the hyphae of FON. Meanwhile, the volatile and non-volatile compounds produced by IBFCBF-4 were able to suppress the mycelial growth of FON. Furthermore, PCR analysis confirmed the presence of ituA, ituB, ituC, ituD, bamD, bmyB, fenB, srfAB, surf, bacA, bacD, bacAB, tasA, mrsA and mrsM genes involved in the biosynthesis of iturin A, bacillomycin, fengycin, surfactin, bacilysin, tasA and mersacidin, respectively. Taken together, B. subtilis IBFCBF-4 can be used as a potential biocontrol agent against Fusarium wilt of watermelon, but further work is required to improve its efficacy.

Biological control of Fusarium wilt on watermelon by fluorescent pseudomonads

ABSTRACTFusarium wilt caused by Fusarium oxysporum f. sp. niveum (FON) is a destructive soilborne disease commonly found in watermelon producing areas throughout the world. At present, the control of Fusarium wilt depends heavily on host resistance and chemical fungicide application. In this study, we isolated fluorescent pseudomonads from the rhizosphere of healthy watermelon and evaluated their biocontrol capacity against FON race 2. Biochemical assays indicated that all 14 fluorescent Pseudomonas strains were able to produce indole-3-acetic acid and at least one type of biosurfactants. Both sulphur oxidising and proteolytic activities were exhibited in 2 of the 14 strains. However, none of them could synthesise phenazine antibiotics. Seven strains with different morphological and biochemical characteristics were identified by sequencing the rpoB gene. Among those, one was Pseudomonas resinovorans (WMT16-1-1), two were P. putida (WMC16-1-1 and WMC16-2-5), one was P. fluorescens (WMC16-1-8) and others were Pseudomonas sp. In vitro studies indicated that WMC16-1-1, WMC16-1-8 and WMC16-2-5 inhibited mycelial growth of FON significantly. Detection of biosynthetic loci of 2,4-diacetylphloroglucinol (DAPG), pyrrolnitrin and pyoluteorin using specific primers indicated that the three strains could not produce the antibiotics. In greenhouse studies, WMC16-1-1, WMC16-1-8 and WMC16-2-5 reduced the severity of Fusarium wilt significantly with WMC16-1-1 being the most effective. Inoculation with WMC16-2-5 resulted in the significantly greater weight of both stems and roots compared to the nontreated control, while WMC16-1-1 resulted in greater stem weight. Pseudomonas strains WMC16-1-1, WMC16-1-8 and WMC16-2-5 are potential candidates for controlling Fusarium wilt and promoting the growth of watermelon.

Genetic and phenotypic diversity of Fusarium oxysporum f. sp. niveum populations from watermelon in the southeastern United States.

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (FON), occurs worldwide and is responsible for substantial yield losses in watermelon-producing areas of the southeastern United States. Management of this disease largely relies on the use of integrated pest management (i.e., fungicides, resistant cultivars, crop rotation, etc.). Knowledge about race structure and genetic diversity of FON in the southeastern US is limited. To determine genetic diversity of the pathogen, FON isolates were collected from symptomatic watermelon plants in commercial fields in Georgia and Florida, USA, and identified based on morphological characteristics and PCR analysis using FON-specific primers. Discriminant analysis of principal components (DAPC) of 99 isolates genotyped with 15 simple sequence repeat (SSR) markers grouped the isolates in eight distinct clusters with two prominent clusters (clusters 1 and 8). Cluster 1 consisted of a total of 14 isolates, out of which 85.7% of the isolates were collected in Florida. However, most of the isolates (92.4%) in cluster 8 were collected in Georgia. Both DAPC and pairwise population differentiation analysis (ФPT) revealed that the genetic groups were closely associated with geographical locations of pathogen collection. Three races of FON (races 0, 2 and 3) were identified in the phenotypic analysis; with race 3 identified for the first time in Georgia. Overall, 5.1%, 38.9% and 55.9% of the isolates were identified as race 0, race 2 and race 3, respectively. The majority of the isolates in cluster 1 and cluster 8 belonged to either race 2 (35.6%) or race 3 (45.8%). Additionally, no relationship between genetic cluster assignment and races of the isolates was observed. The information obtained on genotypic and phenotypic diversity of FON in the southeastern US will help in development of effective disease management programs to combat Fusarium wilt.

Meloidogyne incognita intensifies the severity of Fusarium wilt on watermelon caused by Fusarium oxysporum f. sp. niveum

Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (FON), is a devastating soil-borne disease in watermelon production. Race 1 and race 2 of the pathogen are widely distributed in different watermelon producing regions. To investigate whether the root-knot nematodes break resistance of watermelon genotypes with resistance to FON race 1 and race 2, four greenhouse experiments were conducted on two watermelon cultivars ‘Calhoun Gray’ and ‘Fascination’ that are resistant to race 1, and an accession PI 296 341-FR that is resistant to race 2. The treatments included seedlings of ‘Calhoun Gray’ and ‘Fascination’ inoculated with Meloidogyne incognita (1000 J2/plant) or FON race 1 alone, nematodes 5 days before FON, FON 5 days before nematodes, nematodes and FON simultaneously, and non-inoculated control. PI 296 341-FR was inoculated with FON race 2, with or without M. incognita inoculation. The presence of M. incognita enhanced the susceptibility of all watermelon genotypes to Fusarium wilt. Co-inoculation with M. incognita led to an early development of wilt symptoms and increased disease severity. Galls were observed on roots of all nematode-inoculated plants, and sequential inoculation of FON followed by nematodes resulted in numerically lower galling indices compared with other inoculation methods. Whereas inoculation of the nematodes alone did not reduce plant growth, growth suppression was evident when seedlings were inoculated with both pathogens. The results indicated that M. incognita could enhance susceptibility of resistant watermelon genotypes to respective FON races, and host resistance alone is not sufficient for managing Fusarium wilt on watermelon in soils infested with root-knot nematodes.

Soil fumigation with ammonium bicarbonate or metam sodium under high temperature alleviates continuous cropping-induced Fusarium wilt in watermelon

The induction of Fusarium wilt by continuous cropping has become a severe problem for watermelon cultivation around the world. Here, the feasibility and underlying mechanism of soil fumigation with ammonium bicarbonate (AB) or metam sodium (MS) under high temperature (HT) were investigated for the control of Fusarium wilt during watermelon growth on continuously cropped soils. Both AB + HT and MS + HT decreased the density of Fusarium oxysporum f. sp. niveum (FON) in soil, improved the soil properties (i.e. increased the pH, reduced salinity, improved soil nutrients, balanced soil micro-ecology, and increased soil enzyme activities), and consequently decreased the Fusarium wilt incidence and improved the watermelon fruit quality. In vitro, MS treatment stopped the growth of FON on medium, while AB mixed with lime (but not AB) alone inhibited FON growth. AB or MS fumigated soils under HT also decreased Fusarium wilt incidence of watermelon seedlings after artificial FON infection. Consequently, AB + HT and MS + HT can control continuous cropping-induced Fusarium wilt by both inhibiting FON growth and improving the soil environment.

Assay development and marker validation for marker assisted selection of Fusarium oxysporum f. sp. niveum race 1 in watermelon

Fusarium wilt, Fusarium oxysporum f. sp. niveum (FON), of watermelon (Citrullus lanatus) is a fungal pathogen that causes significant yield losses in the US watermelon industry. FON damages watermelon through invasion of the root system and remains a difficult pathogen to manage due to its long-lasting survival spores which persist in the soil. Chemical control options for this pathogen are lacking, making development of genetic resistance the best option. There are four known races of FON (0, 1, 2, and 3) which are distinguished based on their pathogenicity of differential cultivars. Most modern cultivar releases have FON race 1 (FON-1) resistance, which has been mapped on the end of chromosome 1. Application of marker assisted selection (MAS) would improve the efficiency of FON-1 resistance breeding. In order to identify markers for selection in the FON-1 region, the QTL-seq method was utilized on an F2 population segregating for FON-1 resistance. Single nucleotide polymorphism (SNP) markers in the region were developed into Kompetitive allele-specific PCR (KASP™) assays and tested for trait association on the segregating F2:3 population. Marker validation was done using an F2 population from a cross between FON-1 susceptible “New Hampshire Midget” and FON-1-resistant “Calhoun Gray.” Further validation on a panel of susceptible and resistant cultivars and Plant Introductions identified SNP marker UGA1_502161 as a useful marker for selection of FON-1 resistance from Calhoun Gray.

Performance of Grafted Seedless Watermelon Plants with and without Root Excision under Inoculation with Fusarium oxysporum f. sp. niveum Race 2

Fusarium wilt of watermelon can be effectively managed by grafting with resistant rootstocks. Excision and regeneration of grafted seedling roots is a common practice among cucurbit-grafting nurseries that has not been thoroughly examined. The objectives of this study were to compare the performance of grafted and nongrafted watermelon plants under both greenhouse and field conditions when inoculated with Fusarium oxysporum f. sp. niveum (FON) race 2, and assess the effect of root excision on growth of grafted plants with Cucurbita moschata and Cucurbita maxima × C. moschata rootstocks. Two greenhouse experiments (Fall 2015 and Spring 2016) and one field trial (Spring 2016) of seedless watermelon ‘Melody’ were conducted in this study. In both greenhouse experiments, inoculated, nongrafted watermelon plants showed a significantly higher percentage of recovered Fusarium spp. colonies (70% to 75%) compared with grafted treatments (0% to 7.5%). Some plant growth measurements, including the longest vine length and aboveground fresh and dry weight, indicated less vigorous growth for nongrafted plants compared with the grafted treatments. Significantly higher percent recovery of Fusarium spp. below the graft union was observed in the grafted plants with root excision and regeneration treatment (3.7%) in contrast to the intact root treatment (0.5%), suggesting that the root excision method may possibly create entry points for FON infections. Overall, the root excision treatment showed little influence on aboveground growth and root characteristics of grafted plants. Yield of grafted watermelon with FON inoculation in the fumigated field trial was significantly higher than that of noninoculated, nongrafted ‘Melody’ (NGM) control as reflected by the increase of fruit number and size. Averaged over all the grafted treatments, the increase in marketable fruit number and weight reached 108.3% and 240.9%, respectively, and the total fruit number and weight increase was at 80.0% and 237.2%, respectively. However, grafted plants also exhibited greater levels of root-knot nematode infestation as indicated by the significantly higher root galling ratings. Results from this study demonstrated that grafting with squash rootstocks can effectively limit FON colonization in seedless watermelon plants, although more research in rootstock selection and testing is needed to optimize the use of grafted plants for improving plant growth and fruit yield.

Antifungal Activity and Action Mode of Cuminic Acid from the Seeds of Cuminum cyminum L. against Fusarium oxysporum f. sp. Niveum (FON) Causing Fusarium Wilt on Watermelon.

In order to develop a novel biofungicide, the antifungal activity and action mode of cuminic acid from the seed of Cuminum cyminum L. against Fusarium oxysporum f. sp. niveum (FON) on watermelon was determined systematically. In this study, the median effective concentration (EC50) value for cuminic acid in inhibiting mycelial growth of FON was 22.53 μg/mL. After treatment with cuminic acid, the mycelial morphology was seriously influenced; cell membrane permeability and glycerol content were increased markedly, but pigment and mycotoxin (mainly fusaric acid) were significantly decreased. Synthesis genes of bikaverin (Bike1, Bike2 and Bike3) and fusaric acid (FUB1, FUB2, FUB3 and FUB4) both were downregulated compared with the control, as confirmed by quantitative RT-PCR. In greenhouse experiments, cuminic acid at all concentrations displayed significant bioactivities against FON. Importantly, significant enhancement of activities of SOD, POD, CAT and decrease of MDA content were observed after in vivo cuminic acid treatment on watermelon leaves. These indicated that cuminic acid not only showed high antifungal activity, but also could enhance the self-defense system of the host plant. Above all, cuminic acid showed the potential as a biofungicide to control FON.

Inhibitory effect and mechanism of Tagetes erecta L. fungicide on Fusarium oxysporum f. sp. niveum

Botanical fungicides comprise attractive alternatives to chemical fungicides because of their environmental compatibility. Flavonoids extracted from Tagetes erecta L. have an inhibitory effect on fusarium wilt in watermelons caused by Fusarium oxysporum f. sp. niveum (FON). In this study, we synthesized one of these flavonoids, 2,5-dicyclopentylidene cyclopentanone (Tagetes erecta L. fungicide (TEF)) and assessed its activity against FON. In vitro, TEF inhibited FON growth and killed FON cells directly. TEF also affected FON cell physiology and mycelial structure. In watermelon plants with fusarium wilt, TEF protected the leaf cell structure and improved the germination rate of infected seeds while increasing overall plant resistance. A TEF-resistant mutant (FONM) was created by chemical mutagenesis. FON and FONM were analysed using iTRAQ and RNA-Seq, which identified 422 differentially expressed proteins and 7817 differentially expressed mRNAs in the proteome and transcriptome, respectively. The FONM mutations caused changes in the cell membrane and cell wall, which may constitute the site of action of TEF. Together, these results demonstrate that TEF could effectively control the watermelon fusarium wilt caused by FON, possibly through the inhibition of sterol biosynthesis. The data presented here suggest that TEF represents a new potential botanical anti-fungal drug.

Antagonistic Activity of Pseudomonas Fluorescens Against Fusarium Oxysporum f. sp. Nievum Isolated from Soil Samples in Palestine

Watermelon is an important summer crop in Palestine, for several decades filling the needs of local market and some Arab countries. The yield of watermelon decreased dramatically in recent years due to severe infections with the soil borne fungal pathogen Fusarium oxysporum f. sp. niveum (FON). Soil fumigation with methyl bromide was commonly applied by Palestinian farmers until it was recently legally banned. Different control mechanisms were not feasible to overcome problems caused by the disease resulting in decreased watermelon cultivation in Palestine for the past 30 years. In this work, we have experimentally shown that Pseudomonas fluorescens was efficient in controlling FON infection and allowing normal seedling growth of both the root and shoot systems. Field experiments are necessary to further confirm the efficacy of biocontrol application.

A Genetic Locus Associated with Resistance to Fusarium oxysporum f. sp. niveum Race 2 in Citrullus lanatus-type Watermelon

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.

Yield and Quality of Grafted Watermelon Grown in a Field Naturally Infested with Fusarium Wilt

Grafting of seedlings is a technique used for watermelon (Citrullus lanatus) production in many countries. Because of higher costs involved, the use of grafted seedlings can only be recommended if it provides clear biological and economic benefits. Since rootstock performance is influenced by compatibility with the cultivar, the existing disease pressure, and climatic conditions, it is necessary to evaluate rootstocks with current cultivars to appraise possible benefits in a given area. Two experiments were carried out in two consecutive seasons with the objective of evaluating the benefits of grafting under Chilean conditions. The rootstocks used were ‘Marathon’ (Cucurbita maxima × Cucurbita moschata) and ‘Macis’ (Lagenaria siceraria) with different scions, including some seedless cultivars. In both experiments, grafted plants increased their yield compared with nongrafted plants (136% and 159% in Expts. 1 and 2, respectively). This effect was due to an increased number of fruit per plant (P &lt; 0.01), and the weight gain of the fruit (P &lt; 0.01). Plants presented with fusarium wilt [Fusarium oxysporum f. sp. niveum (FON)] in both experiments, which seemed to be the main limitation for nongrafted plant production. In the evaluation of quality attributes [soluble solid concentration (SCC), firmness, color, polar diameter, equatorial diameter, and rind thickness], positive effects were observed in the fruit of grafted plants. For the conditions of these experiments, the increase in yield of grafted plants would be associated with an economic benefit that exceeds its additional cost.

Genetic analysis and chromosome mapping of resistance to Fusarium oxysporum f. sp. niveum (FON) race 1 and race 2 in watermelon (Citrullus lanatus L.).

Fusarium wilt (FW) caused by Fusarium oxysporum f. sp. niveum (FON) is the major soilborne disease of watermelon (Citrullus lanatus L.). The development and deployment of resistant cultivars is generally considered to be an effective approach to control FW. In this study, an F8 population consisting of 103 recombinant inbred lines derived from a cross between the cultivar 97103 and a wild accession PI 296341-FR was used for FON race 1 and race 2 fungal inoculations. One major QTL on chromosome 1 for FON race 1 resistance was detected with a logarithm of odds of 13.2 and explained phenotypic variation R2 = 48.1 %; two QTLs of FON race 2 resistance on chromosomes 9 and 10 were discovered based on the high-density integrated genetic map we constructed. The nearest molecular marker should be useful for marker-assisted selection of FON race 1 and race 2 resistance. One receptor kinase, one glucan endo-1,3-β-glucosidase precursors and three acidic chitinase located in the FON-1 QTL genomic region. In Qfon2.1 QTL region, one lipoxygenase gene, five receptor-like kinases and four glutathione S-transferase genes are discovered. One arginine biosynthesis bifunctional protein, two receptor kinase proteins and one lipid-transfer protein located in Qfon2.2 QTL region. Based on SNP analysis by using 20 re-sequenced accessions of watermelon and 231-plant F2 population generated from Black Diamond × Calhoun Grey, we developed a SNP marker Chr1SNP_502124 for FON-1 detection.Electronic supplementary materialThe online version of this article (doi:10.1007/s11032-015-0375-5) contains supplementary material, which is available to authorized users.

Characterization of the watermelon seedling infection process by Fusarium oxysporum f. sp. niveum

Fusarium oxysporum f. sp. niveum (FON) is the causal pathogen of fusarium wilt in watermelon. Understanding the interactions of watermelon seedlings and FON is of scientific and practical importance for disease control. The colonization dynamics of a susceptible watermelon cultivar, Sumi 1 (SM 1), were visualized using a green fluorescent protein (GFP)‐expressing strain of FON race 1 (FON 1‐GFP). Within 2 days post‐inoculation (dpi), hyphae adhered to and grew on epidermal cells. At 3 dpi, hyphae crossed the epidermis, penetrated into the cortex and progressed through parenchymal cell borders reaching the xylem vessels at 7 dpi. Growth of hyphae was restricted to the xylem vessels even in the colonized lower hypocotyl. The responses of SM 1 seedlings to infection were analysed. In the roots of SM 1, infected xylem vessels were plugged with gums and tyloses and some occluded vessels were shed from the root vascular tissue. Parenchyma cells surrounding the vascular cylinder showed wall thickening and cell disintegration, which eventually formed cavities. Hypocotyls of most SM 1 plants were also infected. Vascular tissues were densely colonized by hyphae until the vascular bundle became hollow and plants withered. Moreover, the expression patterns of six defence‐related genes in roots of SM 1 were analysed using real‐time quantitative PCR. Transcript levels of plant defensin‐like genes ClPDF2.1 and ClPDF2.4, phenylalanine ammonia lyase (PAL), chitinase (CHI) and ascorbate peroxidase (APX) were significantly induced in SM 1 roots during FON 1 infection, while polyphenoloxidase (PPO) did not show significant responses to FON 1 infection.