Managing Fusarium Wilt of Watermelon with Delayed Transplanting and Cultivar Resistance

Global characteristics and trends of researches on watermelon: Based on bibliometric and visualized analysis

Watermelon is an important horticultural plant. A bibliometric analysis of the watermelon literature was carried out in order to analyze the research state, hotspots, and trends, as well as to highlight the overall watermelon research development from a holistic viewpoint. The summary of watermelon research is given via metrological analysis based on a set of indices using a newly built Bibliometrix R-package tool. This study gathered 6,632 documents indexed in the Core Collection of Web of Science (WoS) in the domain of watermelon from 1992 to 2022 using bibliometrix. The results indicated that the number of published articles showed an apparently upward trend. The United States was in the first place, with Plant Disease being the most productive journal. Levi A from the United States Department of Agriculture-Agricultural Research Service is the most prolific author, and Levi A is the most cited; The most frequently used keywords by authors are "growth", "resistance", "identification", "yield", "quality" "plants", "watermelon stomach" and "expression"; The most talked-about issues in this subject are resistance, yield, and quality, which highlight the crucial research areas. To effectively comprehend the turning moments for future research, it is useful to monitor the hotspots and frontiers of watermelon studies. The results highlight the future paths for study in the field of watermelon and provide useful information for researchers interested in the topic.

Differential Susceptibility of Two Citrullus amarus Pollenizer Watermelons to Five Species of Pythium and Globisporangium

Pollenizer watermelon (Citrullus amarus) must be planted alongside triploid watermelon cultivars to provide a source of pollen. Early season death of pollenizer watermelon cultivars SP-6 and SP-7 was observed in a research field in Charleston County, SC, in April 2022, 3 to 4 weeks after transplanting. Disease incidence was 15 and 12% for the two cultivars, respectively. Two species of Pythium (P. myriotylum and P. aphanidermatum) and three species of Globisporangium (G. ultimum, G. irregulare, and G. spinosum) were recovered from 13 of 17 plants cultured and identified based on the sequences of the cytochrome c oxidase subunit I gene. Pythium spp. were recovered from both cultivars, while Globisporangium spp. were recovered only from 'SP-7'. In pathogenicity tests, 'SP-7' had lower area under the disease progress curve (AUDPC) values than 'SP-6' with one isolate each of four species and lower AUDPC values than seedless watermelon 'Estrella' with one isolate each of two species. Mean AUDPC values did not differ between 'Estrella' and 'SP-6' with any isolate. AUDPC values were greater with G. ultimum on all three cultivars than with one isolate of G. spinosum and both isolates of P. myriotylum and P. aphanidermatum. Susceptibility of the new cultivar SP-7 to Pythium and Globisporangium spp. differs from susceptibility of 'SP-6'. These results will help growers choose which pollenizer cultivar to plant.

Micronutrients Affect Expression of Induced Resistance Genes in Hydroponically Grown Watermelon against Fusarium oxysporum f. sp. niveum and Meloidogyne incognita

The soil-borne pathogens, particularly Fusarium oxysporum f. sp. niveum (FON) and southern root-knot nematode (RKN, Meloidogyne incognita) are the major threats to watermelon production in the southeastern United States. The role of soil micronutrients on induced resistance (IR) to plant diseases is well-documented in soil-based media. However, soil-based media do not allow us to determine the contribution of individual micronutrients in the induction of IR. In this manuscript, we utilized hydroponics-medium to assess the effect of controlled application of micronutrients, including iron (Fe), manganese (Mn), and zinc (Zn) on the expression of important IR genes (PR1, PR5, and NPR1 from salicylic acid (SA) pathway, and VSP, PDF, and LOX genes from jasmonic acid (JA) pathway) in watermelon seedlings upon inoculation with either FON or RKN or both. A subset of micronutrient-treated plants was inoculated (on the eighth day of micronutrient application) with FON and RKN (single or mixed inoculation). The expression of the IR genes in treated and control samples was evaluated using qRT-PCR. Although, significant phenotypic differences were not observed with respect to the severity of wilt symptoms or RKN galling with any of the micronutrient treatments within the 30-day experimental period, differences in the induction of IR genes were considerably noticeable. However, the level of gene expression varied with sampling period, type and concentration of micronutrients applied, and pathogen inoculation. In the absence of pathogens, micronutrient applications on the seventh day, in general, downregulated the expression of the majority of the IR genes. However, pathogen inoculation preferentially either up- or down-regulated the expression levels of the IR genes at three days post-inoculation depending on the type and concentration of micronutrients. The results demonstrated here indicate that micronutrients in watermelon may potentially make watermelon plants susceptible to infection by FON and RKN. However, upon infection the IR genes are significantly up-regulated that they may potentially aid the prevention of further infection via SA- and JA-pathways. This is the first demonstration of the impact of micronutrients affecting IR in watermelon against FON and RKN infection.

Fermentation: An Unreliable Seed Treatment for Bacterial Fruit Blotch of Watermelon

Acidovorax citrulli is a seedborne pathogen that causes bacterial fruit blotch (BFB), a global threat to watermelon production. Treating watermelon seeds to eliminate A. citrulli is a critical component of BFB management, and several strategies have been evaluated to mitigate the impact of the disease. In China, watermelon seed producers routinely incubate seeds in watermelon juice (fermentation) to reduce the risk of seed infection by A. citrulli and seedling transmission of BFB. However, there has been limited effort to evaluate the efficacy of fermentation in mitigating A. citrulli seed infection. The current study showed that fermented watermelon fruit juice could inhibit A. citrulli population growth and demonstrated that the low pH conditions, not the temperature dynamic, generated during fermentation might play a major role in A. citrulli growth inhibition and could induce the viable but nonculturable (VBNC) state in A. citrulli. We developed an effective method that was based on propidium monoazide PCR to detect viable A. citrulli cells under low pH conditions or in fermented watermelon fruit juice. We also provided evidence that VBNC A. citrulli cells induced by fermented watermelon fruit juice could not be resuscitated and did not retain their virulence on watermelon seedlings. However, VBNC A. citrulli cells could be resuscitated in Luria-Bertani medium. Based on these observations, we conclude that fermentation in watermelon fruit juice may not be an effective seed treatment for BFB because it may increase the seed infection by A. citrulli.

Phylogenetic and phenotypic characterization of Fusarium oxysporum f. sp. niveum isolates from Florida-grown watermelon

Fusarium wilt of watermelon (Citrullus lanatus) caused by Fusarium oxysporum f. sp. niveum (Fon), has become an increasing concern of farmers in the southeastern USA, especially in Florida. Management of this disease, most often through the use of resistant cultivars and crop rotation, requires an accurate understanding of an area’s pathogen population structure and phenotypic characteristics. This study improved the understanding of the state’s pathogen population by completing multilocus sequence analysis (MLSA) of two housekeeping genes (BT and TEF) and two loci (ITS and IGS), aggressiveness and race-determining bioassays on 72 isolates collected between 2011 and 2015 from major watermelon production areas in North, Central, and South Florida. Multilocus sequence analysis (MLSA) failed to group race 3 isolates into a single large clade; moreover, clade membership was not apparently correlated with aggressiveness (which varied both within and between clades), and only slightly with sampling location. The failure of multilocus sequence analysis using four highly conserved housekeeping genes and loci to clearly group and delineate known Fon races provides justification for future whole genome sequencing efforts whose more robust genomic comparisons will provide higher resolution of intra-species genetic distinctions. Consequently, these results suggest that identification of Fon isolates by race determination alone may fail to detect economically important phenotypic characteristics such as aggressiveness leading to inaccurate risk assessment.

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.

Pathogen Infection and Host-Resistance Interactively Affect Root-Associated Fungal Communities in Watermelon

Interactions of pathogen infection, host plant resistance, and fungal communities are poorly understood. Although the use of resistant watermelon cultivars is an effective control measure of watermelon wilt disease, fungal communities may also have significant effects on the development of the soil-borne pathogen complexes. We characterized the root and rhizosphere fungal communities associated with healthy and diseased watermelons of three different cultivars with different susceptibilities toward wilt disease by paired-end Illumina MiSeq sequencing. Thirty watermelon plants including highly wilt-resistant, moderately resistant, and susceptible cultivars were collected from a greenhouse, half of which showing clear wilt symptoms and the other half with no symptoms. Patterns of watermelon wilt disease and the response of the fungal communities varied among the three cultivars. The amount of the pathogen Fusarium oxysporum f. sp. niveum was higher in diseased root and rhizosphere samples, particularly in the susceptible cultivar, and was significantly positively correlated with the disease index of Fusarium wilt. Plant health had significant effects on root-associated fungal communities, whereas only the highly resistant cultivar had significant effects only on the rhizosphere fungal communities. Co-occurrence networks revealed a higher complexity of fungal communities in the symptom-free roots compared to diseased roots. In addition, networks from roots of the highly resistant plants showing symptoms had a higher complexity compared to the susceptible cultivars. Keystone species were identified for the plants with different symptom severity and the different cultivars in the root and rhizosphere, such as Fusarium oxysporum, Monosporascus cannonballus, and Mortierella alpina. Overall, the most important factor determining fungal communities in the roots was plant symptom severity, whereas in the rhizosphere, plant genotype was the most important factor determining fungal communities.

Characterization of Pythium Species Collected from a Multiple Time-Point Sampling of Cucurbits in South Carolina

Species of Pythium cause root and stem rot in cucurbits, but no formal surveys have been conducted in the United States to identify which species are responsible. The cucurbit hosts bottle gourd, cucumber, Hubbard squash, and watermelon were transplanted in May, July, September, and November into sentinel plots in four and five different fields in 2017 and 2018, respectively, in South Carolina. Eight of the nine fields were replanted in March 2019. Isolates (600) were collected and identified by sequencing DNA of the mitochondrial cytochrome oxidase I region. The four most common species were P. spinosum (45.6% of all isolates), P. myriotylum (20.0%), P. irregulare (15.3%), and P. aphanidermatum (12.8%). P. myriotylum and P. aphanidermatum were predominantly isolated in May, July, and September, whereas P. spinosum and P. irregulare were predominantly isolated in November and March. Isolates of P. ultimum, P. irregulare, and P. spinosum were more virulent than isolates of P. myriotylum and P. aphanidermatum at 25°C. Representative isolates were screened in vitro for sensitivity to three fungicides: mefenoxam, propamocarb, and oxathiapiprolin. All isolates were sensitive to mefenoxam and propamocarb, but these same isolates were insensitive to oxathiapiprolin, except those classified taxonomically in Pythium clade I.

Identifying Races of Fusarium oxysporum f. sp. niveum in South Carolina Recovered From Watermelon Seedlings, Plants, and Field Soil

Fusarium wilt of watermelon (Citrullus lanatus), caused by the soilborne fungus Fusarium oxysporum f. sp. niveum, is the most serious disease of watermelon in South Carolina and other southeastern U.S. states. Isolates of F. oxysporum collected from field-grown plants, greenhouse-grown seedlings, and field soil between 1999 and 2018 were inoculated onto three differential watermelon cultivars to identify races. Of 197 isolates obtained from plants, 12% were nonpathogenic, 2% were race 0, 23% were race 1, and 63% were race 2. One collection of isolates from greenhouse seedlings was exclusively race 1 and the other was exclusively race 2. Seventeen of 81 soil isolates were pathogenic: five were race 1 and 12 were race 2. Reactions of C. amarus PI 296341-FR, Carolina Strongback, and SP-6, cultigens with resistance to race 2, did not differ significantly among five highly virulent race 2 isolates and a standard race 2 isolate, indicating a lack of a race 3 phenotype. Forma specialis-specific primers matched phenotypic race identification for 74% of the isolates. Race-specific primers based on a secreted-in-xylem elicitor present in race 0 and 1 isolates matched phenotypic race identification for 66% of the isolates. Because a majority of the F. oxysporum f. sp. niveum isolates from South Carolina were race 2, integrated management practices should be used until commercial cultivars with resistance to race 2 are available.

Cucurbit Rootstocks Resistant to Fusarium oxysporum f. sp. niveum Remain Resistant When Coinfected by Meloidogyne incognita in the Field

Interspecific hybrid squash (Cucurbita maxima × Cucurbita moschata) rootstocks used to graft watermelon (Citrullus lanatus var. lanatus) are resistant to Fusarium oxysporum f. sp. niveum, the fungus that causes Fusarium wilt of watermelon, but they are susceptible to Meloidogyne incognita, the southern root knot nematode. A new citron (Citrullus amarus) rootstock cultivar Carolina Strongback is resistant to F. oxysporum f. sp. niveum and M. incognita. The objective of this study was to determine if an interaction between M. incognita and F. oxysporum f. sp. niveum race 2 occurred on grafted or nongrafted triploid watermelon susceptible to F. oxysporum f. sp. niveum race 2. In 2016 and 2018, plants of nongrafted cultivar Fascination and Fascination grafted onto Carolina Strongback and interspecific hybrid squash cultivar Carnivor were inoculated or not inoculated with M. incognita before transplanting into field plots infested or not infested with F. oxysporum f. sp. niveum race 2. Incidence of Fusarium wilt and area under the disease progress curve did not differ when hosts were inoculated with F. oxysporum f. sp. niveum alone or F. oxysporum f. sp. niveum and M. incognita together. Fusarium wilt was greater on nongrafted watermelon (78% mean incidence) than on both grafted rootstocks and lower on Carnivor (1% incidence) than on Carolina Strongback (12% incidence; P ≤ 0.01). Plants not inoculated with F. oxysporum f. sp. niveum did not wilt. At the end of the season, Carnivor had a greater percentage of the root system galled than the other two hosts, whereas galling did not differ on Fascination and Carolina Strongback. F. oxysporum f. sp. niveum reduced marketable weight of nongrafted Fascination with and without coinoculation with M. incognita. M. incognita reduced marketable weight of Fascination grafted onto Carnivor compared with noninoculated, nongrafted Fascination. In conclusion, cucurbit rootstocks that are susceptible and resistant to M. incognita retain resistance to F. oxysporum f. sp. niveum when they are coinfected with M. incognita.