Progressive metabolic impairment underlies the novel nematicidal action of fluensulfone on the potato cyst nematode Globodera pallida

Transcriptomic and physiological analysis of the effect of octanoic acid on Meloidogyne incognita

Root knot nematodes are the most devastating root pathogens, causing severe damage and serious economic losses to agriculture worldwide. Octanoic acid has been reported as one of the nematicides, and its mode of action is not fully understood. The main objective of this study was to elucidate the effect of octanoic acid on Meloidogyne incognita by transcriptomic analysis combined with physiological and biochemical assays. In the toxicity assays with octanoic acid, the threshold concentration with nematicidal activity and the maximum concentration to which nematodes could respond were 0.03 μL/mL and 0.08 μL/mL respectively. Microscopic observation combined with protein and carbohydrates assays confirmed that the structure of the second-stage juveniles (J2s) was severely disrupted after 72 h of immersion in octanoic acid. Transcriptome analysis has shown that octanoic acid can interfere with the nematode energy metabolism, lifespan and signaling. Although the effects are multifaceted, the findings strongly point to the cuticle, lysosomes, and extracellular regions and spaces as the primary targets for octanoic acid. In addition, nematodes can withstand the negative effects of low concentration of octanoic acid to some extent by up-regulating the defense enzyme system and heterologous metabolic pathways. These findings will help us to explore the nematicidal mechanism of octanoic acid and provide important target genes for the development of new nematicides in the future.

Attraction of second-stage juveniles of Meloidogyne species to fluopyram

BACKGROUND

Several benzenoid aromatic compounds were found to attract second-stage juveniles (J2) of Meloidogyne species in previous studies. Here, the attraction of Meloidogyne J2 to the nematicides fluopyram and fluensulfone, with and without aromatic attractants, was evaluated on agar plates and in sand.

RESULTS

Fluensulfone mixed with 2-methoxybenzaldehyde, carvacrol, trans-cinnamic acid, and 2-methoxycinnamaldehyde, attracted Meloidogyne javanica J2 on an agar plate, whereas fluensulfone alone did not. In contrast, fluopyram alone attracted J2 of M. javanica, Meloidogyne hapla, and Meloidogyne marylandi, although higher numbers of M. javanica J2 were attracted to the nematicide with the aromatic compounds. Trap tubes loaded with 1 and 2 μg fluopyram attracted M. javanica, Meloidogyne incognita, M. hapla, and M. marylandi J2 in the sand. Fluopyram-treated tubes attracted 4.4-6.3 times higher numbers of M. javanica and M. marylandi J2 than fluensulfone. Potassium nitrate (KNO₃ ), a Meloidogyne J2 repellent, did not abolish fluopyram's attractiveness to M. marylandi. These results indicate that high numbers of Meloidogyne J2 near fluopyram on an agar plate or in sand are caused by the attractiveness of the nematicide and not by the accumulation of dead J2 after their random encounter with the nematicide.

CONCLUSION

Aromatic attractants have the potential to attract Meloidogyne J2 to nematicides; however, fluopyram itself was attractive to Meloidogyne J2. The attractiveness of fluopyram to Meloidogyne J2 might contribute to its high control efficacy, and elucidation of the attraction mechanism could be useful for nematode-control strategies. © 2023 Society of Chemical Industry.

Belonolaimus longicaudatus management using metam potassium and fluensulfone in potato

Belonolaimus longicaudatus (sting nematode) is an important pest in Florida potato production and is managed primarily by fumigation using 1,3-dichloropropene (1,3-D). Other effective nematicides are needed for more flexibility in managing this pest. The objective of this study was to evaluate fluensulfone, metam potassium, and mixtures of the two products, relative to 1,3-D and untreated control, for efficacy at managing sting nematode, and for non-target effects on free-living nematodes in potato. To test this objective, a small-plot field experiment was conducted in northeast Florida in 2020 and repeated in 2021. Metam potassium fumigation (390 kg a.i./treated ha)-with or without fluensulfone-managed sting nematode soil abundances but was phytotoxic to potato. Strategies that mitigate metam potassium phytotoxicity, such as reduced application rates, are needed before efficacy of metam potassium in this system can be determined. As a preplant soil spray, fluensulfone alone (403 g a.i./treated ha) did not manage sting nematode abundances and had an inconsistent effect on yield. Fumigation with 1,3-D (88.3 kg a.i./treated ha) was the only treatment that consistently managed sting nematode and increased potato yield. Nematicides did not consistently affect free-living nematodes.

Effect of fumigants and non-fumigants on nematode and weed control, crop yield, and soil microbial diversity and predicted functionality in a strawberry production system

Fumigants are commonly used to control soil-borne pathogens of high-value crops, but they may also impact non-target soil microorganisms. Increasing interest in the use of sustainable management practices to control plant- and root-parasitic nematodes has resulted in the formulation of non-fumigant nematicides (chemicals or bionematicides) which are considered environmentally friendly alternatives to fumigants. However, the impact of these new products compared to standard fumigants on soil-borne pathogens, plant production, and the diversity and composition of non-target microbial communities in all crops remains unclear. To begin to address this knowledge gap, we examined the effect of fumigants commonly used in Florida (United States) strawberry production and newly formulated non-fumigant nematicides on nematode and weed control, plant growth, crop yield, and bacterial and fungal community diversity and predicted functionality. We found the standard fumigants increased crop yields and reduced weed pressure more than non-fumigants. Both fumigants and non-fumigants were an efficient management strategy to control sting nematodes. Treatments also impacted the abundance of specific beneficial and antagonistic taxa. Both fumigants and non-fumigants reduced soil bacterial and fungal diversity, an effect that remained for six months, thus suggesting a potential residual impact of these products on soil microorganisms. However, only fumigants altered soil microbial community composition and reduced network complexity, inducing a decrease or even a loss of some predicted bacterial and fungal functions, particularly during the first weeks after fumigation. Nevertheless, soil collected at the end of the season showed significant levels of root-knot nematode suppression in a growth chamber experiment, irrespective of the previous treatment. By linking the effect of fumigants and non-fumigants on soil-borne pests, plant and production, and the soil microbiome, this study increases our knowledge regarding the environmental impact of these products.

Transcriptional changes of biochemical pathways in Meloidogyne incognita in response to non-fumigant nematicides

Meloidogyne incognita is a destructive and economically important agricultural pest. Similar to other plant-parasitic nematodes, management of M. incognita relies heavily on chemical controls. As old, broad spectrum, and toxic nematicides leave the market, replacements have entered including fluensulfone, fluazaindolizine, and fluopyram that are plant-parasitic nematode specific in target and less toxic to applicators. However, there is limited research into their modes-of-action and other off-target cellular effects caused by these nematicides in plant-parasitic nematodes. This study aimed to broaden the knowledge about these new nematicides by examining the transcriptional changes in M. incognita second-stage juveniles (J2) after 24-h exposure to fluensulfone, fluazaindolizine, and fluopyram as well as oxamyl, an older non-fumigant nematicide. Total RNA was extracted and sequenced using Illumina HiSeq to investigate transcriptional changes in the citric acid cycle, the glyoxylate pathway, [Formula: see text]-fatty acid oxidation pathway, oxidative phosphorylation, and acetylcholine neuron components. Observed transcriptional changes in M. incognita exposed to fluopyram and oxamyl corresponded to their respective modes-of-action. Potential targets for fluensulfone and fluazaindolizine were identified in the [Formula: see text]-fatty acid oxidation pathway and 2-oxoglutarate dehydrogenase of the citric acid cycle, respectively. This study provides a foundation for understanding how potential nematicide resistance could develop, identifies cellular pathways as potential nematicide targets, and determines targets for confirming unknown modes-of-action.

Role of Indole Derivatives in Agrochemistry: Synthesis and Future Insights

Abstract:

Heterocycles constitute a wider class of organic compounds which contribute significantly in every facet of pure and applied chemistry. Indole, one of the bicyclic heterocyclic compounds containing nitrogen atom, witnessed unparalleled biological activity such as antiviral, antibacterial, anticancer, anti-depressant and antifungal activities. Different biological activities exhibited by indole derivatives provide the impulsion to explore its activity against anti-phytopathogenic microbes to save the plants from pests and disease, as food security will once again become a rigid demand. This review mainly focuses on various methods related to the synthesis of indole derivatives and its role in agriculture.

[New genetic tools for plant defense against parasitic nematodes]

Нематоды относятся к числу значимых вредителей сельскохозяйственных растений. В обзоре рассмотрены последние данные о молекулярных механизмах устойчивости растений к цистообразующим и галловым нематодам, среди которых одни из наиболее вредоносных видов: Globodera rostochiensis, G. pallida, Heterodera schachtii, Meloidogyne chitwoodi и M. incognita. Например, золотистая картофельная нематода G. rostochiensis, зарегистрированная в 61 субъекте РФ на общей площади 1.8 млн га, способна приводить к потере от 19 до 90 % урожая картофеля. Биологические особенности нематод затрудняют разработку агротехнических способов борьбы с ними: цисты G. rostochiensis сохраняют жизнеспособность в почве в течение многих лет, нематициды токсичны или малоэффективны, поэтому предпочтительным методом борьбы с ними является интрогрессия генов устойчивости от родственных культурных и дикорастущих видов. Стратегия жизненного цикла цистообразующих и галловых нематод основана на способности личинок проникать в корни восприимчивых видов растений, репрограммировать клетки растения-хозяина, формирующие гигантские клетки или синцитии в качестве питающих структур, а также ингибировать иммунный ответ. Молекулярные механизмы, лежащие в основе такого взаимодействия в системе «патоген–хозяин», вызывают значительный интерес как с точки зрения управления морфогенезом растений, так и в аспекте разработки безопасных и эффективных способов борьбы с паразитическими нематодами. В обзоре рассмотрены данные об эффекторах, с помощью которых разные виды нематод контролируют иммунный ответ растения-хозяина, а также гены устойчивости (R-гены) и некоторые молекулярные механизмы, прерывающие формирование питающих структур и развитие паразита. Приведены новые данные о способах генетического контроля, основанных на одном из активно обсуждаемых в последнее время варианте механизма РНК-интерференции – HIGS (host induced gene silencing), представляющем собой адресное выключение экспрессии гена-мишени в клетках личинки нематоды с помощью специфических двуцепочечных РНК, синтезирующихся в клетках растения-хозяина. Индукция РНК-интерференции в клетках растений приводит к появлению молекул-медиаторов, способных инициировать аналогичный процесс в клетках фитофагов, взаимодействующих с растением, в том числе у личинок нематод. Описаны случаи, в которых такое адресное выключение экспрессии генов-мишеней приводило к нарушениям развития личинок и высокому уровню защиты сельскохозяйственных растений от наиболее опасных видов нематод.

Effect of fluensulfone on different functional genes of root-knot nematode Meloidogyne incognita

Meloidogyne incognita is an obligate plant-parasitic nematode causing serious damage to agricultural crops. Major constraints in nematode management arose due to the limited availability of non-fumigant nematicides in conjunction with the considerable ill effects of fumigants on human and non-target organisms. Recently, fluensulfone has been reported to be an effective non-fumigant nematicide against plant-parasitic nematodes and the model nematode Caenorhabditis elegans. The nematicidal efficacy varies according to its concentration at the time of application, exposure timing, nematode species variability, and even across subpopulations within the same species. It interferes with the key physiological processes of nematodes, like motility, behavior, chemosensation, stylet thrusting, infectivity, metabolism, lipid consumption, tissue integrity, oviposition, egg hatching, and survival. However, the molecular basis of these multivariate physiological anomalies is still largely unknown. Quantitative real-time PCR was carried out to understand the acute transcriptional perturbation of 30 functional genes associated with key physiological and life processes in a M. incognita population, following exposure of 10, 50, and 100 ppm of fluensulfone for 5 and 10 hr. The chemical treatment resulted in significant downregulation of all the neuropeptidergic genes, with concomitant repression of majority of genes related to chemosensation, esophageal gland secretion, parasitism, fatty acid metabolism, and G-protein coupled receptors. Collectively, the parasitism genes were found to be perturbed at highest magnitude, followed by the GPCRs and neuropeptidergic genes. These results establish the wide ranging effect of fluensulfone on various metabolic and physiological pathways of nematode.

Potato Cyst Nematode Egg Viability Assessment and Preparasitic Juvenile Screening Using a Large Particle Flow Cytometer and Sorter

Potato cyst nematode (PCN) cysts consist of heterogenous populations of eggs, juveniles, and eggshells that make manual sorting of individual life stages cumbersome. The number of viable PCN eggs is a major determinant of crop damage. An accurate high-throughput PCN egg viability assay is useful for developing effective management and eradication plans. In this study, we present a method for rapid and precise enumeration and sorting of PCN eggs and juveniles, along with an egg viability assessment by staining eggs with the fluorescent stain, acridine orange, and sorting with the Complex Object Parametric Analyzer and Sorter (COPAS) system, a large particle flow cytometer. Both size sorting and fluorescent sorting capabilities of the COPAS were explored. By using the COPAS, sorting efficiency for eggs and preparasitic second-stage juveniles (J2s) was 97.6 and 97.2%, respectively, with 99% recovery at a flow rate of 15 events/s. Purity of sorted live and dead eggs was 95.5 and 94.1%, respectively. Sorting of J2s by size indicated that 15 to 16.4% of Globodera ellingtonae or G. pallida had an average body length of 436.1 ± 3.4 µm compared with an average size of 512.9 ± 4.4 µm for the majority of the J2 population for both species. A red autofluorescing J2 population was also identified through sorting. Sorting of eggs by flow cytometry did not significantly affect hatching (55.1 ± 1.2 and 53.9 ± 1.6%, respectively, for sorted or nonsorted eggs) or juvenile motility (91.3 ± 1.0 or 90.1 ± 1.1%, respectively), thus confirming that the method does not impair the biological activity of the nematode.

Chemical Nematicides: Recent Research Progress and Outlook

Plant-parasitic nematodes have caused huge economic losses to agriculture worldwide and seriously threaten the sustainable development of modern agriculture. Chemical nematicides are still the most effective means to manage nematodes. However, the long-term use of organophosphorus and carbamate nematicides has led to a lack of field control efficacy and increased nematode resistance. To meet the huge market demand and slow the growth of resistance, new nematicides are needed to enter the market. The rational design and synthesis of new chemical scaffolds to screen for new nematicides is still a difficult task. We reviewed the latest research progress of nematicidal compounds in the past decade, discussed the structure-activity relationship and mechanism of action, and recommended some nematicidal active fragments. It is hoped that this review can update the recent progress on nematicide discoveries and provide new ideas for the design and mechanism of action studies of nematicides.

The mechanism of potato resistance to Globodera rostochiensis: comparison of root transcriptomes of resistant and susceptible Solanum phureja genotypes

BACKGROUND

Globodera rostochiensis belongs to major potato pathogens with a sophisticated mechanism of interaction with roots of the host plants. Resistance of commercial varieties is commonly based on specific R genes introgressed from natural populations of related wild species and from native potato varieties grown in the Andean highlands. Investigation of molecular resistance mechanisms and screening the natural populations for novel R genes are important for both fundamental knowledge on plant pathogen interactions and breeding for durable resistance. Here we exploited the Solanum phureja accessions collected in South America with contrasting resistance to G. rostochiensis.

RESULTS

The infestation of S. phureja with G. rostochiensis juveniles resulted in wounding stress followed by activation of cell division and tissue regeneration processes. Unlike the susceptible S. phureja genotype, the resistant accession reacted by rapid induction of variety of stress response related genes. This chain of molecular events accompanies the hypersensitive response at the juveniles' invasion sites and provides high-level resistance. Transcriptomic analysis also revealed considerable differences between the analyzed S. phureja genotypes and the reference genome.

CONCLUSION

The molecular processes in plant roots associated with changes in gene expression patterns in response to G. rostochiensis infestation and establishment of either resistant or susceptible phenotypes are discussed. De novo transcriptome assembling is considered as an important tool for discovery of novel resistance traits in S. phureja accessions.

Identification and characterisation of serotonin signalling in the potato cyst nematode Globodera pallida reveals new targets for crop protection

Plant parasitic nematodes are microscopic pathogens that invade plant roots and cause extensive damage to crops. We have used a chemical biology approach to define mechanisms underpinning their parasitic behaviour: We discovered that reserpine, a plant alkaloid that inhibits the vesicular monoamine transporter (VMAT), potently impairs the ability of the potato cyst nematode Globodera pallida to enter the host plant root. We show this is due to an inhibition of serotonergic signalling that is essential for activation of the stylet which is used to access the host root. Prompted by this we identified core molecular components of G. pallida serotonin signalling encompassing the target of reserpine, VMAT; the synthetic enzyme for serotonin, tryptophan hydroxylase; the G protein coupled receptor SER-7 and the serotonin-gated chloride channel MOD-1. We cloned each of these molecular components and confirmed their functional identity by complementation of the corresponding C. elegans mutant thus mapping out serotonergic signalling in G. pallida. Complementary approaches testing the effect of chemical inhibitors of each of these signalling elements on discrete sub-behaviours required for parasitism and root invasion reinforce the critical role of serotonin. Thus, targeting the serotonin signalling pathway presents a promising new route to control plant parasitic nematodes.

From Old-Generation to Next-Generation Nematicides

The phaseout of methyl bromide and the ban on, or withdrawal of, other toxic soil fumigants and non-fumigant nematicides belonging to the organophosphate and carbamate groups are leading to changes in nematode-control strategies. Sustainable nematode-control methods are available and preferred, but not always effective enough, especially for cash crops in intensive agriculture. A few non-fumigant nematicides, which have a relatively high control efficacy with a low toxicity to non-target organisms, have been released to the market or are in the process of being registered for use. Fluensulfone, fluopyram, and fluazaindolizine are the three main and most promising next-generation nematicides. In this paper, several aspects of these non-fumigant nematicides are reviewed, along with a brief history and problems of old-generation nematicides.

New reduced-risk agricultural nematicides - rationale and review

The last decade has seen a sharp increase in nematicide research in the agricultural industry. As a result, several new synthetic nematicides have become available to growers, and several more are expected in the near future. This new interest in nematicides is directly related to the growing demand for safer and more selective products, and the increasing regulatory pressure on many of the traditional nematicides. This has led to a ban of several widely used fumigant (e.g. methyl bromide) and non-fumigant (e.g. aldicarb) nematicides. The loss of traditional nematicides, combined with a lack of replacement products and awareness of the damage that nematodes can cause, has not only raised concern among growers, but has also created new opportunities for the crop protection industry. Nematicides have become a priority, and many companies are now allocating significant research dollars to discover new nematicides. The new nematicides are very different from previous products: (i) they are more selective, often only targeting nematodes, and (ii) they are less toxic, and safer to use. This review article describes these new developments by discussing the challenges that are associated with finding new nematicides, reviewing the nature, characteristics, and efficacy of new nematicides, and discussing the impact they could have on future nematode management.

Sensitivity to fluensulfone of inactivated Meloidogyne spp. second-stage juveniles

BACKGROUND

Nonfumigant nematicide efficacy is affected by several factors, such as nematode species and environmental conditions. However, the influence of nematodes' physiological status on nematicide efficacy is unknown. Inactive nematodes, such as those in quiescence or dormancy, seem to be more tolerant to nematicides than active ones. Second-stage juveniles of Meloidogyne species were inactivated by low temperatures and reversible nematicides before and during exposure to fluensulfone. The sensitivity of inactive juveniles to fluensulfone the nematicide was compared to that of active juveniles by EC₅₀ (median effective concentration) for juvenile immobilization and root gall reduction.

RESULTS

Inactivating Meloidogyne hapla and Meloidogyne javanica juveniles at 5 °C increased the EC₅₀ (median effective concentration) of fluensulfone for immobilization of and root galling by Meloidogyne spp. 3.6 to 9.5 times. When the exposure temperature was decreased from 25 to 15 °C, EC₅₀ for M. javanica root gall reduction after 24 and 48 h exposure increased 3.1 and 4.9 times, respectively, whereas for M. hapla, it increased 2.3 and 2.0 times, respectively. Juveniles of M. javanica and M. incognita immobilized by fenamiphos were as sensitive to fluensulfone as active juveniles based on the number of root galls. However, juveniles of these species immobilized by fluopyram were more tolerant to fluensulfone than untreated active juveniles. An interaction of fluopyram and fluensulfone activities is suggested.

CONCLUSION

Changes in the sensitivity of inactive Meloidogyne spp. juveniles to fluensulfone depend on the inactivation method. Fluensulfone could be better applied when nematodes are active in the soil. © 2020 Society of Chemical Industry.

The distinct profiles of the inhibitory effects of fluensulfone, abamectin, aldicarb and fluopyram on Globodera pallida hatching

BACKGROUND

Fluensulfone is a nematicide with a novel mode of action against plant parasitic nematodes. Here, we utilize in vitro hatching assays to investigate fluensufone's ability to inhibit Globodera pallida hatching, relative to the efficacy of other distinct classes of nematicides.

RESULTS

Fluensulfone, abamectin, aldicarb and fluopyram inhibit G. pallida hatching from cysts more potently than from isolated eggs. At 1 μM for cysts, the order of potency is fluensulfone> fluopyram> abamectin> aldicarb. At low concentrations of fluensulfone, inhibition of hatching is reversible, however, more than 50% of the juveniles that hatch from cysts pre-treated with fluensulfone have reduced motility. This is observed to a lesser extent with abamectin, fluopyram and aldicarb. When cysts are exposed to higher concentrations of fluensulfone (≥500 μM), abamectin (≥100 μM) and fluopyram (≥50 μM) inhibition of hatching is irreversible. This results from the loss of encysted juvenile structure giving rise to a granulated appearance consistent with necrosis, suggesting a nematicidal effect. Intriguingly, hatching initiated by root diffusate is arrested when egg populations are subsequently exposed to fluensulfone.

CONCLUSION

Fluensulfone, abamectin, fluopyram and aldicarb inhibit G. pallida hatching. Fluensulfone is a potent inhibitor of hatching and impacts on the viability of the J2 s emerging from the cysts. This activity, and the previously described impaired motility and metabolism of hatched juveniles, show that fluensulfone's distinct mode of action among existing nematicides intersects at two pivotal steps of the parasitic life cycle.

Multi-Year Field Evaluation of Fluorinated Nematicides Against Meloidogyne incognita in Carrots

California grows approximately 80% of the U.S. carrot production. The primary production challenges derive from root-knot nematodes (Meloidogyne spp.). Between 2013 and 2016, we evaluated three novel fluorinated nonfumigant nematicides in five field trials. Fluensulfone, fluopyram, and fluazaindolizine were applied as product-ready formulations at various rates, dates, and formulations. They were rated for their efficacy against the Southern root-knot nematode (M. incognita), their ability to mitigate nematode-caused crop damage, and potential to produce marketable carrot yield under high disease pressure. All trials were conducted in randomized complete block designs in M. incognita-infested, sandy-loam fields. Soil population of M. incognita at seeding and harvest, midseason plant vigor and fibrous root galling, harvest taproot galling, and marketable carrot yield were analyzed by ANOVA. Midseason gall ratings were indicative of disease ratings at harvest. All fluazaindolizine and fluensulfone treatments reduced at-harvest galling compared with the untreated controls. Fluopyram resulted in less galling but did not sufficiently protect the lower part of the taproot. Overall, fluazaindolizine at 2.24 kg/ha resulted in the most consistent and highest marketable carrot yield, followed by fluensulfone at 2.95 kg/ha. Both fluazaindolizine and fluensulfone will likely provide effective and target-selective crop protection against root-knot nematodes in fresh carrot production.

Short-Term Effects of Sublethal Doses of Nematicides on Meloidogyne incognita

Meloidogyne species are one of the most important groups of plant-parasitic nematodes globally because of their ability to damage most cultivated plants. Although they are widespread and economically important, there are limited control measures to combat these nematodes. New nonfumigant nematicides have been discovered that have the potential to be widely utilized for the management of plant-parasitic nematodes. Because of the longer half-lives in soil and lower toxicity of new nematicides compared with traditional fumigant and nonfumigant nematicides, understanding how nematodes respond to sublethal doses of nematicides is imperative to understanding whether nematicide resistance has the potential to develop. Characterizing responses of nematodes to sublethal doses will provide the foundation for future work, such as gene expression studies. In this study, the nematicides oxamyl (Vydate), fluazaindolizine (Salibro), fluensulfone (Nimitz), and fluopyram (Velum), were evaluated to understand how sublethal doses affect the fecundity and mobility of Meloidogyne incognita second-stage juveniles (J2). Using a microwell assay system, dose-response curves for each nematicide were established for M. incognita J2. Fluopyram was the most toxic nematicide, with effective doses up to 230 times lower than that of other nematicides. The other nematicides had predicted ED₅₀ values (effective doses that resulted in 50% of the population becoming inactive) of 89.4, 131.7, and 180.6 ppm for oxamyl, fluensulfone, and fluazaindolizine, respectively. The 24-h ED₅₀ of each nematicide was then used in both motility and infectivity assays. The motility and activity of M. incognita J2 exposed to ED₅₀ doses of fluazaindolizine and fluensulfone was significantly reduced, with nematodes initially being motile but eventually becoming inactive. However, the motility of M. incognita J2 exposed to ED₅₀ doses of fluopyram and oxamyl was not different from a water control. In a pot assay, M. incognita J2 exposed to ED₅₀ doses of fluazaindolizine, oxamyl, and fluensulfone were unable to reproduce on tomato, with reproduction factors (RF = final population density/initial population density) of 0 to 0.03. Fluopyram did not reduce reproduction of M. incognita, with a mean RF of 38.7 ± 4.5, which was similar to the RF of 46.3 ± 4.6 for the water control. This study is the first comprehensive evaluation of M. incognita activity, motility, and fecundity after exposure to the traditional nematicide, oxamyl, as well as three new nematicides, fluazaindolizine, fluopyram, and fluensulfone.

Effect of fluensulfone and fluopyram on the mobility and infection of second-stage juveniles of Meloidogyne incognita and M. javanica

BACKGROUND

Fluensulfone and fluopyram are new nematicides whose mode of action is not fully understood. Differences in the nematicidal activity of these compounds among two Meloidogyne incognita populations and a Meloidogyne javanica population, and the effect of sublethal exposure to the nematicide on their infection process were studied in vitro.

RESULTS

The M. incognita populations were more sensitive to fluensulfone than M. javanica, whereas M. javanica was more sensitive to fluopyram. A more than 10-fold difference in median lethal concentration (LC₅₀ ) was observed between the M. incognita populations after 17-h exposure to fluensulfone. Exposure of M. incognita and M. javanica to 4 mg L^-1 fluopyram for 48 h resulted in irreversible immobilization, whereas lower concentrations or 17-h exposure to fluopyram caused reversible immobilization. Pre-exposure of M. javanica to fluensulfone at sublethal concentrations reduced the number of juveniles attracted to root tips and caused smaller galls. Pre-exposure to fluopyram delayed the nematodes' attraction. Presence of fluopyram in a Pluronic gel at concentrations that immobilized the nematodes in water showed no or only slight inhibition of the nematodes' attraction to root tips and gall formation.

CONCLUSION

Different sensitivities to nematicides were observed among Meloidogyne species and populations. Sublethal exposure to fluensulfone reduced nematode attraction to root tips and infection. Pluronic gel negatively affected nematicidal activity, especially for fluopyram. © 2019 Society of Chemical Industry.

Estimation of lipid reserves in different life stages of Meloidogyne incognita using image analysis of Nile Red-stained nematodes

Biochemical analyses of nematodes have revealed that neutral lipids (especially triglycerides) are the main source of energy reserves, which is depleted as the nematodes age. Several methodologies have been developed to visualise triglyceride-rich fat stores in plant-parasitic nematodes using non-fluorescent, lipophilic dyes, such as Oil Red O. Here, we propose a robust and reproducible fluorescence-based Nile Red staining method (followed by image analysis) for rapid detection of neutral lipid droplets in Meloidogyne incognita. This unique lipophilic dye selectively fluoresces in red and green spectra in a lipid-rich environment. The neutral lipid content of M. incognita juveniles gradually diminished during different periods of food deprivation, and this was significantly correlated with reduction in parasitic success of M. incognita in eggplant. Additionally, variation in fat reserves in different developmental stages of M. incognita infecting adzuki bean was also demonstrated. This investigation may aid future metabolic research, including functional analysis of lipid regulatory genes in plant-parasitic nematodes.

Fluensulfone and 1,3-dichloroprene for plant-parasitic nematode management in potato production

Florida produces 35% of the spring potato (Solanum tuberosum) crop in the USA, but plant-parasitic nematodes suppress yield in the region. The stubby-root nematodes, Paratrichodorus (Nanidorus) spp. and Trichodorus spp., vectors for corky ringspot disease, and sting nematode (Belonolaimus longicaudatus) are among the most damaging nematodes in Florida potato production. Nematicide application is an important component of nematode management in this system, but relatively few nematicides are currently available. Therefore, pre-plant applications of fluensulfone nematicide at various rates (3, 4, 6, and 8 l/ha) and the commercial standard fumigant 1,3-dichloropropene (1,3-D) were tested for management of plant-parasitic nematodes in three field trials from 2016 to 2018. Both fluensulfone, at all rates, and 1,3-D consistently decreased sting nematode abundance relative to the untreated control at harvest. Neither fluensulfone nor 1,3-D affected stubby-root nematode abundances at harvest. Efficacy of fluensulfone and 1,3-D for lesion nematode (Pratylenchus sp.) management varied by year. In 2016 and 2018, fluensulfone at most rates and 1,3-D increased marketable potato yield relative to the untreated control with increases by 49 to 66% and 33 to 55% in 2016 and 2018, respectively. In 2017, fluensulfone at lower rates (3, 4, and 6 l/ha) increased marketable potato yield relative to the untreated control by 41 to 61%, but fluensulfone at 8 l/ha and 1,3-D had similar yields to the untreated control. Results suggest that nematicidal activity of fluensulfone and 1,3-D varies by target nematode with both products effective against sting nematode, ineffective against stubby-root nematodes, and inconsistent against lesion nematode. In conclusion, fluensulfone and 1,3-D are effective options for sting nematode management in Florida potato production.