Incognita Second-stage Juveniles Research Articles

Biological Control of a Root-Knot Nematode Meloidogyne incognita Infection of Tomato (Solanum lycopersicum L.) by the Oomycete Biocontrol Agent Pythium oligandrum.

The biocontrol agent Pythium oligandrum, which is a member of the phylum Oomycota, can control diseases caused by a taxonomically wide range of plant pathogens, including fungi, bacteria, and oomycetes. However, whether P. oligandrum could control diseases caused by plant root-knot nematodes (RKNs) was unknown. We investigated a recently isolated P. oligandrum strain GAQ1, and the P. oligandrum strain CBS530.74, for the control of an RKN Meloidogyne incognita infection of tomato (Solanum lycopersicum L.). Initially, P. oligandrum culture filtrates were found to be lethal to M. incognita second-stage juveniles (J2s) with up to 84% mortality 24 h after treatment compared to 14% in the control group. Consistent with the lethality to M. incognita J2s, tomato roots treated with P. oligandrum culture filtrates reduced their attraction of nematodes, and the number of nematodes penetrating the roots was reduced by up to 78%. In a greenhouse pot trial, the P. oligandrum GAQ1 inoculation of tomato plants significantly reduced the gall number by 58% in plants infected with M. incognita. Notably, the P. oligandrum GAQ1 mycelial treatment significantly increased tomato plant height (by 36%), weight (by 27%), and root weight (by 48%). A transcriptome analysis of tomato seedling roots inoculated with the P. oligandrum GAQ1 strain identified ~2500 differentially expressed genes. The enriched GO terms and annotations in the up-regulated genes suggested a modulation of the plant hormone-signaling and defense-related pathways in response to P. oligandrum. In conclusion, our results support that P. oligandrum GAQ1 can serve as a potential biocontrol agent for M. incognita control in tomato. Multiple mechanisms appear to contribute to the biocontrol effect, including the direct inhibition of M. incognita, the potential priming of tomato plant defenses, and plant growth promotion.

Rhizosphere-Associated Microbiota Strengthen the Pathogenicity of Meloidogyne incognita on Arabidopsis thaliana

Microorganisms associated with nematodes or enriched in galls have been reported previously to aid plant-parasitic nematodes (PPNs) in infecting and establishing parasitism in the host plants. However, the rhizosphere-associated microbiota, which strengthens the pathogenicity of PPNs, remains largely unknown. This study illustrated rhizosphere bacteria enhancing Meloidogyne incognita infection on Arabidopsis thaliana by comparing the gall numbers of the treatments between natural soil and the sterile soil or soils drenched with antibiotics. By culture-dependent and pot testing methods, sixteen bacterial combinations from rhizosphere soils of A. thaliana were demonstrated to enhance M. incognita pathogenicity, including the most effective Nocardioides. Single-strain inoculation from the Nocardioides combination significantly resulted in M. incognita forming more galls on roots than the control, in which N. nematodiphilus R-N-C8 was the most effective strain. Strain R-N-C8 could substantially facilitate the M. incognita second-stage juveniles (J2s) moving towards the roots of A. thaliana and infecting the roots by releasing chemoattractant to attract J2s. The chemoattractant from strain R-N-C8 was determined to be L-lysine. This study furnishes vital insights for understanding the infection of root-knot nematodes associated with rhizosphere microbes.

Effect of short-term growth of mung bean and its incorporation with nitrogen fertiliser on the density of the root-knot nematode, Meloidogyne incognita, in soil

Summary Soybean is attacked by multiple plant-parasitic nematodes and the major ones are soybean cyst nematode (SCN) and root-knot nematodes. Our previous study reported that 2 weeks growth of mung bean and its soil incorporation markedly reduced SCN density in soil via hatching stimulation by mung bean residue. This study aimed: i) to evaluate whether incorporation of mung bean in combination with N fertiliser decreased the density of Meloidogyne incognita, which attacks both soybean and mung bean; and ii) to determine the direct effect of ammonium solutions on M. incognita J2 and hatching. The results showed that after the growth period of mung bean for 2 weeks and 4 weeks of its incorporation, the density of M. incognita was significantly reduced by 54 and 72% in the mung bean treatment with and without N fertiliser (140 mg N (kg soil)−1), respectively, compared with the control. Soil urease activity was significantly higher in the mung bean with N fertiliser than in the control. Meloidogyne incognita second-stage juveniles were less mobile in NH4Cl solutions with 100-250 mg N l−1 than in 0 mg N l−1. Moreover, NH4Cl solutions with 140 mg N l−1 and 250 mg N l−1 significantly inhibited the hatching of M. incognita. This study demonstrated that 2 weeks growth of mung bean may be a potential trap crop for M. incognita and N metabolism might be involved to a certain extent in the suppression of M. incognita.

Sulfoxide-Containing Bisabolane Sesquiterpenoids with Antimicrobial and Nematicidal Activities from the Marine-Derived Fungus Aspergillus sydowii LW09.

Phytopathogens, such as phytopathogenic bacteria, fungi, and nematodes, have caused great losses of crops every year, seriously threatening human health and agricultural production. Moreover, marine-derived fungi are abundant sources of structurally unique and bioactive secondary metabolites that could be potential candidates for anti-phytopathogenic drugs. One new sulfoxide-containing bisabolane sesquiterpenoid aspersydosulfoxide A (1) and nine known analogues (2-10) were isolated from the marine-derived A. sydowii LW09. The absolute configuration of the sulfur stereogenic center in 1 was determined by electronic circular dichroism (ECD) calculations. Compound 5 showed inhibition activity against Pseudomonas syringae, with a minimum inhibitory concentration (MIC) value of 32 μg/mL, whereas, compounds 2, 7, and 8 showed antibacterial activities toward Ralstonia solanacarum, with the same MIC value at 32 μg/mL. Meanwhile, compounds 3, 7, and 8 inhibited the fungal spore germination of Fusarium oxysporum, with the half maximal effective concentration (EC50) values of 54.55, 77.16, and 1.85 μg/mL, respectively, while compounds 2, 3, 7, and 8 inhibited the fungal spore germination of Alternaria alternata, which could be induced by vacuolization of germ tubes, with EC50 values of 34.04, 44.44, 26.02, and 46.15 μg/mL, respectively. In addition, compounds 3, 7, and 8 exhibited nematicidal activities against Meloidogyne incognita second-stage juveniles (J2s). In addition, compound 8 possessed the strongest nematicidal activity of nearly 80% mortality at 60 h with the half lethal concentration (LC50) values of 192.40 μg/mL. Furthermore, compounds 3, 7, and 8 could paralyze the nematodes and then impair their pathogenicity.

Methylorubrum rhodesianum M520 as a biocontrol agent against Meloidogyne incognita (Tylenchida: Heteroderidae) J2s infecting cucumber roots.

Root-knot nematodes (RKNs) are plant pathogens that cause huge economic losses worldwide. The biological management of RKNs may be a sustainable alternative to chemical control methods. Here, the biocontrol potential of Methylorubrum rhodesianum M520 against the RKN Meloidogyne incognita was investigated to theoretically support its application as a biocontrol agent in field production. In-vitro assays showed 91.9% mortality of M. incognita second-stage juveniles in the presence of strain M520 and that the hatching rate of M. incognita eggs was 21.7% lower than that of eggs treated with sterile water. In pot experiments, the M520 treatment caused 70.8% reduction in root-knots and increased plant shoot length and stem and root fresh weights, compared to control plant values. In split-root experiments, cucumber roots treated with M520 showed 25.6% decrease in root gall number, compared to that in control roots. M520 has multiple mechanisms against RKNs and might be used as a biocontrol agent against M. incognita in cucumber, laying a foundation for further studying M520 biocontrol against RKNs.

(R)-Carvone is a potential soil fumigant against Meloidogyne incognita whose likely enzymatic target in the nematode is acetylcholinesterase

To contribute to the development of new fumigant nematicides for the control of the plant-parasitic nematode Meloidogyne incognita, this study started with 31 volatile organic compounds reported as toxic to nematodes. At 500 μg/mL, α-ionone, (S)-carvone, (R)-carvone, 2-methylpropyl acetate, undecan-2-one, decan-2-one, and dodecan-2-one caused mortalities to M. incognita second-stage juveniles (J2) that were similar to those obtained with the commercial nematicides carbofuran (170 μg/mL) and fluensulfone (42.2 μg/mL). (R)-carvone, with a lethal concentration to 50% J2 (LC50) equal to 524 μg/mL, was selected for subsequent studies. When J2 were exposed to the (R)-carvone solution, the infectivity and reproduction on tomato were reduced. In the M. incognita egg hatching assay, (R)-carvone behaved like a true ovicide. When employed as a fumigant, (R)-carvone (3.9 g/L) was as efficient as the soil fumigant dazomet (0.245 g/L) in eliminating eggs of the nematode in a substrate to be used for tomato planting. According to in silico studies employing pharmacophoric searches and molecular docking, acetylcholinesterases are the target of (R)-carvone in the nematode.

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, upbeta-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 upbeta-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.

Transcriptional response of Meloidogyne incognita to non-fumigant nematicides

There is limited research about the impacts of new nematicides, including fluazaindolizine, fluopyram, and fluensulfone, on the plant-parasitic nematode Meloidogyne incognita, despite it being a pervasive agricultural pest. In this study, M. incognita second-stage juveniles were exposed for 24-h to fluensulfone, fluazaindolizine, fluopyram, and oxamyl and total RNA was extracted and sequenced using next-generation sequencing to determine gene expression. The effects of nematicide exposure on cellular detoxification pathways, common differentially expressed (DE) genes, and fatty acid and retinol-binding genes were examined. Fluopyram and oxamyl had the smallest impacts on the M. incognita transcriptome with 48 and 151 genes that were DE, respectively. These compounds also elicited a weak response in the cellular detoxification pathway and fatty acid and retinol-binding (FAR) genes. Fluensulfone and fluazaindolizine produced robust transcriptional responses with 1208 and 2611 DE genes, respectively. These compounds had strong impacts on cellular detoxification, causing differential regulation of transcription factors and genes in the detox pathway. These compounds strongly down-regulated FAR genes between 52–85%. Having a greater understanding of how these compounds function at a molecular level will help to promote proper stewardship, aid with nematicide discovery, and help to stay a step ahead of nematicide resistance.

Methyl Esters of (E)-Cinnamic Acid: Activity against the Plant-Parasitic Nematode Meloidogyne incognita and In Silico Interaction with Histone Deacetylase

(E)-Cinnamaldehyde is very active against Meloidogyne incognita but has low persistence in soil. To circumvent this problem, esters of cinnamic acid were evaluated as a substitute for (E)-cinnamaldehyde. The best results under assays with M. incognita second-stage juveniles (J2) were obtained for the methyl esters of (E)-p-fluoro- (13), (E)-p-chloro- (14), and (E)-p-bromocinnamic acid (15), which showed lethal concentrations to 50% (LC50) J2 of 168, 95, and 216 μg/mL, respectively. Under the same conditions, the LC50 values for the nematicides carbofuran and fluensulfone were 160 and 34 μg/mL, respectively. Substances 13-15 were also active against nematode eggs, which account for most of the M. incognita population in the field. According to an in silico study, substances 13-15 can act against the nematode through inhibition of histone deacetylase. Therefore, esters 13-15 and histone deacetylase are potentially useful for the rational design of new nematicides for the control of M. incognita.

Meloidogyne-SP4 effector gene silencing reduces reproduction of root-knot nematodes in rice (Oryza sativa)

Summary The root-knot nematodes (RKN) Meloidogyne graminicola and M. incognita are responsible for rice yield losses worldwide, particularly in Asia and Africa. Previous studies demonstrated that nematode-secreted proteins are crucial for root invasion and establishment in the host. We present some characteristics of a pioneer effector, M. incognita-secreted protein 4 (Mi-SP4), which is conserved in RKN and required for infection in compatible rice-RKN interactions. In situ hybridisation assays revealed Mi-SP4 expression in the dorsal pharyngeal gland of M. incognita second-stage juveniles (J2). Meloidogyne-SP4 transcripts strongly accumulated in pre-parasitic J2 and decreased in later parasitic stages of M. incognita and M. graminicola. Transient expression of the nematode effector gene in Nicotiana benthamiana leaves and onion cells indicated that GFP-tagged Mi-SP4 was present in the cytoplasm and accumulated in the nucleus of the plant cells. In vitro RNA interference (RNAi) gene silencing, obtained by soaking J2 with small-interfering (si)RNA si4-1, decreased Mi -SP4 expression in J2 by 35% and significantly reduced M. incognita reproduction in rice by at least 30%. Similarly, host-mediated gene silencing of the nematode SP4 effector candidate gene in transgenic rice plants significantly reduced M. graminicola reproduction by 26% to 47%. The data obtained demonstrate that Mi -SP4 is a pioneer virulence effector, which plays an essential role in both M. incognita and M. graminicola pathogenicity on rice.

Attraction and toxicity: Ways volatile organic compounds released by Pochonia chlamydosporia affect Meloidogyne incognita

The production of volatile organic compounds (VOCs) acting against plant-parasitic nematodes has been characterized in different fungi; however, the role of VOCs emitted by Pochonia chlamydosporia in its trophic interaction with Meloidogyne incognita is still unknown. The aim of this study was to determine the effects of VOCs emitted by P. chlamydosporia strain Pc-10 on different stages (eggs, juveniles and female) of the M. incognita life cycle. Exposure of M. incognita eggs to VOCs released by Pc-10 resulted in a reduction up to 88 % in the nematode egg hatching, when compared to the control treatments. The VOCs emitted by Pc-10 also attracted M. incognita second-stage juveniles (J2). Through gas chromatography-mass spectrometry (GC–MS), three molecules were identified from the volatiles of the strain Pc-10, with 1,4-dimethoxybenzene being the major compound. In tests performed in vitro, 1,4-dimethoxybenzene at a concentration of 1050 μg mL−1 inhibited M. incognita egg hatching by up to 78.7 % compared to the control (0 μg mL−1) and attracted M. incognita J2 in all concentrations evaluated (1, 10, 100, 1000, and 10000 μg mL-1). The 1,4-dimethoxybenzene also showed fumigant and non-fumigant nematicidal activity against M. incognita. This compound presented lethal concentration for 50 % (LC50) of M. incognita J2 ranged from 132 to 136 μg mL−1. Fumigation with 1,4-dimethoxybenzene (100 mg) reduced egg hatching by up to 89 % and killed up to 86 % of M. incognita J2 compared to the control (0 μg mL−1). In vivo, the VOCs produced by Pc-10, 1,4-dimethoxybenzene, and the combination of both (Pc-10 + 1,4-dimethoxybenzene) attracted the M. incognita J2, compared to the respective controls. To the best of our knowledge, this is the first report on the attraction of M. incognita J2 and the toxicity to eggs and J2 by VOCs from P. chlamydosporia in which 1,4-dimethoxybenzene is the main toxin and attractant.

Biocontrol of the root-knot nematode Meloidogyne incognita by a nematicidal bacterium Pseudomonas simiae MB751 with cyclic dipeptide.

Root-knot nematodes (RKNs) are harmful plant-parasitic nematodes that cause serious damage to plant hosts. In the long-term practice of RKN management, bacterial nematicides have attracted increasing attention as an effective biocontrol means. Here we determined the active substances against Meloidogyne incognita from a nematicidal bacterium, developed a biocontrol agent (BCA) based on optimized culture processes. The effects of the BCA on RKN control and plant growth-promotion were evaluated in tomato pot trials. Pseudomonas simiae strain MB751 exhibiting significant nematicidal activity against M. incognita second-stage juveniles (J2) with approximately 80% mortality (with culture supernatant, 96% volume percentage) was isolated from a vineyard. A set of purification and identification experiments was performed to determine the main nematicidal component in MB751. A cyclic dipeptide Cyclo(L-Pro-L-Leu) was identified with a lethal concentration necessary to kill 50% of the population (LC50 ) of 65.3μg mL-1 against M. incognita J2. Following optimization trials on culture medium/fermentation conditions, such as the single factor test, Plackett-Burman test, steepest ascent, and response surface methodology experiments, the MB751 fermentation broth was then prepared as a BCA via a cold-air drying process. The BCA and was evaluated in tomato pot experiments for effectiveness in suppressing M. incognita. Significant effects on M. incognita suppression and plant-growth promotion as well as induced systemic resistance to M. incognita of tomato, were observed. The cyclic dipeptide-producing bacterium P. simiae MB751 exhibited high nematicidal activity and performance. Further development of this BCA should be pursued for the management of M. incognita in agriculture. © 2021 Society of Chemical Industry.

Differential response of cowpea towards the CuO nanoparticles under Meloidogyne incognita stress

The application of nanoparticles as potent nematicides along with the plant growth-promoting effects is quite intriguing to meet the need for an alternative management strategy for nematode management. Here we reported the nematicidal effects of synthesized Copper oxide nanoparticles (CuO) against Meloidogyne incognita second-stage juveniles (J2S) under in vitro and in planta conditions at different concentrations. Scanning Electron Micrograph of CuO nanoparticles treated J₂S revealed the distorted, crinkled cuticle in comparison to control J₂S. CuO nanoparticles in graded concentrations (50, 100 and 200 ppm) applied concomitantly, three days before and after the nematode inoculation was assessed in terms of morphology, yield and biochemical attributes of infected Cowpea (Vigna unguiculata L. Gomti.). In planta, trials revealed that the application of CuO nanoparticles before the nematode inoculation at 100 ppm concentration exhibited healthier growth attributes besides concurrent reduction in nematode parameters in comparison to plants treated with M. incognita.

Volatile fatty acids from whey volatilome as potential soil fumigants to control Meloidogyne incognita

Organic residues emit volatile organic compounds (VOCs) that are toxic to plant-parasitic nematodes (PPNs). In this study, we demonstrated that whey produced by cow milk emit VOCs that are toxic to Meloidogyne incognita second-stage juveniles (J2), causing immobility above 80% and significant (P < 0.05) mortality compared to controls. The analysis of the volatilome done by gas chromatography coupled with mass spectrometry revealed 28 compounds, predominantly carboxylic acids and esters. Four of those volatile fatty acids (VFAs), namely acetic acid, octanoic acid, ethyl octanoate, and isovaleric acid, were chosen to further studies. The four tested VFAs were toxic to M. incognita J2, with lethal concentration values required to kill 50% of the nematode population (LC50), ranging from 134.30 to 236.08 μg mL−1. However, on eggs, only acetic acid and ethyl octanoate were consistent with J2 hatch inhibition, reaching 80% at a concentration of 1000 μg mL−1. In a greenhouse assay, when the VFAs were applied as soil fumigant, M. incognita infectivity and reproduction were significantly (P < 0.05) reduced compared to the negative control (water). Among them, ethyl octanoate application reduced (P < 0.05) the number of eggs to the level of the commercial fumigant dazomet. Overall, the volatile compounds released by whey were toxic to M. incognita J2 and contained a great diversity of molecules. Among the VFAs, ethyl octanoate stood out and showed the potential to be used in future field studies.

The host status of glyphosate-tolerant soybean genotypes to Meloidogyne incognita and Pratylenchus infection

The host status of South African adapted, genetically modified (GM) glyphosate-tolerant soybean genotypes to root-knot (field and glasshouse) and lesion (field) nematodes were assessed. Analyses of root and soil samples of 29 genotypes (collected from seven production areas during the 2014/15 season) enabled the identification of nine plant-parasitic nematode genera and 10 species. Predominant endoparasitic genera in root samples were Meloidogyne (Meloidogyne incognita and M. javanica) and Pratylenchus (Pratylenchus brachyurus, P. zeae and P. teres). Rotylenchulus parvus was the predominant semi-endoparasite in soil, followed by Scutellonema brachyurus and Helicotylenchus sp. Only ‘PAN 1583 R’ and ‘PAN 1521 R’ maintained less than 10% of the Meloidogyne spp. densities present in roots of the most susceptible genotype, while all genotypes were susceptible to the Pratylenchus spp. The host status of 36 soybean genotypes to M. incognita infection, evaluated in two follow-up glasshouse experiments terminated 56 days after inoculation of ca. 1000 M. incognita eggs and second-stage juveniles (J2) per seedling, varied substantially for final population density (Pf), reproduction factor (Rf) and relative percentage susceptibility (%S). Only ‘PRF-GCI7’ and the resistant reference ‘LS 5995’ had Rfs < 1 for both experiments, despite higher minimum and maximum temperatures recorded for the second experiment. Continuous evaluation of soybean genotypes for their host status to predominant nematode pests and their use to reduce densities of such species in producer’s fields are crucial to enable sustainable crop production, and contribute towards food provision and security.

Chemotaxis response of Meloidogyne incognita to volatiles and organic acids from root exudates

Chemotaxis plays an important role in food-finding, mate-finding, response to host plants and other aspects of nematode interactions. In this study, chemotaxis responses of Meloidogyne incognita second-stage juveniles (J2) to seven plant hosts were investigated. Chemotaxis volatiles from root exudates were analyzed by gas chromatography-mass spectrometry, from which four attractive compounds were identified as the tridecane, 1-docosene, 1-octadecene and 2-hexyl-1-decanol. 1-docosene was distributed in root exudates of all seven plants, while the other compounds were found only in some root exudates. In addition, organic acids commonly found in plant root exudates were screened. Among the 16 types of organic acids, six organic acids could attract J2 of M. incognita. Malic acid, oxalic acid and lactic acid were found to demonstrate chemotactic effect on M. incognita J2 and were distributed differently in the seven plants. These results indicate that volatiles and organic acids secreted from different plants possess the ability to attract the J2 of M. incognita. This study provides insight into the signaling mechanism between root-knot nematodes and host plants, which may thus help develop more effective biological control methods.

Morphometric and total protein responses in Meloidogyne incognita second-stage juveniles to Nemafric-BL phytonematicide

After hatch, second-stage juveniles (J2) of root-knot (Meloidogyne species) nematodes could spend at least 12 weeks in soil solutions searching for penetration sites of suitable host plants. The external covering of nematodes, the cuticle, consists of various layers that contain glycoproteins, lipids, soluble proteins (collagens) and insoluble proteins (cuticulins). Generally, cucurbitacins are lipophilic, but there is scant information on how cuticular proteins relate to these complex terpenoids. A study was conducted to investigate the nature and extent of damage post-exposure of J2 to a wide range of Nemafric-BL phytonematicide concentrations. Post-72 h exposure to Nemafric-BL phytonematicide, nematode morphometrics versus phytonematicides exhibited either negative quadratic, positive quadratic, or negative linear relations, with the models explained by significant (P < 0.05) associations (R-squared). Similarly, total proteins versus phytonematicide exhibited significant negative quadratic relations. The principal component analysis indicated that concentration level of 2–4% of Nemafric-BL phytonematicide have the highest impact on the morphometric changes of J2. In conclusion, the nature and extent of damage suggested that Nemafric-BL phytonematicide was highly nematicidal as opposed to being nematostatic, thereby explaining its potent suppressive effects on nematode population densities.

Activity against Meloidogyne incognita of volatile compounds produced during amendment of soil with castor bean cake

Summary Incorporation of castor bean cake into the soil results in the emission of four main compounds: skatole, γ-decalactone, 4-methylphenol and phenol. The toxic effects of these compounds on the life cycle of Meloidogyne incognita were studied here. Only phenol did not reduce hatching of M. incognita second-stage juveniles (J2) at any of the concentrations tested. The other compounds reduced J2 hatching by 42.1 to 59.3% at the highest concentrations. When J2 were immersed in Tween 80® solutions of each compound, at their respective LC50 values and inoculated into soil with tomato plants, reductions in the numbers of galls (48.5-69.3%) and eggs (42.8-62.5%) were observed in comparison with the control; this result was similar to that of carbofuran. The highest reductions in the numbers of galls (21.8-88.5%) and eggs (49.6-98.9%) occurred with all compounds when the highest concentration (500 mg l−1) was applied to the substrate at the moment of J2 inoculation. Volatile compounds of γ-decalactone and skatole failed to cause J2 immobility or mortality. However, phenol caused 95.8% and 4-methylphenol caused 100% mortality. With fumigation, the highest concentration (1000 mg (l substrate)−1) of any tested compound reduced gall numbers at a similar rate to the nematicide dazomet (98.7-100%) and reduced egg numbers (87.1-99.7%). The volatile compounds skatole, γ-decalactone, 4-methylphenol and phenol have a nematicidal effect on the life cycle of M. incognita and alter its pathogenic behaviour on the plant.

Τhe Nematicidal Potential of Bioactive Streptomyces Strains Isolated from Greek Rhizosphere Soils Tested on Arabidopsis Plants of Varying Susceptibility to Meloidogyne spp.

A total of 461 indigenous Streptomycetes strains recovered from various Greek rhizosphere habitats were tested for their bioactivity. All isolates were examined for their ability to suppress the growth of 12 specific target microorganisms. Twenty-six were found to exert antimicrobial activity and were screened for potential nematicidal action. S. monomycini ATHUBA 220, S. colombiensis ATHUBA 438, S. colombiensis ATHUBA 431, and S. youssoufensis ATHUBA 546 were proved to have a nematicidal effect and thus were further sequenced. Batch culture supernatants and solvent extracts were assessed for paralysis on Meloidogyne javanica and Meloidogyne incognita second-stage juveniles (J2). The solvent extracts of S. monomycini ATHUBA 220 and S. colombiensis ATHUBA 438 had the highest paralysis rates, so these Streptomycetes strains were further on tested for nematodes’ biological cycle arrest on two Arabidopsis thaliana plants; the wild type (Col-0) and the katanin mutant fra2, which is susceptible to M. incognita. Interestingly, S. monomycini ATHUBA 220 and S. colombiensis ATHUBA 438 were able to negatively affect the M. incognita biological cycle in Col-0 and fra2 respectively, and increased growth in Col-0 upon M. incognita infection. However, they were ineffective against M. javanica. Fra2 plants were also proved susceptible to M. javanica infestation, with a reduced growth upon treatments with the Streptomyces strains. The nematicidal action and the plant-growth modulating abilities of the selected Streptomycetes strains are discussed.

Synergism of citral from Cymbopogon citratus essential oil and undecan-2-one against Meloidogyne incognita

Summary The nematicidal activity of the essential oil of Cymbopogon citratus and its main components were studied. During in vitro assays with Meloidogyne incognita second-stage juveniles (J2), the oil showed an LC50 of 166 μg ml−1. In vivo assays that exposed J2 to the LC50 oil concentration significantly reduced galls and eggs on tomato roots. According to gas chromatography-mass spectrometry analyses, the main components of the essential oil were geranial (42.7%) and neral (33.0%). The combination of these two isomers, known as citral, had an enhanced nematicidal activity at low concentrations as shown by in vitro tests through a synergistic interaction with undecan-2-one. In assays with tomato plants inoculated with M. incognita J2, the combination of citral (600 μg ml−1) and undecan-2-one (300 μg ml−1) showed a high reduction in the number of galls (63.4%) and eggs (56.3%) in relation to Tween 80® control. The citral/undecan-2-one combination showed a synergistic nematicidal activity against M. incognita during in vitro and in vivo assays.