Managing nematodes without methyl bromide

Comparative transcriptomics of susceptible and resistant Cucumis metuliferus upon Meloidogyne incognita infection

MAIN CONCLUSION

Comparative transcriptomics has identified several candidate genes contributing to the resistance of Cucumis metuliferus against Meloidogyne incognita. The Southern root-knot nematode (Meloidogyne incognita) is a significant threat to Cucurbitaceae crops. The African horned melon (Cucumis metuliferus), a wild relative, exhibits high resistance to this nematode. To explore the resistance mechanism, phenotypic analyses were conducted on a susceptible inbred line (CM27) and a resistant inbred line (CM3). CM3 exhibited enhanced root biomass and significantly higher resistance compared to CM27, with poor nematode development observed in CM3 roots. Transcriptomic profiling at multiple post-infection time points revealed 2243 and 3700 differentially expressed genes (DEGs) in CM3 and CM27, respectively. Among these, the top ten DEGs upregulated exclusively in CM3 were functionally analyzed using virus-induced gene silencing (VIGS). Silencing of EVM0019904 or EVM0017058 in CM3 led to susceptibility to M. incognita. These findings provide novel insights into the resistance mechanisms of M. incognita in C. metuliferus and offer potential resources for breeding nematode-resistant Cucurbitaceae crops.

Preplant Soil Treatments Influence Tree Performance and Nematode Dynamics in Replanted Cherry Orchards

In this 2-year field study, the impacts of preplant soil management strategies, including soil fumigation, nematicide application, and organic amendments, on the growth and nematode community dynamics on cherry cultivars 'Emperor Francis' and 'Ulster' grafted to 'Mahaleb' rootstock were investigated in a replanted orchard site. In the first year, fumigation with 1,3-dichloropropene-chloropicrin mixture (Telone C-35) led to significantly increased trunk cross-sectional area and canopy height in both cultivars. Pratylenchus penetrans population densities were suppressed only short term. Plots treated with the fungicide/nematicide fluopyram (Velum Prime) had P. penetrans reproduction factors below one throughout both years independent of the scion. Additionally, the combined application of Seed Starter 101, Dairy Doo compost, and straw mulch reduced the reproduction factor of P. penetrans to below one in the first year. In the same time period, this combinatory treatment had the highest reproduction factor for bacterivore and fungivore nematodes. Based on the results of this study, fumigation with Telone C-35 resulted in improvement of tree establishment and provided effective short-term suppression of P. penetrans. Velum Prime exhibited longer-term efficacy for the suppression of P. penetrans.

Genetic dissection of Meloidogyne incognita resistance genes based on VIGS functional analysis in Cucumis metuliferus

The southern root-knot nematode, Meloidogyne incognita, is a highly serious plant parasitic nematode species that causes significant economic losses in various crops, including cucumber (Cucumis sativus L.). Currently, there are no commercial cultivars available with resistance to M. incognita in cucumber. However, the African horned melon (Cucumis metuliferus Naud.), a semi-wild relative of cucumber, has shown high resistance to M. incognita. In this study, we constructed an ultrahigh-density genetic linkage bin-map using low-coverage sequences from an F2 population generated through the cross between C. metuliferus inbred lines CM3 and CM27. Finally, we identified a QTL (quantitative trait locus, QTL3.1) with a LOD (logarithm of the odds) score of 3.84, explaining 8.4% of the resistance variation. Subsequently, by combining the results of qPCR (quantitative PCR) and VIGS (virus-induced gene silencing), we identified two genes, EVM0025394 and EVM0006042, that are potentially involved in the resistance to M. incognita in CM3. The identification of QTLs and candidate genes in this study serve as a basis for further functional analysis and lay the groundwork for harnessing this resistance trait.

Biochar combined with humic acid improves the soil environment and regulate microbial communities in apple replant soil

Apple replant disease (ARD) negatively affects plant growth and reduces yields in replanted orchards. In this study, biochar and humic acid were applied to apple replant soil. We aimed to investigate whether biochar and humic acid could promote plant growth and alleviate apple replant disease by reducing the growth of harmful soil microorganisms, changing soil microbial community structure, and improving the soil environment. This experiment included five treatments: apple replant soil (CK), apple replant soil with methyl bromide fumigation (FM), replant soil with biochar addition (2 %), replant soil with humic acid addition (1.5 ‰), and replant soil with biochar combined with humic acid. Seedling biomass, the activity of antioxidant enzymes in the leaves and roots, and soil environmental variables were measured. Microbial community composition and structure were analyzed using ITS gene sequencing. Biochar and humic acid significantly reduced the abundance of Fusarium and promoted the recovery of replant soil microbial communities. Biochar and humic acid also increased the soil enzymes activity (urease, invertase, neutral phosphatase, and catalase), the plant height, fresh weight, dry weight, the activity of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase), and root indexes of apple seedlings increased in replant soil. In sum, We can use biochar combined with humic acid to alleviate apple replant disease.

The Effects of Bacillus subtilis Expressing a Plant Elicitor Peptide on Nematode Infection on Soybean

There is a pressing need to develop alternative management strategies for the soybean cyst nematode (Heterodera glycines), the most costly pathogen to soybeans. Plant elicitor peptides (PEPs), which are produced by plants in response to stress and stimulate broad-spectrum disease resistance, were previously shown to reduce soybean cyst nematode infection on soybeans when applied as a seed treatment. Here, we introduce an alternative method to deliver PEPs to soybean using a common plant growth-promoting rhizobacterium, Bacillus subtilis, as a bacterial expression system. Similar to the empty vector control, B. subtilis engineered to express a PEP from soybean (GmPEP3) was able to colonize soybean roots and persisted on roots more than a month after treatment. Compared with water or the empty vector control, plants that received a seed treatment with B. subtilis expressing GmPEP3 (B.+GmPEP3) were significantly taller early in vegetative growth (V1 stage) and had lower chlorophyll content in the reproductive stage (R3/R4); these results suggest that GmPEP3 may hasten growth and subsequent senescence. When plants were inoculated with soybean cyst nematode at the V1 stage, those pretreated with B.+GmPEP3 supported significantly fewer nematode eggs at the reproductive stage (R3/R4) than plants treated with water or the empty vector. The effects of B.+GmPEP3 on nematode infection and plant growth appeared to be due primarily to the peptide itself because no significant differences were observed between plants treated with water or with B. subtilis expressing the empty vector. These results indicate the ability of B. subtilis to deliver defense activators for nematode management on soybean.

Combining Masculinizing Resistance, Rotation, and Biocontrol to Achieve Durable Suppression of the Potato Pale Cyst Nematode: A Model

The pale cyst nematode, Globodera pallida, is a pest that poses a significant threat to potato crops worldwide. The most effective chemical nematicides are toxic to nontarget organisms and are now banned. Alternative control methods are therefore required. Crop rotation and biological control methods have limitations for effectively managing nematodes. The use of genetically resistant cultivars is a promising alternative, but nematode populations evolve, and virulent mutants can break resistance after just a few years. Masculinizing resistances, preventing avirulent nematodes from producing females, might be more durable than blocking resistances, preventing infection. Our demo-genetic model, tracking both nematode population densities and virulence allele frequencies, shows that virulence against masculinizing resistance may not be fixed in the pest population under realistic agricultural conditions. Avirulence may persist despite the uniform use of resistance. This is because avirulent male nematodes may transmit avirulent alleles to their progeny by mating with virulent females. Additionally, because avirulent nematodes do not produce females themselves, they weaken the reproductive rate of the nematode population, leading to a reduction in its density by at least 20%. This avirulence load can even lead to the collapse of the nematode population in theory. Overall, our model showed that combining masculinizing resistance, rotation, and biocontrol may achieve durable suppression of G. pallida in a reasonable time frame. Our work is supported by an online interactive interface allowing users (i.e., growers, plant health authorities, researchers) to test their own control combinations.

Protective role of native root-associated bacterial consortium against root-knot nematode infection in susceptible plants

Root-knot nematodes (RKNs) are a global menace to agricultural crop production. The role of root-associated microbes (RAMs) in plant protection against RKN infection remains unclear. Here we observe that cucumber (highly susceptible to Meloidogyne incognita) exhibits a consistently lower susceptibility to M. incognita in the presence of native RAMs in three distinct soils. Nematode infection alters the assembly of bacterial RAMs along the life cycle of M. incognita. Particularly, the loss of bacterial diversity of RAMs exacerbates plant susceptibility to M. incognita. A diverse range of native bacterial strains isolated from M. incognita-infected roots has nematode-antagonistic activity. Increasing the number of native bacterial strains causes decreasing nematode infection, which is lowest when six or more bacterial strains are present. Multiple simplified synthetic communities consisting of six bacterial strains show pronounced inhibitory effects on M. incognita infection in plants. These inhibitory effects are underpinned via multiple mechanisms including direct inhibition of infection, secretion of anti-nematode substances, and regulation of plant defense responses. This study highlights the role of native bacterial RAMs in plant resistance against RKNs and provides a useful insight into the development of a sustainable way to protect susceptible plants.

Cyprocide selectively kills nematodes via cytochrome P450 bioactivation

Left unchecked, plant-parasitic nematodes have the potential to devastate crops globally. Highly effective but non-selective nematicides are justifiably being phased-out, leaving farmers with limited options for managing nematode infestation. Here, we report our discovery of a 1,3,4-oxadiazole thioether scaffold called Cyprocide that selectively kills nematodes including diverse species of plant-parasitic nematodes. Cyprocide is bioactivated into a lethal reactive electrophilic metabolite by specific nematode cytochrome P450 enzymes. Cyprocide fails to kill organisms beyond nematodes, suggesting that the targeted lethality of this pro-nematicide derives from P450 substrate selectivity. Our findings demonstrate that Cyprocide is a selective nematicidal scaffold with broad-spectrum activity that holds the potential to help safeguard our global food supply.

Upgrading Strategies for Managing Nematode Pests on Profitable Crops

Plant-parasitic nematodes (PPNs) reduce the high profitability of many crops and degrade their quantitative and qualitative yields globally. Traditional nematicides and other nematode control methods are being used against PPNs. However, stakeholders are searching for more sustainable and effective alternatives with limited side effects on the environment and mankind to face increased food demand, unfavorable climate change, and using unhealthy nematicides. This review focuses on upgrading the pre-procedures of PPN control as well as novel measures for their effective and durable management strategies on economically important crops. Sound and effective sampling, extraction, identification, and counting methods of PPNs and their related microorganisms, in addition to perfecting designation of nematode-host susceptibility/resistance, form the bases for these strategies. Therefore, their related frontiers should be expanded to synthesize innovative integrated solutions for these strategies. The latter involve supplanting unsafe nematicides with a new generation of safe and reliable chemical nematicidal and bionematicidal alternatives. For better efficacy, nematicidal materials and techniques should be further developed via computer-aided nematicide design. Bioinformatics devices can reinforce the potential of safe and effective biocontrol agents (BCAs) and their active components. They can delineate the interactions of bionematicides with their targeted PPN species and tackle complex diseases. Also, the functional plan of nematicides based on a blueprint of the intended goals should be further explored. Such goals can currently engage succinate dehydrogenase, acetylcholinesterase, and chitin deacetylase. Nonetheless, other biochemical compounds as novel targets for nematicides should be earnestly sought. Commonly used nematicides should be further tested for synergistic or additive function and be optimized via novel sequential, dual-purpose, and co-application of agricultural inputs, especially in integrated pest management schemes. Future directions and research priorities should address this novelty. Meanwhile, emerging bioactivated nematicides that offer reliability and nematode selectivity should be advanced for their favorable large-scale synthesis. Recent technological means should intervene to prevail over nematicide-related limitations. Nanoencapsulation can challenge production costs, effectiveness, and manufacturing defects of some nematicides. Recent progress in studying molecular plant-nematode interaction mechanisms can be further exploited for novel PPN control given related topics such as interfering RNA techniques, RNA-Seq in BCA development, and targeted genome editing. A few recent materials/techniques for control of PPNs in durable agroecosystems via decision support tools and decision support systems are addressed. The capability and effectiveness of nematicide operation harmony should be optimized via employing proper cooperative mechanisms among all partners.

Impacts of the Ban on the Soil-Applied Fumigant Methyl Bromide

The loss of the soil fumigant methyl bromide (MeBr) and adoption of soil fumigant alternatives has been challenging for farmers, particularly for those crops in which pathogens previously controlled by MeBr have emerged as significant problems, but it has resulted in some unanticipated benefits for the scientific community and the environment. Applauded as one of the most effective environmental agreements to date, the universally accepted Montreal Protocol on Ozone Depleting Substances has had a significant impact on the environment, reducing the release of halogenated compounds from anthropogenic sources enough to mitigate global warming by an estimated 1.1°C by 2021. The funding associated with various MeBr transition programs has increased collaboration across scientific disciplines, commodity groups, industry, and regulatory agencies. Chemical alternatives and improved application strategies, including the development of gas-retentive agricultural films, coupled with sound efficacy data and grower ingenuity have resulted in the sustained production of many of the impacted crops; although there has been some loss of acreage and value, particularly for Florida fumigated crops, for some, value has continued to increase, allowing production to continue. The loss of a single, broad-spectrum tool for pest control has led to a deeper understanding of the specific pest complexes impacting these at-risk crops, as well as the development of new, biologically based management tools for their control while increasing our understanding of the role of the soil microbiome in pest control and crop production.

Do biological control agents adapt to local pest genotypes? A multiyear test across geographic scales

Parasite local adaptation has been a major focus of (co)evolutionary research on host-parasite interactions. Studies of wild host-parasite systems frequently find that parasites paired with local, sympatric host genotypes perform better than parasites paired with allopatric host genotypes. In contrast, there are few such tests in biological control systems to establish whether biological control parasites commonly perform better on sympatric pest genotypes. This knowledge gap prevents the optimal design of biological control programs: strong local adaptation could argue for the use of sympatric parasites to achieve consistent pest control. To address this gap, we tested for local adaptation of the biological control bacterium Pasteuria penetrans to the root-knot nematode Meloidogyne arenaria, a global threat to a wide range of crops. We measured the probability and intensity of P. penetrans infection on sympatric and allopatric M. arenaria over the course of 4 years. Our design accounted for variation in adaptation across scales by conducting tests within and across fields, and we isolated the signature of parasite adaptation by comparing parasites collected over the course of the growing season. Our results are largely inconsistent with local adaptation of P. penetrans to M. arenaria: in 3 of 4 years, parasites performed similarly well in sympatric and allopatric combinations. In 1 year, however, infection probability was 28% higher for parasites paired with hosts from their sympatric plot, relative to parasites paired with hosts from other plots within the same field. These mixed results argue for population genetic data to characterize the scale of gene flow and genetic divergence in this system. Overall, our findings do not provide strong support for using P. penetrans from local fields to enhance biological control of Meloidogyne.

Evaluation of Soybean Genotypes (Glycine max and G. soja) for Resistance to the Root-Knot Nematode, Meloidogyne enterolobii

Potential resistance to the root-knot nematode (RKN) Meloidogyne enterolobii in 72 Glycine soja and 44 G. max soybean genotypes was evaluated in greenhouse experiments. Approximately 2,500 eggs of M. enterolobii were inoculated on each soybean genotype grown in a steam sterilized 1:1 sand to soil mixture. Sixty days postinoculation, plants were destructively harvested to determine the host status. The host status of each soybean genotype was determined by assessing root galling severity and calculating the final eggs per root system divided by the initial inoculum, or the reproduction factor (Rf). Five G. soja soybean genotypes were identified as resistant (Rf < 1) to M. enterolobii: '407202', '407239', '424083', '507618', and '639621'. None of the tested G. max soybean genotypes were identified as resistant to M. enterolobii. Some of the G. max genotypes determined to be susceptible to M. enterolobii include 'Hagood', 'Avery', 'Rhodes', 'Santee', and 'Bryan'. The genotype 'Bryan' had the lowest Rf values among the group at 5.06 and 6.67 in two independent trials, respectively, which represents a five- to sixfold increase in reproduction of M. enterolobii. Plant genotypes resistant to RKNs are effective in managing the disease and preserving yield, cost-efficient, and environmentally sustainable, and host resistance is often regarded as the most robust management tactic for controlling plant-parasitic nematodes. Resistance to RKNs in soybean genotypes has been identified for other Meloidogyne species, yet there is currently limited data regarding soybean host status to the highly aggressive nematode M. enterolobii. This study adds to the knowledge of potential native resistance to M. enterolobii in wild and cultivated soybean.

Evaluation of nematicides for Meloidogyne enterolobii management in sweetpotato

Sweetpotato is an important crop whose roots are consumed by people worldwide. Meloidogyne enterolobii stands out as a highly deleterious variant among the species of root-knot nematode that causes significant damage in sweetpotato. In the present study, the activity of four nematicides against M. enterolobii was assessed both in vitro and in growth cabinet experiments. After 48 hours of exposure, fluopyram and cyclobutrifluram had a greater negative effect on the motility of M. enterolobii second-stage juveniles (J2s) compared to fluensulfone and hymexazol, with respective median effective concentration (EC50) values of 0.204, 0.423, 22.335 and 216.622 mg L-1. When M. enterolobii eggs were incubated for 72 hours at the highest concentration of each nematicides, the inhibitory hatching effect of cyclobutrifluram (2.5 mg L-1), fluopyram (1.25 mg L-1) and fluensulfone (80 mg L-1) surpassed 85%, whereas hymexazol (640 mg L-1) was only 67%. Similar results were observed in growth cabinet experiments as well. The disease index (DI) and gall index (GI) were significantly decreased by all four nematicides compared to the control. However, the application of hymexazol did not yield a statistically significant difference in the egg masses index compared to the control, a finding which may be attributed to its potentially limited penetrability through the eggshell barrier. Overall, this study has demonstrated that all four nematicides effectively suppress M. enterolobii in sweetpotato, and this is the first report on the nematicidal activity of cyclobutrifluram and hymexazol against M. enterolobii.

Evaluation of Weed Species for Host Status to the Root-Knot Nematodes Meloidogyne enterolobii and M. incognita Race 4

Weeds that compete with valuable crops can also host plant-parasitic nematodes, acting as a source of nematode inoculum in a field and further damaging crops. The host status of 10 weed species commonly found in North Carolina, USA, was determined for the root-knot nematodes Meloidogyne enterolobii and M. incognita race 4 in the greenhouse. Each weed species was challenged with 5,000 eggs/plant of either M. enterolobii or M. incognita race 4, with five replicate plants per treatment in two separate greenhouse trials. Root galling severity and total number of nematode eggs per root system were recorded 60 days after inoculation. Reproduction factor (Rf = final nematode population/initial nematode population) was calculated to determine the host status of each weed species to M. enterolobii and M. incognita race 4. Four weed species (Datura stramonium, Digitaria sanguinalis, Senna obtusifolia, and Cyperus esculentus) were poor hosts (Rf < 1) to both nematode species, and roots of these weed plants did not display galling. Four weed species (Ipomoea hederacea, Amaranthus palmeri, Portulaca pilosa, and Ipomoea lacunosa) were hosts (Rf > 1) to both nematode species, and all had observable root gall formation. Sida rhombifolia and Cyperus rotundus were poor hosts to M. enterolobii but susceptible hosts to M. incognita. This study documents a differential host status of some common weeds to M. enterolobii and M. incognita race 4, and these results highlight the necessity of managing root-knot nematodes through controlling weeds in order to protect valuable crops.

Biocontrol potential of endophytic Pseudomonas strain IALR1619 against two Pythium species in cucumber and hydroponic lettuce

The use of fungicides to manage disease has led to multiple environmental externalities, including resistance development, pollution, and non-target mortality. Growers have limited options as legacy chemistry is withdrawn from the market. Moreover, fungicides are generally labeled for traditional soil-based production, and not for liquid culture systems. Biocontrol agents for disease management are a more sustainable and environmentally friendly alternative to conventional agroprotectants. Pythium ultimum is a soil borne oomycete plant pathogen with a broad taxonomic host range exceeding 300 plants. Cucumber seedlings exposed to P. ultimum 1 day after a protective inoculation with bacterial endophyte accession IALR1619 (Pseudomonas sp.) recorded 59% survival; with the control assessed at 18%. When the pathogen was added 5 days post endophyte inoculation, 74% of the seedlings treated survived, compared to 36% of the control, indicating a longer-term effect of IALR1619. Under hydroponic conditions, IALR1619 treated leaf type lettuce cv. 'Cristabel' and Romaine cv. 'Red Rosie' showed 29% and 42% higher shoot fresh weight compared to their controls, respectively. Similar results with less growth decline were observed for a repeat experiment with IALR1619. Additionally, an experiment on hydroponic lettuce in pots with perlite was carried out with a mixture of P. ultimum and P. dissotocum after IALR1619 inoculation. The endophyte treated 'Cristabel' showed fresh weight gain, but the second cultivar 'Pensacola' yielded no increase. In summary, the endophyte IALR1619 provided short term as well as medium-term protection against Pythium blight in cucumber seedlings and may be used as an alternative to conventional fungicides in a greenhouse setting. This study also demonstrated the potential of ALR1619 as a biocontrol agent against Pythium blight in hydroponic lettuce.

Revolutionizing nematode management to achieve global food security goals - An overview

Nematodes are soil-dwelling organisms that inflict substantial damage to crops, resulting in significant declines in agricultural productivity. Consequently, they are recognized as one of the primary contributors to global crop damage, with profound implications for food security. Nematology research assumes a pivotal role in tackling this issue and safeguarding food security. The pursuit of nematology research focused on mitigating nematode-induced crop damage and promoting sustainable agriculture represents a fundamental strategy for enhancing food security. Investment in nematology research is crucial to advance food security objectives by identifying and managing nematode species, developing novel technologies, comprehending nematode ecology, and strengthening the capabilities of researchers and farmers. This endeavor constitutes an indispensable step toward addressing one of the most pressing challenges in achieving global food security and promoting sustainable agricultural practices. Primarily, research endeavors facilitate the identification of nematode species responsible for crop damage, leading to the development of effective management strategies. These strategies encompass the utilization of resistant crop varieties, implementation of cultural practices, biological control, and chemical interventions. Secondly, research efforts contribute to the development of innovative technologies aimed at managing nematode populations, such as gene editing techniques that confer resistance to nematode infestations in crops. Additionally, the exploration of beneficial microbes, such as certain fungi and bacteria, as potential biocontrol agents against nematodes, holds promise. The study of nematode ecology represents a foundational research domain that fosters a deeper comprehension of nematode biology and ecological interactions. This knowledge is instrumental in devising precise and efficacious management strategies.

Metabolic variations in root tissues and rhizosphere soils of weak host plants potently lead to distinct host status and chemotaxis regulation of Meloidogyne incognita in intercropping

Root-knot nematodes (RKNs) inflict extensive damage to global agricultural production. Intercropping has been identified as a viable agricultural tool for combating RKNs, but the mechanisms by which intercropped plants modulate RKN parasitism are still not well understood. Here, we focus on the cucumber-amaranth intercropping system. We used a range of approaches, including the attraction assay, in vitro RNA interference (RNAi), untargeted metabolomics, and hairy root transformation, to unveil the mechanisms by which weak host plants regulate Meloidogyne incognita chemotaxis towards host plants and control infection. Amaranth roots showed a direct repellence to M. incognita through disrupting its chemotaxis. The in vitro RNAi assay demonstrated that the Mi-flp-1 and Mi-flp-18 genes (encoding FMRFamide-like peptides) regulated M. incognita chemotaxis towards cucumber and controlled infection. Moreover, M. incognita infection stimulated cucumber and amaranth to accumulate distinct metabolites in both root tissues and rhizosphere soils. In particular, naringenin and salicin, enriched specifically in amaranth rhizosphere soils, inhibited the expression of Mi-flp-1 and Mi-flp-18. In addition, overexpression of genes involved in the biosynthesis of pantothenic acid and phloretin, both of which were enriched specifically in amaranth root tissues, delayed M. incognita development in cucumber hairy roots. Together, our results reveal that both the distinct host status and disruption of chemotaxis contribute to M. incognita inhibition in intercropping.

Efficacy of five nematicides against root-knot nematode when applied via single and double drip tapes in a Florida sandy soil

BACKGROUND

The efficacy of drip-applied nematicides depends on adequate product distribution, which can be difficult in sandy soils. Three new non-fumigant nematicides (fluazaindolizine, fluensulfone, fluopyram), together with two old nematicides, oxamyl and metam potassium, were evaluated when applied via single and double drip tapes to control root-knot nematode in cucumber and squash in Florida between February 2020 and December 2022.

RESULTS

Nematicide applications via double drip tapes resulted in lower root gall infection (and tend to have higher yield) as compared to a single tape for fluopyram, but no difference was noted between single and double tapes for oxamyl and fluazaindolizine. Fluensulfone response was somewhere in between and metam potassium had higher squash yield when applied with double tapes. Root-knot infection was higher in cucumber than in squash, and metam potassium had the highest yields and lowest nematode infection compared to other nematicide treatments.

CONCLUSION

The benefit of double versus single drip tapes depended on the type of nematicide that was applied and was evident for nematicides that have poor water solubility like fluopyram. Some benefit was noted for metam potassium, but no or limited benefit was noted for oxamyl, fluazaindolizine and fluensulfone. © 2023 Society of Chemical Industry.

The New Nematicide Cyclobutrifluram Targets the Mitochondrial Succinate Dehydrogenase Complex in Caenorhabditis elegans

Today, agriculture around the world is challenged by parasitic nematode infections. Plant-parasitic nematodes (PPNs) can cause significant damage and crop loss and are a threat to food security. For a long time, the management of PPN infection has relied on nematicides that impact not only parasitic nematodes but also other organisms. More recently, new nematicides have been developed that appear to specifically target PPN. Cyclobutrifluram belongs to this new category of nematicides. Using the nematode Caenorhabditis elegans as a model organism, we show here that cyclobutrifluram strongly impacts the survival and fertility rates of the worm by decreasing the number of germ cells. Furthermore, using a genetic approach, we demonstrate that cyclobutrifluram functions by inhibiting the mitochondrial succinate dehydrogenase (SDH) complex. Transcriptomic analysis revealed a strong response to cyclobutrifluram exposure. Among the deregulated genes, we found genes coding for detoxifying proteins, such as cytochrome P450s and UDP-glucuronosyl transferases (UGTs). Overall, our study contributes to the understanding of the molecular mode of action of cyclobutrifluram, to the finding of new approaches against nematicide resistance, and to the discovery of novel nematicides. Furthermore, this study confirms that C. elegans is a suitable model organism to study the mode of action of nematicides.

Biocontrol Efficacy of Bacillus methylotrophicus TA-1 Against Meloidogyne incognita in Tomato

Root-knot nematodes (RKNs) are harmful plant-parasitic nematodes of tomatoes which can cause significant yield losses. Therefore, there is increasing interest in exploring the application of bacterial nematicides. The bacterium Bacillus methylotrophicus TA-1 is a broad-spectrum biological control agent; however, its effect on RKNs control remains largely unclear. In this study, the toxicity of B. methylotrophicus TA-1 against Meloidogyne incognita was investigated in vitro, and the potential of B. methylotrophicus TA-1 to decrease infection of RKNs in tomato were evaluated in pot and field trials. Results showed that B. methylotrophicus TA-1 exhibited high nematicidal activity against second-stage juveniles (J2s) and eggs of M. incognita with 50% lethal concentration (LC50) values of 5.80 and 7.00 × 108 colony forming units (CFU)/ml, respectively. In the pot experiments and field trials conducted in 2020 and 2021, tomato plants treated with B. methylotrophicus TA-1 soil drench applied once at 3, 6, and 9 × 108 CFU/plant had significantly higher plant height and greater yield compared with the untreated control. Tomato yields of the treated plots with B. methylotrophicus TA-1 in 2 consecutive years' field trials were between 53.4 to 66.1 and 52.8 to 61.5 t/ha, while they were 49.7 and 48.2 t/ha in the untreated control for each year, respectively. The lowest population densities of M. incognita at 30 and 60 days after treatment were 119 and 135 J2s per 100 g soil in 2020 and 43 and 118 J2s in 2021 in TA-1-treated plots. The lowest gall index of 4.7 and 3.3 in 2020 and 2021, respectively, and the highest yield were all observed in the TA-1 at 9 × 108 CFU/plant treated plants, with no significant differences with the commercial control abamectin. These results provided a basis for further studies of B. methylotrophicus TA-1 formulations, application doses, frequencies, and mechanisms of action, which are necessary before it could be used as a component of integrated management programs to manage RKNs in tomato production.

Nematicidal Coumarins from Cnidium monnieri Fruits and Angelica dahurica Roots and Their Physiological Effect on Pine Wood Nematode (Bursaphelenchus xylophilus)

Pine wood nematode (PWN), Bursaphelenchus xylophilus, is a major pathogen of pine wilt disease (PWD), which is a devastating disease affecting pine trees. Eco-friendly plant-derived nematicides against PWN have been considered as promising alternatives to control PWD. In this study, the ethyl acetate extracts of Cnidium monnieri fruits and Angelica dahurica roots were confirmed to have significant nematicidal activity against PWN. Through bioassay-guided fractionations, eight nematicidal coumarins against PWN were separately isolated from the ethyl acetate extracts of C. monnieri fruits and A. dahurica roots, and they were identified to be osthol (Compound 1), xanthotoxin (Compound 2), cindimine (Compound 3), isopimpinellin (Compound 4), marmesin (Compound 5), isoimperatorin (Compound 6), imperatorin (Compound 7), and bergapten (Compound 8) by mass and nuclear magnetic resonance (NMR) spectral data analysis. Coumarins 1-8 were all determined to have inhibitory effects on the egg hatching, feeding ability, and reproduction of PWN. Moreover, all eight nematicidal coumarins could inhibit the acetylcholinesterase (AChE) and Ca²⁺ ATPase of PWN. Cindimine 3 from C. monnieri fruits showed the strongest nematicidal activity against PWN, with an LC₅₀ value of 64 μM at 72 h, and the highest inhibitory effect on PWN vitality. In addition, bioassays on PWN pathogenicity demonstrated that the eight nematicidal coumarins could effectively relieve the wilt symptoms of black pine seedlings infected by PWN. The research identified several potent botanical nematicidal coumarins for use against PWN, which could contribute to the development of greener nematicides for PWD control.

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.

Plant-Parasitic Nematodes and their Effects on Ornamental Plants: A Review

Worldwide, the ornamental plant industry is estimated to be valued at $70 billion, with the United States' ornamental plant industry valued at $4.8 billion in 2020. Ornamental plants are cultivated for numerous reasons worldwide, such as decorative, medicinal, social, and utility purposes, making the ornamental field a high growth industry. One of the main pathogen groups affecting the yield and growth of the ornamental plant industry is plant-parasitic nematodes, which are microscopic roundworms that feed on plant parts causing significant yield loss. There are many kinds of plant-parasitic nematodes that affect ornamental plants, with the main genera being Meloidogyne spp., Aphelenchoides spp., Paratylenchus spp., Pratylenchus spp., Helicotylenchus spp., Radopholus spp., Xiphinema spp., Trichodorus spp., Paratrichodorus spp., Rotylenchulus spp., and Longidorus spp. The aim of this review is to focus on the effects, hosts, and symptoms of these major plant-parasitic nematodes on ornamental plants and synthesize current management strategies in the ornamental plant industry.

Seed Treatments for Management of Soybean Cyst Nematode, Heterodera glycines, in Mid-Atlantic Soybean Production

Soybean Cyst Nematode (SCN), Heterodera glycines Ichinohe, is the most important pathogen of soybean in the Mid-Atlantic region. In recent decades, a decline in the effectiveness of genetic resistance has been observed and additional management approaches are needed. Seed treatments are of rising interest, but no local data on product response exists for the region. In 2020-2021, two experiments were conducted to observe the effects of chemical and biological seed treatment options. In one experiment, chemical seed treatments pydiflumetofen (Saltro®) and fluopyram (ILEVO®) were screened against nontreated plain seed for SCN suppression. In a second experiment, pydiflumetofen, fluopyram and four biological nematode-protectant seed treatments with a standard base insecticide and fungicide treatment were compared to nontreated plain seed and seed with only the standard base treatment to test product efficacy against SCN. Seed treatments increased the percent emergence over plain seed. Nematode reproductive factors and female counts from roots were collected, but did not statistically differ between seed treatments or plain seed. Yield differences were observed in one of the five trials, where pydiflumetofen + base seed treatment yielded the highest (p < 0.001) at 3813.1 kg/ha. Response from seed treatments varied, with no specific seed treatment consistently reducing SCN populations or increasing yield across trials. Seed treatments may have potential as an element of an integrated management approach for SCN.

Comparative Effectiveness of Filamentous Fungi in Biocontrol of Meloidogyne javanica and Activated Defense Mechanisms on Tomato

The nematicidal potential of five filamentous fungi as biological control agents (BCAs) against the root-knot nematode (RKN), Meloidogyne javanica, infecting tomato was assessed in vitro and in pot experiments. The five promising native taxa, namely Trichoderma longibrachiatum, T. harzainum, T. asperellum, Lecanicillium spp., and Metacordyceps chlamydosporia, were selected to compare their effectiveness against both chemical (Mocap, 10% ethoprophos) and biological (abamectin) nematicides on M. javanica reproduction indices and plant growth parameters. The stimulation of defense mechanisms was assessed by monitoring changes in the enzymatic activities of the polyphenol oxidase (PPO), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), lipid peroxidation (MDA), phenols, and proteins content of tomato roots. The laboratory assays revealed that T. longibrachiatum, M. chlamydoporia, and Lecanicillium spp. seemed to be the most effective under laboratory conditions, with more than 60% of juvenile mortality. The egg infection rate was above 62%, and the egg hatching rate was below 32%. The direct parasitism by the five taxa was confirmed by scanning electron microscope observation. The results of this study found a similar parasitism mechanism for T. longibrachiatum, T. harzianum, and M. chlamydosporia, where their hyphae and spores adhered to the M. javanica juveniles cuticle layer and formed trapping rings around them. The pot experiment results showed that T. harzianum and Lecanicillium spp. enhanced the plant growth parameters. Trichoderma longibrachiatum, abamectin, and the ethoprophos-based nematicides effectively decreased the reproduction rates of the nematode. The Trichoderma species and M. chlamydosporia significantly reduced the gall index and female fecundity of RKN. The treatment with BCAs and chemical nematicides involved a significant increase in the antioxidant activities of nematode-infected plants. The ethoprophos and fungal treatments decreased the MDA and total phenols content compared with the nematode-infested seedlings. This paper analyzes the advancements made towards the effective and efficient biocontrol of M. javanica using different fungal taxa, especially T. longibrachiatum and M. chlamydosporia, and the implications of these advancements for sustainable agriculture and food security.

Exploiting Plant-Phytonematode Interactions to Upgrade Safe and Effective Nematode Control

Plant-parasitic nematodes (PPNs) bring about substantial losses of economic crops globally. With the environmental and health issues facing the use of chemical nematicides, research efforts should focus on providing economically effective and safe control methods. The sound exploitation of plant-PPN interactions is fundamental to such efforts. Initially, proper sampling and extraction techniques should be followed to avoid misleading nematode data. Recent evolutions in plant-PPN interactions can make use of diverse non-molecular and molecular approaches to boost plant defenses. Therefore, PPN control and increasing crop yields through single, sequential, dual-purpose, and simultaneous applications of agricultural inputs, including biocontrol agents, should be seriously attempted, especially within IPM schemes. The use of biologicals would ideally be facilitated by production practices to solve related issues. The full investment of such interactions should employ new views of interdisciplinary specialties in the relevant modern disciplines to optimize the PPN management. Having an accurate grasp of the related molecular events will help in developing tools for PPN control. Nonetheless, the currently investigated molecular plant-PPN interactions favoring plant responses, e.g., resistance genes, RNA interference, marker-assisted selection, proteinase inhibitors, chemo-disruptive peptides, and plant-incorporated protectants, are key factors to expanding reliable management. They may be applied on broader scales for a substantial improvement in crop yields.

(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.

Application Potential of Bacterial Volatile Organic Compounds in the Control of Root-Knot Nematodes

Plant-parasitic nematodes (PPNs) constitute the most damaging group of plant pathogens. Plant infections by root-knot nematodes (RKNs) alone could cause approximately 5% of global crop loss. Conventionally, chemical-based methods are used to control PPNs at the expense of the environment and human health. Accordingly, the development of eco-friendly and safer methods has been urged to supplement or replace chemical-based methods for the control of RKNs. Using microorganisms or their metabolites as biological control agents (BCAs) is a promising approach to controlling RKNs. Among the metabolites, volatile organic compounds (VOCs) have gained increasing attention because of their potential in the control of not only RKNs but also other plant pathogens, such as insects, fungi, and bacteria. This review discusses the biology of RKNs as well as the status of various control strategies. The discovery of VOCs emitted by bacteria from various environmental sources and their application potential as BCAs in controlling RKNs are specifically addressed.

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.

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.

Juglone and 1,4-Naphthoquinone-Promising Nematicides for Sustainable Control of the Root Knot Nematode Meloidogyne luci

The scarce availability of efficient and eco-friendly nematicides to control root-knot nematodes (RKN), Meloidogyne spp., has encouraged research toward the development of bionematicides. Naphthoquinones, juglone (JUG) and 1,4-naphthoquinone (1,4-NTQ), are being explored as alternatives to synthetic nematicides to control RKN. This study expands the knowledge on the effects of these natural compounds toward M. luci life cycle (mortality, hatching, penetration, reproduction). M. luci second-stage juveniles (J2)/eggs were exposed to each compound (250, 150, 100, 50, and 20 ppm) to monitor nematode mortality and hatching during 72 h and 15 days, respectively. Tomato seedlings were then inoculated with 200 J2, which had been exposed to JUG/1,4-NTQ for 3 days. The number of nematodes inside the roots was determined at 3 days after inoculation, and the final population density was assessed at 45 days after inoculation. Moreover, the potential mode of action of JUG/1,4-NTQ was investigated for the first time on RKN, through the assessment of reactive oxygen species (ROS) generation, acetylcholinesterase (AChE) in vitro inhibitory activity and expression analysis of ache and glutathione-S-transferase (gst) genes. 1,4-NTQ was the most active compound, causing ≥50% J2 mortality at 250 ppm, within 24 h. At 20 and 50 ppm, hatching was reduced by ≈50% for both compounds. JUG showed a greater effect on M. luci penetration and reproduction, decreasing infection by ≈80% (50 ppm) on tomato plants. However, 1,4-NTQ-induced generation of ROS and nematode vacuolization was observed. Our study confirms that JUG/1,4-NTQ are promising nematicidal compounds, and new knowledge on their physiological impacts on Meloidogyne was provided to open new avenues for the development of innovative sustainable nematicides.

Bacterial Volatile-Mediated Suppression of Root-Knot Nematode (Meloidogyne incognita)

Root-knot nematodes (Meloidogyne spp.) are obligate plant parasites that cause severe economic losses to agricultural crops worldwide. Because of serious health and environmental concerns related to the use of chemical nematicides, the development of efficient alternatives is of great importance. Biological control through exploiting the potential of rhizosphere microorganisms is currently accepted as an important approach for pest management in sustainable agriculture. In our research, during screening of rhizosphere bacteria against the root-knot nematodes Meloidogyne incognita, Ochrobactrum pseudogrignonense strain NC1 from the rhizosphere of healthy tomatoes showed strong nematode inhibition. A volatile nematicidal assay showed that the cell-free fermentation filtrate in the first-row wells of 12-well tissue culture plates caused M. incognita juvenile mortality in the second-row wells. Gas chromatography-mass spectrometry analysis revealed that dimethyl disulfide (DMDS) and benzaldehyde were the main volatile compounds produced by strain NC1. The nematicidal activity of these compounds indicated that the lethal concentration 50 against the M. incognita juveniles in the second-row wells and the fourth-row wells were 23.4 μmol/ml and 30.7 μmol/ml for DMDS and 4.7 μmol/ml and 15.2 μmol/ml for benzaldehyde, respectively. A greenhouse trial using O. pseudogrignonense strain NC1 provided management efficiencies of root-knot nematodes of 88 to 100% compared with the untreated control. This study demonstrated that nematode-induced root-gall suppression mediated by the bacterial volatiles DMDS and benzaldehyde presents a new opportunity for root-knot nematode management.

Sensitivity of Meloidogyne enterolobii and M. incognita to fluorinated nematicides

BACKGROUND

Meloidogyne enterolobii (Yang and Eisenback) was recently introduced into Louisiana on contaminated sweetpotato planting material. Given the known variation in sensitivity to nematicides within the genus Meloidogyne, there is question as to whether fluorinated nematicides will be as efficacious toward M. enterolobii as they are with M. incognita (Chitwood). Using a series of in vitro and growth cabinet experiments, this study compared the sensitivity of M. enterolobii and M. incognita to four synthetic non-fumigant nematicides (fluopyram, fluensulfone, fluazaindolizine, and oxamyl).

RESULTS

Meloidogyne enterolobii had lower sensitivity to nematicides than M. incognita in the majority of the in vitro exposure assays. Similar levels of reduction in root infectivity were observed after nematicide exposure among both nematode species. Fluopyram showed high hatching inhibition for both Meloidogyne species at low concentrations [median effective concentration (EC₅₀ ) values of 0.273 to 0.018 mg L^-1 ], whereas fluensulfone showed high root penetration inhibition at low concentrations (EC₅₀ values of 0.151 to 0.065 mg L^-1 ) relative to that of other evaluated nematicides. For both Meloidogyne species, each of the four non-fumigant nematicides reduced root galling (58-96% reduction for M. enterolobii, 71-100% reduction for M. incognita) and egg production (63-99% reduction for M. enterolobii, 58-96% reduction for M. incognita) on sweetpotato when applied at the label recommended rate.

CONCLUSION

Fluorinated nematicides and oxamyl show capacity to suppress M. enterolobii on sweetpotato. © 2021 Society of Chemical Industry.

Germination Stimulant Activity of Isothiocyanates on Phelipanche spp

The root parasitic weed broomrapes, Phelipanche spp., cause severe damage to agriculture all over the world. They have a special host-dependent lifecycle and their seeds can germinate only when they receive chemical signals released from host roots. Our previous study demonstrated that 2-phenylethyl isothiocyanate is an active germination stimulant for P. ramosa in root exudates of oilseed rape. In the present study, 21 commercially available ITCs were examined for P. ramosa seed germination stimulation, and some important structural features of ITCs for exhibiting P. ramosa seed germination stimulation have been uncovered. Structural optimization of ITC for germination stimulation resulted in ITCs that are highly active to P. ramosa. Interestingly, these ITCs induced germination of P. aegyptiaca but not Orobanche minor or Striga hermonthica. P. aegyptiaca seeds collected from mature plants parasitizing different hosts responded to these ITCs with different levels of sensitivity. ITCs have the potential to be used as inducers of suicidal germination of Phelipanche seeds.

Management of Meloidogyne incognita on Cucumber with a New Nonfumigant Nematicide Fluopimomide

Cucumber (Cucumis sativus L.) is an economically important vegetable crop in China. Southern root-knot nematode (Meloidogyne incognita) is a significant obstacle in cucumber production, causing severe root damage and yield losses. Moreover, resistance development to fosthiazate, and poor mobility of abamectin, have led to failure to control this nematode. It is of great interest to growers and the vegetable industry to explore novel nonfumigant nematicides that can provide adequate control in an environmentally friendly manner. Fluopimomide (FM), a new chemical having a similar structure to fluopyram, was shown to exhibit toxic effects on fungi and nematodes. The efficacy of FM to reduce infection of M. incognita in cucumber was evaluated under greenhouse and field conditions. In the greenhouse, FM at all test rates resulted in a 22.5 to 39.6% and 31.3 to 55.0% reduction in the population density of M. incognita in the soil at 30 and 60 days after treatment (DAT), respectively, compared with the nontreated control. FM at 500 and 750 g ha^-1 reduced (P < 0.05) root galling, meanwhile increasing plant height compared with the nontreated control at 30 and 60 DAT. In the field trials, FM at 500 and 750 g ha^-1 decreased the population density of M. incognita and root galling 57.2 to 69.9% compared with the untreated control, while enhancing cucumber yield in two consecutive years. Furthermore, FM at 500 g ha^-1 combined with fosthiazate was the most effective treatment showing a synergistic effect on reducing population densities of M. incognita, which was significantly greater than either FM or fosthiazate by themselves. In summary, FM has considerable potential for managing M. incognita on cucumber.

An emerging chemical fumigant: two-sided effects of dazomet on soil microbial environment and plant response

Methyl bromide has been banned worldwide because it causes damage to the ozone layer and the environment. To find a substitute for methyl bromide, the relationships among fumigation, plant growth, and the microbial community in replant soil require further study. We performed pot and field experiments to investigate the effects of dazomet fumigation on soil properties and plant performance. Changes in soil microbial community structure and diversity were assessed using high-throughput sequencing, and plant physiological performance and soil physicochemical properties were also measured. Dazomet fumigation enhanced photosynthesis and promoted plant growth in replant soil; it altered soil physical and chemical properties and reduced soil enzyme activities, although these parameters gradually recovered over time. After dazomet fumigation, the dominant soil phyla changed, microbial diversity decreased significantly, the relative abundance of biocontrol bacteria such as Mortierella increased, and the relative abundance of pathogenic bacteria such as Fusarium decreased. Over the course of the experiment, the soil microbial flora changed dynamically, and soil enzyme activities and other physical and chemical properties also recovered to a certain extent. This result suggested that the effect of dazomet on soil microorganisms was temporary. However, fumigation also led to an increase in some resistant pathogens, such as Trichosporon, that affect soil function and health. Therefore, it is necessary to consider potential negative impacts of dazomet on the soil environment and to perform active environmental risk management in China.

Volatile Organic Compounds from Bacillus aryabhattai MCCC 1K02966 with Multiple Modes against Meloidogyne incognita

Plant-parasitic nematodes cause severe losses to crop production and economies all over the world. Bacillus aryabhattai MCCC 1K02966, a deep-sea bacterium, was obtained from the Southwest Indian Ocean and showed nematicidal and fumigant activities against Meloidogyne incognita in vitro. The nematicidal volatile organic compounds (VOCs) from the fermentation broth of B. aryabhattai MCCC 1K02966 were investigated further using solid-phase microextraction gas chromatography-mass spectrometry. Four VOCs, namely, pentane, 1-butanol, methyl thioacetate, and dimethyl disulfide, were identified in the fermentation broth. Among these VOCs, methyl thioacetate exhibited multiple nematicidal activities, including contact nematicidal, fumigant, and repellent activities against M. incognita. Methyl thioacetate showed a significant contact nematicidal activity with 87.90% mortality at 0.01 mg/mL by 72 h, fumigant activity in mortality 91.10% at 1 mg/mL by 48 h, and repellent activity at 0.01-10 mg/mL. In addition, methyl thioacetate exhibited 80-100% egg-hatching inhibition on the 7th day over the range of 0.5 mg/mL to 5 mg/mL. These results showed that methyl thioacetate from MCCC 1K02966 control M. incognita with multiple nematicidal modes and can be used as a potential biological control agent.

Alleviation of Nematode-Mediated Apple Replant Disease by Pre-Cultivation of Tagetes

Apple replant disease (ARD) is a severe problem in orchards and tree nurseries caused by yet unknown soil biota that accumulate over replanting cycles. This study tested the contribution of nematodes to ARD, and cultivation of Tagetes as a control option. In a pot experiment, Tagetes patula or Tagetes tenuifolia were grown in ARD soil, incorporated or removed. Nematodes extracted from untreated ARD soil and washed on 20-µm sieves induced ARD symptoms when inoculated to apple plantlets growing in a sterile substrate. In contrast, nematodes from Tagetes treated ARD soil did not reduce root growth compared to uninoculated plants, irrespective of Tagetes species and incorporation. In plots of five apple tree nurseries or orchards, either Tagetes or grass was grown on ARD soil. Nematodes extracted from the grass plots and inoculated to apple plantlets significantly reduced plant growth compared to nematodes from Tagetes plots for all five farms. Apple rootstocks showed overall a significantly higher increase in shoot base diameter when grown on Tagetes-treated plots compared to grass plots, while this effect differed among farms. Plant-parasitic nematodes were too low in abundance to explain plant damage. In conclusion, Tagetes alleviated ARD by changing the nematode community in soil.

Isolation, Identification, and Antibacterial Mechanisms of Bacillus amyloliquefaciens QSB-6 and Its Effect on Plant Roots

Apple replant disease (ARD) is a common problem in major apple planting areas, and biological factors play a leading role in its etiology. Here, we isolated the bacterial strain QSB-6 from the rhizosphere soil of healthy apple trees in a replanted orchard using the serial dilution method. Strain QSB-6 was provisionally identified as Bacillus amyloliquefaciens based on its morphology, physiological and biochemical characteristics, carbon source utilization, and chemical sensitivity. Maximum likelihood analysis based on four gene sequences [16S ribosomal RNA gene (16S rDNA), DNA gyrase subunit A (gyrA), DNA gyrase subunit B (gyrB), and RNA polymerase subunit B (rpoB)] from QSB-6 and other strains indicated that it had 100% homology with B. amyloliquefaciens, thereby confirming its identification. Flat standoff tests showed that strain QSB-6 had a strong inhibitory effect on Fusarium proliferatum, Fusarium solani, Fusarium verticillioides, Fusarium oxysporum, Alternaria alternata, Aspergillus flavus, Phoma sp., Valsa mali, Rhizoctonia solani, Penicillium brasilianum, and Albifimbria verrucaria, and it had broad-spectrum antibacterial characteristics. Extracellular metabolites from strain QSB-6 showed a strong inhibitory effect on Fusarium hyphal growth and spore germination, causing irregular swelling, atrophy, rupture, and cytoplasmic leakage of fungal hyphae. Analysis of its metabolites showed that 1,2-benzenedicarboxylic acid and benzeneacetic acid, 3- hydroxy-, methyl ester had good inhibitory effects on Fusarium, and increased the length of primary roots and the number of lateral roots of Arabidopsis thaliana plantlet. Pot experiments demonstrated that a QSB-6 bacterial fertilizer treatment (T2) significantly improved the growth of Malus hupehensis Rehd. seedlings. It increased root length, surface area, tips, and forks, respiration rate, protective enzyme activities, and the number of soil bacteria while reducing the number of soil fungi. Fermentation broth from strain QSB-6 effectively prevented root damage from Fusarium. terminal restriction fragment length polymorphism (T-RFLP) and quantitative PCR (qPCR) assays showed that the T2 treatment significantly reduced the abundance of Fusarium in the soil and altered the soil fungal community structure. In summary, B. amyloliquefaciens QSB-6 has a good inhibitory effect on Fusarium in the soil and can significantly promote plant root growth. It has great potential as a biological control agent against ARD.

Transcriptomic Analysis of Resistant and Susceptible Responses in a New Model Root-Knot Nematode Infection System Using Solanum torvum and Meloidogyne arenaria

Root-knot nematodes (RKNs) are among the most devastating pests in agriculture. Solanum torvum Sw. (Turkey berry) has been used as a rootstock for eggplant (aubergine) cultivation because of its resistance to RKNs, including Meloidogyne incognita and M. arenaria. We previously found that a pathotype of M. arenaria, A2-J, is able to infect and propagate in S. torvum. In vitro infection assays showed that S. torvum induced the accumulation of brown pigments during avirulent pathotype A2-O infection, but not during virulent A2-J infection. This experimental system is advantageous because resistant and susceptible responses can be distinguished within a few days, and because a single plant genome can yield information about both resistant and susceptible responses. Comparative RNA-sequencing analysis of S. torvum inoculated with A2-J and A2-O at early stages of infection was used to parse the specific resistance and susceptible responses. Infection with A2-J did not induce statistically significant changes in gene expression within one day post-inoculation (DPI), but afterward, A2-J specifically induced the expression of chalcone synthase, spermidine synthase, and genes related to cell wall modification and transmembrane transport. Infection with A2-O rapidly induced the expression of genes encoding class III peroxidases, sesquiterpene synthases, and fatty acid desaturases at 1 DPI, followed by genes involved in defense, hormone signaling, and the biosynthesis of lignin at 3 DPI. Both isolates induced the expression of suberin biosynthetic genes, which may be triggered by wounding during nematode infection. Histochemical analysis revealed that A2-O, but not A2-J, induced lignin accumulation at the root tip, suggesting that physical reinforcement of cell walls with lignin is an important defense response against nematodes. The S. torvum-RKN system can provide a molecular basis for understanding plant-nematode interactions.

Long-term effect of fumigation and a sorghum cover crop on broadleaf and grass weeds in plastic-mulched tomato

BACKGROUND

Broadleaf and grass weeds can adversely affect growth and productivity of plastic-mulched tomato (Solanum lycopersicum L.). Two, four-year research trials were conducted in Florida to evaluate the effect of repeated fumigation and chemical fallow versus a sorghum [Sorghum bicolor S. bicolor var. sudanense (Piper) Stapf.] cover crop on broadleaf and grass weeds in tomato plasticulture.

RESULTS

1,3-Dichloropropene (1,3-D) + chloropicrin (Pic), dimethyl disulfide (DMDS) + Pic, and DMDS + metam potassium effectively controlled broadleaf weeds in-crop and reduced densities by 79-98% compared to the non-fumigated control but provided inconsistent control of grass weeds. DMDS + metam potassium was generally the most effective fumigant. During the fallow period, a sorghum cover crop effectively reduced broadleaf weed density than the chemical fallow, while chemical fallow effectively reduced grass weed density than the cover crop. The fallow program did not affect in-crop densities of broadleaf and grass weeds. In some measurements, the evaluated fumigants resulted in taller tomato plants and higher yield compared to the non-fumigated control.

CONCLUSION

We conclude that the evaluated soil fumigants effectively control broadleaf and grass weeds. Planting a sorghum cover crop effectively suppresses broadleaf weeds but not grasses during the fallow period. However, this suppression does not result in reduced weed density in-crop despite the fact that similar weed species were observed in both time periods. © 2020 Society of Chemical Industry.

Differential Response of Meloidogyne, Pratylenchus, Globodera, and Xiphinema Species to the Nematicide Fluazaindolizine

This research focused on the effects of fluazaindolizine on a diversity of plant-parasitic nematodes. In microwell assays, 24-h dose-response curves were generated for several species and populations of Meloidogyne, Pratylenchus neglectus, P. penetrans, Globodera ellingtonae, and Xiphinema americanum. In a greenhouse study, the impact of fluazaindolizine on fecundity of M. incognita, M. hapla, and M. chitwoodi was tested by exposing nematodes for 24 h in solution and inoculating on tomato. The average 24-h ED₅₀s (dose that resulted in the immobility of 50% of exposed nematodes) for M. hapla, M. chitwoodi, and M. incognita were 325.7, 223.4, and 100.7 ppm, respectively. M. hapla had the most variation among populations, with 24-h ED₅₀s ranging from 72 to 788 ppm. G. ellingtonae had the lowest 24-h ED₅₀ at 30 ppm. Pratylenchus spp. were unaffected by fluazaindolizine. X. americanum was the only species where effects of fluazaindolizine were reversible, but had a 24-h ED₅₀ that fell in the range of the Meloidogyne spp. In the greenhouse study, M. chitwoodi was the least sensitive with reproduction reaching 62% of the untreated control after a pre-exposure to 47 ppm, whereas M. incognita and M. hapla at the same exposure dose had reproduction rates of 27 and 36% of the untreated control, respectively. Despite varying in in vitro responses to fluazaindolizine, reproduction of all Meloidogyne spp. was suppressed after only 24 h of exposure. This study expanded our understanding of how G. ellingtonae, P. thornei, P. penetrans, and X. americanum respond to fluazaindolizine.

Meloidogyne enterolobii, a Major Threat to Tomato Production: Current Status and Future Prospects for Its Management

The guava root-knot nematode, Meloidogyne enterolobii (Syn. M. mayaguensis), is an emerging pathogen to many crops in the world. This nematode can cause chlorosis, stunting, and reduce yields associated with the induction of many root galls on host plants. Recently, this pathogen has been considered as a global threat for tomato (Solanum lycopersicum L.) production due to the lack of known resistance in commercially accepted varieties and the aggressiveness of M. enterolobii. Both conventional morphological and molecular approaches have been used to identify M. enterolobii, an important first step in an integrated management. To combat root-knot nematodes, integrated disease management strategies such as crop rotation, field sanitation, biocontrol agents, fumigants, and resistant cultivars have been developed and successfully used in the past. However, the resistance in tomato varieties mediated by known Mi-genes does not control M. enterolobii. Here, we review the current knowledge on geographic distribution, host range, population biology, control measures, and proposed future strategies to improve M. enterolobii control in tomato.

Multi-seasonal modelling of plant-nematode interactions reveals efficient plant resistance deployment strategies

Root-knot nematodes, Meloidogyne spp., are soil-borne polyphagous pests with major impact on crop yield worldwide. Resistant crops efficiently control avirulent root-knot nematodes, but favour the emergence of virulent forms. Since virulence is associated with fitness costs, susceptible crops counter-select virulent root-knot nematodes. In this study, we identify optimal rotation strategies between susceptible and resistant crops to control root-knot nematodes and maximize crop yield. We developed an epidemiological model describing the within-season dynamics of avirulent and virulent root-knot nematodes on susceptible or resistant plant root-systems, and their between-season survival. The model was fitted to experimental data and used to predict yield-maximizing rotation strategies, with special attention to the impact of epidemic severity and genetic parameters. Crop rotations were found to be efficient under realistic parameter ranges. They were characterized by low ratios of resistant plants and were robust to parameter uncertainty. Rotations provide significant gain over resistant-only strategies, especially under intermediate fitness costs and severe epidemic contexts. Switching from the current general deployment of resistant crops to custom rotation strategies could not only maintain or increase crop yield, but also preserve the few and valuable R-genes available.

Organic Amendments for Pathogen and Nematode Control

The loss of methyl bromide as a soil fumigant and minimal advances in the development and registration of new chemical fumigants has resulted in a resurgence of interest in the application of organic amendments (OAs) for soilborne plant pathogen and plant-parasitic nematode management. Significant progress has been made in the characterization of OAs, application of strategies for their use, and elucidation of mechanisms by which they suppress soilborne pests. Nonetheless, their utility is limited by the variability of disease control, expense, and the logistics of introducing them into crop production systems. Recent advances in molecular techniques have led to significant progress in the elucidation of the role of bacteria and fungi and their metabolic products on disease suppression with the addition of OAs. Biosolarization and anaerobic soil disinfestation, developed to manipulate systems and favor beneficial microorganisms to maximize their impact on plant pathogens, are built on a strong historical research foundation in OAs and the physical, chemical, and biological characteristics of disease-suppressive soils. This review focuses on recent applications of OAs and their potential for the management of soilborne plant pathogens and plant-parasitic nematodes, with emphasis primarily on annual fruit and vegetable production systems.

Current Insights into Migratory Endoparasitism: Deciphering the Biology, Parasitism Mechanisms, and Management Strategies of Key Migratory Endoparasitic Phytonematodes

Despite their physiological differences, sedentary and migratory plant-parasitic nematodes (PPNs) share several commonalities. Functional characterization studies of key effectors and their targets identified in sedentary phytonematodes are broadly applied to migratory PPNs, generalizing parasitism mechanisms existing in distinct lifestyles. Despite their economic significance, host-pathogen interaction studies of migratory endoparasitic nematodes are limited; they have received little attention when compared to their sedentary counterparts. Because several migratory PPNs form disease complexes with other plant-pathogens, it is important to understand multiple factors regulating their feeding behavior and lifecycle. Here, we provide current insights into the biology, parasitism mechanism, and management strategies of the four-key migratory endoparasitic PPN genera, namely Pratylenchus, Radopholus, Ditylenchus, and Bursaphelenchus. Although this review focuses on these four genera, many facets of feeding mechanisms and management are common across all migratory PPNs and hence can be applied across a broad genera of migratory phytonematodes.

Evaluation of ethanedinitrile (EDN) as a preplant soil fumigant in Florida strawberry production

BACKGROUND

Ethanedinitrile (EDN) has shown promising efficacy against weeds, plant-parasitic nematodes and soil-borne pathogens. Field experiments were conducted for three strawberry (Fragaria× ananassa D.) seasons in Balm and Dover, FL, USA to evaluate the control efficacy of EDN applied through drip tape on various weed species and the fungus Macrophomina phaseolina, the causal agent of charcoal rot of strawberry.

RESULTS

Results revealed that 224, 336, 448 or 560 kg ha^-1 EDN, and 104 kg ha^-1 1,3-dichloropropene (1,3-D) + 176 kg ha^-1 chloropicrin (Pic) applied through drip tapes under totally impermeable film were safe for strawberry production, with no adverse effect on strawberry growth and yield. EDN rates at 224, 336, 448 and 560 kg ha^-1 were highly efficacious and equally effective in reducing purple nutsedge (Cyperus rotundus L.) in two of three growing seasons. Compared with the efficacy on purple nutsedge, the evaluated EDN rates were generally less effective on various broadleaf and grass weeds emerging on the bare ground and planting holes, but at 560 kg ha^-1 consistently exhibited similar levels of control as 1,3-D + Pic. For M. phaseolina, the evaluated EDN rates were more efficacious than 1,3-D + Pic and significantly reduced inoculum buried in different bed locations in plastic-mulched beds.

CONCLUSION

EDN has potential to be an efficacious tool for soil disinfestation and weed control in plasticulture strawberry production. © 2019 Society of Chemical Industry.

Screening for Resistance to Meloidogyne enterolobii in Capsicum annuum Landraces From Mexico

Meloidogyne enterolobii has become an economically important plant parasitic nematode worldwide because of its high aggressiveness, increasing geographic distribution, wide host range, and pathogenicity in pepper (Capsicum annuum) cultivars carrying resistance genes to Meloidogyne incognita, Meloidogyne arenaria, and Meloidogyne javanica. The objectives of this study were to identify landraces of peppers resistant to M. enterolobii and analyze the relationship between resistance indicators and the phenotype parameters of plant height, stem width, leaf length, leaf width, relative chlorophyll, and number of flowers. Ninety landraces of C. annuum were collected from several states of Mexico and were inoculated with 2,000 eggs of M. enterolobii. Eleven resistant landraces were selected and confirmed with a second inoculation experiment. Seventy-five days after inoculation, in both experiments, the resistance of landraces UTC66, UTC90, UTC67, UTC88, and UTC81 to M. enterolobii was consistent. Although genotypes UTC24, UTC79, UTC65, UTC68, UTC69, and UTC25 were susceptible, these landraces had a significantly higher proportion of resistant plants, less root galling, and a lower reproductive index, in comparison with the rest of the 79 genotypes and the susceptible control, which were highly susceptible in both experiments. There was no correlation between resistant indicators and phenotypic parameters, although plant height, relative chlorophyll, and number of flowers were in general significantly affected compared with noninoculated controls, indicating that the nematodes reduce the growth and yield of peppers. Results indicate that all resistant plants from these landraces are promising sources of resistance for the development of pepper cultivars resistant to M. enterolobii.

Differences in parasitism of root-knot nematodes (Meloidogyne spp.) on oilseed radish and oat

Oilseed radish and oat are cool season annual crops that are potentially used as “trap” or “biofumigant” crops for the suppression of plant-parasitic nematodes in soil. Cultivars of oilseed radish (Carwoodi, Cardinal, Final, Image, Concorde, Control, Eco-Till, Karakter and Cannavaro), white (Tachiibuki) and black (Pratex) oats were evaluated for their ability to reduce reproduction of three root-knot nematode species: Meloidogyne javanica, M. incognita race 3, and M. arenaria race 1. Nematode penetration and development were also evaluated using selected resistant and susceptible cultivars under greenhouse conditions. Root galling severity, number of eggs per gram of fresh root, and rate of reproduction varied among the cultivars in response to nematode infection. Oilseed radish cv. Carwoodi was resistant to M. javanica, whereas Karakter and Concorde were maintenance hosts allowing the nematode to maintain or increase its population on the plants. For M. incognita, Control and Carwoodi oilseed radish and Tachiibuki oat were resistant hosts. The cultivars that supported little reproduction of M. arenaria were Karakter and Carwoodi radish, and Tachiibuki oat. Comparable numbers of nematodes entered the roots of susceptible and resistant cultivars of oilseed radish and oat during early stages of infection. However, the development of the nematodes as evident from counting young and egg-laying females in roots were significantly decreased or inhibited in the resistant cultivars compared to the susceptible cultivars indicating that resistance occurs at post-infection stages. Histopathological examinations of galled-root tissues also revealed the susceptibility and resistance responses of selected cultivars of oilseed radish and oat to these nematode species.