Activity of the novel fungicide oxathiapiprolin against plant-pathogenic oomycetes

Inhibitory activity to Fusarium spp. and control potential for wheat Fusarium crown rot of a novel succinate dehydrogenase inhibitor cyclobutrifluram

BACKGROUND

Wheat Fusarium crown rot (FCR) is a serious problem primarily caused by Fusarium pseudograminearum, a pathogenic agent known to produce mycotoxins, including deoxynivalenol (DON). Cyclobutrifluram, a novel succinate dehydrogenase inhibitor devised by Syngenta, has immense potential to control both nematodes and Fusarium diseases. However, its efficacy in combating Fusarium species, its ability to prevent and reverse the detrimental effects of FCR, and its impact on the production of DON by F. pseudograminearum are yet to be fully ascertained.

RESULTS

Cyclobutrifluram exhibited substantial inhibitory activity against Fusarium species, with half-maximal effective concentration values ranging from 0.0021-0.0647 μg mL-1 . It demonstrated significant inhibitory activity toward three developmental stages of F. pseudograminearum, F. graminearum and F. asiaticum. Furthermore, cyclobutrifluram showed both protective and curative activities against FCR and was rapidly absorbed by roots and transported to wheat stems and leaves. Cyclobutrifluram could also decrease DON production by F. pseudograminearum.

CONCLUSION

This investigation has revealed the potential of cyclobutrifluram as a formidable candidate fungicide, particularly in its ability to effectively combat FCR and other Fusarium-related ailments. This novel compound has exceptional pathogen-fighting capabilities, coupled with remarkable systemic translocation properties and a notable ability to reduce the production of DON. © 2023 Society of Chemical Industry.

Characterizing the Dynamics of Virulence and Fungicide Resistance of Phytophthora capsici in Michigan Vegetable Fields Reveals Loci Associated with Virulence

The oomycete Phytophthora capsici is a destructive pathogen infecting more than 50 plant species and is one of the most serious threats to cucurbit production. Phytophthora blight caused by Phytophthora capsici can affect all plant growth stages, and fungicides and cultural controls are used to limit losses. Dissecting pathogen virulence and fungicide resistance can provide insights into pathogenic mechanisms and inform effective management practices to control P. capsici. In this study, we assessed virulence, mefenoxam sensitivity, and genetic diversity of nine P. capsici populations collected from Cucurbitaceae, Solanaceae, and Fabaceae host families in Michigan from 2002 to 2016. We developed 992 simple sequence repeats (SSRs) in the P. capsici genome and identified 60 SSRs located within or close to RXLR-class (Arginine-any amino acid-Leucine-Arginine) effectors and 29 SSRs within or close to effector CRN (CRinkling and Necrosis) family protein, which represent 62 RXLR and 34 putative CRNs. Population structure analysis shows that mefenoxam resistance was not associated with the year of collection, host type, or location, but there were significant differences in virulence among the populations. Using the general linear model and mixed linear model-based association analyses with all effector-related SSR markers, we identified four SSR markers significantly associated with at least one of the virulence-related parameters. Of these, one (Pce_SC18) was in a predicted CRN effector and had high identity with the putative PhCRN37 effector in the pathogen Plasmopara halstedii, which can be further verified for virulence identification in P. capsici.

Synthesis of Acrylopimaric Acid Triazole Derivatives and Their Antioomycete Activity against Phytophthora capsici

To develop new antioomycete agents against plant pathogens, two series of acrylopimaric acid triazole derivatives from rosin were synthesized. The in vitro antioomycete activity of these derivatives was evaluated and screened against Pseudoperonospora cubensisi, Plasmopara viticola, Phytophthora sojae, Phytophthora infestans, and Phytophthora capsici. Compound 5m showed the highest antioomycete activity against P. capsici, with a half-maximal effective concentration (EC50) value that was lower than that of the positive control metalaxyl (1.391 and 1.815 mg/L, respectively). Compound 5m demonstrated satisfactory protective and curative efficacy against P. capsici in pepper in in vivo antioomycete activity studies. Physiological and biochemical testing showed that the action mechanism of compound 5m on P. capsici involved altering the morphology and ultrastructure of the mycelium, increasing cell membrane permeability, inducing dysfunction of the nucleus and mitochondria, and ultimately causing cell necrosis. In addition, the analysis of three-dimensional quantitative structure-activity relationship (3D-QSAR) revealed the significance of the molecular structure and charge distribution in the interaction between compound 5m and its target. Collectively, these findings indicate that compound 5m has the potential as an antioomycete candidate.

Design, Synthesis and Antifungal Activities of Novel Pyrazole Analogues Containing the Aryl Trifluoromethoxy Group

On the basis of the three-component synthetic methodology developed by us, a total of twenty-six pyrazole compounds bearing aryl OCF3 were designed and synthesized. Their chemical structures were characterized by 1H and 13C nuclear magnetic resonance and high-resolution mass spectrometry. These compounds were evaluated systematically for antifungal activities in vitro against six plant pathogenic fungi by the mycelium growth rate method. Most of the compounds showed some activity against each of the fungi at 100 μg/mL. Compounds 1t and 1v exhibited higher activity against all the tested fungi, and 1v displayed the highest activity against F. graminearum with an EC50 value of 0.0530 μM, which was comparable with commercial pyraclostrobin. Structure-activity relationship analysis showed that, with respect to the R1 substituent, the straight chain or cycloalkyl ring moiety was a key structural moiety for the activity, and the R2 substituent on the pyrazole ring could have significant effects on the activity. Simple and readily available pyrazoles with potent antifungal activity were obtained, which are ready for further elaboration to serve as a pharmacophore in new potential antifungal agents.

Activity of the new OSBP inhibitor Y18501 against Pseudoperonospora cubensis and its application for the control of cucumber downy mildew

Y18501 is a new oxysterol-binding protein inhibitor (OSBPI) with a similar structure to oxathiapiprolin. Y18501 showed strong inhibitory activities against Phytophthora spp. and Pseudoperonospora cubensis, with EC50 ranging from 0.0005 to 0.0046 μg/mL. It also had good control efficacy on cucumber downy mildew (CDM) in the green house and in the field, and could effectively inhibit different development stages of P. cubensis, especially for sporangiophore production, sporangial production, mycelium extension, and elongation of germ tube. In addition, Y18501 showed excellent protective and curative activities against P. cubensis. It also had acropetal systemic mobility in the cucumber leaves, and could be taken up and translocated to the upper leaves more effectively from the lower leaves than from the roots. Y18501 had poorer permeability in cucumber leaves compared to oxathiapiprolin. The simultaneous application of Y18501 and chlorothalonil could significantly promote the inhibition of P. cubensis.

Design, Synthesis, and Antifungal/Antioomycete Activity of Thiohydantoin Analogues Containing Spirocyclic Butenolide

Novel fungicidal agents were designed based on the combination of two privileged scaffolds, thiohydantoin and spirocyclic butenolide, which are widely found in natural products. The synthesized compounds were characterized by ¹H NMR, ¹³C NMR, and high-resolution electrospray ionisation mass spectrometry. The in vitro antioomycete activity evaluation showed that most of the compounds exhibited excellent inhibitory activities against different developmental stages in the life cycle of pathogenic oomycete Phytophthora capsici. Compound 5j could inhibit the mycelial growth, sporangium production, zoospore release, and cystospore germination significantly with EC₅₀ values of 0.38, 0.25, 0.11, and 0.026 μg/mL, respectively. The in vivo antifungal/antioomycete bioassay results revealed that the series of compounds generally showed outstanding control efficacies against the pathogenic oomycete Pseudoperonospora cubensis, and compounds 5j, 5l, 7j, 7k, and 7l possessed broad-spectrum antifungal activities against the test phytopathogens. The in vivo protective and curative efficacies against P. capsici of the representative compound 5j were excellent, which were better than those of azoxystrobin. More prominently, 5j significantly promoted the biomass accumulation of the root system and reinforced the cell wall by callose deposition. The pronounced upregulation of immune response-related genes indicated that the active oomycete inhibitor 5j also functioned as a plant elicitor. Transmission electron microscopy observation and the enzyme activity test demonstrated that the mechanism of action of 5j was to bind to the pivotal protein, complex III on the respiratory chain, which resulted in a shortage of energy supply. Molecular docking results exhibited that compound 5j appropriately matched with the Qo pocket and had no interaction with the most commonly mutated site Gly-142, which may be of significant benefit in Qo fungicide resistance management. Compound 5j showed great advantages and potential in oomycete control, resistance management, and induction of disease resistance. A further investigation of 5j with a unique structure might have direct implications for the creation of novel oomycete inhibitors against plant-pathogenic oomycetes.

Discovery of Novel Isoxazoline Compounds that Incorporate a para-Diamide Moiety as Potential Insecticidal Agents against Fall Armyworm (Spodoptera frugiperda)

Spodoptera frugiperda is a major migratory agricultural pest, which seriously impedes agricultural production around the world. To discover potent compounds against S. frugiperda, a number of novel isoxazoline derivatives were designed and synthesized and created on account of the identified lead compound F32 (4-(5-(3,5-dichlorophenyl)-5-(trifluoromethyl)-4,5-dihydroisoxazol-3-yl)-2-methyl-N-(3-propionamidophenyl)benzamide). Based on the three-dimensional quantitative structure-activity relationship of those compounds, the compound G22 (N-(4-acetamidophenyl)-4-(5-(3,5-dichlorophenyl)-5-(trifluoromethyl)-4,5-dihydroisoxazol-3-yl)-2-methylbenzamide) was developed. A bioassay showed that G22 is highly lethal to S. frugiperda (LC₅₀ = 1.57 mg/L), a more effective control than insecticides fipronil (LC₅₀ = 78.8 mg/L) and chlorantraniliprole (LC₅₀ = 1.60 mg/L). Field trials were also implemented to identify candidate agents. Furthermore, from the insect γ-aminobutyric acid (GABA) enzyme-linked immunosorbent assay, it is obvious that G22 could up-regulate the expression of GABA of insects, which showed a similar result to fipronil. The analysis of molecular docking exhibited that the hydrophobic effect and hydrogen bonds play key roles in the combination between G22 with GABA receptors. This study provides a potent isoxazoline candidate compound for the S. frugiperda control.

Root Absorption and Limited Mobility of Mandipropamid as Compared to Oxathiapiprolin and Mefenoxam After Soil Treatment of Citrus Plants for Managing Phytophthora Root Rot

Phytophthora root rot can greatly impact citrus production worldwide, especially in newly established orchards by reducing crop yield and increasing the cost of disease management. Mandipropamid is an Oomycota fungicide that is currently registered as a soil treatment for citrus nursery container plants to manage Phytophthora root rot. In this study, we investigated the uptake of mandipropamid into citrus roots and its translocation to stems and leaves after soil application and evaluated its mobility in roots as compared to oxathiapiprolin and mefenoxam using split-root potted plants and trees in the field. A bioassay and liquid chromatography-tandem mass spectrometry were used to detect and quantify fungicides in citrus tissues, and overall, similar results were obtained using the two methods. When applied to the soil of potted, 6- to 7-month-old citrus plants using labeled rates, the majority of mandipropamid was found in root tissues (4.9 to 18.1 μg/g), but small amounts were also present in stems (0.18 to 0.32 μg/g) and leaves (0.03 to 0.22 μg/g). There was no significant increase in concentrations in all three tissues between 1 and 4 weeks after application. Concentrations in all tissues exceeded established EC₅₀ values for mycelial growth inhibition of P. citrophthora and P. nicotianae, the main citrus root rot pathogens in California. In a split-root study where the root systems of single plants were separated, no basipetal phloem-based mobility of mandipropamid or oxathiapiprolin was observed, but relative uptake into roots was higher for mandipropamid. In contrast, low amounts of mefenoxam were also present in roots in the untreated soil. Similar results were obtained in a field study where part of the root system was treated, and fungicides were extracted from nontreated roots. All three fungicides persisted inside roots over the 8-week period of this study. Uptake and persistence inside roots, as well as the previously reported high efficacy against citrus root rot in greenhouse and field studies support the use of mandipropamid in citrus nurseries and potentially in the orchard.

Resistance Risk Assessment for the New OSBP Inhibitor Y18501 in Pseudoperonospora cubensis and Point Mutations (G705V, L798W, and I812F) in PscORP1 that Confer Resistance

Y18501 is a new oxysterol-binding protein inhibitor (OSBPI) that shows strong inhibitory activity against Pseudoperonospora cubensis. In this study, the sensitivities of 159 Ps. cubensis isolates to Y18501 were determined, with EC₅₀ values ranging from 0.001 to 11.785 μg/mL, indicating that a Y18501-resistant subpopulation has appeared in the field. Ten Y18501-resistant mutants were obtained by fungicide adaptation and displayed fitness equal to or stronger than their parental isolates, which suggests that the resistance risk of Ps. cubensis to Y18501 is high. The consecutive applications of Y18501 in the field resulted in the rapid resistance of Ps. cubensis and decreased control efficacy of cucumber downy mildew (CDM), which could be alleviated by compounding with mancozeb. A positive cross-resistance was detected between Y18501 and oxathiapiprolin. The amino acid substitutions G705V, L798W, and I812F in PscORP1 conferred resistance to Y18501 in Ps. cubensis, which was validated by molecular docking and molecular dynamics simulations.

Risk Evaluation of Benzovindiflupyr Resistance of Verticillium dahliae Population in Maine

Verticillium dahliae causes Verticillium wilt, resulting in significant losses to potato production. Benzovindiflupyr, a succinate dehydrogenase inhibitor, effectively controls V. dahliae. However, frequent applications of the chemical may expedite the development of fungicide resistance in the pathogen population. To evaluate the risk of benzovindiflupyr resistance, 38 V. dahliae strains were obtained from diseased potatoes in Maine. The sensitivity of the field population was determined based on effective concentration for 50% inhibition (EC₅₀), which ranged from 0.07 to 11.28 μg ml^-1 with a median of 1.08. Segregated clusters of EC₅₀ values indicated that Maine V. dahliae populations have developed benzovindiflupyr resistance. By exposing conidia of V. dahliae to a high concentration of benzovindiflupyr, 18 benzovindiflupyr-resistant mutants were obtained. To examine their fitness, the mutants were continuously subculture-transferred for up to 10 generations. Mycelial growth, conidial production, competitiveness, pathogenicity, and cross resistance of the 10th generation mutants were examined. Results showed that 50% of the resistant mutants retained an adaptive level in mycelial growth, and 60% maintained conidial production similar to their parents. Pathogenicity did not change for any of the mutants. No cross resistance was detected between benzovindiflupyr and either azoxystrobin, boscalid, fluopyram, or pyrimethanil. Thus, the resistance risk in V. dahliae to benzovindiflupyr should be considered in Maine potato production.

Characterization of Italian Plasmopara viticola populations for resistance to oxathiapiprolin

BACKGROUND

Oxathiapiprolin is a novel fungicide and the first of the piperidinyl-thiazole-isoxazoline class to be discovered. This fungicide has been reported to have high activity against Plasmopara viticola, the grapevine downy mildew agent, and other plant-pathogenic oomycetes. In this study, the baseline sensitivity of Italian P. viticola populations towards oxathiapiprolin was established on 29 samples collected in 10 different viticultural areas. Two insensitive strains were characterized for their mechanism of resistance.

RESULTS

Oxathiapiprolin exhibited substantial inhibitory activity against 27 of the 29 populations tested, with EC₅₀ values ranging from a minimum of under 4 × 10^-5  mg L^-1 to over 4 × 10^-1  mg L^-1 , with an average value of 3.2 × 10^-2  mg L^-1 . Two stable suspected oxathiapiprolin-resistant mutants were isolated from population exhibiting reduced sensitivity, and sequenced for the oxathiapiprolin target gene PvORP1. The comparison with wild-type isolates revealed that the resistant isolates possessed a heterozygous mutation causing the amino acid substitution N837I, recently reported in the literature.

CONCLUSION

The results obtained indicate a risk for Italian P. viticola populations to develop resistance to oxathiapiprolin in association with the N837I mutation at PvORP1. Anti-resistance strategies should be carefully implemented and the sensitivity levels to this molecule should be monitored accurately in future to preserve its effectiveness. © 2022 Society of Chemical Industry.

Design, Synthesis, and Insecticidal Activity of Novel Isoxazoline Compounds That Contain Meta-diamides against Fall Armyworm (Spodoptera frugiperda)

Fall armyworm (Spodoptera frugiperda) is a major migratory pest around the entire world that causes severe damage to agriculture. We designed and synthesized a series of novel isoxazoline derivatives based on the previously discovered active compound H13 to find new and effective candidates against S. frugiperda. Most of them showed excellent insecticidal activity. In addition, a three-dimensional quantitative structure-activity relationship model was established, and compound F32 was designed and synthesized based on the results. The bioassay result showed that compound F32 exhibited excellent activity against S. frugiperda (LC₅₀ = 3.46 mg/L), which was substantially better than that of the positive control fipronil (LC₅₀ = 78.8 mg/L). Furthermore, an insect γ-aminobutyric acid (GABA) enzyme-linked immunosorbent assay indicated that F32 can upregulate the content of GABA in insects in a manner similar to that of fipronil. Molecular docking showed that the hydrophobic effect and hydrogen-bond interactions are vital factors between the binding of F32 and receptors. All of these results suggest that compound F32 could be employed as a novel isoxazoline lead compound to control S. frugiperda.

Use of oxathiapiprolin for controlling soybean root rot caused by Phytophthora sojae: efficacy and mechanism of action

BACKGROUND

Oxathiapiprolin is a new isoxazoline fungicide developed by DuPont to control oomycete diseases. Although oxathiapiprolin has shown strong inhibitory activity against oomycete pathogens, little is known about its ability to control Phytophthora sojae.

RESULTS

Oxathiapiprolin showed high inhibitory activity against Phytophthora sojae, with 50% effective concentration (EC₅₀ ) values ranging from 1.15 × 10^-4 to 4.43 × 10^-3  μg mL^-1 . Oxathiapiprolin inhibited various stages of Phytophthora sojae development, including mycelial growth, sporangium formation, oospore production, and zoospore release. Electron microscopy studies revealed that oxathiapiprolin caused severe morphological and ultrastructural damage to Phytophthora sojae. Oxathiapiprolin affected the cell membrane and wall of Phytophthora sojae, making it more sensitive to osmotic and cell wall stress. Oxathiapiprolin exhibited translocation activity; it was absorbed by soybean roots and then translocated to the leaves. It was effective at reducing soybean Phytophthora root rot under glasshouse and field conditions. Both fungicide seed treatment and foliar spray significantly reduced disease incidence and yield losses compared with untreated controls in the field.

CONCLUSION

Oxathiapiprolin exhibits high inhibitory activity against Phytophthora sojae, and has multiple mechanisms of action including severe mycelial damage and modulation of osmotic and cell wall stress. These results indicate that oxathiapiprolin can be used at low concentrations for highly effective management of soybean Phytophthora root rot caused by Phytophthora sojae. © 2022 Society of Chemical Industry.

Efficacy and Timing of Application of Fungicides, Biofungicides, Host-Plant Defense Inducers, and Fertilizer to Control Phytophthora Root Rot of Flowering Dogwood in Simulated Flooding Conditions in Container Production

Phytophthora root rot, caused by Phytophthora cinnamomi Rands, is one of the major diseases of flowering dogwood (Cornus florida L.). The severity of root rot disease increases when the plants are exposed to flooding conditions. A study was conducted to determine the efficacy and timing of application of different fungicides, biofungicides, host-plant defense inducers, and fertilizer to manage Phytophthora root rot in month-old seedlings in simulated flooding events for 1, 3, and 7 days. Preventative treatments were drench applied 3 weeks and 1 week before flooding whereas curative treatments were applied 24 h after flooding. Dogwood seedlings were inoculated with P. cinnamomi 3 days before the flooding. Plant height and width were recorded at the beginning and end of the study. At the end of the study, plant total weight and root weight were recorded and disease severity in the root was assessed using a scale of 0 to 100%. Root samples were plated using PARPH-V8 medium to determine the percent recovery of the pathogen. Empress Intrinsic, Pageant Intrinsic, Segovis, and Subdue MAXX, as preventative and curative applications, were able to suppress the disease severity compared with the inoculated control in all flooding durations. All treatments, with the exception of Stargus as a preventative application 3 weeks before flooding and Orkestra Intrinsic as a curative application, were able to suppress the disease severity compared with the inoculated control for a 1-day flooding event. Aliette and ON-Gard were effective in the first trial when applied preventatively at both 1 week and 3 weeks before flooding but not in the second trial. Signature Xtra was effective as a preventative application but not as a curative application. Interface was effective as a curative application but not as a preventative application. The findings of this study will help nursery growers to understand the performance of fungicides, biofungicides, host-plant defense inducers, and fertilizer at different time intervals and repeated applications to manage Phytophthora root rot in flooding conditions.

Activity and Point Mutation G699V in PcoORP1 Confer Resistance to Oxathiapiprolin in Phytophthora colocasiae Field Isolates

The oxysterol-binding protein inhibitor oxathiapiprolin is a new fungicide for controlling oomycetes diseases. Besides, laboratory mutagenesis oxathiapiprolin-resistance among phytopathogenic oomycetes in the field remains unknown. Here, the sensitivity of 97 P. colocasiae isolates to oxathiapiprolin was examined that were collected between 2011 and 2016. We obtained a baseline sensitivity with a mean EC₅₀ value of 5.2639 × 10^-4 μg mL^-1. We showed that 6/32 isolates collected in Fujian Province from 2019 to 2020 were resistant to oxathiapiprolin without a significant fitness penalty on sporulation, vegetative growth, and virulence of the field isolates. The oxathiapiprolin resistance field isolates contained the point mutation glycine to valine at 699 in PcoORP1. The point mutation G699V was verified to confer resistance of P. colocasiae to oxathiapiprolin using the CRISPR/Cas9 system. The mutation G699V decreased the binding affinity between oxathiapiprolin and PcoORP1. These results will improve our understanding of the mechanism of P. colocasiae resistance to oxathiapiprolin under field conditions.

Selection of an Appropriate In Vitro Susceptibility Test for Assessing Anti-Pythium insidiosum Activity of Potassium Iodide, Triamcinolone Acetonide, Dimethyl Sulfoxide, and Ethanol

The orphan but highly virulent pathogen Pythium insidiosum causes pythiosis in humans and animals. Surgery is a primary treatment aiming to cure but trading off losing affected organs. Antimicrobial drugs show limited efficacy in treating pythiosis. Alternative drugs effective against the pathogen are needed. In-house drug susceptibility tests (i.e., broth dilution, disc diffusion, and radial growth assays) have been established, some of which adapted the standard protocols (i.e., CLSI M38-A2 and CLSI M51) designed for fungi. Hyphal plug, hyphal suspension, and zoospore are inocula commonly used in the drug susceptibility assessment for P. insidiosum. A side-by-side comparison demonstrated that each method had advantages and limitations. Minimum inhibitory and cidal concentrations of a drug varied depending on the selected method. Material availability, user experience, and organism and drug quantities determined which susceptibility assay should be used. We employed the hyphal plug and a combination of broth dilution and radial growth methods to screen and validate the anti-P. insidiosum activities of several previously reported chemicals, including potassium iodide, triamcinolone acetonide, dimethyl sulfoxide, and ethanol, in which data on their anti-P. insidiosum efficacy are limited. We tested each chemical against 29 genetically diverse isolates of P. insidiosum. These chemicals possessed direct antimicrobial effects on the growth of the pathogen in a dose- and time-dependent manner, suggesting their potential application in pythiosis treatment. Future attempts should focus on standardizing these drug susceptibility methods, such as determining susceptibility/resistant breakpoints, so healthcare workers can confidently interpret a result and select an effective drug against P. insidiosum.

Uptake, Translocation, and Subcellular Distribution of Oxathiapiprolin and Famoxadone in Tomato Plants (Lycopersicon esculentum Miller)

The uptake, translocation, and subcellular distribution of oxathiapiprolin and famoxadone in tomato plants were investigated using hydroponic experiments. Oxathiapiprolin and famoxadone mainly accumulated in the tomato roots with limited translocation capacity from the roots to the upper part. The root absorption and inhibitor results noted the dominance of the apoplastic and symplastic pathways in the oxathiapiprolin and famoxadone uptake by the tomato roots, respectively. Furthermore, the uptake process for the two fungicides followed passive and aquaporin-dependent transport. Insoluble cell components (cell organelles and walls) were the dominant storage compartments for oxathiapiprolin and famoxadone. In the protoplast, oxathiapiprolin in the soluble fraction had a higher proportion than that of famoxadone. Finally, the uptake and distribution of the two fungicides by the tomato plants was accurately predicted using a partition-limited model. Thus, this study provides an in-depth understanding of the transfer of oxathiapiprolin and famoxadone from the environment to tomato plants.

Advances in the green synthesis and agrichemical applications of oxathiapiprolin derivatives

Oxathiapiprolin was developed with high antifungal activity and novel target protein and is used in the oomycetes control for crop protection. The structural modifications of oxathiapiprolin are summarized. The achievements and challenges in the structural modification of oxathiapiprolin are also discussed in this mini review. The outlook in this field is perspected according to our own opinion and understanding on the development of oxysterol binding protein inhibition fungicides.

Oxathiapiprolin Alone or Mixed with Metalaxyl Seed Treatment for Management of Soybean Seedling Diseases Caused by Species of Phytophthora, Phytopythium, and Pythium

Species of Phytophthora, Phytopythium, and Pythium affect soybean seed and seedlings each year, primarily through reduced plant populations and yield. Oxathiapiprolin is effective at managing several foliar diseases caused by some oomycetes. The objectives of these studies were to evaluate oxathiapiprolin in a discriminatory dose assay in vitro; evaluate oxathiapiprolin as a soybean seed treatment on a moderately susceptible cultivar in 10 environments; compare the impact of seed treatment on plant populations and yields in environments with low and high precipitation; and compare a seed treatment mixture on cultivars with different levels of resistance in four environments. There was no reduction in growth in vitro among 13 species of Pythium at 0.1 µg ml-1. Soybean seed treated with the base fungicide plus oxathiapiprolin (12 and 24 µg a.i. seed-1) alone, oxathiapiprolin (12 µg a.i. seed-1) plus mefenoxam (6 µg a.i. seed-1), or oxathiapiprolin (24 µg a.i. seed-1) plus ethaboxam (12.1 µg a.i. seed-1) had greater yields in environments that received ≥50 mm of precipitation within 14 days after planting compared with those that received less. Early plant population and yield were significantly higher for seed treated with oxathiapiprolin (24 µg a.i. seed-1) + metalaxyl (13.2 µg a.i. seed-1) compared with nontreated for six of seven cultivars in at least one of four environments. Oxathiapiprolin combined with another Oomycota fungicide applied to seed has the potential to be used to protect soybean plant establishment and yield in regions prone to poor drainage after high levels of precipitation.

Discovery of Tropolone Stipitaldehyde as a Potential Agent for Controlling Phytophthora Blight and Its Action Mechanism Research

The fermentation of endophytic Nigrospora chinensis GGY-3 resulted in the isolation of tropolone stipitaldehyde (1), which exhibited broad-spectrum inhibition activity against fungi and bacteria, especially against Phytophthora capsici, with an EC₅₀ value of 0.83 μg/mL and Xanthomonas oryzae pv. oryzicola, with a minimum inhibitory concentration value of 4.0 μg/mL. The in vitro and in vivo assays demonstrated that 1 had a significant protective effect on P. capsici. Furthermore, 1 inhibited the spore germination of P. capsici and damaged the plasma membrane structure. As observed by SEM and TEM, after exposure to 1, mycelia exhibited swelling, shrunken, branch-increasing phenomena, cell wall and membrane damage, and disordered content. Transcriptome analysis revealed that 1 might affect starch and sucrose metabolism and fatty acid biosynthesis by suppressing the expression of genes relevant to cell wall synthetases and cell membrane-associated genes. These findings indicate that 1 may be a potential agrochemical fungicide for controlling phytophthora blight.

iTRAQ proteomic analysis of the inhibitory effect of 1,6-O,O-diacetylbritannilactone on the plant pathogenic oomycete Phytophthora capsici

Phytophthora capsici is a highly destructive oomycete of vegetables; its management is challenging due to its broad host range, rapid dispersion, resilient spores and severe fungicide resistance. Identifying an effective alternative fungicide is important for the control of P. capsici. 1,6-O,O-diacetylbritannilactone (ABLOO), one of the secondary metabolites of Inula Britannica, showed a favorable inhibitory activity against P. capsici at different developmental stages, with a sensitivity order as follows: sporangia formation (30.45 mg/L) > zoospore discharge (77.69 mg/L) > mycelial growth (93.18 mg/L) > cystospore germination (591.48 mg/L). To investigate the mode of action of ABLOO in P. capsici, iTRAQ-based quantitative proteomic analysis was performed by comparing the expression levels of proteins in the control and ABLOO-treated (400 mg/L, inhibition rate of 80%) mycelial groups. A total of 65 downregulated and 75 upregulated proteins were identified in the proteomic analysis. Functional enrichment analyses showed that proteins with transmembrane transport activity were significantly inhibited, while proteins involved in energy production were significantly increased, including proteins involved in ubiquinone and other terpenoid-quinone biosynthesis, oxidative phosphorylation, and glycolysis/gluconeogenesis. The morphological results indicated that ABLOO treatment could decrease the thickness of the cell walls of P. capsici mycelia. Correspondingly, biochemical results showed that ABLOO treatment reduced the β-1,3-glucan contents (the key component of the cell wall of P. capsici) and increased the cell membrane permeability of P. capsici. ABLOO may exhibit antioomycete activity by destroying the cell membrane of P. capsici. This study provides new evidence regarding the inhibitory mechanisms of ABLOO against P. capsici.

Resistance Risk and Novel Resistance-Related Point Mutations in Target Protein PiORP1 of Fluoxapiprolin in Phytophthora infestans

Fluoxapiprolin is a new oxysterol binding protein inhibitor (OSBPI), which showed excellent inhibitory activity to plant pathogenic oomycetes. Its resistance risk and mechanism in Phytophthora infestans are unclear. In the current study, the sensitivities of 103 P. infestans isolates to fluoxapiprolin were investigated, and a unimodal distribution with a mean EC₅₀ value of 0.00035 μg/mL was observed. Four types of resistant mutants, with a resistance factor from 14 to more than 1000, and point mutations S768I+N837I, S768I+L860I, S768I, and I877F in PiORP1, were acquired using fungicide adaption. The fitness of the mutants was similar to or lower than that of the corresponding parental isolate. Positive cross-resistance was detected between fluoxapiprolin and oxathiapiprolin. The point mutations were verified in P. sojae homologue positions using the CRISPR/Cas9 genome editing system. Transformants containing S768I+N837I or S768I+L860I, showed high fluoxapiprolin resistance (RF > 1000). In conclusion, the risk of P. infestans resistance to fluoxapiprolin is moderate, and novel point mutation types S768I+N837I or S768I+L860I could cause high fluoxapiprolin resistance in P. infestans.

Identification and Management of Phytophthora Aerial Blight Caused by Phytophthora nicotianae on Catharanthus roseus

Phytophthora nicotianae is the most common pathogen in nurseries and gardens, infecting both woody and herbaceous ornamental plants. Phytophthora aerial blight symptoms such dull water-soaked lesions on shoot tips and leaf petioles, girdling on the main stem, necrosis, and wilting of annual vinca were observed in a commercial greenhouse in Warren County, TN, U.S.A., in May 2016. The objective of this study was to identify the causal agent of Phytophthora aerial blight and develop a fungicide management recommendation for ornamental producers. Attempts to isolate the pathogen from symptomatic leaf tissue were conducted, and excised leaf pieces were embedded in V8 agar medium. Morphological characterization, PCR, sequencing, and pathogenicity test of the isolate FBG2016_444 were conducted to confirm the pathogen identification. The sequence identity was 100% identical to P. nicotianae, and a combined phylogenetic tree (internal transcribed spacer, large subunit of rDNA, and ras-related protein gene) grouped isolate FBG2016_444 within the clade of P. nicotianae. In the pathogenicity study, all inoculated annual vinca plant showed Phytophthora aerial blight symptoms, and P. nicotianae was reisolated, whereas noninoculated annual vinca plant remained symptomless. These findings confirmed P. nicotianae as the causal agent of Phytophthora aerial blight of annual vinca. In addition, two rates (0.078 and 0.156 ml·liter^-1) and three application intervals (7, 14, and 21 days before inoculation [DBI]) of oxathiapiprolin (Segovis) were evaluated for their ability to reduce the Phytophthora aerial blight severity on annual vinca plants. The control groups were positive (nontreated inoculated) and negative (nontreated noninoculated) plants. Both rates and application timings of oxathiapiprolin significantly reduced Phytophthora aerial blight severity and disease progress (area under disease progress curve [AUDPC]) on annual vinca plants compared with the positive control. However, 0.078 and 0.156 ml·liter^-1 of oxathiapiprolin applied at 7 or 14 DBI were the most effective treatments in reducing the disease severity and AUDPC on annual vinca plants. The plant growth parameters such as increase in height and width, total plant weight, and root weight were not influenced by the application of oxathiapiprolin. The findings reported in this study will help ornamental producers with better management of Phytophthora aerial blight of annual vinca.

Phytophthora sojae Pathotype Distribution and Fungicide Sensitivity in Michigan

Identifying the pathotype structure of a Phytophthora sojae population is crucial for the effective management of Phytophthora stem and root rot of soybean (PRR). P. sojae has been successfully managed with major resistance genes, partial resistance, and fungicide seed treatments. However, prolonged use of resistance genes or fungicides can cause pathogen populations to adapt over time, rendering resistance genes or fungicides ineffective. A statewide survey was conducted to characterize this pathotype structure and fungicide sensitivity of P. sojae within Michigan. Soil samples were collected from 69 fields with a history of PRR and fields having consistent plant stand establishment issues. Eighty-three isolates of P. sojae were obtained, and hypocotyl inoculations were performed on 14 differential soybean cultivars, all of which carry a single Rps gene or no resistance gene. The survey identified a loss of effectiveness of Rps genes 1b, 1k, 3b, and 6, compared with a previous survey conducted in Michigan from 1993 to 1997. Three effective resistance genes were identified for P. sojae management in Michigan; Rps 3a, 3c, and 4. Additionally, the effective concentration of common seed treatment fungicides to inhibit mycelial growth by 50% (EC50) was determined. No P. sojae isolates were insensitive to the tested chemistries with mean EC50 values of 2.60 × 10-2 μg/ml for ethaboxam, 3.03 × 10-2 μg/ml for mefenoxam, 2.88 × 10-4 μg/ml for oxathiapiprolin, and 5.08 × 10-2 μg/ml for pyraclostrobin. Results suggest that while there has been a significant shift in Rps gene effectiveness, seed treatments are still effective for early season management of this disease.

Discovery of zeylenone from Uvaria grandiflora as a potential botanical fungicide

BACKGROUND

Botanical pesticides play an important role in organic agricultural practices and are widely used in integrated pest management (IPM). Uvaria grandiflora was mainly reported as traditional medicines and possessed antibacterial, antioxidant, and antiprotozoal activities. Therefore, important biological activities of U. grandiflora may suggest that they have the potential to be used as botanical pesticides.

RESULTS

The extract of U. grandiflora exhibited broad-spectrum inhibitory activity toward phytopathogenic fungi and oomycetes, particularly against Colletotrichum musae and Phytophthora capsici, and its secondary metabolite zeylenone also displayed strong antifungal and anti-oomycete activities against phytopathogens. Particularly, half maximal effective concentration (EC₅₀ ) values of zeylenone against Phytophthora capsici and C. musae were 6.98 and 3.37 μg mL^-1 , showing better inhibitory effects than those of commercial fungicides (azoxystrobin and osthole). Additionally, the pot experiments showed that the extract of U. grandiflora could effectively control Pseudoperonospora cubensis, Phytophthora infestans, Phytophthora capsici and Podosphaera xanthii. In the field experiment, 5% microemulsion of U. grandiflora extract exhibited 79.72% efficacy against cucumber powdery mildew at 87.5 g ha^-1 on the 14th day after two sprayings, which was better than that of 21.5% trifloxystrobin and 21.5% fluopyram SC at 200.9 g ha^-1 . Surprisingly, 5% microemulsion of U. grandiflora extract could promote cucumber growth significantly. Furthermore, the action mechanism analysis indicated that zeylenone may damage the cytoderm and affect energy metabolism of Phytophthora capsici.

CONCLUSION

It is the first time that the extract of U. grandiflora and zeylenone have been discovered leading to broad application prospects in the development as botanical fungicides. © 2021 Society of Chemical Industry.

Assessing the effectiveness of oxathiapiprolin toward Phytophthora agathidicida, the causal agent of kauri dieback disease

Phytophthora species cause disease and devastation of plants in ecological and horticultural settings worldwide. A recently identified species, Phytophthoraagathidicida, infects and ultimately kills the treasured kauri trees (Agathis australis) that are endemic to New Zealand. Currently, there are few options for managing kauri dieback disease. In this study, we sought to assess the efficacy of the oomycide oxathiapiprolin against several life cycle stages of two geographically distinct P. agathidicida isolates. The effective concentration to inhibit 50% of mycelial growth (EC50) was determined to be ∼0.1 ng/ml, indicating that P. agathidicida mycelia are more sensitive to oxathiapiprolin than those from most other Phytophthora species that have been studied. Oxathiapiprolin was also highly effective at inhibiting the germination of zoospores (EC50 = 2–9 ng/ml for the two isolates) and oospores (complete inhibition at 100 ng/ml). In addition, oxathiapiprolin delayed the onset of detached kauri leaf infection in a dose-dependent manner. Collectively, the results presented here highlight the significant potential of oxathiapiprolin as a tool to aid in the control of kauri dieback disease.

The Bioactivity and Efficacy of Benzovindiflupyr Against Corynespora cassiicola, the Causal Agent of Cucumber Corynespora Leaf Spot

Corynespora cassiicola, which causes Corynespora leaf spot, results in considerable yield loss of cucumber grown in greenhouses. Frequent reports of reduced efficacy and control failure of fungicides warrant new, efficient alternative chemistries. In this study, the sensitivity of C. cassiicola to benzovindiflupyr was evaluated using a collection of 81 isolates collected from Shandong, China. The mean EC₅₀ values for mycelial growth, spore germination, and germ tube elongation of C. cassiicola were 0.69 ± 0.44, 0.12 ± 0.063, and 0.13 ± 0.076 µg ml^-1, respectively. Benzovindiflupyr treatment led to a reduced respiration rate and ATP production of C. cassiicola and decreased spore pathogenicity by 21.9% on average. Additionally, detached cucumber leaves sprayed with fungicides before or after inoculation were used to assess the efficacy of benzovindiflupyr against C. cassiicola. Benzovindiflupyr (150 µg ml^-1) exhibited preventive and curative efficacies of 86.9 and 77.1%, respectively. Benzovindiflupyr at 150 g a.i. ha^-1 provided over 70% efficacy in field trials performed in 2018 and 2019, which was significantly higher than that of the reference fungicides fluopyram and fluxapyroxad at the same dose. Furthermore, the yield of commercial cucumber increased as disease incidence decreased. Our findings pave the way for the introduction of benzovindiflupyr in the integrated management of Corynespora leaf spot.

Inhibitory Effects of the Natural Product Esculetin on Phytophthora capsici and Its Possible Mechanism

Esculetin is an important plant-derived natural product that has multiple bioactivities and applications. Phytophthora capsici is a notorious plant pathogen capable of infecting a broad range of hosts. In this study, we evaluated the antifungal activity of esculetin against P. capsici. The baseline sensitivity of P. capsici to esculetin was established using 108 isolates collected from various geographical regions in the Jiangsu and Shandong Provinces of China. The median effective concentration (EC₅₀) values for esculetin ranged from 2.08 to 16.46 μg/ml (mean, 6.87 ± 2.70 μg/ml) and were normally distributed. Furthermore, both zoospore production and germination were strongly inhibited by esculetin. Importantly, esculetin exhibited protective as well as curative activities against P. capsici on tomato and was capable of restricting the early infection of P. capsici on Nicotiana benthamiana. We found that the esculetin treatment led to cell membrane damage of P. capsici, as revealed by morphological observations and measurements of relative conductivity and malondialdehyde (MDA). Finally, our results also suggested that esculetin may adversely affect P. capsici by inhibiting its DNA and protein synthesis. These findings will contribute to the broader evaluation of the use of esculetin to control diseases caused by P. capsici and toward a better understanding of its mode of action as a potential fungicide.

Activity of the Novel Fungicide Mefentrifluconazole Against Colletotrichum scovillei

The prevalence and destructiveness of anthracnose, caused by Colletotrichum scovillei, in pepper production regions seriously affects pepper yield and quality. Mefentrifluconazole, the first of the isopropanol-azole subgroup of triazole fungicides, was introduced for the control of pepper anthracnose. However, the growth characteristics of pepper fruit and rapid spread of anthracnose suggest that the fungicide application method must be optimized to enhance fungicide efficacy. The sensitivity of C. scovillei to mefentrifluconazole was determined by mycelial growth and germ tube elongation assays using 157 single-spore isolates with mean 50% effective concentration values of 0.462 ± 0.138 and 0.359 ± 0.263 mg/liter, respectively. The in vivo data also showed that mefentrifluconazole had favorable protective and curative effects against pepper anthracnose. Mefentrifluconazole significantly affected C. scovillei infection on pepper by reducing appressorium formation and sporulation, shriveling spores and germ tubes, and causing the abnormal development of appressoria and conidiophores. Mefentrifluconazole could move acropetally, horizontally, and basipetally in pepper plants. Compared with a knapsack sprayer, mefentrifluconazole applied by mist sprayer exhibited significantly better activity against pepper anthracnose. Additionally, as the spray volume increased from 45 to 150 liters/ha, the control efficacy of mefentrifluconazole first increased and then tended to be steady, with an optimal spray volume of 90 liters/ha. The difference in disease control efficacy was related to the deposition and droplet distribution of mefentrifluconazole on the pepper fruit. These results provide scientific guidance for the application of mefentrifluconazole in pepper fields and improved fungicide utilization.

Activity and Resistance-Related Point Mutations in Target Protein PcORP1 of Fluoxapiprolin in Phytophthora capsici

Fluoxapiprolin is a new piperidinyl thiazole isoxazoline fungicide developed by Bayer Crop Science in 2012, but the sensitivity and resistance mechanism of fluoxapiprolin are unclear. In this study, the sensitivities of 130 Phytophthora capsici isolates to fluoxapiprolin were determined, and a unimodal distribution was observed with a mean EC₅₀ value of 0.00043 μg/mL. Nine stable fluoxapiprolin-resistant mutants were obtained by fungicide adaption. The fitness of mutants was similar to or lower than that of the corresponding parental isolate. Seven heterozygous point mutations in the target protein PcORP1 were found in these mutants. These point mutations were confirmed in PsORP1 of P. sojae homologue positions using the CRISPR/Cas9 system. G770V and N835S+I877F do confer high fluoxapiprolin resistance (resistance factor, RF > 1000), and ΔN835, N767I, and N837T+S910C lead to low resistance (RF < 100). P. sojae transformants containing L733W, S768F, S768Y, ΔG818/F819, N837Y, N837F, P861H, L863W, and I877Y in PsORP1 were also associated with high fluoxapiprolin resistance (RF > 100). In conclusion, the resistance risk of P. capsici to fluoxapiprolin is moderate, and multiple point mutations in PcORP1 could confer different resistance levels to fluoxapiprolin.

Moments of weaknesses - exploiting vulnerabilities between germination and encystment in the Phytomyxea

Attempts at management of diseases caused by protozoan plant parasitic Phytomyxea have often been ineffective. The dormant life stage is characterised by long-lived highly robust resting spores that are largely impervious to chemical treatment and environmental stress. This review explores some life stage weaknesses and highlights possible control measures associated with resting spore germination and zoospore taxis. With phytomyxid pathogens of agricultural importance, zoospore release from resting spores is stimulated by plant root exudates. On germination, the zoospores are attracted to host roots by chemoattractant components of root exudates. Both the relatively metabolically inactive resting spore and motile zoospore need to sense the chemical environment to determine the suitability of these germination stimulants or attractants respectively, before they can initiate an appropriate response. Blocking such sensing could inhibit resting spore germination or zoospore taxis. Conversely, the short life span and the vulnerability of zoospores to the environment require them to infect their host within a few hours after release. Identifying a mechanism or conditions that could synchronise resting spore germination in the absence of host plants could lead to diminished pathogen populations in the field.

Enantioselective Detection, Bioactivity, and Degradation of the Novel Chiral Fungicide Oxathiapiprolin

Oxathiapiprolin is a novel chiral piperidine thiazole isooxazoline fungicide that contains a pair of enantiomers. An effective analytical method was established for the enantioselective detection of oxathiapiprolin in fruit, vegetable, and soil samples using ultraperformance liquid chromatography-tandem triple quadrupole mass spectrometry. The optimal enantioseparation was achieved on a Chiralpak IG column at 35 °C using acetonitrile and 0.1% formic acid aqueous solution (90:10, v/v) as the mobile phase. The absolute configuration of the oxathiapiprolin enantiomers was identified with the elution order of R-(-)-oxathiapiprolin and S-(+)-oxathiapiprolin by electron circular dichroism spectra. The bioactivity of R-(-)-oxathiapiprolin was 2.49 to 13.30-fold higher than that of S-(+)-oxathiapiprolin against six kinds of oomycetes. The molecular docking result illuminated the mechanism of enantioselectivity in bioactivity. The glide score (-8.00 kcal/mol) for the R-enantiomer was better with the binding site in Phytophthora capsici than the S-enantiomer (-7.50 kcal/mol). Enantioselective degradation in tomato and pepper under the field condition was investigated and indicated that R-(-)-oxathiapiprolin was preferentially degraded. The present study determines the enantioselectivity of oxathiapiprolin about enantioselective detection, bioactivity, and degradation for the first time. The R-enantiomer will be a better choice than racemic oxathiapiprolin to enhance the bioactivity and reduce the pesticide residues at a lower application rate.

The consensus Nglyco -X-S/T motif and a previously unknown Nglyco -N-linked glycosylation are necessary for growth and pathogenicity of Phytophthora

Asparagine (Asn, N)-linked glycosylation within N_glyco -X-S/T; X ≠ P motif is a ubiquitously distributed post-translational modification that participates in diverse cellular processes. In this work, N-glycosylation inhibitor was shown to prevent Phytophthora sojae growth, suggesting that N-glycosylation is necessary for oomycete development. We conducted a glycoproteomic analysis of P. sojae to identify and map N-glycosylated proteins and to quantify differentially expressed glycoproteins associated with mycelia, asexual cyst, and sexual oospore developmental stages. A total of 355 N-glycosylated proteins was found, containing 496 glycosites, potentially involved in glycan degradation, carbon metabolism, glycolysis, or other metabolic pathways. Through PNGase F deglycosylation assays and site-directed mutagenesis of a GPI transamidase protein (GPI16) upregulated in cysts and a heat shock protein 70 (HSP70) upregulated in oospores, we demonstrated that both proteins were N-glycosylated and that the N_glyco -N motif is a target site for asparagine - oligosaccharide linkage. Glycosite mutations of Asn 94 N_glyco -X-S/T in the GPI16 led to impaired cyst germination and pathogenicity, while mutation of the previously unknown Asn 270 N_glyco -N motif in HSP70 led to decreased oospore production. In addition to providing a map of the oomycete N-glycoproteome, this work confirms that P. sojae has evolved multiple N-glycosylation motifs essential for growth.

Fungicide Resistance Evolution and Detection in Plant Pathogens: Plasmopara viticola as a Case Study

The use of single-site fungicides to control plant pathogens in the agroecosystem can be associated with an increased selection of resistance. The evolution of resistance represents one of the biggest challenges in disease control. In vineyards, frequent applications of fungicides are carried out every season for multiple years. The agronomic risk of developing fungicide resistance is, therefore, high. Plasmopara viticola, the causal agent of grapevine downy mildew, is a high risk pathogen associated with the development of fungicide resistance. P. viticola has developed resistance to most of the fungicide classes used and constitutes one of the most important threats for grapevine production. The goals of this review are to describe fungicide resistance evolution in P. viticola populations and how to conduct proper monitoring activities. Different methods have been developed for phenotyping and genotyping P. viticola for fungicide resistance and the different phases of resistance evolution and life cycles of the pathogen are discussed, to provide a full monitoring toolkit to limit the spread of resistance. A detailed revision of the available tools will help in shaping and harmonizing the monitoring activities between countries and organizations.

4-Ethylphenol, A Volatile Organic Compound Produced by Disease-Resistant Soybean, Is a Potential Botanical Agrochemical Against Oomycetes

Oomycetes, represented by Phytophthora, are seriously harmful to agricultural production, resulting in a decline in grain quality and agricultural products and causing great economic losses. Integrated management of oomycete diseases is becoming more challenging, and plant derivatives represent effective alternatives to synthetic chemicals as novel crop protection solutions. Biologically active secondary metabolites are rapidly synthesized and released by plants in response to biotic stress caused by herbivores or insects, as well as pathogens. In this study, we identified groups of volatile organic compounds (VOCs) from soybean plants inoculated with Phytophthora sojae, the causal agent of soybean root rot. 4-Ethylphenol was present among the identified VOCs and was induced in the incompatible interaction between the plants and the pathogen. 4-Ethylphenol inhibited the growth of P. sojae and Phytophthora nicotianae and had toxicity to sporangia formation and zoospore germination by destroying the pathogen cell membrane; it had a good control effect on soybean root rot and tobacco black shank in the safe concentration range. Furthermore, 4-Ethylphenol had a potent antifungal activity against three soil-borne phytopathogenic fungi, Rhizoctonia solani, Fusarium graminearum, and Gaeumannomyces graminis var tritici, and four forma specialis of Fusarium oxysporum, which suggest a potential to be an eco-friendly biological control agent.

Sensitivity of Pythium spp. and Phytopythium spp. and tolerance mechanism of Pythium spp. to oxathiapiprolin

BACKGROUND

Oxathiapiprolin, developed by DuPont, is the only commercial oxysterol-binding protein inhibitor (OSBPI) of oomycete pathogens. Although the activity of oxathiapiprolin on some Pythium spp. and Phytopythium spp. has been reported, it has not been tested on many other species, and little is known about the mechanisms of Pythium spp. that are tolerant to it.

RESULTS

Oxathiapiprolin exhibited a strong inhibitory effect on mycelial growth of Phy. litorale, Phy. helicoides and Phy. chamaehyphon, with EC₅₀ values ranging from 0.002 to 0.013 μg mL^-1 . It also showed good effectiveness against Py. splendens and two Py. ultimum isolates, with EC₅₀ values ranging from 0.167 to 0.706 μg mL^-1 , but showed no activity against 14 other Pythium spp. Oxathiapiprolin provoked a slight upregulation of PuORP1 in Py. ultimum, but it did not lead to PaORP1-1 or PaORP1-2 overexpression in Py. aphanidermatum. Transformation and expression of PuORP1, PaORP1-1 or PaORP1-2 in the sensitive wild-type Phytophthora sojae isolate P6497 confirmed that either the PuORP1, PaORP1-1 or PaORP1-2 was responsible for the observed oxathiapiprolin tolerance.

CONCLUSION

This study showed that oxathiapiprolin had excellent activity against Phytopythium spp. but displayed a differentiated activity against different Pythium spp. ORP1s in Pythium spp. are positively related to the tolerance of Pythium species to oxathiapiprolin. © 2020 Society of Chemical Industry.

Activity and Resistance Assessment of a New OSBP Inhibitor, R034-1, in Phytophthora capsici and the Detection of Point Mutations in PcORP1 that Confer Resistance

R034-1 is a new member of the piperidinyl thiazole isoxazoline class of fungicides that shows high activity against most plant-pathogenic oomycetes and could effectively inhibit several developmental stages of Phytophthora capsici. Here, the potential resistance risk for R034-1 was evaluated in P. capsici. The baseline sensitivities of 135 isolates to R034-1 showed a unimodal curve, with a mean EC₅₀ value of 0.004 μg/mL. Twelve resistant mutants were generated by fungicide adaptation and displayed lower fitness compared to parental isolates, which suggests that the resistance risk of P. capsici to R034-1 is low. R034-1 and oxathiapiprolin are structurally related, and resistant isolates display cross-resistance to both compounds, suggesting that these fungicides may target the same oxysterol binding protein. Comparison of PcORP1 genes in the resistant mutants and their parental isolates revealed (N767S, N767I, and G700V) amino acid substitutions in the R034-1 resistant mutant. Causality was functionally validated using site-directed mutagenesis of the target gene using the CRISPR/Cas9 system.

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

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

Enantioselective residue analysis of oxathiapiprolin and its metabolite in tea and other crops by ultra-high performance liquid chromatography-tandem mass spectrometry

Oxathiapiprolin is the first chiral piperidinyl thiazole isoxazoline fungicide developed to control downy mildew and other diseases, and there were no prior reports on its enantiomeric residue. In this study, a modified quick, easy, cheap, effective, rugged, and safe extraction and purification method followed by ultra-high performance liquid chromatography-tandem mass spectrometry determination was first developed and validated for the residue analysis of oxathiapiprolin enantiomers and its metabolite IN-E8S72 in green tea and other crops. Oxathiapiprolin enantiomers and IN-E8S72 were separated on a chiral Lux Cellulose-3 column with the use of 0.1% formic acid in acetonitrile and 5 mmol/L ammonium acetate in water as mobile phases. IN-E8S72 was eluted first, followed by (-)-oxathiapiprolin, and then (+)-oxathiapiprolin. The recoveries ranged from 53.3 to 125.3% with relative standard deviations ranging from 1.4 to 16.0%. The limits of quantification for (-)-oxathiapiprolin and (+)-oxathiapiprolin were 0.005 mg/kg in romaine lettuce, head cabbage, potato, grape, and garlic, 0.01 mg/kg in soybean and pea, and 0.025 mg/kg in green tea and dry pepper. The limits of quantification of IN-E8S72 were twice those of (-)-oxathiapiprolin. Screening results with real market samples indicated that there was no enantiomeric excess in the oxathiapiprolin residue in romaine lettuce.

Design, synthesis, and fungicidal evaluation of novel oxysterol binding protein inhibitors for combatting resistance associated with oxathiapiprolin

Oxathiapiprolin, the first successful oxysterol binding protein (OSBP) inhibitor for oomycete control, is regarded as an important milestone in the history of fungicide discovery. However, its interaction with OSBP remain unclear. Moreover, some plant pathogenic oomycetes have developed medium to high resistance to oxathiapiprolin. In this paper, the three-dimensional (3D) structure of OSBP from Phytophthora capsici (pcOSBP) was built, and its interaction with oxathiapiprolin was systematically investigated by integrating molecular docking, molecular dynamics simulations, and molecular mechanics Poisson-Boltzmann surface area (MM/PBSA) calculations. The computational results showed that oxathiapiprolin bound to pcOSBP forms H-bonds with Leu73, Lys74, Ser69, and water molecules. Then, based on its interaction with pcOSBP, oxathiapiprolin was structurally modified to discover new analogs with high fungicidal activity and a low risk of resistance. Fortunately, compound 1e was successfully designed and synthesized as the most potent candidate, and it showed a much lower resistance risk (RF < 1) against LP3-M and LP3-H in P. capsici. The present work indicated that the piperidinyl-thiazole-isoxazoline moiety is useful for further optimization. Furthermore, compound 1e could be used as a lead compound for the discovery of new OSBP inhibitors.

Detection and Characterization of Carboxylic Acid Amide-Resistant Plasmopara viticola in China Using a TaqMan-MGB Real-Time PCR

Grape production is increasing globally and so are problems with downy mildew, one of the main constraints in grape production. Downy mildew on grape is caused by Plasmopara viticola, an obligate biotrophic pathogen belonging to the oomycetes. Control of the disease is usually performed by fungicide applications, of which carboxylic acid amide (CAA) fungicides represent one of the most widely used groups of fungicides. Our previous research showed that the extensive application of CAA fungicides can result in fungicide resistance and in China, CAA-resistant isolates of P. viticola were collected from the field in 2014. To monitor the distribution and spread of CAA fungicide resistance, we developed a TaqMan-minor groove binder (MGB) real-time PCR-based method designed on a functional mutation in the PvCesA3 gene that allows efficient identification of CAA fungicide resistant and sensitive genotypes. The assay was validated on 50 isolates using Sanger sequencing and fungicide bioassays and exploited in a comprehensive survey comprising 2,227 single-sporangiophore isolates from eight major grapevine regions in China. We demonstrate that CAA fungicide resistance in P. viticola is widespread in China. On average, 53.3% of the isolates were found to be resistant, but marked differences were found between locations with percentages of resistant isolates varying from 0.3 to 96.6%. Furthermore, the frequency of CAA-resistant isolates was found to be significantly correlated with the exposure to CAA fungicides (P < 0.05). We further discussed the possibilities to apply the TaqMan-MGB real-time PCR assay to assess the frequency of fungicide-resistant P. viticola isolates in each region or vineyard, which would facilitate the correct choice of fungicide for grape downy mildew and resistance management strategies.

Bioactivity of the Novel Fungicide SYP-14288 Against Plant Pathogens and the Study of its Mode of Action Based on Untargeted Metabolomics

Plant disease is a major threat to crop production, and fungicide application is one of the most effective methods to control plant disease. With emerging issues related to toxic residues and pathogen resistance, new fungicides with novel modes of action are urgently needed. SYP-14288 is a novel fungicide that could efficiently promote respiration and inhibit ATP biosynthesis in target organisms, but its bioactivity against various plant pathogens and exact mode of action are still unknown. In this study, we found that SYP-14288 is highly effective against 31 important plant pathogens belonging to a range of taxonomic groups. In addition, SYP-14288 has demonstrated excellent activity against all life stages of the important fungal plant pathogen Magnaporthe oryzae and is especially effective during the pathogen's high energy consumption stages. SYP-14288 showed good preventative control efficacy against pepper blight and rice blast in the greenhouse and field, respectively. In an untargeted metabolomics assay designed to determine the exact mode of action of SYP-14288, significant changes occurred in 25 metabolites, with the accumulation of seven fatty acid metabolites and a decrease in 18 starch and sugar metabolites (e.g., from the tricarboxylic acid cycle). This suggests that SYP-14288 is an uncoupling agent similar to 2,4-dinitrophenol, which can allow for accumulation of various fatty acids after destroying oxidative phosphorylation coupling, thereby inhibiting the growth of the phytopathogen. These results indicate that the novel uncoupler SYP-14288 is a promising agrochemical in plant disease management.

Multiple point mutations in PsORP1 gene conferring different resistance levels to oxathiapiprolin confirmed using CRISPR-Cas9 in Phytophthora sojae

BACKGROUND

Oxathiapiprolin is among the first commercial oxysterol-binding protein inhibitors (OSBPIs) developed by DuPont Corporation and shows excellent activity against plant-pathogenic oomycetes. Although more than 21 target site mutations have been identified in insensitive oomycetes, only G770V, G839W, and ΔN837 have been verified to confer oxathiapiprolin resistance in Phytophthora capsici or P. sojae. The effect of other mutations on OSBPIs sensitivity requires urgent investigation.

RESULTS

P. sojae transformants containing 16 mutations of PsORP1 were recovered using the CRISPR-Cas9 system. Transformants containing L733W, S768F, S768Y, N837Y, N837F, P861H, L863W, or I877Y showed high oxathiapiprolin resistance, with resistant factors (RFs) > 3000. Point mutations S768K, S768I, G770L, G770P, G770A, ΔG818/F819, N837I, and I877F exhibited low resistance, with RFs < 80. Phenotype assays revealed that the most highly resistant transformants showed enhanced or similar pathogenicity, oospore production, and cyst gemination. However, most transformants displayed decreased sporangia and zoospore production compared with parental wild-type P6497.

CONCLUSION

This study demonstrated that L733W, S768F, S768Y, N837Y, N837F, P861H, L863W, and I877Y in PsORP1 confer high oxathiapiprolin resistance in P. sojae.

Oxathiapiprolin, a Novel Chemical Inducer Activates the Plant Disease Resistance

Oxathiapiprolin was developed as a specific plant pathogenic oomycete inhibitor, previously shown to have highly curative and protective activities against the pepper Phytophthora blight disease under field and greenhouse tests. Therefore, it was hypothesized that oxathiapiprolin might potentially activate the plant disease resistance against pathogen infections. This study investigated the potential and related mechanism of oxathiapiprolin to activate the plant disease resistance using the bacterium Pseudomonas syringae pv tomato (Pst) and plant Arabidopsis interaction as the targeted system. Our results showed that oxathiapiprolin could activate the plant disease resistance against Pst DC3000, a non-target pathogen of oxathiapiprolin, in Arabidopsis, tobacco, and tomato plants. Our results also showed the enhanced callose deposition and H2O2 accumulation in the oxathiapiprolin-treated Arabidopsis under the induction of flg22 as the pathogen-associated molecular pattern (PAMP) treatment. Furthermore, increased levels of free salicylic acid (SA) and jasmonic acid (JA) were detected in the oxathiapiprolin-treated Arabidopsis plants compared to the mock-treated ones under the challenge of Pst DC3000. Besides, the gene expression results confirmed that at 24 h after the infiltration with Pst DC3000, the oxathiapiprolin-treated Arabidopsis plants had upregulated expression levels of the respiratory burst oxidase homolog D (RBOHD), JA-responsive gene (PDF1.2), and SA-responsive genes (PR1, PR2, and PR5) compared to the control. Taken together, oxathiapiprolin is identified as a novel chemical inducer which activates the plant disease resistance against Pst DC3000 by enhancing the callose deposition, H2O2 accumulation, and hormone SA and JA production.

Etiology of Cyclocarya paliurus Anthracnose in Jiangsu Province, China

Cyclocarya paliurus is an extremely valuable and multifunctional tree species whose leaves have traditionally been used in used in medicine or as a medicinal tea in China. In recent years, anthracnose has been frequently observed on young leaves of C. paliurus in several nurseries located in Jiangsu Province, resulting in great yield and quality losses. To date, no information is available about the prevalence of C. paliurus anthracnose in China. The main purpose of the present study was to characterize the etiology of C. paliurus anthracnose. Phylogenetic analysis of the eight-loci concatenated dataset revealed that all 44 single-spore Colletotrichum isolates belonged to three species in the Colletotrichum gloeosporioides species complex, namely, Colletotrichum aenigma, Colletotrichum fructicola, and C. gloeosporioides sensu stricto. Phenotypic features, including the colony appearance and the morphology of conidia, appressoria, and ascospores, were consistent with the phylogenetic grouping. Virulence tests validated that the three Colletotrichum species could cause typical symptoms of anthracnose on C. paliurus leaves, similar to those observed in the field. The optimum mycelial growth temperature ranged from 25 to 30°C for all representative isolates, while C. gloeosporioides s. s. isolates exhibited greater tolerance to high temperature (40°C). Fungicide sensitivity assays indicated that all three Colletotrichum species were sensitive to tetramycin, which may be a potential alternative for the management of C. paliurus anthracnose. To our knowledge, this study provides the first report of C. aenigma, C. fructicola, and C. gloeosporioides s. s. causing C. paliurus anthracnose in China as well as in the world.

Residue Analysis and Risk Assessment of Oxathiapiprolin and Its Metabolites in Cucumbers under Field Conditions

In this study, a rapid, sensitive, and selective method was established for the detection of oxathiapiprolin and the metabolite IN-E8S72, as well as its glucose conjugate IN-SXS67 in cucumber using modified QuEChERS procedure combined with HPLC-MS/MS. The LOQs for all compounds were 0.02 mg kg^-1, and the average recoveries were 77.4-111.3% with RSDs of 1.0-8.5%. Under the optimized conditions, the established method was successfully used to determine field samples in dissipation and terminal residue studies. The dissipation study results showed that oxathiapiprolin dissipated rapidly in cucumber with half-lives of 2.4-4.0 days. On the basis of the terminal residue results, the risk assessment was conducted, and both the international estimated daily intake (IEDI) or national estimated daily intake (NEDI) of oxathiapiprolin were much less than 100% which indicate a low health risk to consumers. This work provides guidance for establishing MRL of oxathiapiprolin in China and is of great significance for evaluating its dietary risk in cucumber.

PcMuORP1, an Oxathiapiprolin-Resistance Gene, Functions as a Novel Selection Marker for Phytophthora Transformation and CRISPR/Cas9 Mediated Genome Editing

Phytophthora, a genus of oomycetes, contains many devastating plant pathogens, which cause substantial economic losses worldwide. Recently, CRISPR/Cas9-based genome editing tool was introduced into Phytophthora to delineate the functionality of individual genes. The available selection markers for Phytophthora transformation, however, are limited, which can restrain transgenic manipulation in some cases. We hypothesized that PcMuORP1, an endogenous fungicide resistance gene from P. capsici that confers resistance to the fungicide oxathiapiprolin via an altered target site in the ORP1 protein, could be used as an alternative marker. To test this hypothesis, the gene PcMuORP1 was introduced into the CRISPR/Cas9 system and complementation of a deleted gene in P. capsici was achieved using it as a selection marker. All of the oxathiapiprolin-resistant transformants were confirmed to contain the marker gene, indicating that the positive screening rate was 100%. The novel selection marker could also be used in other representative Phytophthora species including P. sojae and P. litchii, also with 100% positive screening rate. Furthermore, comparative studies indicated that use of PcMuORP1 resulted in a much higher efficiency of screening compared to the conventional selection marker NPT II, especially in P. capsici. Successive subculture and asexual reproduction in the absence of selective pressure were found to result in the loss of the selection marker from the transformants, which indicates that the PcMuORP1 gene would have little long term influence on the fitness of transformants and could be reused as the selection marker in subsequent projects. Thus, we have created an alternative selection marker for Phytophthora transformation by using a fungicide resistance gene, which would accelerate functional studies of genes in these species.

New Oomycota Fungicides With Activity Against Phytophthora cinnamomi and Their Potential Use for Managing Avocado Root Rot in California

Phytophthora root rot (PRR), caused by Phytophthora cinnamomi, is the most destructive disease of avocado worldwide. In the United States, mefenoxam and phosphonate products are currently the only registered fungicides for managing avocado PRR. Four new Oomycota-specific and two registered fungicides, all with different modes of action, were evaluated. Seventy-one isolates of P. cinnamomi from avocado in California, most of them collected between 2009 to 2017, were tested for their in vitro sensitivity to the six fungicides. Baseline sensitivity ranges and mean values (in parentheses) of effective concentrations to inhibit mycelial growth by 50% (EC₅₀) for the new fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin were 0.017 to 0.069 μg/ml (0.035), 0.046 to 0.330 μg/ml (0.133), 0.003 to 0.011 μg/ml (0.005), and 0.0002 to 0.0007 μg/ml (0.0004), respectively. In comparison, the EC₅₀ value range (mean) was 0.023 to 0.138 μg/ml (0.061) for mefenoxam and 12.9 to 361.2 μg/ml (81.5) for potassium phosphite. Greenhouse soil inoculation trials with 8-month-old Zutano seedlings and 10-month-old Dusa and PS.54 clonal rootstocks were conducted to assess the efficacy of these fungicides for managing PRR. Mefenoxam and potassium phosphite were effective treatments; however, oxathiapiprolin, fluopicolide, and mandipropamid were more effective. Ethaboxam was effective in reducing PRR on the rootstocks evaluated. Oxathiapiprolin reduced PRR incidence and pathogen population size in the soil by >90%, and plant shoot growth and root dry weight were significantly increased compared with the control; thus, oxathiapiprolin was one of the best treatments overall. The high activity and performance of these new fungicides supports their registrations on avocado for use in rotation and mixture programs, including with previously registered compounds, to reduce the risk of development and spread of resistance in pathogen populations.

Evaluation of New Oomycota Fungicides for Management of Phytophthora Root Rot of Citrus in California

Phytophthora root rot, caused by several species of Phytophthora, is an important disease of citrus in California and other growing regions. For chemical management, mefenoxam and potassium phosphite have been available for many years, and resistance in Phytophthora spp. has been reported for both compounds. We evaluated the efficacy of the new Oomycota fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin, each with a different mode of action, against Phytophthora root rot of citrus in field and greenhouse studies. Root balls of navel orange trees on 'Carrizo citrange' rootstock were inoculated with P. nicotianae at planting in the field in fall 2013. Applications with 11 fungicide treatments were made 5 weeks after planting, in spring and fall 2014, and in spring 2015. Feeder roots and adjacent soil were collected before or after application. All of the new fungicides significantly reduced root rot incidence and Phytophthora soil populations to very low levels as compared with the control starting after the first application. Mefenoxam was only effective when a high label rate was used in the fourth application. Selected treatments also increased tree canopy size, trunk diameter, and fruit yield as compared with the control. A rate comparison with the four new fungicides was initiated in summer 2016 in another field trial using navel orange trees inoculated with P. citrophthora. Minimum effective rates to reduce Phytophthora root rot incidence and pathogen soil populations were determined after one and two applications in fall 2016 and summer 2017, respectively. Greenhouse studies confirmed the efficacy of the new fungicides. Based in part on our studies, fluopicolide recently received a federal and oxathiapiprolin a full registration for use on citrus, and registrations for ethaboxam and mandipropamid have been requested. These new compounds will provide highly effective treatment options and resistance management strategies using rotation and mixture programs for the control of Phytophthora root rot of citrus.