Development and implementation of a novel CAPS assay reveals high prevalence of a boscalid resistance marker and its co-occurrence with an azole resistance marker in Erysiphe necator

Succinate dehydrogenase inhibitors (SDHIs), are frequently used against powdery mildew (PM) fungi, such as Erysiphe necator, the causal agent of grapevine PM. Fungicide resistance, however, hinders effective control. DNA-based monitoring facilitates the recognition of resistance. We aimed (i) to adapt an effective method to detect a widespread genetic marker of resistance to boscalid, a commonly used SDHI, and (ii) to study the co-occurrence of the marker with a marker of resistance to demethylase inhibitor (DMI) fungicides. Sequencing of the sdhB gene identified a non-synonymous substitution, denoted as sdhB-A794G, leading to an amino acid change (H242R) in the sdhB protein. In vitro fungicide resistance tests showed that E. necator isolates carrying sdhB-A794G were resistant to boscalid. We adopted a cleaved amplified polymorphic sequence-based method and screened more than 500 field samples collected from five Hungarian wine regions in two consecutive years. The sdhB-A794G marker was detected in all wine regions and in both years, altogether in 61.7% of samples, including 20.5% in which both sdhB-A794G and the wild-type were present. The frequency of sdhB-A794G was higher in SDHI-treated vineyards than in vineyards without any SDHI application. A significant difference in the presence of the marker was detected among wine regions; its prevalence ranged from none to 100%. We identified significant co-occurrence of sdhB-A794G with the CYP51-A495T (Y136F) mutation of the CYP51 gene, a known marker of resistance to DMIs. The monitoring of fungicide resistance is fundamental for the successful control of E. necator. Our rapid, cost-effective diagnostic method will support decision-making and fungicide resistance monitoring and management.

Design, synthesis, antifungal activity and molecular docking of novel pyrazole-4-carboxamides containing tertiary alcohol and difluoromethyl moiety as potential succinate dehydrogenase inhibitors

BACKGROUND

Resistance problems with the long-term and frequent use of existing fungicides, and the lack of structure diversity of traditional pyrazole-4-carboxamide succinate dehydrogenase inhibitors, it is highly required to design and develop new fungicides to address the resistance issue.

RESULTS

Different from previous pyrazole-4-carboxamide succinate dehydrogenase inhibitors by breaking the norm of difluoromethyl at the C-3 position of pyrazole and introducing a tertiary alcohol group at the C-3 position, 27 novel pyrazole-4-carboxamide derivatives were designed, synthesized and characterized by proton (1 H) nuclear magnetic resonance (NMR), carbon-13 (13 C) NMR, fluorine-19 (19 F) NMR and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). The crystal structures of compounds A14 and C5 were analyzed by single crystal X-ray diffraction. Their in vitro antifungal activities were evaluated against phytopathogen Fusarium graminearum, Botrytis cinerea, Phytophthora capsica, Sclerotinia sclerotiorum, Thanatephorus cucumeris. The results displayed that most of them exhibited significant antifungal activities against S. sclerotiorum at 50 mg/L, the half maximal effective concentration (EC50 ) data of A8 and A14 were 3.96 and 2.52 mg/L, respectively. Their in vivo antifungal activities were evaluated against Pseudoperonospora cubensis, Puccinia sorghi Schw, Colletotrichum gloeosporioides, F. graminearum, Erysiphe graminis, Thanatephorus cucumeris, the control efficacies of A6, B3, C3, and C6 against E. graminis reached 100% at a concentration of 400 mg/L. The molecular docking results showed that the binding mode of the target compounds containing tertiary alcohols were similar to that of fluxapyroxad in succinate dehydrogenase. In addition, tertiary alcohols were involved in the formation of hydrogen bonds.

CONCLUSION

The excellent in vitro and in vivo inhibitory activities of novel pyrazole-4-carboxamide derivatives against succinate dehydrogenase were reported for the first time, and they could be used as the potential lead compounds. © 2023 Society of Chemical Industry.

Design, Synthesis, and Fungicidal Activities of Novel N-(Pyrazol-5-yl)benzamide Derivatives Containing a Diphenylamine Moiety

To accelerate the development of novel fungicides, a variety of N-(pyrazol-5-yl)benzamide derivatives with a diphenylamine moiety were designed and synthesized using a pharmacophore recombination strategy based on the structure of pyrazol-5-yl-aminophenyl-benzamides. The bioassay results demonstrated that most of the target compounds had excellent in vitro antifungal activities against Sclerotinia sclerotiorum, Valsa mali, and Botrytis cinerea. In particular, compound 5IIIh exhibited remarkable activity against S. sclerotiorum (EC50 = 0.37 mg/L), which was similar to that of fluxapyroxad (EC50 = 0.27 mg/L). In addition, compound 5IIIc (EC50 = 1.32 mg/L) was observed to be more effective against V. mali than fluxapyroxad (EC50 = 12.8 mg/L) and comparable to trifloxystrobin (EC50 = 1.62 mg/L). Furthermore, compound 5IIIh demonstrated remarkable in vivo protective antifungal properties against S. sclerotiorum, with an inhibition rate of 96.8% at 100 mg/L, which was close to that of fluxapyroxad (99.6%). Compounds 5IIIc (66.7%) and 5IIIh (62.9%) exhibited good in vivo antifungal effects against V. mali at 100 mg/L, which were superior to that of fluxapyroxad (11.1%) but lower than that of trifloxystrobin (88.9%). The succinate dehydrogenase (SDH) enzymatic inhibition assay was conducted to confirm the mechanism of action. Molecular docking analysis further revealed that compound 5IIIh has significant hydrogen-bonding, π-π, and p-π conjugation interactions with ARG 43, SER 39, TRP 173, and TYR 58 in the binding site of SDH, and the binding mode was similar to that of the commercial fungicide fluxapyroxad. All of the results suggest that compound 5IIIh could be a potential SDH inhibitor, offering a valuable reference for future studies.

Novel quinolin-2(1H)-one analogues as potential fungicides targeting succinate dehydrogenase: design, synthesis, inhibitory evaluation and molecular modeling

BACKGROUND

Succinate dehydrogenase is an important target of fungicides. Succinate dehydrogenase inhibitors (SDHIs) have widely been used to combat destructive plant pathogenic fungi because they possess efficient and broad-spectrum antifungal activities and as well as unique mode of action. The research and development of novel SDHIs have been ongoing.

RESULTS

Thirty-six novel quinolin-2(1H)-one derivatives were designed, synthesized and characterized. The single crystal structure of compound 3c was determined through the X-ray diffraction of single crystals. The bioassay results displayed that most compounds had good antifungal activities at 16 μg mL-1 against Rhizoctonia cerealis, Erysiphe graminis, Botrytis cinerea, Penicillium italicum and Phytophthora infestans. Compounds 6o, 6p and 6r had better antifungal activities than the commercialized fungicide pyraziflumid against Botrytis cinerea. Their half maximal effective concentration (EC50 ) values were 0.398, 0.513, 0.205 and 0.706 μg mL-1 , respectively. Moreover, the inhibiting activities of the bioactive compounds were tested against succinate dehydrogenase. The results indicated that they possessed outstanding activities. Compounds 6o, 6p and 6r also exhibited better inhibiting activities than pyraziflumid against succinate dehydrogenase. Their half maximal inhibitory concentration (IC50 ) values were 0.450, 0.672, 0.232 and 0.858 μg mL-1 , respectively. The results of molecular dynamic (MD) simulations indicated that compound 6r displayed stronger affinity to succinate dehydrogenase than pyraziflumid.

CONCLUSION

The results of the present study displayed that quinolin-2(1H)-one derivative could be one scaffold of potential SDHIs and will provide some valuable information for the research and development of new SDHIs. © 2022 Society of Chemical Industry.

Potential Succinate Dehydrogenase Inhibitors Bearing a Novel Pyrazole-4-sulfonohydrazide Scaffold: Molecular Design, Antifungal Evaluation, and Action Mechanism

Aiming to develop novel antifungal agents with a distinctive molecular scaffold targeting succinate dehydrogenase (SDH), 24 N'-phenyl-1H-pyrazole-4-sulfonohydrazide derivatives were first devised, synthesized, and verified by ¹H NMR, ¹³C NMR, high-resolution mass spectrometry (HRMS), and single-crystal X-ray diffraction analysis. The bioassays revealed that the target compounds possessed highly efficient and broad-spectrum antifungal activities against four tested plant pathogenic fungi Rhizoctonia solani (R. solani), Botrytis cinerea, Fusarium graminearum, and Alternaria sonali. Strikingly, compound B6 was assessed as the selective inhibitor against R. solani, with an in vitro EC₅₀ value (0.23 μg/mL) that was similar to that of thifluzamide (0.20 μg/mL). The in vivo preventative effect of compound B6 (75.76%) at 200 μg/mL against R. solani was roughly comparable to thifluzamide (84.31%) under the same conditions. The exploration of morphological observations indicated that compound B6 could strongly damage the mycelium morphology, obviously increase the permeability of the cell membrane, and dramatically increase the number of mitochondria. Compound B6 also significantly inhibited SDH enzyme activity with an IC₅₀ value of 0.28 μg/mL, and its fluorescence quenching dynamic curves were similar to that of thifluzamide. Molecular docking and molecular dynamics simulations demonstrated that compound B6 could strongly interact with similar residues around the SDH active pocket as thifluzamide. The present study revealed that the novel N'-phenyl-1H-pyrazole pyrazole-4-sulfonohydrazide derivatives are worthy of being further investigated as the promising replacements of traditional carboxamide derivatives targeting SDH of fungi.

Design, synthesis, and antifungal activity of novel dithiin tetracarboximide derivatives as potential succinate dehydrogenase inhibitors

BACKGROUND

Succinate dehydrogenase inhibitor (SDHI) fungicides are an important class of agricultural fungicides with the advantages of high efficiency and a broad bactericidal spectrum. To pursue novel SDHIs, a series of N-substituted dithiin tetracarboximide derivatives were designed, synthesized, and characterized by ¹ H NMR, ¹³ C NMR, and high resolution mass spectrum (HRMS).

RESULTS

These engineered compounds displayed potent fungicidal activity against phytopathogens, including Sclerotinia sclerotiorum, Botrytis cinerea, and Rhizoctonia solani, comparable with that of the commercial SDHI fungicide boscalid. In particular, compound 18 stood out with prominent activity against S. sclerotiorum with a half-maximal effective concentration (EC₅₀ ) value of 1.37 μg ml^-1 . Compound 1 exhibited the most potent antifungal activity against B. cinerea with EC₅₀ values of 5.02 μg ml^-1 . As for R. solani, 12 and 13 exhibited remarkably inhibitory activity with EC₅₀ values of 4.26 and 5.76 μg ml^-1 , respectively. In the succinate dehydrogenase (SDH) inhibition assay, 13 presented significant inhibitory activity with a half-maximal inhibitory concentration (IC₅₀ ) value of 15.3 μm, which was approximately equivalent to that of boscalid (14.2 μm). Furthermore, molecular docking studies revealed that 13 could anchor in the binding site of SDH.

CONCLUSION

Taken together, results suggested that the dithiin tetracarboximide scaffold possessed a huge potential to be developed as novel fungicides and SDHIs. © 2023 Society of Chemical Industry.

Discovery of N-Phenylpropiolamide as a Novel Succinate Dehydrogenase Inhibitor Scaffold with Broad-Spectrum Antifungal Activity on Phytopathogenic Fungi

Based on the structural features of both succinate dehydrogenase inhibitors (SDHIs) and targeted covalent inhibitors, a series of N-phenylpropiolamides containing a Michael acceptor moiety were designed to find new antifungal compounds. Nineteen compounds showed potent inhibition activity in vitro on nine species of plant pathogenic fungi. Compounds 9 and 13 showed higher activity on most of the fungi than the standard drug azoxystrobin. Compound 13 could completely inhibit Physalospora piricola infection on apples at 200 μg/mL concentration over 7 days and showed high safety to seed germination and seedling growth of plants at ≤100 μg/mL concentration. The action mechanism showed that 13 is an SDH inhibitor with a median inhibitory concentration, IC₅₀, value of 0.55 μg/mL, comparable with that of the positive drug boscalid. Molecular docking studies revealed that 13 can bind well to the ubiquinone-binding region of SDH by hydrogen bonds and undergoes π-alkyl interaction and π-cation interaction. At the cellular level, 1 as the parent compound could destruct the mycelial structure of P. piricola and partly dissolve the cell wall and/or membrane. Structure-activity relationship analysis showed that the acetenyl group should be a structure determinant for the activity, and the substitution pattern of the phenyl ring can significantly impact the activity. Thus, N-phenylpropiolamide emerged as a novel and promising lead scaffold for the development of new SDHIs for plant protection.

Design, Synthesis, and Fungicidal Evaluation of Novel N-Methoxy Pyrazole-4-Carboxamides as Potent Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH, EC 1.3.5.1, also known as complex II) has been recognized as one of the most promising targets of fungicides. Here, based on the binding mode of pydiflumetofen with SDH, the carbon-carbon double bond was introduced into the chemical of pydiflumetofen and then produced the target compounds 6a-6o. The enzymatic inhibitory activity and structure-activity relationship (SAR) study showed that the 2-position and 4-position in terminal benzene were positive to increasing activity. Furthermore, fungicidal activity results in greenhouses indicated that compound 6o showed good control effects against wheat powdery mildew (WPM), cucumber powdery mildew (CPM), and southern corn rust (SCR), showing its broad-spectrum property. Especially, compound 6o exhibited 95 and 75% control effects against CPM and SCR at 6.25 mg/L, respectively, which are better than pydiflumetofen (80% control effects against CPM and 15% against SCR), indicating its potency that is worthy of further development.

Novel Camphor Sulfonohydrazide and Sulfonamide Derivatives as Potential Succinate Dehydrogenase Inhibitors against Phytopathogenic Fungi/Oomycetes

To discover novel fungicidal agrochemicals for treating wheat scab, 39 novel camphor sulfonohydrazide/sulfonamide derivatives 4a-4t and 6a-6s were designed and synthesized. In the in vitro antifungal/antioomycete assay, compounds 4g, 4n, and 4o displayed significant inhibitory activities against Fusarium graminearum, Botryosphaeria dothidea, and Phytophthora capsici. Among them, 4n exhibited the best antifungal activity against F. graminearum with an EC₅₀ value of 0.41 mg/L, which was at the same level as that of pydiflumetofen. The in vivo experiment revealed that 4n presented excellent protective and curative efficacy toward F. graminearum. In the antifungal mechanism study, 4n could increase the cell membrane permeability and reduce the exopolysaccharide and ergosterol content of F. graminearum. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed that 4n could significantly damage the surface morphology and the cell ultrastructure of mycelia to interfere with the growth of F. graminearum. Furthermore, 4n exhibited potent succinate dehydrogenase (SDH) inhibitory activity in vitro with an IC₅₀ value of 3.94 μM, which was equipotent to pydiflumetofen (IC₅₀ = 4.07 μM). The molecular dynamics simulation and docking study suggested that compound 4n could well occupy the active site and form strong interactions with the key residues of SDH. The above-mentioned results demonstrated that the title camphor sulfonohydrazide/sulfonamide derivatives could be promising lead compounds for further succinate dehydrogenase inhibitor (SDHI) fungicide development.

Discovery of N-Methoxy-(biphenyl-ethyl)-pyrazole-carboxamides as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) inhibitor is one of the research hotspots for the development of fungicides. Herein, we describe the design and synthesis of N-methoxy-(biphenyl-ethyl)-pyrazole-carboxamide derivatives with enhanced fungicidal activity by employing fragment combination strategy. The SDH enzymatic activity was evaluated for 24 title compounds, and compound 7s was identified as the highest activity against porcine SDH with an IC₅₀ value of 0.014 μM, 205-fold greater than that of fluxapyroxad. Furthermore, the greenhouse experiments showed that compound 7u exhibited potent fungicidal activity against wheat powdery mildew with an EC₅₀ value of 0.633 mg/L, higher activity than fluxapyroxad and benzovindiflupyr. The computational results showed that the fluorine atom substituted on the pyrazole ring formed an extra dipolar-dipolar interaction with C_S42 and then increased the van der Waals interaction between the compound and SDH. The structural and mechanistic insights obtained from the present work will provide a valuable clue to developing novel SDH inhibitors.

Design, Synthesis, and Fungicidal Evaluation of Novel 1,3-Benzodioxole-Pyrimidine Derivatives as Potential Succinate Dehydrogenase Inhibitors

A series of novel 1,3-benzodioxole-pyrimidine derivatives were designed and synthesized. The in vitro bioassay indicated that compounds 4e, 4g, 4n, 5c, and 5e displayed excellent fungicidal activities against test fungal strains. Especially, in the in vitro experiments, 5c exhibited a broad spectrum of fungicidal activity against Botrytis cinerea, Rhizoctonia solani, Fusarium oxysporum, Alternaria solani, and Gibberella zeae with EC₅₀ values of 0.44, 6.96, 6.99, 0.07, and 0.57 mg/L, respectively, which were significantly more potent than those of positive control boscalid (EC₅₀: 5.02, >50, >50, 0.16, and 1.28 mg/L). In vivo testing on tomato fruits and leaves showed that 5c displayed considerable protective and curative efficacy against A. solani. Scanning electron microscopy analysis indicated that 5c possessed a strong ability to destroy the surface morphology of mycelia and seriously interfere with the growth of the fungal pathogen. In the in vitro enzyme inhibition assay, 5c exhibited pronounced succinate dehydrogenase (SDH) inhibitory activity with an IC₅₀ value of 3.41 μM, equivalent to that of boscalid (IC₅₀: 3.40 μM). In addition, fluorescence quenching experiment further confirmed the strong interaction of 5c with SDH. Through chiral resolution, 5c was separated into two enantiomers. Among them, (S)-5c exhibited stronger fungicidal activity (EC₅₀: 0.06 mg/L) and SDH inhibitory (2.92 μM) activity than the R-enantiomer (EC₅₀: 0.17 mg/L and SDH IC₅₀: 3.68 μM), which was in accordance with the molecular docking study (CDOCKER Interaction Energy for (R)-5c and (S)-5c: -28.23 and -29.98 kcal/mol, respectively). These results presented a promising lead for the discovery of novel SDHIs as antifungal pesticides.

Design, Synthesis, and Antifungal Activity of Novel Thiophene/Furan-1,3,4-Oxadiazole Carboxamides as Potent Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) is known as an ideal target for the investigations of fungicides. To develop novel SDH inhibitors, 30 novel thiophene/furan-1,3,4-oxadiazole carboxamide derivatives were designed and synthesized. In the in vitro antifungal assay, a majority of the target compounds demonstrated fair to potent antifungal activity against seven tested phytopathogenic fungi. Compounds 4b, 4g, 4h, 4i, and 5j showed remarkable antifungal activity against Sclerotinia sclerotiorum, affording EC50 values ranging from 0.1∼1.1 mg/L. In particular, compound 4i displayed the most potent activity against S. sclerotiorum (EC₅₀ = 0.140 ± 0.034 mg/L), which was superior to that of boscalid (EC₅₀ = 0.645 ± 0.023 mg/L). A further morphological investigation revealed the abnormal mycelia and damaged cell structures of compound 4i-treated S. sclerotiorum by scanning electron microscopy. Furthermore, the in vivo antifungal assay against S. sclerotiorum revealed that compounds 4g and 4i were effective for suppressing rape Sclerotinia rot at a dosage of 200 mg/L. In the SDH inhibition assay, compounds 4g and 4i also presented significant inhibitory activity with IC₅₀ values of 1.01 ± 0.21 and 4.53 ± 0.19 μM, respectively, which were superior or equivalent to that of boscalid (3.51 ± 2.02 μM). Molecular docking and molecular dynamics simulation of compound 4g with SDH revealed that compound 4g could form strong interactions with the key residues of the SDH. These results indicated that this class of derivatives could be a promising scaffold for the discovery and development of novel SDH inhibitors.

Resistance to Boscalid, Fluopyram and Fluxapyroxad in Blumeriella jaapii from Michigan (U.S.A.): Molecular Characterization and Assessment of Practical Resistance in Commercial Cherry Orchards

Management of cherry leaf spot disease, caused by the fungus Blumeriella jaapii, with succinate dehydrogenase inhibitor (SDHI) fungicides has been ongoing in Michigan tart cherry orchards for the past 17 years. After boscalid-resistant B. jaapii were first isolated from commercial orchards in 2010, premixes of SDHI fungicides fluopyram or fluxapyroxad with a quinone outside inhibitor were registered in 2012. Here, we report widespread resistance to fluopyram (Fluo^R), fluxapyroxad (Flux^R), and boscalid (Bosc^R) in commercial orchard populations of B. jaapii in Michigan from surveys conducted between 2016 and 2019. A total of 26% of 1610 isolates from the 2016-2017 surveys exhibited the fully-resistant Bosc^R Fluo^R Flux^R phenotype and only 7% were sensitive to all three SDHIs. Practical resistance to fluopyram and fluxapyroxad was detected in 29 of 35 and 14 of 35 commercial tart cherry orchards, respectively, in surveys conducted in 2018 and 2019. Sequencing of the SdhB, SdhC, and SdhD target genes from 22 isolates with varying resistance phenotypes showed that Bosc^S Fluo^R Flux^S isolates harbored either an I262V substitution in SdhB or an S84L substitution in SdhC. Bosc^R Fluo^R Flux^R isolates harbored an N86S substitution in SdhC, or contained the N86S substitution with the additional I262V substitution in SdhB. One Bosc^R Fluo^R Flux^R isolate contained both the I262V substitution in SdhB and the S84L substitution in SdhC. These mutational analyses suggest that Bosc^R Fluo^R Flux^R isolates evolved from fully sensitive Bosc^S, Fluo^S, Flux^S isolates in the population and not from boscalid-resistant isolates that were prevalent in the 2010-2012 time period.

Design, Synthesis, Biological Evaluation, and Molecular Modeling of Novel 4H-Chromene Analogs as Potential Succinate Dehydrogenase Inhibitors

Thirty-one new 4H-chromene derivatives were designed and synthesized. Their structures were identified with IR, ¹H NMR, ¹³C NMR, and HRMS. The crystal structure of compound 2a was determined by single-crystal X-ray diffraction. Their antifungal activities were evaluated against Pyricularia oryzae, Erysiphe graminis, Coniella diplodiella, Pseudoperonospora cubensis, and Sclerotinia sclerotiorum. These results demonstrated that most compounds exhibited remarkable inhibitory activities at 20 μg/mL. Compounds 4b and 4c displayed excellent antifungal activity against S. sclerotiorum and possessed better efficacy than fluopyram. At the same time, the inhibitory activity of the bioactive compounds was evaluated against succinate dehydrogenase (SDH). The results showed that these compounds possessed outstanding activity. Compounds 4b and 4c displayed better inhibitory activity than fluopyram. The molecular modeling results revealed that compound 4c had stronger affinity to SDH than fluopyram. It is the first time that the inhibitory activity of 4H-chromene analogs against SDH has been reported.

Fungicide Sensitivity in Strawberry Powdery Mildew Caused by Podosphaera aphanis in California

Field observations suggest that reduced fungicide sensitivity exists in field populations of Podosphaera aphanis, the causal agent of strawberry powdery mildew (SPM). SPM is one of the most common diseases in strawberry production and is controlled with foliar fungicide applications. This study characterizes the sensitivity of 19 P. aphanis isolates to the most common fungicides used against SPM in California. Isolates were collected from commercial fruit production fields in Oxnard, Ventura, Santa Maria, Salinas, and Watsonville and from a plant nursery in Balico, California. Healthy, unfurled strawberry leaves (cultivar Monterey) free of visual disease symptoms were removed from actively growing plants and treated with one of six commercially formulated fungicides at the minimum labeled rate and inoculated with conidia of P. aphanis. Inoculated leaves were incubated at 20°C under 16/8 h of day/night lighting and assessed for disease incidence after 14 days. Pathogen growth on the treated leaflets constituted a measure of insensitivity to the fungicide. The six fungicide treatments and their average disease incidence on treated leaves for the 19 isolates are penthiopyrad (51.4%), quinoxyfen (41.5%), myclobutanil (39.8%), trifloxystrobin (19.8%), cyflufenamid (19.3%), and fluopyram + trifloxystrobin (3.5%). The average disease incidence for the trifloxystrobin treatment was raised significantly by two isolates considered to be resistant to the product (disease incidence >66.6%). Two isolates collected from organic production systems were sensitive to all fungicides. We document compromised fungicide efficacy due to resistance to most of the fungicides currently used for control of SPM in California. This is the first report of resistance in P. aphanis to any fungicide in California and the first report of resistance in P. aphanis to penthiopyrad and quinoxyfen worldwide.

Design, synthesis and antifungal/anti-oomycete activity of pyrazolyl oxime ethers as novel potential succinate dehydrogenase inhibitors

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs) play an increasingly important role in controlling plant diseases. However, the similar structures of SDHIs result in rapid development of cross-resistance development and a clear bottleneck of poor activity against oomycetes, therefore the need to seek new SDHI fungicides with novel structures is urgent.

RESULTS

Innovative pyrazolyl oxime ethers were designed by replacing amide with oxime ether based on the succinate dehydrogenase (SDH) structure, and 19 pairs of Z- and E-isomers were efficiently prepared for the discovery of SDHI compounds with a novel bridge. Their biological activities against four fungi and two oomycetes were evaluated, and substantial differences were observed between the Z- and E- isomers of the title compounds. Furthermore, most of these compounds exhibited remarkable activities against Rhizoctonia solani with EC₅₀ values of less than 10 mg L^-1 in vitro, and bioassay in vivo further confirmed that E-I-6 exhibited good protective efficacy (76.12%) at 200 mg L^-1 . In addition, Z-I-12 provided better activity against the oomycetes Pythium aphanidermatum and Phytophthora capsici (EC₅₀  = 1.56 and 0.93 mg L^-1 ) than those of boscalid. Moreover, E-I-12 exhibited excellent SDH inhibition (IC₅₀  = 0.21 mg L^-1 ) thanks to its good binding ability to the SDH by hydrogen-bonding interactions, π-cation interaction and hydrophobic interactions.

CONCLUSION

Novel pyrazolyl oxime ethers have the potential as SDHI compounds for future development, and the strategy of replacing an amide bond with oxime ether may offer an alternative option in SDHI fungicide discovery.

Novel Pyrazole-4-acetohydrazide Derivatives Potentially Targeting Fungal Succinate Dehydrogenase: Design, Synthesis, Three-Dimensional Quantitative Structure-Activity Relationship, and Molecular Docking

Succinate dehydrogenase inhibitors (SDHIs) have emerged in fungicide markets as one of the fastest-growing categories that are widely applied in agricultural production for crop protection. Currently, the structural modification focusing on the flexible amide link of SDHI molecules is being gradually identified as one of the innovative strategies for developing novel highly efficient and broad-spectrum fungicides. Based on the above structural features, a series of pyrazole-4-acetohydrazide derivatives potentially targeting fungal SDH were constructed and evaluated for their antifungal effects against Rhizoctonia solani, Fusarium graminearum, and Botrytis cinerea. Strikingly, the in vitro EC₅₀ values of constructed pyrazole-4-acetohydrazides 6w against R. solani, 6c against F. graminearum, and 6f against B. cinerea were, respectively, determined as 0.27, 1.94, and 1.93 μg/mL, which were obviously superior to that of boscalid against R. solani (0.94 μg/mL), fluopyram against F. graminearum (9.37 μg/mL), and B. cinerea (1.94 μg/mL). Concurrently, the effects of the substituent steric, electrostatic, hydrophobic, and hydrogen-bond fields on structure-activity relationships were elaborated by the reliable comparative molecular field analysis and comparative molecular similarity index analysis models. Subsequently, the practical value of pyrazole-4-acetohydrazide derivative 6w as a potential SDHI was ascertained by the relative surveys on the in vivo anti-R. solani preventative efficacy, inhibitory effects against fungal SDH, and molecular docking studies. The present results provide an indispensable complement for the structural optimization of antifungal leads potentially targeting SDH.

Field Assessment of Six Point-Mutations in SDH Subunit Genes Conferring Varying Resistance Levels to SDHIs in Clarireedia spp

Dollar spot, caused by Clarireedia spp. (formerly Sclerotinia homoeocarpa F.T. Bennett), is the most economically important turfgrass disease causing considerable damage on golf courses. While cultural practices are available for reducing dollar spot infection, chemical fungicide use is often necessary for maintaining optimal turf quality. Since the release of boscalid in 2003, the succinate dehydrogenase inhibitor (SDHI) class has become an invaluable tool for managing dollar spot. However, resistance to this class has recently been reported in Clarireedia spp. and many other plant pathogenic fungi. After SDHI field failure on four golf courses and one university research plot, a total of six unique SDH mutations conferring differential in vitro sensitivities to SDHIs have been identified in Clarireedia spp. In 2018 and 2019, turf research plots were inoculated with sensitive, non-mutated isolates of Clarireedia spp., as well as resistant isolates harboring each unique identified mutation. Fungicide efficacy trials were conducted on inoculated plots to assess differential sensitivity to five SDHI active ingredients (boscalid, fluxapyroxad, isofetamid, fluopyram, and pydiflumetofen) across mutations under field conditions. Results indicate unique mutations are associated with distinct SDHI field efficacy profiles as shown in in-vitro sensitivity assays. Isolate populations with B subunit mutations (H267Y/R) were more sensitive to fluopyram, whereas isolate populations with C subunit mutations (C-G91R, C-G150R) showed resistance to all SDHIs tested. Mutation-associated differential sensitivity observed under field conditions indicates a need for a nation-wide survey and frequent monitoring of SDHI sensitivity of dollar spot populations on golf courses in the USA. Further, the information gained from this study will be useful in providing sustainable management recommendations for controlling site-specific resistant populations of Clarireedia spp.

A Rapid Molecular Detection System for SdhB and SdhC Point Mutations Conferring Differential Succinate Dehydrogenase Inhibitor Resistance in Clarireedia Populations

Dollar spot, caused by the ascomycete fungus Clarireedia (formerly Sclerotinia), is one of the most resource-demanding diseases on amenity turfgrasses in North America. Differential resistance to the succinate dehydrogenase inhibitor (SDHI) fungicide class, conferred by singular point mutations on the SdhB, SdhC, and SdhD subunits of the succinate dehydrogenase enzyme (SDH), has been reported in dollar spot as well as many other plant-pathogenic fungal diseases. Four unique mutations were previously reported from Clarireedia field isolates collected from two different cool-season golf courses in Japan and Rhode Island: an amino acid substitution H267Y and a silent mutation (CTT to CTC) at codon 181 on the SdhB subunit gene, and amino acid substitutions G91R and G150R on the SdhC subunit gene. To properly diagnose and monitor SDHI resistance in the field, a rapid detection system for known mutations is crucial. As part of this study, additional SDHI-resistant Clarireedia isolates were collected from Rutgers University research plots and in vitro sensitivity to four SDHI active ingredients was assessed. SdhB, SdhC, and SdhD subunits of these isolates were sequenced to reveal an additional mutation on the SdhB subunit gene, H267R, not previously observed in Clarireedia. Cleaved amplified polymorphic sequence (CAPS) and derived CAPS molecular markers were developed to detect five mutations conferring SDHI resistance in Clarireedia isolates and validated using samples from two additional golf courses in Connecticut and Wisconsin experiencing SDHI field failure. This PCR-based molecular detection system will be useful for continued monitoring, assessment, and delay of SDHI resistance in the field.

Baseline and Temporal Changes in Sensitivity of Zymoseptoria tritici Isolates to Benzovindiflupyr in Oregon, U.S.A., and Cross-Sensitivity to Other SDHI Fungicides

Zymoseptoria tritici is the causal agent of Septoria tritici blotch (STB), a disease of wheat (Triticum aestivum) that results in significant yield loss worldwide. Z. tritici's life cycle, reproductive system, effective population size, and gene flow put it at high likelihood of developing fungicide resistance. Succinate dehydrogenase inhibitor (SDHI) fungicides (FRAC code 7) were not widely used to control STB in the Willamette Valley until 2016. Field isolates of Z. tritici collected in the Willamette Valley at dates spanning the introduction of SDHI (2015 to 2017) were screened for sensitivity to four SDHI active ingredients: benzovindiflupyr, penthiopyrad, fluxapyroxad, and fluindapyr. Fungicide sensitivity changes were determined by the fungicide concentration at which fungal growth is decreased by 50% (EC₅₀) values. The benzovindiflupyr EC₅₀ values increased significantly, indicating a reduction in sensitivity, following the adoption of SDHI fungicides in Oregon (P < 0.0001). Additionally, significant reduction in cross-sensitivity among SDHI active ingredients was also observed with a moderate and significant relationship between penthiopyrad and benzovindiflupyr (P = 0.0002) and a weak relationship between penthiopyrad and fluxapyroxad (P = 0.0482). No change in cross-sensitivity was observed with fluindapyr, which has not yet been labeled in the region. The results document a decrease in SDHI sensitivity in Z. tritici isolates following the introduction of the active ingredients to the Willamette Valley. The reduction in cross-sensitivity observed between SDHI active ingredients highlights the notion that careful consideration is required to manage fungicide resistance and suggests that within-group rotation is insufficient for resistance management.

Expedient Discovery for Novel Antifungal Leads Targeting Succinate Dehydrogenase: Pyrazole-4-formylhydrazide Derivatives Bearing a Diphenyl Ether Fragment

The pyrazole-4-carboxamide scaffold containing a flexible amide chain has emerged as the molecular skeleton of highly efficient agricultural fungicides targeting succinate dehydrogenase (SDH). Based on the above vital structural features of succinate dehydrogenase inhibitors (SDHI), three types of novel pyrazole-4-formylhydrazine derivatives bearing a diphenyl ether moiety were rationally conceived under the guidance of a virtual docking comparison between bioactive molecules and SDH. Consistent with the virtual verification results of a molecular docking comparison, the in vitro antifungal bioassays indicated that the skeleton structure of title compounds should be optimized as an N'-(4-phenoxyphenyl)-1H-pyrazole-4-carbohydrazide scaffold. Strikingly, N'-(4-phenoxyphenyl)-1H-pyrazole-4-carbohydrazide derivatives 11o against Rhizoctonia solani, 11m against Fusarium graminearum, and 11g against Botrytis cinerea exhibited excellent antifungal effects, with corresponding EC₅₀ values of 0.14, 0.27, and 0.52 μg/mL, which were obviously better than carbendazim against R. solani (0.34 μg/mL) and F. graminearum (0.57 μg/mL) as well as penthiopyrad against B. cinerea (0.83 μg/mL). The relative studies on an in vivo bioassay against R. solani, bioactive evaluation against SDH, and molecular docking were further explored to ascertain the practical value of compound 11o as a potential fungicide targeting SDH. The present work provided a non-negligible complement for the structural optimization of antifungal leads targeting SDH.

Synthesis and Biological Evaluation of Novel Benodanil-Heterocyclic Carboxamide Hybrids as a Potential Succinate Dehydrogenase Inhibitors

In order to discover new antifungal agents, twenty novel benodanil-heterocyclic carboxamide hybrids were designed, synthesized, and characterized by ¹H NMR and HRMS. In vitro, their antifungal activities against four phytopathogenic fungi were evaluated, as well as some of the target compounds at 50 mg/L demonstrated significant antifungal activities against Rhizoctonia solani. Especially, compounds 17 (EC₅₀ = 6.32 mg/L) and 18 (EC₅₀ = 6.06 mg/L) exhibited good antifungal activities against R. solani and were superior to the lead fungicide benodanil (a succinate dehydrogenase inhibitor, SDHI) (EC₅₀ = 6.38 mg/L). Furthermore, scanning electron microscopy images showed that the mycelia on treated media with the addition of compound 17 grew abnormally as compared with the negative control with tenuous, wizened, and overlapping colonies, and compounds 17 (IC₅₀ = 52.58 mg/L) and 18 (IC₅₀ = 56.86 mg/L) showed better inhibition abilities against succinate dehydrogenase (SDH) than benodanil (IC₅₀ = 62.02 mg/L). Molecular docking revealed that compound 17 fit in the gap composed of subunit B, C, and D of SDH. Furthermore, it was shown that the main interaction, one hydrogen bond interaction, was observed between compound 17 and the residue C/Trp-73. These studies suggested that compound 17 could act as a potential fungicide to be used for further optimization.

The Effects of Succinate Dehydrogenase Inhibitor Fungicide Dose and Mixture on Development of Resistance in Venturia inaequalis

Understanding how fungicide application practices affect selection for fungicide resistance is imperative for continued sustainable agriculture. Here, we examined the effect of field applications of the succinate dehydrogenase inhibitor (SDHI) fluxapyroxad at different doses and mixtures on the SDHI sensitivity of Venturia inaequalis, the apple scab pathogen. Fungicide applications were part of selection programs involving different doses (high or low) and mixtures (with a second single-site fungicide or a multisite fungicide). These programs were tested in two apple orchards over 4 years to determine potential cumulative selection effects on resistance. Each year after program applications, apple scab lesions were collected, and relative growth assays were conducted to understand shifts in fluxapyroxad sensitivity. After 4 years, there was a trend toward a reduction in sensitivity to fluxapyroxad for most selection programs in comparison to that in the non-selective-pressure control. In most years, the selection program plots treated with low-dose fluxapyroxad applications resulted in a larger number of isolates with reduced sensitivity, supporting the use of higher doses for disease management. Few significant differences (P < 0.05) in fungicide sensitivity were observed between isolates collected from plots where fungicide mixtures were applied compared to that in untreated plots, supporting the use of multiple modes of action in field applications. In all, appropriate doses and mixtures may contribute to increased longevity of SDHI fungicides used on perennial crops like apples.IMPORTANCE Of much debate is the effect of fungicide application dose on resistance development, as fungicide resistance is a critical barrier to effective disease management in agricultural systems. Our field study in apples investigated the effect of fungicide application dose and mixture on the selection of succinate dehydrogenase inhibitor resistance in Venturia inaequalis, a fungal pathogen that causes the economically important disease apple scab. Understanding how to best delay the development of resistance can result in increased efficacy, fewer applications, and sustainable fungicide use. Results from this study may have relevance to other perennial crops that require multiple fungicide applications and that are impacted by the development of resistance.

Novel 1,3,4-Oxadiazole-2-carbohydrazides as Prospective Agricultural Antifungal Agents Potentially Targeting Succinate Dehydrogenase

A novel simple 1,3,4-oxadiazole-2-carbohydrazide was reported to discover low-cost and versatile antifungal agents. Bioassay results suggested that a majority of the designed compounds were extremely bioactive against four types of fungi and two kinds of oomycetes. This extreme bioactivity was highlighted by the applausive inhibitory effects of compounds 4b, 4h, 5c, 5g, 5h, 5i, 5m, 5p, 5t, and 5v against Gibberella zeae, affording EC₅₀ values ranging from 0.486 to 0.799 μg/mL, which were superior to that of fluopyram (2.96 μg/mL) and comparable to those of carbendazim (0.947 μg/mL) and prochloraz (0.570 μg/mL). Meanwhile, compounds 4g, 5f, 5i, and 5t showed significant actions against Fusarium oxysporum with EC₅₀ values of 0.652, 0.706, 0.813, and 0.925 μg/mL, respectively. Pharmacophore exploration suggested that the N'-phenyl-1,3,4-oxadiazole-2-carbohydrazide pattern is necessary for the bioactivity. Molecular docking of 5h with succinate dehydrogenase (SDH) indicated that it can completely locate the inside of the binding pocket via hydrogen-bonding and hydrophobic interactions, revealing that this novel framework might target SDH. This result was further verified by the significant inhibitory effect on SDH activity. In addition, scanning electron microscopy patterns were performed to elucidate the anti-G. zeae mechanism. Given these features, this type of framework is a suitable template for future exploration of alternative SDH inhibitors against plant microbial infections.

Differences in fungicide resistance profiles and multiple resistance to a quinone-outside inhibitor (QoI), two succinate dehydrogenase inhibitors (SDHI), and a demethylation inhibitor (DMI) for two Stagonosporopsis species causing gummy stem blight of cucurbits

BACKGROUND

Gummy stem blight (GSB) is a devastating disease of cucurbits that has been effectively managed with fungicide applications. However, the Stagonosporopsis spp. that cause GSB have rapidly evolved resistance to multiple classes of fungicides. To better understand the evolution and persistence of fungicide resistance in field populations, resistance profiles of unique and clonal genotypes of 113 Stagonosporopsis citrulli and 19 S. caricae isolates to four different fungicides were determined based on in vitro mycelial growth assays and molecular markers based on genes encoding fungicide targets.

RESULTS

All 19 S. caricae isolates screened were resistant to tebuconazole and azoxystrobin, and sensitive to boscalid and fluopyram. All 113 S. citrulli isolates were sensitive to tebuconazole and sensitive to fluopyram, with one exception that was fluopyram-resistant. All isolates of S. citrulli except two were resistant to azoxystrobin. Phenotypic differences in response to boscalid were detected among S. citrulli isolates, but the phenotypes were not associated with multilocus genotypes (MLG) determined by 16 microsatellite loci. Additionally, isolates sharing the same MLG varied by SdhB genotype. A unique mutation of I229V in SdhB, a target of succinate dehydrogenase inhibitor fungicides, was detected for the fluopyram-resistant isolate of S. citrulli.

CONCLUSION

Both the lack of association of fungicide resistance profiles with genetic similarity of isolates based on microsatellite loci and the finding that widely distributed MLG varied in fungicide resistance profiles suggest that independent evolutionary events for resistance to boscalid have likely occurred. Frequent genetic recombination within populations may be responsible for resistance to multiple fungicides. This study provides useful information for effectively managing both species of GSB fungi present in the southeastern USA and understanding the evolution of fungicide resistance within populations of plant-pathogenic fungi. © 2019 Society of Chemical Industry.

Favorable Bioactivity of the SDHI Fungicide Benzovindiflupyr Against Sclerotinia sclerotiorum Mycelial Growth, Sclerotial Production, and Myceliogenic and Carpogenic Germination of Sclerotia

Sclerotinia sclerotiorum, which can cause Sclerotinia stem rot, is a prevalent plant pathogen. This study aims to evaluate the application potential of benzovindiflupyr, a new generation of succinate dehydrogenase inhibitor (SDHI), against S. sclerotiorum. In our study, 181 isolates collected from different crops (including eggplant [n = 34], cucumber [n = 27], tomato [n = 29], pepper [n = 35], pumpkin [n = 32], and kidney bean [n = 25]) in China were used to establish baseline sensitivity to benzovindiflupyr. The frequency distribution of the 50% effective concentration (EC₅₀) values of benzovindiflupyr was a unimodal curve, with mean EC₅₀ values of 0.0260 ± 0.011 μg/ml, and no significant differences in mean EC₅₀ existed among the various crops (P > 0.99). Benzovindiflupyr can effectively inhibit mycelial growth, sclerotial production, sclerotial shape, and myceliogenic and carpogenic germination of the sclerotia of S. sclerotiorum. In addition, benzovindiflupyr showed good systemic translocation in eggplant. Using benzovindiflupyr at 100 μg/ml yielded efficacies of 71.3 and 80.5% for transverse activity and cross-layer activity, respectively, which were higher than those of acropetal and basipetal treatments (43.6 and 44.7%, respectively). Greenhouse experiments were then carried out at two experimental sites for verification. Applying benzovindiflupyr at 200 g a.i. ha^-1 significantly reduced the disease incidence and severity of Sclerotinia stem rot. Overall, the results demonstrated that benzovindiflupyr is a potential alternative product to control Sclerotinia stem rot.

Impact of Five Succinate Dehydrogenase Inhibitors on DON Biosynthesis of Fusarium asiaticum, Causing Fusarium Head Blight in Wheat

Deoxynivalenol (DON) is a class of mycotoxin produced in cereal crops infected with Fusarium graminearum species complex (FGSC). In China, FGSC mainly includes Fusarium asiaticum and F. graminearum. DON belongs to the trichothecenes and poses a serious threat to the safety and health of humans and animals. Succinate dehydrogenase inhibitors (SDHIs) are a class of fungicides that act on succinate dehydrogenase and inhibit the respiration of pathogenic fungi. In this study, the fungicidal activities of five SDHIs, including fluopyram, flutolanil, boscalid, benzovindiflupyr, and fluxapyroxad, against FGSC were determined based on mycelial growth and spore germination inhibition methods. The five SDHIs exhibited better inhibitory activities in spore germination than mycelial growth. Fluopyram exhibited a higher inhibitory effect in mycelial growth and spore germination in comparison to the other four SDHIs. In addition, the biological characteristics of F. asiaticum as affected by the five SDHIs were determined. We found that these five SDHIs decreased DON, pyruvic acid and acetyl-CoA production, isocitrate dehydrogenase mitochondrial (ICDHm) and SDH activities, and NADH and ATP content of F. asiaticum but increased the citric acid content. In addition, TRI5 gene expression was inhibited, and the formation of toxisomes was disrupted by the five SDHIs, further confirming that SDHIs can decrease DON biosynthesis of F. asiaticum. Thus, we concluded that SDHIs may decrease DON biosynthesis of F. asiaticum by inhibiting glycolysis and the tricarboxylic acid (TCA) cycle. Overall, the findings from the study will provide important references for managing Fusarium head blight (FHB) caused by FGSC and reducing DON contamination in F. asiaticum-infected wheat grains.

Discovery of Novel Thiazole Carboxamides as Antifungal Succinate Dehydrogenase Inhibitors

To contribute molecular diversity for novel fungicide development, a series of novel thiazole carboxamides were rationally designed, synthesized, and characterized with the succinate dehydrogenase (SDH) as target. Bioassay indicated that compound 6g showed the similar excellent SDH inhibition as that of Thifluzamide with IC₅₀ of 0.56 mg/L and 0.55 mg/L, respectively. Some derivatives displayed improved in vitro fungicidal activities against Rhizoctonia cerealis and Sclerotinia sclerotiorum with EC₅₀ of 1.2-16.4 mg/L and 0.5-1.9 mg/L. Surprisingly, 6g showed promising in vitro fungicidal activities against R. cerealis and S. sclerotiorum with EC₅₀ of 6.2 and 0.6 mg/L, respectively, which was superior to Thifluzamide with the EC₅₀ of 22.1 and 4.4 mg/L, respectively. Additionally, compounds 6c and 6g displayed excellent in vivo fungicidal activities against S. sclerotiorum on Brassica napus L. leaves with protective activity of 75.4% and 67.3% at 2.0 mg/L, respectively, while Thifluzamide without activity at 5.0 mg/L. Transcriptomic analysis of S. sclerotiorum treated with 6g by RNA sequencing indicated the down-regulation of succinate dehydrogenase gene SDHA and SDHB, and the inhibition of the TCA-cycle.

Foliar application of benzovindiflupyr shows non-fungicidal effects in wheat plants

BACKGROUND

The fungicide benzovindiflupyr belongs to the class of succinate dehydrogenase inhibitors (SDHIs). Certain SDHIs have shown plant physiological effects, so-called secondary effects, that appeared to be related to the plant water status. Therefore, the effect of benzovindiflupyr on transpiration of leaves and whole wheat plants was studied under controlled conditions. Furthermore, wheat yield trials under controlled and natural drought stress in the field were conducted.

RESULTS

Transpiration of detached wheat leaves was reduced by benzovindiflupyr in a dose-dependent manner. Similarly, whole-plant transpiration decreased for several days following application of this fungicide. In 16 field trials under drought stress conditions that were classified as disease-free, treatment of wheat plants at the flag leaf stage or at heading with benzovindiflupyr showed a grain yield increase (+5.2%; P ≤ 0.01) that was partially attributed to an increased thousand-grain weight.

CONCLUSIONS

Water saving during pre-anthesis as a result of benzovindiflupyr application may be associated with better seed setting and filling under dry field conditions in wheat. The results of this research provide new insights into secondary effects of SDHIs that lead directly to yield improvements. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Non-target site SDHI resistance is present as standing genetic variation in field populations of Zymoseptoria tritici

BACKGROUND

A new generation of more active succinate dehydrogenase (Sdh) inhibitors (SDHIs) is currently widely used to control Septoria leaf blotch in northwest Europe. Detailed studies were conducted on Zymoseptoria tritici field isolates with reduced sensitivity to fluopyram and isofetamid; SDHIs which have only just or not been introduced for cereal disease control, respectively.

RESULTS

Strong cross-resistance between fluopyram and isofetamid, but not with other SDHIs, was confirmed through sensitivity tests using laboratory mutants and field isolates with and without Sdh mutations. The sensitivity profiles of most field isolates resistant to fluopyram and isofetamid were very similar to a lab mutant carrying SdhC-A84V, but no alterations were found in SdhB, C and D. Inhibition of mitochondrial Sdh enzyme activity and control efficacy in planta for those isolates was severely impaired by fluopyram and isofetamid, but not by bixafen. Isolates with similar phenotypes were not only detected in northwest Europe but also in New Zealand before the widely use of SDHIs.

CONCLUSION

This is the first report of SDHI-specific non-target site resistance in Z. tritici. Monitoring studies show that this resistance mechanism is present and can be selected from standing genetic variation in field populations. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Detection of Zymoseptoria tritici SDHI-insensitive field isolates carrying the SdhC-H152R and SdhD-R47W substitutions

BACKGROUND

Succinate dehydrogenase inhibitor (SDHI) fungicides are important in the management of Zymoseptoria tritici in wheat. New active ingredients from this group of fungicides have been introduced recently and are widely used. Because the fungicides act at a single enzyme site, resistance development in Z. tritici is classified as medium-to-high risk.

RESULTS

Isolates from Irish experimental plots in 2015 were tested against the SDHI penthiopyrad during routine monitoring. The median of the population was approximately 2 times less sensitive than the median of the baseline population. Two of the 93 isolates were much less sensitive to penthiopyrad than the least sensitive of the baseline isolates. These isolates were also insensitive to most commercially available SDHIs. Analysis of the succinate dehydrogenase coding genes confirmed the presence of the substitutions SdhC-H152R and SdhD-R47W in the very insensitive isolates.

CONCLUSION

This is the first report showing that the SdhC-H152R mutation detected in laboratory mutagenesis studies also exists in the field. The function and relevance of this mutation, combined with SdhD-R47W, still needs to be determined. © 2016 Society of Chemical Industry.

The effect of succinate dehydrogenase inhibitor/azole mixtures on selection of Zymoseptoria tritici isolates with reduced sensitivity

BACKGROUND

Combining fungicides with different modes of action is regarded as one of the most effective means of slowing the selection of resistance. Field trials were used to study the effects of such mixtures on selection for Zymoseptoria tritici with reduced sensitivity to the succinate dehydrogenase inhibitors (SDHIs) and azole fungicides. The SDHI isopyrazam and the azole epoxiconazole were applied individually as solo products, and together in a preformulated mixture. All fungicide treatments were included at both full and half the recommended doses.

RESULTS

Compared with using epoxiconazole alone, mixing epoxiconazole with isopyrazam led to an increase in epoxiconazole-sensitive isolates. In contrast, all treatments containing isopyrazam reduced the sensitivity of Z. tritici to isopyrazam compared with those without. Reducing doses to half the recommended rate had no effect on sensitivity of isolates to either active ingredient. In a subgroup of isolates least sensitive to isopyrazam, non-synonymous mutations were found in the SdhC and SdhD subunits, but their presence was unrelated to sensitivity.

CONCLUSION

Mixing an azole and SDHI was clearly beneficial for the azole, but not for the SDHI component. This dynamic might change if strains conferring reduced sensitivity to the SDHIs were to arise. © 2015 Society of Chemical Industry.