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

High-throughput semi-automated emulsive liquid-liquid microextraction for detecting SDHI fungicides in water, juice, and alcoholic beverage samples via UHPLC-MS/MS

Herein, a High-Throughput Semi-automated Emulsive Liquid-Liquid Microextraction (HTSA-ELLME) method was developed to detect Succinate Dehydrogenase Inhibitor (SDHI) fungicides in food samples via UHPLC-MS/MS. The Oil-in-Water (O/W) emulsion comprising a hydrophobic extractant and water was dilutable with the aqueous sample solution. Upon injecting the primary emulsion into the sample solution, a secondary O/W emulsion was formed, allowing SDHI fungicides to be extracted. Subsequently, a NaCl-saturated solution was injected in the secondary O/W emulsion as a demulsifier to rapidly separate the extractant, eliminating the need for centrifugation. A 12-channel electronic micropipette was used to achieve a high-throughput semi-automation of the novel sample pretreatment. The linear range was 0.003-0.3 μg L-1 with R2 > 0.998. The limit of detection was 0.001 μg L-1. The HTSA-ELLME method successfully detected SDHI fungicides in water, juice, and alcoholic beverage samples, with recoveries and relative standard deviations of 82.6-106.9% and 0.8-5.8%, respectively. Unlike previously reported liquid-liquid microextraction approaches, the HTSA-ELLME method is the first to be both high-throughput and semi-automated and may aid in designing pesticide pretreatment processes in food samples.

Postinfection Application of Fenhexamid at Lower Doses in Conjunction with Captan Slowed Fungicide Resistance Selection in Botrytis cinerea on Detached Grape Berries

Fungicide resistance is a limiting factor in sustainable crop production. General resistance management strategies such as rotation and mixtures of fungicides with different modes of action have been proven to be effective in many studies, but guidance on fungicide dose or application timing for resistance management remains unclear or debatable. In this study, Botrytis cinerea and the high-risk fungicide fenhexamid were used to determine the effects of fungicide dose, mixing partner, and application timing on resistance selection across varied frequencies of resistance via detached fruit assays. The results were largely consistent with the recent modeling studies that favored the use of the lowest effective fungicide dose for improved resistance management. In addition, even 10% resistant B. cinerea in the population led to about a 40% reduction of fenhexamid efficacy. Overall, our findings show that application of doses less than the fungicide label dose, mixture with the low-risk fungicide captan, and application postinfection seem to be the most effective management strategies in our controlled experimental settings. This somewhat contradicts the previous assumption that preventative sprays help resistance management.

Evolution of the Genetic Structure of the Didymella tanaceti Population During Development of Succinate Dehydrogenase Inhibitor Resistance

Emergence of pathogens with decreased sensitivity to succinate dehydrogenase inhibitor fungicides is a global agronomical issue. Analysis of Didymella tanaceti isolates (n = 173), which cause tan spot of pyrethrum (Tanacetum cinerariifolium), collected prior to (2004 to 2005) and after (2009, 2010, 2012, and 2014) the commercial implementation of boscalid in Tasmanian pyrethrum fields identified that insensitivity developed over time and has become widespread. To evaluate temporal change, isolates were characterized for frequency of mutations in the succinate dehydrogenase (Sdh) B, C, and D subunits associated with boscalid resistance, mating type, and SSR genotype. All isolates from 2004 and 2005 exhibited wild-type (WT) Sdh alleles. Seven known Sdh substitutions were identified in isolates collected from 2009 to 2014. In 2009, 60.7% had Sdh substitutions associated with boscalid resistance in D. tanaceti. The frequency of WT isolates decreased over time, with no WT isolates identified in 2014. The frequency of the SdhB-H277Y genotype increased from 10.7 to 77.8% between 2009 and 2014. Genotypic evidence suggested that a shift in the population structure occurred between 2005 and 2009, with decreases in gene diversity (uh; 0.51 to 0.34), genotypic evenness (E5; 0.96 to 0.67), genotypic diversity (G; 9.3 to 6.8), and allele frequencies. No evidence was obtained to support the rapid spread of Sdh genotypes by clonal expansion of the population. Thus, insensitivity to boscalid has developed and become widespread within a diverse population within 4 years of usage. These results suggest that D. tanaceti can disperse insensitivity through repeated frequent mutation, sexual recombination, or a combination of both.

Physical Modes of Action of Fungicides Against Apple Scab: Timing Is Everything, but Dose Matters

The efficacy of currently available fungicides against apple scab, caused by the fungal pathogen Venturia inaequalis, was investigated in relation to when growers spray (ahead, during, or after rain) and how the spray reaches the target. The adaxial surface of individual leaves of potted trees were sprayed and then inoculated with ascospores of V. inaequalis, to establish dose-response curves for each fungicide. Discriminatory doses providing 50 and 90% symptom inhibition (EC50 and EC90, respectively) in sprays mimicking applications ahead of rain were used for experiments imitating alternative spray timings. Sprays were either applied during the spore germination phase or early or late after infection onset (either 336 or 672 degree-hours after inoculation, respectively), corresponding to grower spray schedules. Experiments were also carried out with sprays applied on the abaxial leaf surface to investigate fungicide efficacy through the leaf lamina. For all fungicides, the best efficacy was observed when sprays were applied during germination, followed by applications ahead of inoculation. Some products maintained equal or better efficacy at early infection, while efficacy in late infection dropped for all products, clearly indicating that this spray timing should be avoided. Some products with postinfection efficacy also showed translaminar efficacy. The close relationship found between EC50 of the active ingredients on potted trees and the label rate could help improve spraying decisions and reduce costs.

Fluxapyroxad Resistance Mechanisms in Sclerotinia sclerotiorum

The necrotrophic pathogen Sclerotinia sclerotiorum has a global distribution and a wide host range, making it one of the most damaging and economically important of all plant pathogens. The current study found that fluxapyroxad, a typical succinate dehydrogenase inhibitor fungicide, had a strong inhibitory effect against S. sclerotiorum, with mean effective concentration for 50% inhibition (EC₅₀) values ranging from 0.021 to 0.095 µg/ml. Further investigation of five highly resistant S. sclerotiorum mutants, with EC₅₀ values of 12.37 to 31.36 µg/ml, found that fluxapyroxad resistance was accompanied by a certain cost to fitness. All of the mutants were found to have significantly (P < 0.05) reduced mycelial growth and altered sclerotia production in artificial culture, as well as reduced pathogenicity, compared with wild-type isolates, with one mutant completely losing the capacity to infect detached soybean leaves. Sequence analysis demonstrated that four of the mutants had point mutations leading to amino acid changes in the SsSdhB subunit of the fungicide target protein succinate dehydrogenase. In addition, two of the mutants were also found to have amino acid changes in the predicted sequence of their SsSdhD subunit, while the fifth mutant had no changes in any of its SsSdh sequences, indicating that an alternative mechanism might be responsible for the observed resistance in this mutant. No cross-resistance was found between fluxapyroxad and any of the other fungicides tested, including tebuconazole, prochloraz, dimethachlone, carbendazim, procymidone, pyraclostrobin, boscalid, fluazinam, fludioxonil, and cyprodinil, which indicates that fluxapyroxad has great potential as an alternative method of control for the Sclerotinia stem rot caused by S. sclerotiorum, and which could provide ongoing protection to the soybean fields of China.

Synergy of cystamine and pyraclostrobin against Fusarium graminearum involves membrane permeability mitigation and autophagy enhancement

The application of fungicide mixture is one of the most important measures to extend the service life of highly selective fungicides. Pyraclostrobin (PYR), which has been extensively used to control plant diseases by inhibiting mitochondrial respiration of pathogenic fungi, is at a high risk of resistance development. In this study, the potential of PYR alone or in combination with cystamine, an inhibitor of microbial transglutaminase, to suppress Fusarium graminearum was tested in vitro and in vivo. A synergistic effect of PYR/CYS mixture was observed both in vitro and when applied to etiolated wheat coleoptile. The control effect of PYR/CYS mixture on F. graminearum was better than that of PYR alone, which was reflected by the increased protection effect. The discrepancies of membrane permeability and the redox-physiological state were observed between PYR and PYR/CYS treatments, suggesting that an increased PYR availability in F. graminearum mycelia could be related with the observed synergistic action. Moreover, a synergistic profile was observed between PYR and CYS in regard of massive autophagosomes in mycelia, indicating that enhanced autophagy could be involved in the mode of action of PYR/CYS mixture. The differential content of mitochondrial metabolites between PYR and PYR/CYS treatments also provided evidence for CYS contribution to the fungicidal action of PYR/CYS mixture. The results provide insight into the synergistic mechanism of action of PYR/CYS mixture and an effective way to enhance the efficiency of PYR to combat F. graminearum.

Rapidly Increasing Boscalid Resistance in Corynespora cassiicola in China

Corynespora leaf spot caused by Corynespora cassiicola is an important foliar disease in cucumber. Succinate dehydrogenase inhibitors are the main fungicides used to control this disease. With the application of succinate dehydrogenase inhibitors (SDHIs) in the field, boscalid-resistant isolates have been continuously detected in the field. Resistance monitoring programs were performed to investigate the frequency and genotypes of resistant isolates. In our resistance monitoring, the frequency of resistant isolates rapidly increased from 9.68 to 85.88% in 2005 to 2020. Nine genotypes conferring SDHI resistance were found in resistant isolates, with different levels of resistance to SDHIs: B-H278R, B-H278L, B-H278Y, B-I280V, C-N75S, C-S73P, D-D95E, D-H105R, and D-G109V. The first sdh mutation was detected in Hebei Province in China, conferring an amino acid substitution at codon 278 in the sdhB subunit from histidine to tyrosine (B-H278Y), and it was the dominant resistance genotype in 2014 to 2015. Subsequently, other genotypes were gradually detected in the field, and the dominant mutations varied across years and across regions. The newest genotype (B-H278L) conferring SDHI resistance was found in 2020. To the best of our knowledge, this is the first report of C. cassiicola in cucumber. To date, multiple resistance to SDHIs, quinone outside inhibitors, benzimidazole fungicides, and dicarboximide fungicides have been detected, accounting for 75.64% of SDHI-resistant isolates. Therefore, the above four fungicides must be strictly restricted, and further monitoring work in other provinces with more isolates should be performed in the future.

Succinate Dehydrogenase, Succinate, and Superoxides: A Genetic, Epigenetic, Metabolic, Environmental Explosive Crossroad

Research focused on succinate dehydrogenase (SDH) and its substrate, succinate, culminated in the 1950s accompanying the rapid development of research dedicated to bioenergetics and intermediary metabolism. This allowed researchers to uncover the implication of SDH in both the mitochondrial respiratory chain and the Krebs cycle. Nowadays, this theme is experiencing a real revival following the discovery of the role of SDH and succinate in a subset of tumors and cancers in humans. The aim of this review is to enlighten the many questions yet unanswered, ranging from fundamental to clinically oriented aspects, up to the danger of the current use of SDH as a target for a subclass of pesticides.

Resistance to pydiflumetofen in Botrytis cinerea: risk assessment and detection of point mutations in sdh genes that confer resistance

BACKGROUND

Gray mold caused by Botrytis cinerea Pers. is one of the most significant airborne diseases. It can infest a wide range of crops, causing significant losses in yield and quality worldwide. Pydiflumetofen, a new generation succinate dehydrogenase inhibitor (SDHI), is currently being registered in China to control gray mold in a variety of crops. The baseline sensitivity, resistance risk, and resistance mechanism of Botrytis cinerea to pydiflumetofen were assessed in this study.

RESULTS

A total of 138 strains of B. cinerea from 10 different regions were tested for their sensitivity to pydiflumetofen, and the mean EC₅₀ value was 0.0056 μg mL^-1 . Eight mutants were obtained by fungicide adaption from five sensitive parental isolates, and the resistance factor (RF) ranged from 51 to 135. The mutants exhibited strong adaptive traits in conidial production, conidial germination, and pathogenicity. Positive cross-resistance was only observed between other SDHIs (i.e. boscalid, fluopyram, and isopyrazam). Two different types of pydiflumetofen-resistant mutants were identified: point mutation P225L in sdhB and double mutation G85A and I93V in sdhC. The in vivo control efficacy of pydiflumetofen on the resistant mutants carrying P225L in sdhB as well as G85A and I93V in sdhC was significantly decreased to 52.62% and 32.27%, respectively.

CONCLUSION

The fitness was significantly higher for all pydiflumetofen-resistant mutants than the corresponding parental. Two types of point mutations, sdhB-P225L and sdhC-G85A and I93V, might confer resistance to pydiflumetofen in B. cinerea. A precautionary resistance management strategy should be implemented. © 2021 Society of Chemical Industry.

The research progress in and perspective of potential fungicides: Succinate dehydrogenase inhibitors

Succinate dehydrogenase inhibitors (SDHIs) have become one of the fastest growing classes of new fungicides since entering the market, and have attracted increasing attention as a result of their unique structure, high activity and broad fungicidal spectrum. The mechanism of SDHIs is to inhibit the activity of succinate dehydrogenase, thereby affecting mitochondrial respiration and ultimately killing pathogenic fungi. At present, they have become popular varieties researched and developed by major pesticide companies in the world. In the review, we focused on the mechanism, the history, the representative varieties, structure-activity relationship and resistance of SDHIs. Finally, the potential directions for the development of SDHIs were discussed. It is hoped that this review can strengthen the individuals' understanding of SDHIs and provide some inspiration for the development of new fungicides.

Optimizing the Integration of a Biopesticide (Bacillus subtilis QST 713) with a Single-Site Fungicide (Benzovindiflupyr) to Reduce Reliance on Synthetic Multisite Fungicides (Captan and Mancozeb) for Management of Apple Scab

Apple scab is one of the most economically important diseases of apple in temperate production regions. In the absence of durable host resistance in commercially preferred cultivars, considerable applications of fungicides are needed to manage this disease. With the sequential development of resistance to nearly all classes of single-site fungicides in the apple scab pathogen Venturia inaequalis, synthetic multisite fungicides, such as mancozeb and captan, often comprise the core of chemical management programs for apple scab. Although these fungicides have demonstrable benefits for both disease and fungicide resistance management, the sustainability movement within agriculture aims to reduce reliance on such fungicides because of their broader environmental impacts. In this study, we establish a framework to enhance the feasibility of chemical management programs that do not rely on use of synthetic multisite protectant fungicides to manage apple scab. Specifically, we wish to evaluate chemical programs that integrate the biopesticide Bacillus subtilis QST 713 (Serenade Opti) in rotation with benzovindiflupyr (Aprovia), a single-site fungicide belonging to the class of succinate dehydrogenase inhibitors (SDHI), to circumvent the need for applications of synthetic multisite fungicides. During implementation of these programs, disease incidence data were taken at biweekly intervals. Regardless of the seasonal challenges presented in the 2 years of this study, when Bacillus subtilis QST 713 was used in place of captan and mancozeb mixtures, we did not observe any significant differences (P > 0.05) in development of apple scab symptoms between any of the management programs for the vertical axis or super spindle orchards in either year. This potential for substituting synthetic multisite fungicides with biopesticides is best realized when the programs are used with a decision support system in a super spindle planting system, where trees have reduced canopy densities. This 2-year study shows the potential to achieve adequate disease control using the integration of SDHI fungicides and biological controls without the use of synthetic multisite fungicides.

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.