Characterization of sterol demethylation inhibitor-resistant isolates of Fusarium asiaticum and F. graminearum collected from wheat in China

Absorption, distribution, metabolism, and elimination of epoxiconazole enantiomers in lizards (Eremias argus)

Epoxiconazole (EPX) is a world widely used chiral triazole fungicide in the agriculture field. The excessive application of this triazole may cause damage to lizards. However, limited information is known about the toxicokinetics of EPX on lizards. Our study aimed to investigate the enantioselective absorption, distribution, metabolism, and elimination (ADME) of EPX in lizards following low and high dose exposure (10 and 100 mg kg-1 bodyweitht (bw)). The results demonstrated that (+)-EPX was easier absorbed than (-)-EPX in lizard plasma. Both (+)-EPX and (-)-EPX were detected in the liver, gonad, kidney, skin, brain, and intestine, with (+)-EPX preferentially distributed in these tissues. The elimination of (-)-EPX was faster than that of (+)-EPX in lizard liver and kidney in the high dose groups. Chiral conversion was found between EPX enantiomers in lizard skin. Simultaneously, five metabolites including M2, M4, M10, M18 and M19 were detected in lizard liver and kidney after EPX enantiomers exposure. The relative concentrations of M2, M4, and M10 were higher in the liver and kidney of (-)-EPX groups than those produced from (+)-EPX groups. The metabolic enzymes CYP3A4 and SULT1A1 primarily mediated enantioselective metabolism of EPX. The conclusions drawn from this study significantly enhance our understanding of the enantioselective behaviors of chiral triazole fungicides in reptiles, offering essential guidance for assessing the risks associated with different enantiomers of triazole fungicides.

Comparative transcriptome analysis of Fusarium graminearum challenged with distinct fungicides and functional analysis of FgICL gene

Fusarium graminearum is an economically important phytopathogenic fungus. Chemical control remains the dominant approach to managing this plant pathogen. In the present study, we performed a comparative transcriptome analysis to understand the effects of four commercially used fungicides on F. graminearum. The results revealed a significant number of differentially expressed genes related to carbohydrate, amino acid, and lipid metabolism, particularly in the carbendazim and phenamacril groups. Central carbon pathways, including the TCA and glyoxylate cycles, were found to play crucial roles across all treatments except tebuconazole. Weighted gene co-expression network analysis reinforced the pivotal role of central carbon pathways based on identified hub genes. Additionally, critical candidates associated with ATP-binding cassette transporters, heat shock proteins, and chitin synthases were identified. The crucial functions of the isocitrate lyase in F. graminearum were also validated. Overall, the study provided comprehensive insights into the mechanisms of how F. graminearum responds to fungicide stress.

Cumin Seed Oil Induces Oxidative Stress-Based Antifungal Activities on Fusarium graminearum

In this study, the antifungal activity of cumin seed oil (CSO) was tested on Fusarium graminearum. (i) Minimum inhibitory concentrations (MICs) and related concentrations (IC75, IC50, and IC25) were detected; (ii) toxicity was evaluated by a water-soluble tetrazolium salt-1 (WST-1) assay; (iii) genomic/epigenomic alterations were evaluated by the coupled restriction enzyme digestion-random amplification (CRED-RA) method; (iv) oxidative stress was investigated by CAT expression, catalase activity, and DCF-DA staining; (v) deoxynivalenol biosynthesis was evaluated by tri6 expression; (vi) and potential effects of CSO on wheat were tested by a water loss rate (WLR) assay. MIC, IC75, IC50 and IC25 values were detected at 0.5, 0.375, 0.25, and 0.125 mg mL-1. In WST-1 assays, significant decreases (p < 0.001) were detected. Genomic template stability (GTS) related to methylation differences ranged from 94.60% to 96.30%. Percentage polymorphism for HapII/MspI values were as 9.1%/15.8%. CAT (oxidative stress-related catalase) and tri6 (zinc finger motif transcription factor) gene expressions were recorded between 5.29 ± 0.74 and 0.46 ± 0.10 (p < 0.05). Increased catalase activity was detected (p < 0.05) by spectrophotometric assays. DCF-DA-stained (oxidative stressed) cells were increased in response to increased concentrations, and there were no significant changes in WLR values. It was concluded that CSO showed strong antifungal activity on F. graminearum via different physiological levels.

Genome-Wide Association Study of Fungicide Sensitivity in a Fusarium graminearum Population Collected from North Dakota

Fusarium head blight is a destructive disease of small grains. The disease is predominantly caused by the haploid ascomycete fungus Fusarium graminearum in North America. To understand the genetics of quantitative traits for sensitivity to fungicides in this fungal pathogen, we conducted a genome-wide association study of sensitivity to two demethylation inhibition class fungicides, tebuconazole and prothioconazole, using an F. graminearum population of 183 isolates collected between 1981 and 2013 from North Dakota. Baseline sensitivity to tebuconazole and prothioconazole was established using 21 isolates collected between 1981 and 1994. Most fungal isolates were sensitive to both tebuconazole and prothioconazole; however, five isolates showed significantly reduced sensitivity to prothioconazole. The genome-wide association study identified one significant marker-trait association on chromosome 3 for tebuconazole resistance, whereas six significant marker-trait associations, one on chromosome 1, three on chromosome 2, and two on chromosome 4, were detected for prothioconazole resistance. Functional annotation of the marker-trait association for tebuconazole revealed a candidate gene encoding a basic helix-loop-helix domain-containing protein that reinforces sterol in the fungal membrane. Putative genes for prothioconazole resistance were also identified, which are involved in RNA interference, the detoxification by ubiquitin-proteasome pathway, and membrane integrity reinforcement. Considering the potential of the pathogen toward overcoming chemical control, continued monitoring of fungal sensitivities to commercially applied fungicides, especially those containing prothioconazole, is warranted to reduce risks of fungicide resistance in the pathogen populations.

Comparative transcriptome analysis provides insights into the resistance regulation mechanism and inhibitory effect of fungicide phenamacril in Fusarium asiaticum

Fusarium asiaticum is a destructive phytopathogenic fungus that causes Fusarium head blight of wheat (FHB), leading to serious yield and economic losses to cereal crops worldwide. Our previous studies indicated that target-site mutations (K216R/E, S217P/L, or E420K/G/D) of Type I myosin FaMyo5 conferred high resistance to phenamacril. Here, we first constructed one sensitive strain H1S and three point mutation resistant strains HA, HC and H1R. Then we conducted comparative transcriptome analysis of these F. asiaticum strains after 1 and 10 μg·mL-1 phenamacril treatment. Results indicated that 2135 genes were differentially expressed (DEGs) among the sensitive and resistant strains. The DEGs encoding ammonium transporter MEP1/MEP2, nitrate reductase, copper amine oxidase 1, 4-aminobutyrate aminotransferase, amino-acid permease inda1, succinate-semialdehyde dehydrogenase, 2, 3-dihydroxybenzoic acid decarboxylase, etc., were significantly up-regulated in all the phenamacril-resistant strains. Compared to the control group, a total of 1778 and 2097 DEGs were identified in these strains after 1 and 10 μg·mL-1 phenamacril treatment, respectively. These DEGs involved in 4-aminobutyrate aminotransferase, chitin synthase 1, multiprotein-bridging factor 1, transcriptional regulatory protein pro-1, amino-acid permease inda1, ATP-dependent RNA helicase DED1, acetyl-coenzyme A synthetase, sarcoplasmic/endoplasmic reticulum calcium ATPase 2, etc., showed significantly down-regulated expression in phenamacril-sensitive strain but not in resistant strains after phenamacril treatment. In addition, cyanide hydratase, mating-type protein MAT-1, putative purine nucleoside permease, plasma membrane protein yro2, etc., showed significantly co-down-regulated expression in all the strains after phenamacril treatment. Taken together, This study provides deep insights into the resistance regulation mechanism and the inhibitory effect of fungicide phenamacril and these new annotated proteins or enzymes are worth for the discovery of new fungicide targets.

Inhibition mechanism of fusarium graminearum growth by g-C3N4 homojunction and its application in barley malting

The increase of deoxynivalenol (DON) caused by Fusarium graminearum (F. graminearum) during the malting process is a serious safety problem. In our work, the inhibition mechanism of F. graminearum growth by g-C3N4 homojunction and its application in barley malting were studied. The reason why the growth activity of F. graminearum decreased after photocatalysis by g-C3N4 homojunction was that under visible light irradiation, a large amount of •O2- elicited by g-C3N4 homojunction destroyed the cell structure of F. graminearum, leading to the deficiency of cell membrane selective permeability and serious disorder of intracellular metabolism. The application of photocatalysis technology in malting can effectively inhibit the growth of F. graminearum and the accumulation of ergosterol was reduced by 30.55 %, thus reducing the DON content in finished malt by 31.82 %. Meanwhile, the physicochemical indexes of barley malt after photocatalytic treatment still met the requirements of second class barley malt in Chinese light industry standard QB/T 1686-2008. Our work provides a new idea for the control of fungal contamination in barley malt.

Resistance risk, resistance mechanism and the effect on DON production of a new SDHI fungicide cyclobutrifluram in Fusarium graminearum

Fusarium head blight in wheat is caused by Fusarium graminearum, resulting in significant yield losses and grain contamination with deoxynivalenol (DON), which poses a potential threat to animal health. Cyclobutrifluram, a newly developed succinate dehydrogenase inhibitor, has shown excellent inhibition of Fusarium spp. However, the resistance risk of F. graminearum to cyclobutrifluram and the molecular mechanism of resistance have not been determined. In this study, we established the average EC50 of a range of F. graminearum isolates to cyclobutrifluram to be 0.0110 μg/mL. Six cyclobutrifluram-resistant mutants were obtained using fungicide adaptation. All mutants exhibited impaired fitness relative to their parental isolates. This was evident from measurements of mycelial growth, conidiation, conidial germination, virulence, and DON production. Interestingly, cyclobutrifluram did not seem to affect the DON production of either the sensitive isolates or the resistant mutants. Furthermore, a positive cross-resistance was observed between cyclobutrifluram and pydiflumetofen. These findings suggest that F. graminearum carries a moderate to high risk of developing resistance to cyclobutrifluram. Additionally, point mutations H248Y in FgSdhB and A73V in FgSdhC1 of F. graminearum were observed in the cyclobutrifluram-resistant mutants. Finally, an overexpression transformation assay and molecular docking indicated that FgSdhBH248Y or FgSdhC1A73V could confer resistance of F. graminearum to cyclobutrifluram.

Determining the biocontrol capacities of Trichoderma spp. originating from Turkey on Fusarium culmorum by transcriptional and antagonistic analyses

In this study aiming to investigate potential fungal biocontrol agents for Fusarium culmorum, several isolates of Trichoderma spp. were evaluated for their antagonistic effects by means of transcriptional analyses. At first, 21 monosporic Trichoderma spp. isolates were obtained from natural wood debris and wood area soils in Manisa, Turkey. Trichoderma spp. Isolates were identified as belonging to four different species (T. atroviride, T. harzianum, T. koningii, and T. brevicompactum) by tef1-α sequencing. Then, the linear growth rate (LGR) of each species was calculated and determined to be in a range between 13.22 ± 0.71 mm/day (T. atroviride TR2) and 25.06 ± 1.45 mm/day (T. harzianum K30). Inter-simple sequence repeat (ISSR) genotyping validated the tef1-α sequencing results by presenting two sub-clusters in the dendrogram. We determined the genetically most similar (TR1 & TR2; 97.77%) and dissimilar (K9 & K17; 40.40%) individuals belonging to the same and different species, respectively. Dual sandwich culture tests (which are useful for antagonism studies) revealed that T. harzianum K21 (the least suppressive) and T. brevicompactum K26 (the most suppressive) isolates suppressed F. culmorum with growth rates of 3% and 46%, respectively. Expressions of genes previously associated with mycoparasitism-plant protection-secondary metabolism (nag1, tgf-1, and tmk-1) were tested by quantitative real-time polymerase chain reaction (qRT-PCR) in both those isolates. While there were no significant differences (p>0.05) in expression that were present in the K21 isolate, those three genes were upregulated with fold change values of 2.69 ± 0.26 (p<0.001), 2.23 ± 0.16 (p<0.001), and 5.38 ± 2.01 (p<0.05) in K26, meaning that the presence of significant alteration in the physiological processes of the fungus. Also, its mycoparasitism potential was tested on Triticum aestivum L. cv Basribey in planta, which was infected with the F. culmorum FcUK99 strain. Results of the trials, including specific plant growth parameters (weight or length of plantlets), confirmed the mycoparasitic potential of the isolate. It can be concluded that (i) nag1, tgf-1, and tmk-1 genes could be approved as reliable markers for evaluation of BCA capacities of Trichoderma spp. and (ii) the T. brevicompactum K26 strain can be suggested as a promising candidate for combating in F. culmorum diseases following the necessary procedures to ensure it is non-hazardous and safe.

Rapid and Sensitive Detection of Toxigenic Fusarium asiaticum Integrating Recombinase Polymerase Amplification, CRISPR/Cas12a, and Lateral Flow Techniques

Fusarium head blight (FHB) is a global cereal disease caused by a complex of Fusarium species. Both Fusarium graminearum and F. asiaticum are the causal agents of FHB in China. F. asiaticum is the predominant species in the Middle-Lower Reaches of the Yangtze River (MLRYR) and southwest China. Therefore, detecting F. asiaticum in a timely manner is crucial for controlling the disease and preventing mycotoxins from entering the food chain. Here, we combined rapid genomic DNA extraction, recombinase polymerase amplification, Cas12a cleavage, and lateral flow detection techniques to develop a method for the rapid detection of F. asiaticum. The reaction conditions were optimized to provide a rapid, sensitive, and cost-effective method for F. asiaticum detection. The optimized method demonstrated exceptional specificity in detecting F. asiaticum while not detecting any of the 14 other Fusarium strains and 3 non-Fusarium species. Additionally, it could detect F. asiaticum DNA at concentrations as low as 20 ag/μL, allowing for the diagnosis of F. asiaticum infection in maize and wheat kernels even after 3 days of inoculation. The developed assay will provide an efficient and robust detection platform to accelerate plant pathogen detection.

Identification of Fusarium Basal Rot Pathogens of Onion and Evaluation of Fungicides against the Pathogens

Onion (Allium cepa L.) is an economically important vegetable crop worldwide. However, various fungal diseases, including Fusarium basal rot (FBR), neck rot, and white rot, reduce onion production or bulb storage life. FBR caused by Fusarium species is among the most destructive onion diseases. In this study, we identified Fusarium species associated with FBR in Jeolla and Gyeongsang Provinces in South Korea and evaluated fungicides against the pathogens. Our morphological and molecular analyses showed that FBR in onions is associated with Fusarium commune, Fusarium oxysporum, and Fusarium proliferatum. We selected seven fungicides (fludioxonil, hexaconazole, mandestrobin, penthiopyrad, prochloraz-manganese, pydiflumetofen, and tebuconazole) and evaluated their inhibitory effects on mycelial growth of the pathogens at three different concentrations (0.01, 0.1, and 1 mg/mL). We found that prochloraz-manganese was highly effective, inhibiting 100% of the mycelial growth of the pathogens at all concentrations, followed by tebuconazole. Fludioxonil showed < 50% inhibition at 1 mg/mL for the tested isolates.

Fusarium Species and Mycotoxins Associated with Sorghum Grains in Uruguay

Grain mold and stalk rot are among the fungal diseases that cause significant losses in sorghum worldwide and are caused by different Fusarium spp. The presence of Fusarium species in sorghum grains causes yield losses and mycotoxin contamination, which represents a risk to consumers. In this study, Fusarium graminearum species complex (FGSC) had a high incidence, followed by Fusarium fujikuroi species complex (FFSC) and F. incarnatum-equiseti species complex. Within FFSC, F. proliferatum, F. andiyazi, F. fujikuroi, F. thapsinum, F. verticillioides and F. subglutinans were identified, and this was the first report of F. fujikuroi in sorghum. The most frequent toxins found in sorghum samples were deoxynivalenol (DON) and zearalenone (ZEN). The presence of fumonisins and nivalenol (NIV) was detected at low levels. This study adds new knowledge about the occurrence of Fusarium species and mycotoxins in sorghum grains. Furthermore, this is the first report in Uruguay on fungicide sensitivity for Fusarium isolates from sorghum, which constitutes an important starting point for defining management practices to minimize fungal infection and mycotoxin contamination.

Dynamics of Carbendazim-Resistance Frequency of Pathogens Associated with the Epidemic of Fusarium Head Blight

Carbendazim resistance was detected using 4,701 Fusarium graminearum species complex isolates collected from major wheat-producing regions in China from 2018 to 2020. A total of 348 carbendazim-resistant isolates were identified. The majority of carbendazim-resistant isolates were detected in Jiangsu and Anhui Provinces. In total, 227 and 88 isolates were obtained from each of the Jiangsu and Anhui Provinces, with a high resistance frequency of 41.12 and 20.56%, respectively. The predominant resistant isolates harboring point mutations were F167Y (79.31%), followed by E198Q (16.38%) and F200Y (4.31%). Compared with F. graminearum, F. asiaticum isolates were more likely to produce carbendazim resistance. In this study, we first detected carbendazim-resistant isolates in Hebei, Shaanxi, Sichuan, and Hunan Provinces. In Jiangsu, Anhui, and Zhejiang, the frequency of carbendazim-resistant isolates maintained a high level, resulting in stable carbendazim-resistant populations. We also found the dynamic of carbendazim-resistance frequency in most provinces showed similar trends to the epidemic of Fusarium Head Blight (FHB). Our results facilitate the understanding of the current situation of carbendazim resistance of FHB pathogens and will be helpful for fungicides selection in different wheat-producing areas in China.

Antifungal Activities of Fluorinated Pyrazole Aldehydes on Phytopathogenic Fungi, and Their Effect on Entomopathogenic Nematodes, and Soil-Beneficial Bacteria

Fluoro-substituted pyrazoles have a wide range of biological activities, such as antibacterial, antiviral, and antifungal activities. The aim of this study was to evaluate the antifungal activities of fluorinated 4,5-dihydro-1H-pyrazole derivatives on four phytopathogenic fungi: Sclerotinia sclerotiorum, Macrophomina phaseolina, Fusarium oxysporum f. sp. lycopersici, and F. culmorum. Moreover, they were tested on two soil beneficial bacteria-Bacillus mycoides and Bradyrhizobium japonicum-as well as two entomopathogenic nematodes (EPNs)-Heterorhabditis bacteriophora and Steinernema feltiae. The molecular docking was performed on the three enzymes responsible for fungal growth, the three plant cell wall-degrading enzymes, and acetylcholinesterase (AChE). The most active compounds against fungi S. sclerotiorum were 2-chlorophenyl derivative (H9) (43.07% of inhibition) and 2,5-dimethoxyphenyl derivative (H7) (42.23% of inhibition), as well as H9 against F. culmorum (46.75% of inhibition). Compounds were shown to be safe for beneficial soil bacteria and nematodes, except for compound H9 on EPN H. bacteriophora (18.75% mortality), which also showed the strongest inhibition against AChE (79.50% of inhibition). The molecular docking study revealed that antifungal activity is possible through the inhibition of proteinase K, and nematicidal activity is possible through the inhibition of AChE. The fluorinated pyrazole aldehydes are promising components of future plant protection products that could be environmentally and toxicologically acceptable.

Transcriptomic Profiling of Fusarium pseudograminearum in Response to Carbendazim, Pyraclostrobin, Tebuconazole, and Phenamacril

Fusarium pseudograminearum has been identified as a significant pathogen. It causes Fusarium crown rot (FCR), which occurs in several major wheat-producing areas in China. Chemical control is the primary measure with which to control this disease. In this study, transcriptome sequencing (RNA-Seq) was used to determine the different mechanisms of action of four frequently used fungicides including carbendazim, pyraclostrobin, tebuconazole, and phenamacril on F. pseudograminearum. In brief, 381, 1896, 842, and 814 differentially expressed genes (DEGs) were identified under the carbendazim, pyraclostrobin, tebuconazole, and phenamacril treatments, respectively. After the joint analysis, 67 common DEGs were obtained, and further functional analysis showed that the ABC transported pathway was significantly enriched. Moreover, FPSE_04130 (FER6) and FPSE_11895 (MDR1), two important ABC multidrug transporter genes whose expression levels simultaneously increased, were mined under the different treatments, which unambiguously demonstrated the common effects. In addition, Mfuzz clustering analysis and WGCNA analysis revealed that the core DEGs are involved in several critical pathways in each of the four treatment groups. Taken together, these genes may play a crucial function in the mechanisms of F. pseudograminearum's response to the fungicides stress.

Comparative transcriptome analysis reveals the biocontrol mechanism of Bacillus velezensis E68 against Fusarium graminearum DAOMC 180378, the causal agent of Fusarium head blight

Fusarium graminearum is the causal agent of Fusarium Head Blight, a serious disease affecting grain crops worldwide. Biological control involves the use of microorganisms to combat plant pathogens such as F. graminearum. Strains of Bacillus velezensis are common biological control candidates for use against F. graminearum and other plant pathogens, as they can secrete antifungal secondary metabolites. Here we study the interaction between B. velezensis E68 and F. graminearum DAOMC 180378 by employing a dual RNA-seq approach to assess the transcriptional changes in both organisms. In dual culture, B. velezensis up-regulated genes related to sporulation and phosphate stress and down-regulated genes related to secondary metabolism, biofilm formation and the tricarboxylic acid cycle. F. graminearum up-regulated genes encoding for killer protein 4-like proteins and genes relating to heavy metal tolerance, and down-regulated genes relating to trichothecene biosynthesis and phenol metabolism. This study provides insight into the molecular mechanisms involved in the interaction between a biocontrol bacterium and a phytopathogenic fungus.

Monitoring and characterization of prochloraz resistance in Fusarium fujikuroi in China

Rice bakanae disease, caused by Fusarium fujikuroi, is a destructive seed-borne disease throughout the world. Prochloraz, a DMI (C-14α-demethylase inhibitor) fungicide, has been registered in China for >20 years. Prochloraz resistance in F. fujikuroi was severe in China with resistance frequencies of 34.56%, 45.33%, and 48.45% from 2019 to 2021. The fitness of prochloraz-resistant population was lower than that of sensitive population, with an average CFI of 2.86 × 10⁶ and 4.56 × 10⁶, respectively. No cross-resistance was detected between prochloraz and tebuconazole or hexaconazole, and the prochloraz-resistant isolates were still sensitive to fludioxonil, phenamacril, and pydiflumetofen. S312T mutation in Ffcyp51b or overexpression of Ffcyp51a and Ffcyp51b was detected in the highly resistant isolates. AS-PCR primers were designed to detect the prochloraz-resistant isolates with S312T mutation in the field. Resistant isolates carrying S312T mutation were the dominant group in prochloraz-resistant population with frequencies of 43.26%, 23.59%, and 71.20% from 2019 to 2021, which indicated that more attention should be paid to this genotype when monitoring and managing the prochloraz resistance in F. fujikuroi.

Triticonazole enantiomers induced enantioselective metabolic phenotypes in Fusarium graminearum and HepG2 cells

The management of Fusarium head blight relies heavily on triazole fungicides. Most of triazole fungicides are chiral, and their enantioselective effects on metabolic phenotypes are poorly understood. Herein, we analyzed the bioactivity of triticonazole against Fusarium graminearum, and ¹H-nuclear magnetic resonance-based metabolomics was used to assess the metabolic disturbances of triticonazole enantiomers in Fusarium graminearum and human hepatocarcinoma cells. Results indicated that the bioactivity of R-triticonazole was 4.28-fold higher than its antipode since it bound stronger with fungal CYP51B and induced more abnormal metabolic processes of Fusarium graminearum, including lipid metabolism, glycolysis, and amino acid metabolism. In human hepatocarcinoma cells, pathways of "alanine, aspartic acid and glutamate metabolism" and "pyruvate metabolism" were disturbed significantly by R-triticonazole; "phenylalanine metabolism" and "taurine-hypotaurine metabolism" were abnormal in the exposure of S-triticonazole. These results suggested that R- and S-triticonazole could affect different metabolic pathways of human hepatocarcinoma cells, and the massively use of inefficient S-triticonazole should be avoided. Our data will help to better understand the enantioselectivity of chiral pesticides and provide a reference for the development of green pesticides.

Sterol 14α-Demethylase CaCYP51A and CaCYP51B are Functionally Redundant, but Differentially Regulated in Colletotrichum acutatum: Responsibility for DMI-Fungicide Resistance

Colletotrichum acutatum, the main pathogen causing anthracnose on chili worldwide, is controlled by tebuconazole [a sterol C14-demethylation inhibitor (DMI) fungicide, abbreviated as Teb] with excellent efficacy. Our previous study exhibited that all C. acutatum isolates were sensitive to Teb while the Colletotrichum gloeosporioides population had developed resistance to Teb on the same fungicide-pressure selection. Therefore, the assessment of Teb-resistance in C. acutatum is impending. Twenty Teb-resistant (Teb^R) mutants obtained by fungicide domestication and ultraviolet (UV)-mutagenesis displayed similar fitness compared to parental isolates. Data in the current study exhibited that mutations at CaCYP51A and/or overexpression of CaCYP51s were responsible for Teb-resistance. Furthermore, the deletion mutants ΔCaCYP51A and ΔCaCYP51B played different roles in sensitivities to DMIs. Taken together, this study first reported that mutations at CaCYP51A and/or overexpression of CaCYP51s conferred resistance to Teb in C. acutatum, CaCYP51A and CaCYP51B are functionally redundant, but differentially regulated in DMI resistance.

Sensitivity and Resistance Risk Assessment of Fusarium graminearum from Wheat to Prothioconazole

Fusarium head blight (FHB), caused mainly by Fusarium graminearum, is one of the most devastating diseases of wheat. Prothioconazole is a broad-spectrum demethylation inhibitor fungicide with excellent efficacy against FHB. In this study, 235 strains of F. graminearum collected from different regions of Henan Province of China in 2016, 2017, and 2018 were randomly selected. The sensitivity of F. graminearum to prothioconazole was determined by the mycelial growth inhibition method. The results showed that the half maximal effective concentration (EC₅₀) values of F. graminearum to prothioconazole ranged from 0.4742 to 3.4403 μg/ml, and the average EC₅₀ value was 1.7758 ± 0.6667 μg/ml. The sensitivity frequency distribution presented a consequent unimodal curve, and thus the average EC₅₀ value can be established as the baseline sensitivity of F. graminearum to prothioconazole. Ten strains of prothioconazole-resistant mutants were obtained by fungicide taming, and the resistance factor of the mutants ranged from 5.71 to 12.32. The genetic stability assay showed that resistance can be inherited stably for 10 generations. All mutants displayed different degrees of defects in vegetative growth, conidia formation, and pathogenicity compared with the parental strain. These results indicated that F. graminearum has a low risk of resistance to prothioconazole. Cross-resistance assay showed that no cross-resistance was found between prothioconazole and carbendazim, tebuconazole, phenamacril, and pydiflumetofen. Among all mutants, sequence analysis showed that no mutation site was found in cyp51A and cyp51B. Real-time PCR assays showed that the expression levels of cyp51A and cyp51B of the mutants were significantly increased after prothioconazole treatment for 24 h. In summary, our study provided a theoretical basis for the resistance risk assessment of F. graminearum to prothioconazole and scientific application of prothioconazole in controlling FHB.

Fusarium graminearum FgSdhC1 point mutation A78V confers resistance to the succinate dehydrogenase inhibitor pydiflumetofen

BACKGROUND

Fusarium head blight (FHB) caused by Fusarium graminearum complex (Fg) is a devastating disease of cereal crops worldwide. The succinate dehydrogenase inhibitor, pydiflumetofen, was registered for management of FHB in China in 2019. Previously, laboratory-induced pydiflumetofen-resistant (PyR) mutants of Fg have been characterized. However, resistance situation of Fg to pydiflumetofen in the field remains largely unknown.

RESULTS

After screening 6468 isolates of Fg from various regions of China, six PyR isolates were identified. All six resistant isolates exhibited no fitness penalties based on mycelial growth, conidiation and virulence. However, no cross-resistance between pydiflumetofen and azoxystrobin, tebuconazole or fludioxonil in Fg was detected. Genome-sequencing revealed that all six PyR isolates contained a point mutation A78V in FgSdhC1 (FgSdhC1^A78V ). Genetic replacement assay further confirmed that FgSdhC1^A78V conferred resistance of Fg to pydiflumetofen. Based on this, a mismatch allele-specific polymerase chain reaction was developed for rapidly detecting the PyR isolates containing the FgSdhC1^A78V mutation in Fg.

CONCLUSION

This is the first time that resistance of Fg to pydiflumetofen in the field was reported and point mutation FgSdhC1^A78V conferring resistance of Fg to pydiflumetofen was confirmed. This study provides critical information for monitoring and managing pydiflumetofen resistance in Fg.

The Importin FgPse1 Is Required for Vegetative Development, Virulence, and Deoxynivalenol Production by Interacting with the Nuclear Polyadenylated RNA-Binding Protein FgNab2 in Fusarium graminearum

Karyopherins are involved in transport through nuclear pore complexes. Karyopherins are necessary for nuclear import and export pathways and bind to their cargos. Polyadenylation of messenger RNA (mRNA) is necessary for various biological processes, regulating gene expression in eukaryotes. Until now, the association of karyopherin with mRNA polyadenylation has been less understood in plant pathogenic fungi. In our study, we focused on the biological functions of the karyopherin FgPse1 in Fusarium graminearum. The results showed that FgPse1 is involved in mycelial growth, asexual reproduction, virulence, and deoxynivalenol (DON) production. Co-immunoprecipitation and bimolecular fluorescence complementation showed that FgPse1 interacts with the nuclear polyadenylated RNA-binding protein FgNab2. Moreover, a fluorescence localization assay indicated that FgPse1 is necessary for the nuclear import of FgNab2. The nuclear import of FgNab2 regulates the expression of FgTri4, FgTri5, and FgTri6, which are essential for DON production. Thus, ΔFgPse1 and ΔFgNab2 showed consistent defects in DON production. In summary, our data indicated that FgPse1 is necessary for mycelial growth, virulence, and DON production, interacting with FgNab2 in F. graminearum. These results contribute to our understanding of the functions of importins in phytopathogenic fungi.

Endophytic Fungi as a Promising Biocontrol Agent to Protect Wheat from Fusarium graminearum Head Blight

The search for beneficial endophytes that can be part of a constructed microbial community has increased in recent years. We characterized three endophytic fungi previously isolated from wheat for their in vitro and in planta antagonism toward the Fusarium head blight pathogen, Fusarium graminearum. The endophytes were phylogenetically characterized and shown to be Alternaria destruens, Fusarium commune, and Fusarium oxysporum. Individual fungal endophytes significantly increased seed weight and lowered the accumulation of the mycotoxin deoxynivalenol compared with F. graminearum-infected wheat heads without endophyte pretreatment. Investigation into the mechanism of competition in vitro showed that endophytes competitively excluded F. graminearum by preemptive colonization and possible inhibition over a distance. Investigations on the use of these endophytes in the field are in progress. Identification of these three endophytes highlights a common quandary in searching for beneficial microbes to use in agriculture: species definitions often do not separate individual isolates' lifestyles. A greater understanding of the risks in using intraspecies variants for biocontrol is needed and should be examined in the context of the ecology of the individuals being investigated.

Characterization of Prochloraz Resistance in Fusarium fujikuroi from Heilongjiang Province in China

Prochloraz is widely used to control rice bakanae disease caused by Fusarium fujikuroi. The current study was aimed at monitoring the development of F. fujikuroi resistance to prochloraz in the Heilongjiang Province and analyzing the fitness of F. fujikuroi strains with different resistance levels. The results indicated that most of the 89 F. fujikuroi strains collected from the Heilongjiang Province were resistant to prochloraz, with resistance frequency reaching 92.1%. To assess the field resistance risk of prochloraz, 21 F. fujikuroi strains with different resistance levels were selected to investigate their biological characteristics and assess their fitness. Mycelial growth, sporulation, and germination rates were significantly different among the tested strains. However, when grouped into two subpopulations, no significant difference was tested between prochloraz-resistant and prochloraz-sensitive strains. Pathogenicity assays revealed that the disease severity index of prochloraz-resistant strains was higher than that of prochloraz-sensitive strains. Cross-resistance assays showed no cross-resistance between prochloraz and five other fungicides, namely phenamacril, ipconazole, tebuconazole, carbendazim, and fluopyram. Ffcyp51A gene overexpression was observed in the prochloraz-resistant F. fujikuroi strains after exposure to prochloraz. Collectively, these results indicated that F. fujikuroi resistance against prochloraz was severe. Furthermore, prochloraz-resistant strains were highly fit and could potentially become a dominant population in rice fields, consequently resulting in yield loss.

Point Mutations in FgSdhC2 or in the 5' Untranslated Region of FgSdhC1 Confer Resistance to a Novel Succinate Dehydrogenase Inhibitor Flubeneteram in Fusarium graminearum

Fusarium graminearum is one of the phytopathogenic fungi causing cereal fusarium head blight worldwide. Flubeneteram (Flu) is a novel succinate dehydrogenase inhibitor (SDHI) which exhibits strong fungicidal activity against F. graminearum. In this study, four Flu-resistant (Flu^R) mutants were generated by fungicide domestication from the wildtype strain PH-1. Sequencing alignment results of FgSdh from PH-1 and Flu^R mutants showed that all the mutations could be categorized into three resistant genotypes. Genotype I had an A-to-T mutation at the -57 bp of the 5' untranslated region (5'UTR) of FgSdhC₁, while genotypes II and III carried nonsynonymous mutations conferring T77I or R86C in FgSdhC₂, respectively. All the mutations conferring the Flu resistance and causing fitness penalty were validated. The genotype I mutant showed high Flu-resistance, while genotype II and III mutants exhibited low Flu resistance. Additionally, all the Flu^R genotypes showed distinct cross-resistance patterns among the five SDHIs.

The antioxidant methyl gallate inhibits fungal growth and deoxynivalenol production in Fusarium graminearum

Abstract

Production of the Fusarium toxin deoxynivalenol (DON) is associated with oxidative stress and has been indicated to be part of an adaptive response to oxidative stress in the important wheat fungus Fusarium graminearum. In this study, we found that the antioxidant methyl gallate (MG) displays inhibitory effects against mycelial growth, conidial formation and germination, and DON biosynthesis in F. graminearum in a dose-dependent manner. Treatment with 0.05% (w/v) MG resulted in an abnormal swollen conidial morphology. The expression of the TRI genes involved in DON biosynthesis was significantly reduced, and the induction of Tri1-GFP green fluorescence signals in the spherical and crescent-shaped toxisomes was abolished in the MG-treated mycelium. RNA-Seq analysis of MG-treated F. graminearum showed that 0.5% (w/v) MG inhibited DON production by possibly altering membrane functions and oxidoreductase activities. Coupled with the observations that MG treatment decreases catalase, POD and SOD activity in F. graminearum. The results of this study indicated that MG displays antifungal activity against DON production by modulating its oxidative response. Taken together, the current study revealed the potential of MG in inhibiting mycotoxins in F. graminearum.

Graphical abstract

The FgCYP51B Y123H Mutation Confers Reduced Sensitivity to Prochloraz and Is Important for Conidiation and Ascospore Development in Fusarium graminearum

Fusarium graminearum is one of the most important causal agents of Fusarium head blight disease and is controlled mainly by chemicals such as demethylation inhibitor (DMI) fungicides. FgCYP51B is one of the DMI targets in F. graminearum, and Tyrosine123 (Y123) is an important amino acid in F. graminearum CYP51B, located in one of predicted substrate binding pockets based on the binding mode between DMIs and CYP51B. Previous studies suggest that resistance to DMI fungicides is attributed primarily to point mutations in the CYP51 gene and that the Y123H mutation in F. verticillioides CYP51 confers prochloraz resistance in the laboratory. To investigate the function of FgCYP51B Y123 residue in the growth and development, pathogenicity, and DMI resistance, we generated and analyzed the FgCYP51B Y123H mutant. Results revealed that the Y123H mutation led to reduced conidial sporulation and affected ascospore development; moreover, the mutation conferred reduced sensitivity to prochloraz. Quantitative PCR and molecular docking were performed to investigate the resistance mechanism. Results indicated that Y123H mutation changed the target gene expression and decreased the binding affinity of FgCYP51 to prochloraz. These results will attract more attention to the potential DMI-resistant mutation of F. graminearum and increase our understanding of the DMI resistance mechanism.

Sensitivity of Pyricularia oryzae Populations to Fungicides Over a 26-Year Time Frame in Brazil

The long-term dynamics of fungicide resistance of the rice blast fungus Pyricularia oryzae was monitored by examining the reaction of the fungal field isolates, collected over a period of 26 years, to the active ingredients of commercially relevant fungicides. The in vitro sensitivity of all isolates was measured against quinone outside inhibitors (QoI), melanin biosynthesis inhibitors, and sterol demethylation inhibitor (DMI) fungicides, namely azoxystrobin (as a QoI), tricyclazole (as a melanin biosynthesis inhibitor), tebuconazole (as a DMI), and trifloxystrobin + tebuconazole (QoI + DMI). Over the 26-year collection period, a gradual rise in the EC₅₀ estimates for mycelial growth sensitivity was observed for all fungicides, but most strikingly for azoxystrobin. A rise in conidial germination and appressorium formation was also noted, most markedly for azoxystrobin. Consistently, the earlier isolates were much more sensitive to the active ingredients than the more contemporary isolates. The sequencing of the amplified cyt b fragment distinguished two haplotypes, H1 and H2. Haplotype H1 (six isolates) contained the G to C transversion at codon 143 (resulting in change G143A), linked to the resistant phenotype QoI-R. Haplotype H2 (40 isolates), gathered the isolates sensitive to QoI. This work documents the gradual rise in the frequency of fungicide-resistant isolates in P. oryzae rice populations on a long-term basis.

Antifungal Activity of Quinofumelin against Fusarium graminearum and Its Inhibitory Effect on DON Biosynthesis

Fusarium graminearum, causal agent of Fusarium head blight (FHB), causes a huge economic loss. No information is available on the activity of quinofumelin, a novel quinoline fungicide, against F. graminearum or other phytopathogens. In this study, we used mycelial growth and spore germination inhibition methods to determine the inhibitory effect of quinofumelin against F. graminearum in vitro. The results indicated that quinofumelin excellently inhibited mycelial growth and spore germination of F. graminearum, with the average EC₅₀ values of 0.019 ± 0.007 μg/mL and 0.087 ± 0.024 μg/mL, respectively. In addition, we found that quinofumelin could significantly decrease deoxynivalenol (DON) production and inhibit the expression of DON-related gene TRI5 in F. graminearum. Furthermore, we found that quinofumelin could disrupt the formation of Fusarium toxisome, a structure for producing DON. Western blot analysis demonstrated that the translation level of TRI1, a marker gene for Fusarium toxisome, was suppressed by quinofumelin. The protective and curative assays indicated that quinofumelin had an excellent control efficiency against F. graminearum on wheat coleoptiles. Taken together, quinofumelin exhibits not only an excellent antifungal activity on mycelial growth and spore germination, but also could inhibit DON biosynthesis in F. graminearum. The findings provide a novel candidate for controlling FHB caused by F. graminearum.

PCR-RFLP for Detection of Fusarium graminearum Genotypes with Resistance to Phenamacril

Phenamacril is a cyanoacrylate fungicide that provides excellent control of Fusarium head blight (FHB) or wheat scab, which is caused predominantly by Fusarium graminearum and F. asiaticum. Previous studies revealed that codon mutations of the myosin-5 gene of Fusarium spp. conferred resistance to phenamacril in in vitro lab experiments. In this study, PCR restriction fragment length polymorphism (RFLP) was developed to detect three common mutations (A135T, GCC to ACC at codon 135; S217L, TCA to TTA at codon 217; and E420K, GAA to AAA at codon 420) in F. graminearum induced by fungicide domestication in vitro. PCR products of 841 bp (for mutation of A135T), 802 bp (for mutation of S217L), or 1,649 bp (for mutation of E420K) in the myosin-5 gene were amplified by appropriate primer pairs. Restriction enzyme KpnI, TasI, or DraI was used to distinguish phenamacril-sensitive and -resistant strains with mutation genotypes of A135T, S217L, and E420K, respectively. KpnI digested the 841-bp PCR products of phenamacril-resistant strains with codon mutation A135T into two fragments of 256 and 585 bp. In contrast, KpnI did not digest the PCR products of sensitive strains. TasI digested the 802-bp PCR products of phenamacril-resistant strains with codon mutation S217L into three fragments of 461, 287, and 54 bp. In contrast, TasI digestion of the 802-bp PCR products of phenamacril-sensitive strains resulted in only two fragments of 515 and 287 bp. DraI digested the 1,649-bp PCR products of phenamacril-resistant strains with codon mutation E420K into two fragments of 932 and 717 bp, while the PCR products of phenamacril-sensitive strains was not digested. The three genotypes of resistance mutations were determined by analyzing electrophoresis patterns of the digestion fragments of PCR products. The PCR-RFLP method was evaluated on 48 phenamacril-resistant strains induced by fungicide domestication in vitro and compared with the conventional method (mycelial growth on fungicide-amended agar). The accuracy of the PCR-RFLP method for detecting the three mutation genotypes of F. graminearum resistant to phenamacril was 95.12% compared with conventional method. Bioinformatics analysis revealed that the PCR-RFLP method could also be used to detect the codon mutations of A135T and E420K in F. asiaticum.

Nicotinamide Mononucleotide Potentiates Resistance to Biotrophic Invasion of Fungal Pathogens in Barley

Nicotinamide mononucleotide (NMN), a precursor of nicotinamide adenine dinucleotide (NAD), induces disease resistance to the Fusarium head blight fungus Fusarium graminearum in Arabidopsis and barley, but it is unknown at which stage of the infection it acts. Since the rate of haustorial formation of an obligate biotrophic barley powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh) was significantly reduced in NMN-treated coleoptile epidermal cells, the possibility that NMN induces resistance to the biotrophic stage of F. graminearum was investigated. The results show that NMN treatment caused the wandering of hyphal growth and suppressed the formation of appressoria-like structures. Furthermore, we developed an experimental system to monitor the early stage of infection in real-time and analyzed the infection behavior. We observed that the hyphae elongated windingly by NMN treatment. These results suggest that NMN potentiates resistance to the biotrophic invasion of F. graminearum as well as Bgh.

Resistance risk assessment for a novel succinate dehydrogenase inhibitor pydiflumetofen in Fusarium asiaticum

BACKGROUND

Fusarium asiaticum is one of predominant pathogens of Fusarium head blight (FHB) in China. Pydiflumetofen (Pyd) is a novel succinate dehydrogenase inhibitor (SDHI) which has been commercialized in China for the controlling of wheat FHB since 2019. In the current study, a risk assessment of the pydiflumetofen-resistance selected in Fusarium asiaticum was investigated.

RESULTS

One Pyd^MR mutant [resistance factor (RF) < 80] and four Pyd^HR mutants (RF > 3000) were generated by fungicide-taming from 1000 mycelial discs of the wild-type strain 2021. Nucleotide sequences alignment results of FaSdh from the wild-type strain and resistant mutants showed that all the mutations were categorized into three genotypes, i.e. FaSdhB^H248Y from Pyd^MR mutant, both FaSdhC₁ ^A64V and FaSdhC₁ ^R67K from Pyd^HR mutants. All the resistant mutants possessed no fitness penalty based on the data of mycelial linear growth, conidiation and virulence. In addition, the FaSdhC₁ ^A64V mutants showed positive cross-resistance between pydiflumetofen and boscalid or thifluzamide, but no cross-resistance between pydiflumetofen and Y13149 or Y12196, while the FaSdhC₁ ^R67K mutants exhibited positive cross-resistance between pydiflumetofen and boscalid, thifluzamide or Y12196, and no cross-resistance between pydiflumetofen and Y13149. Furthermore, positive cross-resistance between the five tested SDHIs was detected in the FaSdhB^H248Y mutants.

CONCLUSION

The results suggest a moderate to high resistance risk of F. asiaticum to pydiflumetofen, and provide essential data for monitoring the emergence of resistance and resistance management strategies for pydiflumetofen, which will be useful for scientific application of this fungicide in China.

Fate, residues and dietary risk assessment of the fungicides epoxiconazole and pyraclostrobin in wheat in twelve different regions, China

The fungicides epoxiconazole and pyraclostrobin have been widely used to control wheat fusarium head blight. This study was designed to investigate the dissipation behaviors in different climate regions and provide data for the modification of maximum residue limits of the two fungicides. Wheat samples were collected from field sites in twelve different regions, China and analyzed with an HPLC-MS/MS method for simultaneous detection of epoxiconazole and pyraclostrobin in wheat. The average recoveries of epoxiconazole and pyraclostrobin in wheat matrix were 87-112% and 85-102%, respectively, with the relative standard deviations ≤8.1%. The limits of quantification of epoxiconazole and pyraclostrobin in grain and straw were both 0.01 mg/kg. The dissipations of epoxiconazole and pyraclostrobin followed first-order kinetics, with the half-lives of 10.3 days and 7.6 days, respectively. The terminal residues of epoxiconazole and pyraclostrobin in grain were below 0.034 and 0.028 mg/kg, separately, both lower than the maximum residue limits recommended by China. Based on Chinese dietary pattern and terminal residue distributions, the risk quotients of epoxiconazole and pyraclostrobin were 13.9% and 65.9%, respectively, revealing the evaluated wheat exhibited an acceptably low dietary risk to consumers.

The ASK1 gene regulates the sensitivity of Fusarium graminearum to carbendazim, conidiation and sexual production by combining with β2-tubulin

β-tubulin, a component of microtubules, is involved in a wide variety of roles in cell shape, motility, intracellular trafficking and regulating intracellular metabolism. It has been an important fungicide target to control plant pathogen, for example, Fusarium. However, the regulation of fungicide sensitivity by β-tubulin-interacting proteins is still unclear. Here, ASK1 was identified as a β-tubulin interacting protein. The ASK1 regulated the sensitivity of Fusarium to carbendazim (a benzimidazole carbamate fungicide), and multiple cellular processes, such as chromatin separation, conidiation and sexual production. Further, we found the point mutations at 50th and 198th of β₂-tubulin which caused carbendazim resistance decreased the binding between β₂-tubulin and ASK1, resulting in the deactivation of ASK1. ASK1, on the other hand, competed with carbendazim to bind to β₂-tubulin. The point mutation F167Y in β₂-tubulin broke the intermolecular H-bonds and salt bridges between β₂-tubulin and ASK1, which reduced the competitive effect of ASK1 to carbendazim and resulted in the similar carbendazim sensitivities in F167Y-ΔASK1 and F167Y. These findings have powerful implications for efforts to understand the interaction among β₂-tubulin, its interacting proteins and fungicide, as well as to discover and develop new fungicide against Fusarium.

Flippases play specific but distinct roles in the development, pathogenicity, and secondary metabolism of Fusarium graminearum

The membrane trafficking system is important for compartmentalization of the biosynthesis pathway and secretion of deoxynivalenol (DON) mycotoxin (a virulence factor) in Fusarium graminearum. Flippases are transmembrane lipid transporters and mediate a number of essential physiological steps of membrane trafficking, including vesicle budding, charging, and protein diffusion within the membrane. However, the roles of flippases in secondary metabolism remain unknown in filamentous fungi. Herein, we identified five flippases (FgDnfA, FgDnfB, FgDnfC1, FgDnfC2, and FgDnfD) in F. graminearum and established their specific and redundant functions in the development and pathogenicity of this phytopathogenic fungus. Our results demonstrate that FgDnfA is critical for normal vegetative growth while the other flippases are dispensable. FgDnfA and FgDnfD were found crucial for the fungal pathogenesis, and a remarkable reduction in DON production was observed in ΔFgDNFA and ΔFgDNFD. Deletion of the FgDNFB gene increased DON production to about 30 times that produced by the wild type. Further analysis showed that FgDnfA and FgDnfD have positive roles in the regulation of trichothecene (TRI) genes (TRI1, TRI4, TRI5, TRI6, TRI12, and TRI101) expression and toxisome reorganization, while FgDnfB acts as a negative regulator of DON synthesis. In addition, FgDnfB and FgDnfD have redundant functions in the regulation of phosphatidylcholine transport, and double deletion of FgDNFB and FgDNFD showed serious defects in fungal development, DON synthesis, and virulence. Collectively, our findings reveal the distinct and specific functions of flippase family members in F. graminearum and principally demonstrate that FgDnfA, FgDnfD, and FgDnfB have specific spatiotemporal roles during toxisome biogenesis.

Pathogenomics and Management of Fusarium Diseases in Plants

There is an urgency to supplant the heavy reliance on chemical control of Fusarium diseases in different economically important, staple food crops due to development of resistance in the pathogen population, the high cost of production to the risk-averse grower, and the concomitant environmental impacts. Pathogenomics has enabled (i) the creation of genetic inventories which identify those putative genes, regulators, and effectors that are associated with virulence, pathogenicity, and primary and secondary metabolism; (ii) comparison of such genes among related pathogens; (iii) identification of potential genetic targets for chemical control; and (iv) better characterization of the complex dynamics of host–microbe interactions that lead to disease. This type of genomic data serves to inform host-induced gene silencing (HIGS) technology for targeted disruption of transcription of select genes for the control of Fusarium diseases. This review discusses the various repositories and browser access points for comparison of genomic data, the strategies for identification and selection of pathogenicity- and virulence-associated genes and effectors in different Fusarium species, HIGS and successful Fusarium disease control trials with a consideration of loss of RNAi, off-target effects, and future challenges in applying HIGS for management of Fusarium diseases.

RNA-Spray-Mediated Silencing of Fusarium graminearum AGO and DCL Genes Improve Barley Disease Resistance

Over the last decade, several studies have revealed the enormous potential of RNA-silencing strategies as a potential alternative to conventional pesticides for plant protection. We have previously shown that targeted gene silencing mediated by an in planta expression of non-coding inhibitory double-stranded RNAs (dsRNAs) can protect host plants against various diseases with unprecedented efficiency. In addition to the generation of RNA-silencing (RNAi) signals in planta, plants can be protected from pathogens, and pests by spray-applied RNA-based biopesticides. Despite the striking efficiency of RNA-silencing-based technologies holds for agriculture, the molecular mechanisms underlying spray-induced gene silencing (SIGS) strategies are virtually unresolved, a requirement for successful future application in the field. Based on our previous work, we predict that the molecular mechanism of SIGS is controlled by the fungal-silencing machinery. In this study, we used SIGS to compare the silencing efficiencies of computationally-designed vs. manually-designed dsRNA constructs targeting ARGONAUTE and DICER genes of Fusarium graminearum (Fg). We found that targeting key components of the fungal RNAi machinery via SIGS could protect barley leaves from Fg infection and that the manual design of dsRNAs resulted in higher gene-silencing efficiencies than the tool-based design. Moreover, our results indicate the possibility of cross-kingdom RNA silencing in the Fg-barley interaction, a phenomenon in which sRNAs operate as effector molecules to induce gene silencing between species from different kingdoms, such as a plant host and their interacting pathogens.

Resistance risk assessment for Fusarium graminearum to pydiflumetofen, a new succinate dehydrogenase inhibitor

BACKGROUND

Pydiflumetofen is a new generation succinate dehydrogenase inhibitor currently undergoing the process of registration in China for the control of Fusarium head blight in wheat. A resistance risk assessment of Fusarium graminearum to pydiflumetofen was undertaken in this study.

RESULTS

A total of 75 pydiflumetofen-resistant mutants were generated through spontaneous selection and displayed high resistance with an average resistance factor (RF) value of 78. Four mutants were generated through UV mutagenesis and displayed very high resistance with an RF value >1000. The sequence analysis results for Sdh genes and fitness studies revealed the existence of four types of mutations. In particular, 32 spontaneous selection mutants (SP mutants) had an arginine (R) to histidine (H) transition at position 86 in FGSdhC, resulting in seriously reduced fitness. Seven SP mutants had an R to cysteine (C) transition at position 86 in FGSdhC, resulting in reduced fitness. Thirty-six SP mutants had an alanine (A) to valine (V) transition at position 83 in FGSdhC and had no fitness penalties. The efficacy of pydiflumetofen towards a mutant carrying A83V in FGSdhC in vivo was significantly decreased at 42.7%. Four UV mutants had no mutations on all Sdh genes and no fitness penalties. Cross-resistance among boscalid, fluopyram and pydiflumetofen was observed.

CONCLUSION

Sdhc mutations were found and other target site resistance may be present in laboratory PR mutants of F. graminearum. An overall moderate risk of resistance development in F. graminearum was recommended for pydiflumetofen. © 2019 Society of Chemical Industry.

A New Mutation Genotype of K218T in Myosin-5 Confers Resistance to Phenamacril in Rice Bakanae Disease in the Field

In 2017 and 2018, a total of 294 Fusarium fujikuroi isolates were collected from bakanae-diseased rice plants in Jinhua, Shaoxing, and Jiaxing in Zhejiang Province, China. Phenamacril sensitivity of these isolates was determined by the 50% effective concentration value or minimum inhibitory concentration methods. Our results indicated that the phenamacril resistance frequency of F. fujikuroi increased from 18% in 2017 to 47% in 2018, and rice plants infected with F. fujikuroi-resistant isolates could not be protected effectively with 50 mg/liter of phenamacril. Phenamacril-resistant F. fujikuroi isolates obtained from rice fields showed stable resistance, because their fitness levels (i.e., mycelial growth, sporulation, and pathogenicity) were similar to the phenamacril-sensitive isolates. In addition to the point mutation at codon 219 in the myosin-5 gene that conferred resistance to phenamacril, our results also showed another point mutation at codon 218 (AAG→ACG) in myosin-5 that also conferred resistance to phenamacril. In this study, we found rapid development and persistence of diversified genotypes of phenamacril resistance, highlighting the importance of proper use of phenamacril in rice fields. Our results may also help researchers develop new fungicides or new control strategies using combinations of different fungicides in the control of phenamacril-resistant F. fujikuroi isolates.

Impact of the biofungicide tetramycin on the development of Fusarium head blight, grain yield and deoxynivalenol accumulation in wheat

Fusarium graminearum causes Fusarium head blight (FHB), a devastating disease that leads to extensive yield and quality loss in wheat and barley production. Integrated pest management (IPM) is required to control this disease and biofungicides, such as tetramycin, could be a novel addition to IPM strategies. The current study investigated in vitro tetramycin toxicity in Fusarium graminearum and evaluated its effectiveness for the control of Fusarium head blight FHB. Tetramycin was shown to affect three key aspects of Fusarium pathogenicity: spore germination, mycelium growth and deoxynivalenol (DON) production. The in vitro results indicated that tetramycin had strong inhibitory activity on the mycelial growth and spore germination. Field trials indicated that tetramycin treatment resulted in a significant reduction in both the FHB disease index and the level of DON accumulation. The reduced DON content in harvested grain was correlated with the amount of Tri5 mRNA determined by qRT-PCR. Synergistic effects between tetramycin and metconazole, in both the in vitro and field experiments were found. Tetramycin could provide an alternative option to control FHB.

Paralogous CYP51 Genes of Colletotrichum spp. Mediate Differential Sensitivity to Sterol Demethylation Inhibitors

Colletotrichum spp. isolates contain two paralogous CYP51 genes that encode sterol 14-demethylase enzymes; however, their role in sensitivity to demethylation inhibitor (DMI) fungicides is yet to be determined. In this study, each of the two genes from Colletotrichum fioriniae and C. nymphaeae was able to rescue the function of CYP51 in the yeast Saccharomyces cerevisiae, demonstrating their independent function. Deletion of CYP51A led to increased sensitivity to propiconazole, diniconazole, prothioconazole, cyproconazole, epoxiconazole, flutriafol, prochloraz, and difenoconazole in C. fioriniae, and to the same fungicides and tebuconazole in C. nymphaeae, with the exception of prochloraz. Deletion of CYP51B in C. fioriniae and CYP51B in C. nymphaeae made mutants increasingly sensitive to five of nine DMI fungicides tested. The results suggest species-specific, differential binding of DMI fungicides onto the two CYP51 enzymes. Pairing DMIs with different effects on CYP51A and -B deletion mutants resulted in synergistic effects, as determined in mycelial growth inhibition experiments. Deletion mutants showed no fitness penalty in terms of mycelial growth, sporulation, and virulence. Our study elucidates the effect of CYP51A and CYP51B of Colletotrichum spp. on DMI sensitivity, suggesting that using a mixture of DMIs may improve the efficacy for anthracnose management.

Searching molecular determinants of sensitivity differences towards four demethylase inhibitors in Fusarium graminearum field strains

Demethylase inhibitors (DMIs) also referred to as azoles or triazoles are currently the main fungicides used for controlling Fusarium diseases and associated toxins in cereals. DMIs also represent an important class of fungicides used in the medical domain. The level of sensitivity of a set of F. graminearum strains (n = 23), collected over the period 1994-2010 in Luxembourg, Germany, Canada, USA, Italy and Belgium against three DMIs (cyproconazole, propiconazole, tebuconazole) used in agriculture and one DMI used in medicine (tioconazole) was assessed using a microplate test. Median molar EC₅₀ values varied 113-fold among DMIs and on average 11-fold within DMIs with cyproconazole and tebuconazole being the least and the most effective ones, respectively. The EC₅₀ values of the two DMIs registered for use against Fusarium species on cereals (propiconazole and tebuconazole) were significantly correlated (r = 0.597**), while no evidence for cross-resistance was obtained for other fungicide combinations. Haplotypes for CYP51A and CYP51C were defined based on snps determining amino acid variations in the two genes. EC₅₀ values of strains with the CYP51A haplotype A0 and the CYP51C haplotype D1 varied greatly for the agricultural DMIs tebuconazole, propiconazole and cyproconazole, but not for the medical DMI tioconazole. None of the mutations and snps that were previously reported to be associated with resistance towards propiconazole was unambiguously related with resistance to tioconazole, because the mutations and snps were found in strains with low as well as with high EC₅₀ values. Our results show that (1) DMI sensitivity of F. graminearum mycelium has been largely stable between 1994 and 2010, (2) effects of snps on sensitivity towards one DMI detected in one set of strains cannot be extrapolated to other DMIs and sets of strains and (3) F. graminearum strains responded differently to DMIs used in agriculture and to a representative of a medical DMI with no evidence for cross-resistance.

Three-Locus Sequence Identification and Differential Tebuconazole Sensitivity Suggest Novel Fusarium equiseti Haplotype from Trinidad

The Fusarium incarnatum-equiseti species complex (FIESC) consists of 33 phylogenetic species according to multi-locus sequence typing (MLST) and Genealogical Concordance Phylogenetic Species Recognition (GCPSR). A multi-locus dataset consisting of nucleotide sequences of the translation elongation factor (EF-1α), calmodulin (CAM), partial RNA polymerase largest subunit (RPB1), and partial RNA polymerase second largest subunit (RPB2), was generated to distinguish among phylogenetic species within the FIESC isolates infecting bell pepper in Trinidad. Three phylogenetic species belonged to the Incarnatum clade (FIESC-15, FIESC-16, and FIESC-26), and one species belonged to the Equiseti clade (FIESC-14). Specific MLST types were sensitive to 10 µg/mL of tebuconazole fungicide as a discriminatory dose. The EC50 values were significantly different among the four MLST groups, which were separated into two homogeneous groups: FIESC-26a and FIESC-14a, demonstrating the “sensitive” azole phenotype and FIESC-15a and FIESC-16a as the “less sensitive” azole phenotype. CYP51C sequences of the Trinidad isolates, although under positive selection, were without any signatures of recombination, were highly conserved, and were not correlated with these azole phenotypes. CYP51C sequences were unable to resolve the FIESC isolates as phylogenetic inference indicated polytomic branching for these sequences. This data is important to different research communities, including those studying Fusarium phytopathology, mycotoxins, and public health impacts.

Study on the efficiency of dsRNAs with increasing length in RNA-based silencing of the Fusarium CYP51 genes

Previously, we have demonstrated that transgenic Arabidopsis and barley plants, expressing a 791 nucleotide (nt) dsRNA (CYP3RNA) that targets all three CYP51 genes (FgCYP51A, FgCYP51B, FgCYP51C) in Fusarium graminearum (Fg), inhibited fungal infection via a process designated as host-induced gene silencing (HIGS). More recently, we have shown that spray applications of CYP3RNA also protect barley from fungal infection via a process termed spray-induced gene silencing (SIGS). Thus, RNAi technology may have the potential to revolutionize plant protection in agriculture. Therefore, successful field application will require optimization of RNAi design necessary to maximize the efficacy of the RNA silencing construct for making RNAi-based strategies a realistic and sustainable approach in agriculture. Previous studies indicate that silencing is correlated with the number of siRNAs generated from a dsRNA precursor. To prove the hypothesis that silencing efficiency is correlated with the number of siRNAs processed out of the dsRNA precursor, we tested in a HIGS and SIGS approach dsRNA precursors of increasing length ranging from 400 nt to 1500 nt to assess gene silencing efficiency of individual FgCYP51 genes. Concerning HIGS-mediated disease control, we found that there is no significant correlation between the length of the dsRNA precursor and the reduction of Fg infection on CYP51-dsRNA-expressing Arabidopsis plants. Importantly and in clear contrast to HIGS, we measured a decrease in SIGS-mediated Fg disease resistance that significantly correlates with the length of the dsRNA construct that was sprayed, indicating that the size of the dsRNA interferes with a sufficient uptake of dsRNAs by the fungus.

RNA-Spray-Mediated Silencing of Fusarium graminearum AGO and DCL Genes Improve Barley Disease Resistance

Over the last decade, several studies have revealed the enormous potential of RNA-silencing strategies as a potential alternative to conventional pesticides for plant protection. We have previously shown that targeted gene silencing mediated by an in planta expression of non-coding inhibitory double-stranded RNAs (dsRNAs) can protect host plants against various diseases with unprecedented efficiency. In addition to the generation of RNA-silencing (RNAi) signals in planta, plants can be protected from pathogens, and pests by spray-applied RNA-based biopesticides. Despite the striking efficiency of RNA-silencing-based technologies holds for agriculture, the molecular mechanisms underlying spray-induced gene silencing (SIGS) strategies are virtually unresolved, a requirement for successful future application in the field. Based on our previous work, we predict that the molecular mechanism of SIGS is controlled by the fungal-silencing machinery. In this study, we used SIGS to compare the silencing efficiencies of computationally-designed vs. manually-designed dsRNA constructs targeting ARGONAUTE and DICER genes of Fusarium graminearum (Fg). We found that targeting key components of the fungal RNAi machinery via SIGS could protect barley leaves from Fg infection and that the manual design of dsRNAs resulted in higher gene-silencing efficiencies than the tool-based design. Moreover, our results indicate the possibility of cross-kingdom RNA silencing in the Fg-barley interaction, a phenomenon in which sRNAs operate as effector molecules to induce gene silencing between species from different kingdoms, such as a plant host and their interacting pathogens.

Control of Wheat Fusarium Head Blight by Heat-Stable Antifungal Factor (HSAF) from Lysobacter enzymogenes

Heat-stable antifungal factor (HSAF), which belongs to the polycyclic tetramate macrolactam family, was isolated from Lysobacter enzymogenes fermentations and exhibited inhibitory activities against a wide range of fungal pathogens. In this study, the antifungal activity of HSAF against Fusarium graminearum in vitro and in vivo was investigated. A total of 50% of mycelial growth of F. graminearum was suppressed with 4.1 μg/ml of HSAF (EC₅₀ value). HSAF treatment resulted in abnormal morphology of the hyphae, such as curling, apical swelling, and depolarized growth. Furthermore, HSAF adequately inhibited conidial germination and conidiation of F. graminearum with an inhibition rate of 100% when 1 and 6 μg/ml of HSAF were applied, respectively. HSAF caused ultrastructural changes of F. graminearum, including cell wall thickening and plasmolysis. Moreover, the application of HSAF significantly controlled Fusarium head blight in wheat caused by F. graminearum in the field. Overall, these results indicate that HSAF has potential for development as a fungicide against F. graminearum.

A phosphorylated transcription factor regulates sterol biosynthesis in Fusarium graminearum

Sterol biosynthesis is controlled by transcription factor SREBP in many eukaryotes. Here, we show that SREBP orthologs are not involved in the regulation of sterol biosynthesis in Fusarium graminearum, a fungal pathogen of cereal crops worldwide. Instead, sterol production is controlled in this organism by a different transcription factor, FgSR, that forms a homodimer and binds to a 16-bp cis-element of its target gene promoters containing two conserved CGAA repeat sequences. FgSR is phosphorylated by the MAP kinase FgHog1, and the phosphorylated FgSR interacts with the chromatin remodeling complex SWI/SNF at the target genes, leading to enhanced transcription. Interestingly, FgSR orthologs exist only in Sordariomycetes and Leotiomycetes fungi. Additionally, FgSR controls virulence mainly via modulating deoxynivalenol biosynthesis and responses to phytoalexin.

Polymorphism and expression level of CYP51 (cytochrome P450) and sensitivity to ipconazole in Fusarium fujikuroi isolates

The relationship between the nucleotide sequences of CYP51, its expression level and its sensitivity to ipconazole of Fusarium fujikuroi isolates were investigated. Single nucleotide polymorphisms (SNPs) were observed in the CYP51 of isolates with different sensitivities to ipconazole, but no amino acid substitution was detected in the putative amino acid sequence of the CYP51 protein. On the other hand, the expression of CYP51 was enhanced by the presence of ipconazole, and it tended to be higher in isolates with lower sensitivities and no gibberellin productivity. In the presumed promoter region, the upstream nucleotide sequence of CYP51, several common SNPs and insertions of nucleotides were detected in the lower sensitivity isolates. These results suggest that F. fujikuroi isolates consist of 2 different groups in sensitivity and gibberellin productivity, and no amino acid substitution in CYP51 protein may contribute to the stably high efficacy of ipconazole against rice Bakanae disease for more than 25 years.

In Vitro and in Field Response of Different Fungicides against Aspergillus flavus and Fusarium Species Causing Ear Rot Disease of Maize

Aspergillus flavus, the main aflatoxin B₁ producing fungal species, Fusarium graminearum, a deoxynivalenol producer, and the fumonisin-producing species F. proliferatum and F. verticillioides are the main toxigenic fungi (TF) that colonize maize. Several strategies are available to control TF and related mycotoxins, such as chemical control. However, there is poor knowledge on the efficacy of fungicides on maize plants since few molecules are registered. The sensitivity of F. graminearum, F. proliferatum, F. verticillioides, and A. flavus to eleven fungicides, selected based on their different modes of action, was evaluated in both in vitro assays and, after selection, in the field. In vitro, demethylation inhibitors (DMI) showed excellent performances, followed by thiophanate-methyl and folpet. Among the succinate dehydrogenase inhibitors (SDHI), isopyrazam showed a higher effectiveness against Fusarium species than boscalid, which was ineffective against Fusarium, like the phenyl-pyrrole fludioxonil. Furthermore, both SDHIs and fludioxonil were more active against A. flavus than Fusarium species. In field trials, prothioconazole and thiophanate-methyl were confirmed to be effective to reduce F. graminearum (52% and 48%) and F. proliferatum contamination (44% and 27%). On the other hand, prothioconazole and boscalid could reduce A. flavus contamination at values of 75% and 56%, respectively.