Mutations in the CYP51 gene correlated with changes in sensitivity to sterol 14 alpha-demethylation inhibitors in field isolates of Mycosphaerella graminicola

Population genomics of Fusarium graminearum isolates from the Americas

Fusarium head blight (FHB) is a major disease of wheat and barley worldwide and is caused by different species in the genus Fusarium, Fusarium graminearum being the most important. We conducted population genomics analyses using SNPs obtained through genotyping by sequencing of over 500 isolates of F. graminearum from the US Upper Midwest, New York, Louisiana, and Uruguay. PCA and STRUCTURE analyses group our isolates into four previously described populations: NA1, NA2, Southern Louisiana (SLA) and Gulf Coast (GC). Some isolates were not assigned to populations because of mixed ancestry. Population structure was associated with toxin genotype and geographic origin. The NA1, NA2, and SLA populations are differentiated (FST 0.385 - 0.551) but the presence of admixed isolates indicates that the populations are not reproductively isolated. Patterns of linkage disequilibrium (LD) decay suggest frequent recombination within populations. Fusarium graminearum populations from the US have great evolutionary potential given the high recombination rate and a large proportion of admixed isolates. The NA1, NA2, and Southern Louisiana (SLA) populations separated from their common ancestral population roughly at the same time in the past and are evolving with moderate levels of subsequent gene flow between them. Genome-wide selection scans in all three populations revealed outlier regions with the strongest signatures of recent positive natural selection. These outlier regions include many genes with unknown function and some genes with known roles in plant-microbe interaction, fungicide/drug resistance, cellular transport and genes that are related to cellular organelles. Only a very small proportion of outlier regions are shared as outliers among the three populations, suggesting unique host-pathogen interactions and environmental adaptation.

Fungal Disease Tolerance with a Focus on Wheat: A Review

In this paper, an extensive review of the literature is provided examining the significance of tolerance to fungal diseases in wheat amidst the escalating global demand for wheat and threats from environmental shifts and pathogen movements. The current comprehensive reliance on agrochemicals for disease management poses risks to food safety and the environment, exacerbated by the emergence of fungicide resistance. While resistance traits in wheat can offer some protection, these traits do not guarantee the complete absence of losses during periods of vigorous or moderate disease development. Furthermore, the introduction of individual resistance genes into wheat monoculture exerts selection pressure on pathogen populations. These disadvantages can be addressed or at least mitigated with the cultivation of tolerant varieties of wheat. Research in this area has shown that certain wheat varieties, susceptible to severe infectious diseases, are still capable of achieving high yields. Through the analysis of the existing literature, this paper explores the manifestations and quantification of tolerance in wheat, discussing its implications for integrated disease management and breeding strategies. Additionally, this paper addresses the ecological and evolutionary aspects of tolerance in the pathogen-plant host system, emphasizing its potential to enhance wheat productivity and sustainability.

Effect of Combining Wuyiencin and Pyrimethanil on Controlling Grape Gray Mold and Delaying Resistance Development in Botrytis cinerea

By screening the compounding combination of Wuyiencin and chemical agents, this study aims to delay the emergence of chemical agent resistance, and provide a technical reference for scientific and rational fungicides technology. This study investigated the impacts of the antibiotic wuyiencin derived from Streptomyces albulus var. wuyiensis and its combination with pyrimethanil on the inhibition of Botrytis cinerea. Treatment with wuyiencin (≥80 µg mL-1) strongly inhibited the pathogenicity of B. cinerea and activated the plant defense response against B. cinerea. Application of 80-100 µg mL-1 wuyiencin effectively controlled grape gray mold (by 57.6-88.1% on leaves and 46.7-96.6% on fruits). Consequently, the application of 80-100 µg mL-1 wuyiencin effectively mitigated grape gray mold incidence, leading to a substantial reduction in disease symptoms to nearly imperceptible levels. When wuyiencin (at the median effective concentration [EC50]) was combined with pyrimethanil (EC50) at a ratio of 7:3, it exhibited the highest efficacy in inhibiting B. cinerea growth. This combination was significantly more potent (p < 0.05) than using wuyiencin or pyrimethanil alone in controlling gray mold on grape leaves and fruits. Furthermore, the combination effectively delayed resistance development in gray mold. The experimental results show that wuyiencin can delay resistance development by affecting the expression of methionine biosynthesis genes and reducing the activity of the cell wall-degrading enzyme activity.

Molecular Mechanisms and Biological Characteristics of Botrytis cinerea Field Isolate Resistance to Pyrisoxazole in Liaoning Province

Botrytis cinerea is a broad-host-range necrotrophic phytopathogen responsible for serious diseases in leading crops worldwide. The novel sterol 14α-demethylase inhibitor (DMI) pyrisoxazole was recently registered for the control of tomato gray mold caused by B. cinerea in China. One hundred fifty-seven isolates of B. cinerea were collected from tomato greenhouses in 14 cities of Liaoning Province from 2016 to 2021 and examined for sensitivity to pyrisoxazole, with a mean EC50 value of 0.151 μg/ml. Three highly resistant isolates, XD-5, DG-4, and GQ-3, were screened, and the EC50 values were 0.734, 0.606, and 0.639 μg/ml with corresponding resistance factors of 12.88, 10.63, and 11.21, respectively. Compared with field-sensitive strains, the highly resistant isolate XD-5 exhibited fitness defects in traits, including mycelial growth, conidial production, and pathogenicity, but DG-4 and GQ-3 did not experience fitness costs. Positive cross-resistance was observed only between pyrisoxazole and the DMIs tebuconazole and prochloraz but not between pyrisoxazole and the non-DMIs iprodione, procymidone, pyrimethanil, fludioxonil, fluazinam, and fluopyram. Sequence alignment of the CYP51 gene indicated that three point mutations were observed in the highly resistant mutant, namely, V24I in XD-5, G461S in GQ-3, and R464K in DG-4. When exposed to pyrisoxazole, the induced expression levels of the ABC transporter AtrD and MFS transporter Mfs1 increased in the resistant isolates compared with those in the sensitive isolates, whereas the expression level of the CYP51 gene did not change significantly. Molecular docking suggested that the G461S and R464K mutations both led to a decrease in the binding energy between CYP51 and pyrisoxazole, whereas no change was found with the V24I mutation. Thus, two point mutations in the CYP51 protein combined with induced expression of the Mfs1 and AtrD genes appeared to mediate the pyrisoxazole resistance of the highly resistant mutants DG-4 and GQ-3, while the overexpression of the Mfs1 and AtrD genes was responsible for the highly resistant mutant XD-5.

Survey of prothioconazole sensitivity in Fusarium pseudograminearum isolates from Henan Province, China, and characterization of resistant laboratory mutants

BACKGROUND

Fusarium crown rot (FCR) is one of the most significant diseases limiting crop production in the Huanghuai wheat-growing region of China. Prothioconazole, a triazole sterol 14α-demethylation inhibitor (DMI) fungicide developed by the Bayer Crop Protection Company, is mainly registered for the prevention and control of wheat powdery mildew and stripe rust (China Pesticide Information Network). It is known to exhibit high activity against F. pseudograminearum, but further research, particularly regarding the potential for fungicide resistance, is required before it can be registered for the control of FCR in China.

RESULTS

The current study found that the baseline sensitivity of 67 field isolates of F. pseudograminearum collected between 2019 and 2021 ranged between 0.016-2.974 μg/mL, with an average EC50 value of 1.191 ± 0.720 μg/mL (mean ± SD). Although none of the field isolates exhibited signs of resistance, three highly resistant mutants were produced by repeated exposure to prothioconazole under laboratory conditions. All of the mutants were found to exhibit significantly reduced growth rates on potato dextrose agar (PDA), as well as reduced levels of sporulation, which indicated that there was a fitness cost associated with the resistance. However, inoculation of wounded wheat coleoptiles revealed that the pathogenicity of the resistant mutants was little affected or actually increased. Molecular analysis of the genes corresponding to the prothioconazole target protein, FpCYP51 (FpCYP51A, FpCYP51B, and FpCYP51C), indicated that the resistant mutants contained three conserved substitutions (M63I, A205S, and I246V) that were present in the FpCYP51C sequence of all three mutants, as well as several non-conserved substations in their FpCYP51A and FpCYP51B sequences. Expression analysis revealed that the presence of prothioconazole (0.1 μg/mL) generally resulted in reduced expression of the three FpCYP51 genes, but that the three mutants exhibited more complex patterns of expression that differed in comparison to their parental isolates. The study found no evidence of cross-resistance between prothioconazole and any of the fungicides tested including three DMI fungicides tebuconazole, prochloraz, and flutriafol.

CONCLUSIONS

Taken together these results not only provide new insight into the resistant mechanism and biological characteristics associated with prothioconazole resistance in F. pseudograminearum, but also strong evidence that prothioconazole could provide effective and sustained control of FCR, especially when applied in combination with other fungicides.

Target and non-target site mechanisms of fungicide resistance and their implications for the management of crop pathogens

Fungicides are indispensable for high-quality crops, but the rapid emergence and evolution of fungicide resistance have become the most important issues in modern agriculture. Hence, the sustainability and profitability of agricultural production have been challenged due to the limited number of fungicide chemical classes. Resistance to site-specific fungicides has principally been linked to target and non-target site mechanisms. These mechanisms change the structure or expression level, affecting fungicide efficacy and resulting in different and varying resistance levels. This review provides background information about fungicide resistance mechanisms and their implications for developing anti-resistance strategies in plant pathogens. Here, our purpose was to review changes at the target and non-target sites of quinone outside inhibitor (QoI) fungicides, methyl-benzimidazole carbamate (MBC) fungicides, demethylation inhibitor (DMI) fungicides, and succinate dehydrogenase inhibitor (SDHI) fungicides and to evaluate if they may also be associated with a fitness cost on crop pathogen populations. The current knowledge suggests that understanding fungicide resistance mechanisms can facilitate resistance monitoring and assist in developing anti-resistance strategies and new fungicide molecules to help solve this issue. © 2023 Society of Chemical Industry.

Study on the Antifungal Activity and Potential Mechanism of Natamycin against Colletotrichum fructicola

In this investigation, the antifungal activity, its influence on the quality of apples, and the molecular mechanism of natamycin against Colletotrichum fructicola were systematically explored. Our findings indicated that natamycin showed significant inhibition against C. fructicola. Moreover, it efficaciously maintained the apple quality by modulating the physicochemical index. Research on the antifungal mechanism showed that natamycin altered the mycelial microstructure, disrupted the plasma membrane integrality, and decreased the ergosterol content of C. fructicola. Interestingly, the exogenous addition of ergosterol weakened the antifungal activity of natamycin. Importantly, natamycin markedly inhibited the expression of Cyp51A and Cyp51B genes in C. fructicola, which was contrary to the results obtained after treatment with triazole fungicide flusilazole. All these results exhibited sufficient proof that natamycin had enormous potential to be conducive as a promising biopreservative against C. fructicola on apples, and these findings will advance our knowledge on the mechanism of natamycin against pathogenic fungi.

Fungicidal activity of novel quinazolin-6-ylcarboxylates and mode of action on Botrytis cinerea

BACKGROUND

Fungal diseases remain important causes of crop failure and economic losses. As the resistance toward current selective fungicides becomes increasingly problematic, it is necessary to develop efficient fungicides with novel chemotypes.

RESULTS

A series of novel quinazolin-6-ylcarboxylates which combined the structures of pyridine or heterocyclic motif and the N-(3-chloro-4-fluorophenyl)quinazolin-4-amine moiety, a binding group of ATP-binding site of gefitinib, were evaluated for their fungicidal activity on different phytopathogenic fungi. Most of these compounds showed excellent fungicidal activities against Botrytis cinerea and Exserohilum rostratum, especially compound F17 displayed the highest activity with EC50 values as 3.79 μg mL-1 against B. cinerea and 2.90 μg mL-1 against E. rostratum, which was similar to or even better than those of the commercial fungicides, such as pyraclostrobin (EC50 , 3.68, 17.38 μg mL-1 ) and hymexazol (EC50 , 4.56, 2.13 μg mL-1 ). Moreover, compound F17 significantly arrested the lesion expansion of B. cinerea infection on tomato detached leaves and strongly suppressed grey mold disease on tomato seedlings in greenhouse. The abilities of compound F17 to induce cell apoptosis of the non-germinated spores, to limit oxalic acid production, to reduce malate dehydrogenase (MDH) expression, and to block the active pocket of MDH protein were demonstrated in B. cinerea.

CONCLUSION

The novel quinazolin-6-ylcarboxylates containing ATP-binding site-directed moiety, especially compound F17, could be developed as a potential fungicidal candidate for further study. © 2023 Society of Chemical Industry.

Sensitivity Baselines, Resistance Monitoring, and Molecular Mechanisms of the Rice False Smut Pathogen Ustilaginoidea virens to Prochloraz and Azoxystrobin in Four Regions of Southern China

Rice false smut caused by Ustilaginoidea virens is one of the most devastating fungal diseases of rice (Oryza sativa) worldwide. Prochloraz and azoxystrobin belong to the groups of demethylation inhibitors and quinone outside inhibitors, respectively, and are commonly used for controlling this disease. In this study, we analyzed the sensitivities of 100 U. virens isolates from Yunnan, Sichuan, Chongqing, and Zhejiang in Southern China to prochloraz and azoxystrobin. The ranges of EC50 for prochloraz and azoxystrobin were 0.004-0.536 and 0.020-0.510 μg/mL, with means and standard errors of 0.062 ± 0.008 and 0.120 ± 0.007 μg/mL, respectively. However, the sensitivity frequency distributions of U. virens to prochloraz and azoxystrobin indicated the emergence of subpopulations with decreased sensitivity. Therefore, the mean EC50 values of 74% and 68% of the isolates at the main peak, 0.031 ± 0.001 and 0.078 ± 0.004 μg/mL, were used as the sensitivity baselines of U. virens to prochloraz and azoxystrobin, respectively. We found significant sensitivity differences to azoxystrobin among different geographical populations and no correlation between the sensitivities of U. virens to prochloraz and azoxystrobin. Among 887 U. virens isolates, the isolate 5-3-1 from Zhejiang showed moderate resistance to prochloraz, with a resistance factor of 22.45, while no nucleotide variation in the 1986-bp upstream or 1827-bp gene regions of CYP51 from 5-3-1 was detected. Overexpression of CYP51 is probably responsible for its resistance to prochloraz. Finally, artificial inoculation showed that 5-3-1 was highly pathogenic to rice, suggesting that the resistance of U. virens to prochloraz must be monitored and managed in Zhejiang.

Targeted deletion of three CYP51s in Fusarium fujikuroi and their different roles in determining sensitivity to 14α-demethylase inhibitor fungicides

BACKGROUND

Fusarium fujikuroi is the pathogenic agent of rice bakanae disease and has developed serious resistance to prochloraz, a 14α-demethylase inhibitor (DMI). Prochloraz resistance in F. fujikuroi is caused by cooperation between FfCyp51B with Cyp51A and shows cross-resistance only to prothioconazole but not to tebuconazole, difenoconazole, propiconazole, metconazole, hexaconazole, and triadimefon. This study aimed to analyze the functions of the three Cyp51s in F. fujikuroi, especially their role in determining sensitivity to DMIs.

RESULTS

The respective deletion of FfCyp51A, Cyp51B, and Cyp51C had no obvious effect on morphology, conidium germination, or pathogenicity. The involvement of growth, growth and ergosterol biosynthesis, and conidium production and ergosterol biosynthesis was observed for FfCyp51A, Cyp51B, and Cyp51C, respectively. Compared with the sensitive isolate of F. fujikuroi, the effect on sensitivity to the tested DMIs was divided into four groups: (i) both of Cyp51A and Cyp51B positively regulate the sensitivity to prochloraz and prothioconazole; (ii) Cyp51B positively regulate the sensitivity to tebuconazole and metconazole, but negatively regulate the sensitivity to difenoconazole; (iii) Cyp51A and Cyp51B play opposite roles in the sensitivity to triadimefon. Therefore, deletion of Cyp51A in F. fujikuroi confers a higher sensitivity to triadimefon, while deletion of Cyp51B results in a triadimefon-resistant mutant isolate; (iv) deletion of Cyp51B yielded a mutant isolate that was more resistant to propiconazole and hexaconazole.

CONCLUSION

Sophisticated interactions exist within the three Cyp51 genes to DMIs fungicides sensitivity in F. fujikuroi, and Cyp51B probably plays a more critical role than Cyp51A and Cyp51C. © 2022 Society of Chemical Industry.

Fungicide sensitivity levels in the Lithuanian Zymoseptoria tritici population in 2021

Zymoseptoria tritici causes the disease known as septoria leaf blotch in winter wheat and is a major factor in yield loss worldwide. Farmers are inclined to use fungicides to protect their crops; however, the efficacy of these measures is rapidly decreasing due to the natural mechanisms of mutation emergence in pathogen populations. Increasing fungicide resistance is being recorded worldwide, therefore, screening of the current situation in Lithuania is essential to determine the subsequent steps of crop protection strategies. In this study, in vitro fungicide sensitivity tests, mutation detection, and field experiments were carried out. The mean EC₅₀ values for prothioconazole-desthio and mefentrifluconazole were 0.14 and 0.28 mg/l, respectively. Increased frequency of the mutation S524T, linked to DMIs resistance, was observed. Results revealed that the dominant point mutation in the gene CYP51 was I381V, and the most frequent CYP51 haplotype was D13 (V136C, I381V, Y461H, S524T). The mutation G143A, linked to QoI resistance, was detected in ¾ of the population. Mutations conferring resistance to SDHIs were not detected in single pycnidium isolates. Two-year field experiments likewise showed no decline in field efficacy of SDHI fungicide in Lithuania. Moreover, the baseline sensitivity of the Lithuanian Z. tritici population to QiI fungicide fenpicoxamid was established. The findings of this study provide an update on the current status of fungicide resistance in the Lithuanian Z. tritici population.

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.

A Botybirnavirus Isolated from Alternaria tenuissima Confers Hypervirulence and Decreased Sensitivity of Its Host Fungus to Difenoconazole

Alternaria alternata botybirnavirus 1 (AaBRV1) was isolated from a strain of Alternaria alternata, causing watermelon leaf blight in our previous research. The effect of AaBRV1 on the phenotype of its host fungus, however, was not determined. In the present study, a novel strain of AaBRV1 was identified in A. tenuissima strain TJ-NH-51S-4, the causal agent of cotton Alternaria leaf spot, and designated as AaBRV1-AT1. A mycovirus AaBRV1-AT1-free strain TJ-NH-51S-4-VF was obtained by protoplast regeneration, which eliminated AaBRV1-AT1 from the mycovirus AaBRV1-AT1-infected strain TJ-NH-51S-4. Colony growth rate, spore production, and virulence of strain TJ-NH-51S-4 were greater than they were in TJ-NH-51S-4-VF, while the sensitivity of strain TJ-NH-51S-4 to difenoconazole, as measured by the EC50, was lower. AaBRV1-AT1 was capable of vertical transmission via asexual spores and horizontal transmission from strain TJ-NH-51S-4 to strain XJ-BZ-5-1hyg (another strain of A. tenuissima) through hyphal contact in pairing cultures. A total of 613 differentially expressed genes (DEGs) were identified in a comparative transcriptome analysis between TJ-NH-51S-4 and TJ-NH-51S-4-VF. Relative to strain TJ-NH-51S-4-VF, the number of up-regulated and down-regulated DEGs in strain TJ-NH-51S-4 was 286 and 327, respectively. Notably, the expression level of one DEG-encoding cytochrome P450 sterol 14α-demethylase and four DEGs encoding siderophore iron transporters were significantly up-regulated. To our knowledge, this is the first documentation of hypervirulence and reduced sensitivity to difenoconazole induced by AaBRV1-AT1 infection in A. tenuissima.

Enantioselective effect of chiral fungicide prothioconazole on Fusarium graminearum: Fungicidal activity and DON biosynthesis

Prothioconazole, a chiral triazole fungicide, is widely used to control Fusarium head blight (FHB) of wheat. Fusarium graminearum (F. graminearum), as the main pathogen of FHB, can produce many secondary metabolites including deoxynivalenol (DON), which threatens the health of humans and animals. However, some fungicides may stimulate F. graminearum to synthesize more DON under certain conditions. Until now, the fungicidal activity and enantioselective effect of prothioconazole enantiomers on DON production, transcriptome and metabolome of F. graminearum were unclear. The fungicidal activity of R-(-)-prothioconazole against F. graminearum was 9.12-17.73 times higher than that of S-(+)-prothioconazole under all conditions. Prothioconazole enantiomers can induce F. graminearum to synthesize more DON under 0.99 water activity (a_w) and 30 °C, especially R-(-)-prothioconazole. The expression levels of TRI6, TRI10 and TRI101 under R-(-)-prothioconazole treatment were significantly higher than those under S-(+)-prothioconazole treatment. Most genes in glycolysis, pyruvate metabolism, the target of rapamycin (TOR) signaling transduction pathway and the cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling transduction pathway showed higher expression levels under R-(-)-prothioconazole treatment than uner S-(+)-prothioconazole treatment and the control. The peroxisome pathway displayed higher transcriptional activity under S-(+)-prothioconazole treatment compared with R-(-)-prothioconazole and the control. Based on metabolomic data, R-(-)-prothioconazole can significantly influence phenylalanine metabolism, and no significantly enriched pathway was found under S-(+)-prothioconazole treatment. These results are helpful to understand the risk of prothioconazole enantiomers on DON production of F. graminearum and uncover the relevant underlying mechanisms of prothioconazole enantiomers.

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.

Mechanism of Fusarium graminearum Resistance to Ergosterol Biosynthesis Inhibitors: G443S Substitution of the Drug Target FgCYP51A

Fusarium head blight (FHB), caused by the Fusarium graminearum species complex, is a devastating fungal disease resulting in substantial yield and quality losses. Ergosterol biosynthesis inhibitors (EBIs) are the most popular chemicals for controlling FHB. Recently, the resistance of F. graminearum to EBIs has emerged in the field, and an amino acid substitution (G443S) of the sterol 14α-demethylase FgCYP51A was detected in the field resistant strains. To further illustrate the resistance mechanism of F. graminearum to EBIs, site-directed mutants conferring the G443S substitution of FgCYP51A were generated from the progenitor strain PH-1 via genetic transformation with site-directed mutagenesis. We found that the FgCYP51A-G443S substitution significantly decreased the sensitivity of F. graminearum to EBIs with EC₅₀ values ranging from 0.1190 to 0.2302 μg mL^-1 and EC₉₀ values ranging from 1.3420 to 9.1119 μg mL^-1 for tebuconazole. Furthermore, the FgCYP51A-G443S substitution decreased sexual reproduction and virulence, which will reduce the initial infection source of pathogen populations in the field, while the increase of sporulation capability may enhance the frequencies of the disease cycle, thereby contributing to epidemics of FHB disease. Surprisingly, the FgCYP51A-G443S substitution accelerated DON biosynthesis by upregulating TRI5 expression and enhancing the fluorescence intensity of TRI1-GFP, the marker protein of Fusarium toxisomes. Thus, we concluded that the FgCYP51A-G443S substitution regulates EBI-fungicide resistance and DON biosynthesis, increasing the risk of fungicide resistance development in the field, thereby threatening the control efficacy of EBIs against FHB.

Azole Use in Agriculture, Horticulture, and Wood Preservation - Is It Indispensable?

Plant pathogens cause significant damage to plant products, compromising both quantities and quality. Even though many elements of agricultural practices are an integral part of reducing disease attacks, modern agriculture is still highly reliant on fungicides to guarantee high yields and product quality. The azoles, 14-alpha demethylase inhibitors, have been the fungicide class used most widely to control fungal plant diseases for more than four decades. More than 25 different azoles have been developed for the control of plant diseases in crops and the group has a world market value share of 20-25%. Azoles have proven to provide long-lasting control of many target plant pathogens and are categorized to have moderate risk for developing fungicide resistance. Field performances against many fungal pathogens have correspondingly been stable or only moderately reduced over time. Hence azoles are still, to date, considered the backbone in many control strategies and widely used as solo fungicides or as mixing partners with other fungicide groups, broadening the control spectrum as well as minimizing the overall risk of resistance development. This review describes the historic perspective of azoles, their market shares and importance for production of major crops like cereals, rice, oilseed rape, sugar beet, banana, citrus, and soybeans. In addition, information regarding use in amenity grass, in the wood preservation industry and as plant growth regulators are described. At the end of the review azoles are discussed in a wider context including future threats following stricter requirements for registration and potential impact on human health.

Molecular Characterization of Laboratory Mutants of Fusarium oxysporum f. sp. niveum Resistant to Prothioconazole, a Demethylation Inhibitor (DMI) Fungicide

Fusarium oxysporum f. sp. niveum (FON) is the causal agent of Fusarium wilt in watermelon, an international growth-limiting pathogen of watermelon cultivation. A single demethylation inhibitor (DMI) fungicide, prothioconazole, is registered to control this pathogen, so the risk of resistance arising in the field is high. To determine and predict the mechanism by which FON could develop resistance to prothioconazole, FON isolates were mutagenized using UV irradiation and subsequent fungicide exposure to create artificially resistant mutants. Isolates were then put into three groups based on the EC₅₀ values: sensitive, intermediately resistant, and highly resistant. The mean EC₅₀ values were 4.98 µg/mL for the sensitive, 31.77 µg/mL for the intermediately resistant, and 108.33 µg/mL for the highly resistant isolates. Isolates were then sequenced and analyzed for differences in both the coding and promoter regions. Two mutations were found that conferred amino acid changes in the target gene, CYP51A, in both intermediately and highly resistant mutants. An expression analysis for the gene CYP51A also showed a significant increase in the expression of the highly resistant mutants compared to the sensitive controls. In this study, we were able to identify two potential mechanisms of resistance to the DMI fungicide prothioconazole in FON isolates: gene overexpression and multiple point mutations. This research should expedite growers’ and researchers’ ability to detect and manage fungicide-resistant phytopathogens.

DMI-Fungicide Resistance in Venturia nashicola, the Causal Agent of Asian Pear Scab-How Reliable Are Mycelial Growth Tests in Culture?

Scab, caused by Venturia nashicola, is among the most serious diseases of Asian pears and control of this disease largely relies on sterol demethylation inhibitor (DMI) fungicides. However, pear growers have complained about field performance of DMIs since the mid-2000s. In this study, to evaluate pathogen sensitivity, mycelial growth tests and inoculation tests were conducted using DMI-amended culture medium and fungicide-sprayed potted pear trees, respectively. Results confirmed distribution of isolates resistant to fenarimol, hexaconazole, and difenoconazole in the field populations. Importantly, results from tests in culture did not fully correlate with those from tests in planta. Due to phenotypic instability of resistance and poor sporulation of this pathogen in culture, resistance is generally assessed by laborious and time-consuming inoculation with conidia collected from a field. To improve the result interpretation from in vitro tests, the isolates were genotyped: the CYP51 gene which encodes the target sterol 14α-demethylase was sequenced and various mutations have been detected in the coding sequence of DMI-resistant isolates. In addition to the detected single nucleotide polymorphisms, alternative mechanisms, not based on changes in the structure of the target protein, may also increase DMI resistance. Development of molecular methods for the diagnosis of DMI resistance seems to be challenging in V. nashicola.

Detection and Biological Characteristics of Alternaria alternata Resistant to Difenoconazole from Paris polyphylla var. chinensis, an Indigenous Medicinal Herb

Black spot caused by Alternaria alternata (BSAA) is one of the most common diseases of Paris polyphylla var. chinensis, causing yield losses in China. Demethylation inhibitors (DMIs) have been used to control this disease in China for decades. Some farmers have complained about the decreased efficacy of DMIs against BSAA. The objective of this study was to detect and characterize the resistance of A. alternata against difenoconazole from P. polyphylla var. chinensis during 2018. Of the 22 isolates of A. alternata obtained from Sichuan Province in the southwest of China, 20 were resistant to difenoconazole. Mycelial growth rates and sporulation of the difenoconazole-resistant (Dfn^R) isolates were not different from those of the difenoconazole-sensitive (Dfn^S) isolates. No cross resistance between difenoconazole and tebuconazole or propiconazole was observed. Mutations were identified at gene AaCYP51 of Dfn^R isolates based on the sequence alignment of the Dfn^R and Dfn^S isolates. All of the mutations could be divided into three resistant genotypes, I (K715R + Y781C), II (K715R + D1140G + T1628A), and III (no mutation). The docking total score of the Dfn^S isolates was 5.6020, higher than the resistant genotype I (4.4599) or the resistant genotype II (3.8651), suggesting that the DMI resistance of A. alternata may be caused by the decreased affinity between AaCYP51 and difenoconazole.

Changes in DMI, SDHI, and QoI Fungicide Sensitivity in the Estonian Zymoseptoria tritici Population between 2019 and 2020

Zymoseptoria tritici (Zt) populations adapt under the selection pressure of fungicides applied for disease control. The primary objective of this study was to assess fungicide sensitivity in the Estonian Zt population. A total of 282 Zt isolates from 2019 and 2020 were tested for sensitivity to azoles (DMIs; prothioconazole-desthio, epoxiconazole, mefentrifluconazole) and succinate dehydrogenase inhibitors (SDHIs; boscalid, fluxapyroxad). The efficacy of the tested fungicides varied considerably between the Estonian counties, but the Zt population is mainly sensitive to DMIs. Additionally, the frequencies of CYP51 gene alterations varied; D134G, V136C, A379G, and S524T had increased, but V136A and I381V showed a moderate decrease in 2020 in comparison to 2019. Sensitivity to SDHIs was stable, but boscalid was less effective than fluxapyroxad. SdhC gene mutations C-T33N, C-T34N, and C-N86S were common, but not linked with SDHI fungicide sensitivity assay results. Otherwise, mutation B-N225I in the SdhB subunit occurred in isolates with reduced sensitivity to SDHIs. Sensitivity to strobilurins was evaluated by the mutation G143A in the CytB gene, which was present in nearly half of the population. The data presented confirm the ongoing evolution of fungicide sensitivity in the Zt population in Estonia and highlight the importance of knowledge-based decisions for optimizing anti-resistance strategies in the field.

Mutation in cyp51b and overexpression of cyp51a and cyp51b confer multiple resistant to DMIs fungicide prochloraz in Fusarium fujikuroi

BACKGROUND

Fusarium fujikuroi is a plant pathogen that causes rice bakanae disease. Prochloraz is an imidazole-class sterol, 14α-demethylase inhibitor (DMI), which has been in use for several years as a foliar spray to control Fusarium spp. on agriculturally important monocot crops. F. fujikuroi is highly resistant to prochloraz treatment, and the aim of this study was to clarify the mechanism by which F. fujikuroi renders itself resistant to prochloraz.

RESULTS

Recently, prochloraz-resistant strains were identified over a vast geographical area in the agricultural regions of Zhejiang Province, China. It was found that 21.13% and 3.96% of the strains examined were highly resistant (HR) to prochloraz during 2017 to 2018. The HR strains contained a point mutation (S312T) in the FfCYP51B protein, while the strains identified with prochloraz susceptibility had no such point mutation in FfCYP51A/B/C. To confirm whether the mutations in FfCYP51B confer resistance to prochloraz, we exchanged the CYP51B locus between the sensitive strain and the resistant strain by homologous double exchange. The transformed mutants with a copy of the resistant fragment exhibited resistance to prochloraz, and the transformed mutants with a copy of the sensitive fragment exhibited sensitivity to prochloraz. Furthermore, qRT-PCR analysis of Ffcyp51a/b/c gene expression revealed that Ffcyp51a and Ffcyp51b were significantly up-regulated in the prochloraz-resistant strains relative to the sensitive strains in F. fujikuroi. Contrary to our expectation, docking of prochloraz into the modeled binding pocket of FfCYP51B indicated that the affinity between prochloraz and the FfCYP51B increased after the amino acid at codon 312 changed to Thr.

CONCLUSION

The point mutation S312T in FfCYP51B and overexpression of Ffcyp51a and Ffcyp51b together lead to the prochloraz-resistant phenotype in F. fujikuroi.

Baseline sensitivity and action mechanism of the sterol demethylation inhibitor flusilazole to Valsa mali

The apple Valsa canker caused by Valsa mali is a devastating branch disease that has seriously threatened the development of the apple industry worldwide. In current study, a total of 115 V. mali strains collected from different apple orchards in Shaanxi Province of China during 2016 and 2017 were tested for their sensitivity to flusilazole. The average EC₅₀ (effective concentrations causing 50% mycelial growth inhibition) value of all tested strains for flusilazole was 0.0892 (±0.0036) μg/mL and the frequency distribution of the EC50 values was unimodal. Flusilazole exhibited both excellent protective and curative activity on detached apple branches, which was significantly better than the commonly used fungicide thiophanate-methyl. After flusilazole treatment, mycelia twisted with offshoot of top increased, the V. mali strains lost the ability of fruiting body production, and cell membrane permeability of the mycelia increased while ergosterol content and pectinase activity decreased. The expression of pectinase genes involved in virulence down-regulated after flusilazole treatment. This study is the first report on the baseline sensitivity of V. mali to flusilazole. These results indicated that flusilazole has a great potential to play an important role in the management of Valsa canker.

Wheat phyllosphere yeasts degrade propiconazole

BACKGROUND

Yeasts, which are ubiquitous in agroecosystems, are known to degrade various xenobiotics. The aim of this study was to analyze the effect of fungicides on the abundance of natural yeast communities colonizing winter wheat leaves, to evaluate the sensitivity of yeast isolates to fungicides in vivo, and to select yeasts that degrade propiconazole.

RESULTS

Fungicides applied during the growing season generally did not affect the counts of endophytic yeasts colonizing wheat leaves. Propiconazole and a commercial mixture of flusilazole and carbendazim decreased the counts of epiphytic yeasts, but the size of the yeast community was restored after 10 days. Epoxiconazole and a commercial mixture of fluoxastrobin and prothioconazole clearly stimulated epiphyte growth. The predominant species isolated from leaves were Aureobasidium pullulans and Rhodotorula glutinis. In the disk diffusion test, 14 out of 75 yeast isolates were not sensitive to any of the tested fungicides. After 48 h of incubation in an aqueous solution of propiconazole, the Rhodotorula glutinis Rg 55 isolate degraded the fungicide in 75%. Isolates Rh. glutinis Rg 92 and Rg 55 minimized the phytotoxic effects of propiconazole under greenhouse conditions. The first isolate contributed to an increase in the dry matter content of wheat seedlings, whereas the other reduced the severity of chlorosis.

CONCLUSION

Not sensitivity of many yeast colonizing wheat leaves on the fungicides and the potential of isolate Rhodotorula glutinis Rg 55 to degrade of propiconazole was established. Yeast may partially eliminate the ecologically negative effect of fungicides.

Cercospora beticola: The intoxicating lifestyle of the leaf spot pathogen of sugar beet

Cercospora leaf spot, caused by the fungal pathogen Cercospora beticola, is the most destructive foliar disease of sugar beet worldwide. This review discusses C. beticola genetics, genomics, and biology and summarizes our current understanding of the molecular interactions that occur between C. beticola and its sugar beet host. We highlight the known virulence arsenal of C. beticola as well as its ability to overcome currently used disease management strategies. Finally, we discuss future prospects for the study and management of C. beticola infections in the context of newly employed molecular tools to uncover additional information regarding the biology of this pathogen.

TAXONOMY

Cercospora beticola Sacc.; Kingdom Fungi, Phylum Ascomycota, Class Dothideomycetes, Order Capnodiales, Family Mycosphaerellaceae, Genus Cercospora.

HOST RANGE

Well-known pathogen of sugar beet (Beta vulgaris subsp. vulgaris) and most species of the Beta genus. Reported as pathogenic on other members of the Chenopodiaceae (e.g., lamb's quarters, spinach) as well as members of the Acanthaceae (e.g., bear's breeches), Apiaceae (e.g., Apium), Asteraceae (e.g., chrysanthemum, lettuce, safflower), Brassicaceae (e.g., wild mustard), Malvaceae (e.g., Malva), Plumbaginaceae (e.g., Limonium), and Polygonaceae (e.g., broad-leaved dock) families.

DISEASE SYMPTOMS

Leaves infected with C. beticola exhibit circular lesions that are coloured tan to grey in the centre and are often delimited by tan-brown to reddish-purple rings. As disease progresses, spots can coalesce to form larger necrotic areas, causing severely infected leaves to wither and die. At the centre of these spots are black spore-bearing structures (pseudostromata). Older leaves often show symptoms first and younger leaves become infected as the disease progresses.

MANAGEMENT

Application of a mixture of fungicides with different modes of action is currently performed although elevated resistance has been documented in most employed fungicide classes. Breeding for high-yielding cultivars with improved host resistance is an ongoing effort and prudent cultural practices, such as crop rotation, weed host management, and cultivation to reduce infested residue levels, are widely used to manage disease. USEFUL WEBSITE: https://www.ncbi.nlm.nih.gov/genome/11237?genome_assembly_id=352037.

Sterol Demethylation Inhibitor Fungicide Resistance in Leptosphaeria maculans is Caused by Modifications in the Regulatory Region of ERG11

Blackleg is a worldwide disease of canola (Brassica napus), caused by a complex of fungal species in the genus Leptosphaeria, that impacts canola production and seed quality. Demethylation inhibitor (DMI) fungicides that target sterol 14α-demethylase are an integral part of disease control. Here, we report six DMI-resistant isolates of Leptosphaeria maculans and two different types of genetic modification related to the resistance. Analysis of the regulatory region of the DMI target gene ERG11 (also known as CYP51) revealed a 275-bp insertion in two of the isolates and three long terminal repeat retrotransposons (5,263, 5,267, and 5,248 bp) inserted in the promoter region of three resistant isolates. Genetic approaches confirmed that these elements are responsible for DMI resistance in L. maculans and crosses show segregation consistent with a single locus. Reverse-transcription quantitative PCR assays demonstrated that the 275-bp insertion increases ERG11 gene expression, conferring DMI fungicide resistance both in vitro and in planta. Moreover, transformation of a susceptible isolate of L. maculans with ERG11 driven by a promoter containing the 275-bp insertion increased resistance to tebuconazole. A minimal shift of the values of concentration whereby 50% of the mycelial growth is inhibited in vitro was observed in resistant isolates containing long terminal repeat retrotransposons; nevertheless, these isolates were able to develop significant lesions on cotyledons from fungicide-treated seedlings. This is the first report of genetic modifications in L. maculans relating to DMI fungicide resistance.

Fungicide Sensitivity Shifting of Zymoseptoria tritici in the Finnish-Baltic Region and a Novel Insertion in the MFS1 Promoter

Septoria tritici blotch (STB) is caused by the ascomycete Zymoseptoria tritici and one of the predominating diseases in wheat (Triticum aestivum) in Europe. The control of STB is highly reliant on frequent fungicide applications. The primary objective of this study was to assess sensitivity levels of Z. tritici to different fungicide groups. The fungicides included in this study were epoxiconazole, prothioconazole-desthio, tebuconazole, and fluxapyroxad. A panel of 63 isolates from Estonia, Latvia, and Lithuania, and 10 isolates from Finland were tested. Fungicide sensitivity testing was carried out as a bioassay analyzing single pycnidium isolates on different fungicide concentrations. The average EC₅₀ value in Baltic countries and Finland to epoxiconazole was high ranging from 1.04 to 2.19 ppm. For prothioconazole-desthio and tebuconazole, EC₅₀ varied from 0.01 to 0.24 ppm, and 1.25 to 18.23 ppm, respectively. The average EC₅₀ value for fluxapyroxad varied from 0.07 to 0.33 ppm. To explain the range of sensitivity, the samples were analyzed for CYP51 and Sdh mutations, as well as cytb G143A, CYP51 overexpression, and multidrug resistance (MDR). Frequencies of ZtCYP51 mutations D134G, V136A/C, A379G, I381V, and S524T in the Finnish-Baltic region were lower than in other European countries, but have increased compared to previous years. The frequency of cytb G143A conferring strobilurin resistance also augmented to 50-70% in the Z. tritici populations from Estonia, Finland, Latvia, and Lithuania. No Sdh mutations were found in this study, and neither strains of MDR phenotypes. However, we found a strain harboring a previously unknown transposon insertion in the promoter of the MFS1 gene, involved in drug efflux and multi-drug resistance. This new insert, however, does not confer an MDR phenotype to the strain.

Analysis of mutations in West Australian populations of Blumeria graminis f. sp. hordei CYP51 conferring resistance to DMI fungicides

BACKGROUND

Powdery mildew caused by Blumeria graminis f. sp. hordei (Bgh) is a constant threat to barley production but is generally well controlled through combinations of host genetics and fungicides. An epidemic of barley powdery mildew was observed from 2007 to 2013 in the West Australian grain belt.

RESULTS

We collected isolates across Australia, examined their sensitivity to demethylation inhibitor (DMI) fungicides and sequenced the Cyp51B target gene. Five amino acid substitutions were found, of which four were novel. The most resistant haplotypes increased in prevalence from 0% in 2009 to 16% in 2010 and 90% in 2011. Yeast strains expressing the Bgh Cyp51 haplotypes replicated the altered sensitivity to various DMIs and these results were complemented by in silico protein docking studies.

CONCLUSIONS

The planting of very susceptible cultivars and the use of a single fungicide mode of action was followed by the emergence of a major epidemic of barley powdery mildew. Widespread use of DMI fungicides led to the selection of Bgh isolates carrying both the Y137F and S524T mutations, which, as in Zymoseptoria tritici, account for resistance factors varying from 3.4 for propiconazole to 18 for tebuconazole, the major azoles used at that time in WA. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Mutations and Overexpression of CYP51 Associated with DMI-Resistance in Colletotrichum gloeosporioides from Chili

Chili anthracnose caused by Colletotrichum spp. is an annual production concern for growers in China. Sterol C14-demethylation inhibitors (DMIs, such as tebuconazole) have been widely used to control this disease for more than three decades. In the current study, of 48 isolates collected from commercial chili farms in Jiangsu Province of China during 2018 and 2019, 8 single-spore isolates were identified as Colletotrichum gloeosporioides and the rest were identified as C. acutatum. To determine whether the DMI resistance of isolates develops in the field, mycelial growth of the 48 isolates was measured in culture medium with and without tebuconazole. In all, 6 of the 8 C. gloeosporioides isolates were resistant to tebuconazole, but all 40 of the C. acutatum isolates were sensitive to tebuconazole. The fitness cost of resistance was low based on a comparison of fitness parameters between the sensitive and resistant isolates of C. gloeosporioides. Positive cross-resistance was observed between tebuconazole and difenconazole or propiconazole, but not prochloraz. Alignment results of the CgCYP51 amino acid sequences from the sensitive and resistant isolates indicated that mutations can be divided into three genotypes. Genotype I possessed four substitutions (V18F, L58V, S175P, and P341A) at the CgCYP51A gene but no substitutions at CgCYP51B, while genotype II had five substitutions (L58V, S175P, A340S, T379A, and N476T) at CgCYP51A, concomitant with three substitutions (D121N, T132A, and F391Y) at CgCYP51B. In addition, genotype III contained two substitutions (L58V and S175P) at CgCYP51A, concomitant with one substitution (T262A) at CgCYP51B. Molecular docking models illustrated that the affinity of tebuconazole to the binding site of the CgCYP51 protein from the resistant isolates was decreased when compared with binding site affinity of the sensitive isolates. Our findings provide not only novel insights into understanding the resistance mechanism to DMIs, but also some important references for resistance management of C. gloeosporioides on chili.

Trade-Off Between Triadimefon Sensitivity and Pathogenicity in a Selfed Sexual Population of Puccinia striiformis f. sp. Tritici

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases of wheat and has been largely managed using demethylation inhibitor (DMI) fungicide triadimefon in China. To determine the sensitivity of Pst, a Chinese Pst isolate and its sexually produced progeny isolates were tested with triadimefon using the detached leaf method. The half maximal effective concentration (EC₅₀) values varied greatly among the progeny isolates, ranging from 0.06 mg L^–1 to 7.89 mg L^–1. Twenty-six of the 56 tested progeny isolates were less sensitive to triadimefon than the parental isolate. A single-nucleotide mutation at the 401 position resulting in an amino acid change from tyrosine (Y) to phenylalanine (F) in the 134th codon (Y134F) of the cytochrome P450 sterol 14a-demethylase enzyme (CYP51), the target gene of DMI fungicide, was identified in the parental isolate. The 87 tested progeny isolates segregated into 19 homozygous wild type (AA), 40 heterozygous (AT), and 28 homozygous mutant (TT) genotypes, fitting a 1:2:1 ratio (χ² = 2.43; P = 0.30). The mutant isolates had higher EC₅₀ values than the wild type isolates. Significant differences in logEC₅₀ were found between the mutant isolates and the wild type isolates (P = 2.2e^–16). However, homozygous and heterozygous mutant isolates were not significantly different (P = 0.21), indicating dominant mutation. Twenty-two progeny isolates were used to inoculate a susceptible wheat variety, and latency period and lesion growth were recorded to compare wild type and mutant isolates for the pathogenicity fitness components. A moderate but significant negative correlation was detected between lesion growth and sensitivity to triadimefon (r = −0.53; P = 0.01). No significant variation in lesion growth was found between homozygous and heterozygous mutant isolates (P = 0.83). In the case of latency period and triadimefon sensitivity, no significant correlation was found (P = 0.17). These results are useful for understanding reduced sensitivity in the pathogen population and improving stripe rust management.

Emergent Ascomycetes in Viticulture: An Interdisciplinary Overview

The reduction of pesticide usage is a current imperative and the implementation of sustainable viticulture is an urgent necessity. A potential solution, which is being increasingly adopted, is offered by the use of grapevine cultivars resistant to its main pathogenic threats. This, however, has contributed to changes in defense strategies resulting in the occurrence of secondary diseases, which were previously controlled. Concomitantly, the ongoing climate crisis is contributing to destabilizing the increasingly dynamic viticultural context. In this review, we explore the available knowledge on three Ascomycetes which are considered emergent and causal agents of powdery mildew, black rot and anthracnose. We also aim to provide a survey on methods for phenotyping disease symptoms in fields, greenhouse and lab conditions, and for disease control underlying the insurgence of pathogen resistance to fungicide. Thus, we discuss fungal genetic variability, highlighting the usage and development of molecular markers and barcoding, coupled with genome sequencing. Moreover, we extensively report on the current knowledge available on grapevine-ascomycete interactions, as well as the mechanisms developed by the host to counteract the attack. Indeed, to better understand these resistance mechanisms, it is relevant to identify pathogen effectors which are involved in the infection process and how grapevine resistance genes function and impact the downstream cascade. Dealing with such a wealth of information on both pathogens and the host, the horizon is now represented by multidisciplinary approaches, combining traditional and innovative methods of cultivation. This will support the translation from theory to practice, in an attempt to understand biology very deeply and manage the spread of these Ascomycetes.

Spatiotemporal dynamics of fungicide resistance in the wheat pathogen Zymoseptoria tritici in France

BACKGROUND

Management of pesticide resistance is a major issue in modern agricultural systems, particularly in the context of the broader challenge of reducing pesticide use. However, such management must be adapted to resistance dynamics, which remains challenging to predict due to its dependence on many biological traits of pests, interactions with the environment and pesticide use. We retrospectively studied the evolution of reported resistances to four modes of action (benzimidazoles, quinone outside inhibitors, sterol demethylation inhibitors and succinate dehydrogenase inhibitors), in French populations of the wheat pathogen Zymoseptoria tritici.

RESULTS

We used statistical models to analyse the Performance trial network data set (2004-2017; ∼ 70 locations in France yearly). They highlighted contrasting behaviours between phenotypes, for example: (i) stable spatial distributions and colonization front structures over time, and (ii) different frequency growth rates at the national scale and between regions.

CONCLUSION

We provide a quantitative description of the spatiotemporal patterns of resistance evolution for fungicides with several modes of action. Moreover, we highlight some unexpected resistance dynamics in France, with major differences between the north and south. This complex pattern of resistance evolution in French populations is consistent with previous descriptions of dynamics at the European scale. These results should make it easier to anticipate evolution locally and to improve the management of resistance. © 2019 Society of Chemical Industry.

Sexual reproduction of Zymoseptoria tritici on durum wheat in Tunisia revealed by presence of airborne inoculum, fruiting bodies and high levels of genetic diversity

Septoria tritici blotch (STB) caused by the heterothallic ascomycete Zymoseptoria tritici is currently one of the most devastating diseases of wheat worldwide. The extent of sexual reproduction of this pathogen is well documented on bread wheat, but not on durum wheat. The objective of the present study was to quantify the occurrence of Z. tritici sexual reproduction on durum wheat in the Tunisian environment. The assessment was undertaken using a triple approach combining fruiting body assessment, ascospore trapping and population genetic analyses. The results highlighted the formation of pseudothecia on leaves and stubble from the autumn until the end of the growing season. Likewise, qPCR monitoring highlighted a constant release of Z. tritici airborne inoculum during the wheat-growing season, with a peak of production at the end of the season. Genetic investigations using microsatellites revealed high levels of gene and genotypic diversities, an equal distribution of mating types, and a lack of genetic clustering within and between growing seasons. Taken together, these findings indicate that Z. tritici undergoes sexual reproduction on durum wheat in Tunisia at least to the same extent than on bread wheat in Western Europe, and that the dry and warm climate does not affect the mating process of the fungus. Frequent occurrence of sexual reproduction is a valuable knowledge to take into account in STB control strategies on durum wheat.

Metyltetraprole, a novel putative complex III inhibitor, targets known QoI-resistant strains of Zymoseptoria tritici and Pyrenophora teres

BACKGROUND

Metyltetraprole is a new fungicide with a unique tetrazolinone-moiety and a similar side chain to a known quinone outside inhibitor (QoI), pyraclostrobin. In this study we describe a unique bioactivity of metyltetraprole on QoI-resistant strains of Zymoseptoria tritici and Pyrenophora teres.

RESULTS

Metyltetraprole exhibited potent antifungal activity against Ascomycetes; it was especially effective against Z. tritici and P. teres in seedling pot tests. Metyltetraprole was also effective in field tests with QoI-resistant mutants. Antifungal activity tests using field strains of Z. tritici and P. teres showed that the performance of metyltetraprole was unaltered by QoI, succinate dehydrogenase inhibitor (SDHI), and sterol 14α-demethylation inhibitor (DMI) resistance. However, the mitochondrial activity test indicated that the compound inhibits the respiratory chain via complex III.

CONCLUSION

Metyltetraprole is a novel fungicide that is highly effective against a wide range of fungal diseases, including important cereal diseases. Although metyltetraprole most likely inhibits the respiratory chain via complex III, it remains effective against QoI resistant strains. Therefore, metyltetraprole is considered as a novel fungicidal agent for controlling diseases affecting cereal crops and overcoming pathogen resistance to existing fungicides. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Rapid Parallel Evolution of Azole Fungicide Resistance in Australian Populations of the Wheat Pathogen Zymoseptoria tritici

Zymoseptoria tritici is a globally distributed fungal pathogen which causes Septoria tritici blotch on wheat. Management of the disease is attempted through the deployment of resistant wheat cultivars and the application of fungicides. However, fungicide resistance is commonly observed in Z. tritici populations, and continuous monitoring is required to detect breakdowns in fungicide efficacy. We recently reported azole-resistant isolates in Australia; however, it remained unknown whether resistance was brought into the continent through gene flow or whether resistance emerged independently. To address this question, we screened 43 isolates across five Australian locations for azole sensitivity and performed whole-genome sequencing on 58 isolates from seven locations to determine the genetic basis of resistance. Population genomic analyses showed extremely strong differentiation between the Australian population recovered after azoles began to be used and both Australian populations recovered before azoles began to be used and populations on different continents. The apparent absence of recent gene flow between Australia and other continents suggests that azole fungicide resistance has evolved de novo and subsequently spread within Tasmania. Despite the isolates being distinct at the whole-genome level, we observed combinations of nonsynonymous substitutions at the CYP51 locus identical to those observed elsewhere in the world. We observed nine previously reported nonsynonymous mutations as well as isolates that carried a combination of the previously reported L50S, S188N, A379G, I381V, Y459DEL, G460DEL, and N513K substitutions. Assays for the 50% effective concentration against a subset of isolates exposed to the tebuconazole and epoxiconazole fungicides showed high levels of azole resistance. The rapid, parallel evolution of a complex CYP51 haplotype that matches a dominant European haplotype demonstrates the enormous potential for de novo resistance emergence in pathogenic fungi.IMPORTANCE Fungicides are essential to control diseases in agriculture because many crops are highly susceptible to pathogens. However, many pathogens rapidly evolve resistance to fungicides. A large body of studies have described specific mutations conferring resistance and have often made inferences about the origins of resistance based on sequencing data from the target gene alone. Here, we show the de novo acquisition of resistance to the ubiquitously used azole fungicides in genetically isolated populations of the wheat pathogen Zymoseptoria tritici in Tasmania, Australia. We confirm evidence for parallel evolution through genome-scale analyses of representative worldwide populations. The emergence of complex resistance haplotypes following a well-documented recent introduction of azoles into Australian farming practices demonstrates how rapidly chemical resistance evolves in agricultural ecosystems.

Inherent Resistance to 14α-Demethylation Inhibitor Fungicides in Colletotrichum truncatum Is Likely Linked to CYP51A and/or CYP51B Gene Variants

Anthracnose disease, caused by Colletotrichum truncatum, affects marketable yield during preharvest production and postharvest storage of fruits and vegetables worldwide. Demethylation inhibitor (DMI) fungicides are among the very few chemical classes of single-site mode of action fungicides that are effective in controlling anthracnose disease. However, some species are inherently resistant to DMIs and more information is needed to understand this phenomenon. Isolates of C. truncatum were collected from the United States and China from peach, soybean, citrus, and begonia and sensitivity to six DMIs (difenoconazole, propiconazole, metconazole, tebuconazole, flutriafol, and fenbuconazole) was determined. Compared with DMI sensitive isolates of C. fructicola, C. siamense, and C. fioriniae (EC50 value ranging from 0.03 to 16.2 µg/ml to six DMIs), C. truncatum and C. nymphaeae were resistant to flutriafol and fenbuconazole (with EC50 values of more 50 µg/ml). Moreover, C. truncatum was resistant to tebuconazole and metconazole (with resistance factors of 27.4 and 96.0) and displayed reduced sensitivity to difenoconazole and propiconazole (with resistance factors of 5.1 and 5.2). Analysis of the Colletotrichum spp. genome revealed two potential DMI targets, CYP51A and CYP51B, that putatively encode P450 sterol 14α-demethylases. Both genes were identified and sequenced from C. truncatum and other species and no correlation between CYP51 gene expression levels and fungicide sensitivity was found. Four amino acid variations L208Y, H238R, S302A, and I366L in CYP51A, and three variations H373 N, M376L, and S511T in CYP51B correlated with the DMI resistance phenotype. CYP51A structure model analysis suggested the four alterations may reduce azole affinity. Likewise, CYP51B structure analysis suggested the H373 N and M376L variants may change the conformation of the DMI binding pocket, thereby causing differential sensitivity to DMI fungicides in C. truncatum.

Selection Pressure Pathways and Mechanisms of Resistance to the Demethylation Inhibitor-Difenoconazole in Penicillium expansum

Penicillium expansum causes blue mold, the most economically important postharvest disease of pome fruit worldwide. Beside sanitation practices, the disease is managed through fungicide applications at harvest. Difenoconazole (DIF) is a new demethylation inhibitor (DMI) fungicide registered recently to manage postharvest diseases of pome fruit. Herein, we evaluated the sensitivity of 130 P. expansum baseline isolates never exposed to DIF and determined the effective concentration (EC50) necessary to inhibit 50% germination, germ tube length, and mycelial growth. The respective mean EC50 values of 0.32, 0.26, and 0.18 μg/ml indicate a high sensitivity of P. expansum baseline isolates to DIF. We also found full and extended control efficacy in vivo after 6 months of storage at 1°C. We conducted a risk assessment for DIF-resistance development using ultraviolet excitation combined with or without DIF-selection pressure to generate and characterize lab mutants. Fifteen DIF-resistant mutants were selected and showed EC50 values of 0.92 to 1.4 μg/ml and 1.7 to 3.8 μg/ml without and with a DIF selection pressure, respectively. Resistance to DIF was stable in vitro over a 10-week period without selection pressure. Alignment of the full CYP51 gene sequences from the three wild-type and 15 mutant isolates revealed a tyrosine to phenylalanine mutation at codon 126 (Y126F) in all of the 15 mutants but not in the wild-type parental isolates. Resistance factors increased 5 to 15-fold in the mutants compared to the wild-type-isolates. DIF-resistant mutants also displayed enhanced CYP51 expression by 2 to 14-fold and was positively correlated with the EC50 values (R 2 = 0.8264). Cross resistance between DIF and fludioxonil, the mixing-partner in the commercial product, was not observed. Our findings suggest P. expansum resistance to DIF is likely to emerge in commercial packinghouse when used frequently. Future studies will determine whether resistance to DIF is qualitative or quantitative which will be determinant in the speed at which resistance will develop and spread in commercial packinghouses and to develop appropriate strategies to extend the lifespan of this new fungicide.

Fusarium graminearum ATP-Binding Cassette Transporter Gene FgABCC9 Is Required for Its Transportation of Salicylic Acid, Fungicide Resistance, Mycelial Growth and Pathogenicity towards Wheat

ATP-binding cassette (ABC) transporters hydrolyze ATP to transport a wide range of substrates. Fusarium graminearum is a major causal agent of Fusarium head blight, which is a severe disease in wheat worldwide. FgABCC9 (FG05_07325) encodes an ABC-C (ABC transporter family C) transporter in F. graminearum, which was highly expressed during the infection in wheat and was up-regulated by the plant defense hormone salicylic acid (SA) and the fungicide tebuconazole. The predicted tertiary structure of the FgABCC9 protein was consistent with the schematic of the ABC exporter. Deletion of FgABCC9 resulted in decreased mycelial growth, increased sensitivity to SA and tebuconazole, reduced accumulation of deoxynivalenol (DON), and less pathogenicity towards wheat. Re-introduction of a functional FgABCC9 gene into ΔFgABCC9 recovered the phenotypes of the wild type strain. Transgenic expression of FgABCC9 in Arabidopsis thaliana increased the accumulation of SA in its leaves without activating SA signaling, which suggests that FgABCC9 functions as an SA exporter. Taken together, FgABCC9 encodes an ABC exporter, which is critical for fungal exportation of SA, response to tebuconazole, mycelial growth, and pathogenicity towards wheat.

The Y137H mutation in the cytochrome P450 FgCYP51B protein confers reduced sensitivity to tebuconazole in Fusarium graminearum

BACKGROUND

Fusarium graminearum is the main pathogen of Fusarium head blight (FHB), a worldwide plant disease and one of the most significant wheat diseases in China. Demethylation inhibitor (DMI) fungicides, such as tebuconazole (TEC), are widely used to control FHB, but long-term use leads to low efficacy against FHB. Earlier studies showed that DMI resistance is associated with the fungal sterol 14α-demethylase (cytochrome P450 CYP51) gene, and that point mutations in the CYP51 gene are the primary mechanism of resistance to DMI fungicides. The aims of this study were to clarify the molecular mechanisms of resistance to TEC and identify the binding sites on the FgCYP51B protein.

RESULTS

Site-directed mutagenesis was used to change the FgCYP51B gene of wild-type strain PH-1 from tyrosine to histidine at residue 137 (Y137H) to generate a mutant transformant, which was confirmed to be resistant to TEC compared with the parental strains. A three-dimensional FgCYP51B model was constructed, and molecular docking simulation studies were conducted to identify the optimum binding mode with TEC. The wild-type FgCYP51B protein displayed stronger affinity to TEC than that of the mutated FgCYP51B in the molecular docking analysis.

CONCLUSION

These results indicate that a Tyr137 amino acid mutation in the cytochrome P450 FgCYP51B could lead to resistance to TEC and that Y137 forms part of the tebuconazole-binding pocket. © 2017 Society of Chemical Industry.

Changes in field dose-response curves for demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides against Zymoseptoria tritici, related to laboratory sensitivity phenotyping and genotyping assays

BACKGROUND

Insensitivity of Zymoseptoria tritici to demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides has been widely reported from laboratory studies, but the relationships between laboratory sensitivity phenotype or target site genotype and field efficacy remain uncertain. This article reports field experiments quantifying dose-response curves, and investigates the relationships between field performance and in vitro half maximal effective concentration (EC₅₀ ) values for DMIs, and the frequency of the G143A substitution conferring QoI resistance.

RESULTS

Data were analysed from 83 field experiments over 21 years. Response curves were fitted, expressed as percentage control, rising towards an asymptote with increasing dose. Decline in DMI efficacy over years was associated with a decrease in the asymptote, and reduced curvature. Field ED₅₀ values were positively related to in vitro EC₅₀ values for isolates of Z. tritici collected over a 14-year period. Loss of QoI efficacy was expressed through a change in asymptote. Increasing frequency of G143A was associated with changes in field dose-response asymptotes.

CONCLUSION

New resistant strains are often detected by resistance monitoring and laboratory phenotyped/genotyped before changes in field performance are detected. The relationships demonstrated here between laboratory tests and field performance could aid translation between laboratory and field for other fungicide groups. © 2017 Society of Chemical Industry.

The Point Mutation G461S in the MfCYP51 Gene is Associated with Tebuconazole Resistance in Monilinia fructicola Populations in Brazil

The ascomycete Monilinia fructicola is the causal agent of brown rot of stone fruit in Brazil, causing major pre- and postharvest losses. For many years, the demethylation inhibitor (DMI) fungicide tebuconazole has been used as the most effective active ingredient for controlling brown rot and, as a result, strains of M. fructicola resistant to this ingredient have emerged in many Brazilian states producing stone fruit. The aim of this study was to investigate the mechanisms associated with the resistance of M. fructicola to DMI tebuconazole. By sequencing the M. fructicola CYP51 (MfCYP51) gene, encoding the azole target sterol 14α-demethylase, a mutation was identified at the nucleotide position 1,492, causing the amino acid substitution from glycine to serine at the codon position 461, associated with reduced tebuconazole sensitivity. In addition, it was observed that MfCYP51 gene expression could play a secondary role in DMI fungicide resistance of M. fructicola strains in Brazil. However, for the specific isolate found to exhibit elevated expression levels of MfCYP51, no insertions that would trigger gene expression were found. Based on the point mutation associated with tebuconazole resistance, an allele-specific polymerase chain reaction method was developed to quickly identify resistant genotypes within the Brazilian population. This is the first report determining molecular mechanisms for DMI resistance identification for M. fructicola isolates from Brazil. This information provides an important advancement for risk assessment of DMI fungicides used to manage brown rot of stone fruit.

Mutations in the CYP51 gene reduce DMI sensitivity in Parastagonospora nodorum populations in Europe and China

BACKGROUND

Sterol demethylation inhibitors (DMIs) have been widely used to manage agronomically important fungal diseases in wheat, but reports of DMI-resistant pathogens continue to mount. Parastagonospora nodorum shows a wide range of sensitivity to DMIs, but until now no molecular mechanisms have been identified to explain these differences. The aim of this study was to correlate the DMI sensitivity of a global collection of P. nodorum isolates with mutations in the CYP51 gene that encodes the target of DMI fungicides.

RESULTS

Two non-synonymous mutations connected to DMI resistance in other plant pathogenic fungi were detected for the first time in the CYP51 gene of P. nodorum. The two mutations occurred at amino acid position 144, which is homologous to position 137 in other pathogens. The Y144F mutation was detected in China, Denmark, Sweden and Switzerland while the Y144H mutation was found in China and Switzerland. Both mutations were correlated with significantly reduced sensitivity to the DMI fungicide propiconazole.

CONCLUSION

CYP51 mutations conferred reduced sensitivity against DMIs in field populations of P. nodorum originating from China, Denmark, Sweden and Switzerland. © 2016 Society of Chemical Industry.

Triazole Resistance in Aspergillus Species: An Emerging Problem

Aspergillus species are ubiquitous fungal saprophytes found in diverse ecological niches worldwide. Among them, Aspergillus fumigatus is the most prevalent and is largely responsible for the increased incidence of invasive aspergillosis with high mortality rates in some immunocompromised hosts. Azoles are the first-line drugs in treating diseases caused by Aspergillus spp. However, increasing reports in A. fumigatus azole resistance, both in the clinical setting and in the environment, are threatening the effectiveness of clinical and agricultural azole drugs. The azole target is the 14-α sterol demethylase encoded by cyp51A gene and the main mechanisms of resistance involve the integration of tandem repeats in its promoter and/or single point mutations in this gene. In A. fumigatus, azole resistance can emerge in two different scenarios: a medical route in which azole resistance is generated during long periods of azole treatment in the clinical setting and a route of resistance derived from environmental origin due to extended use of demethylation inhibitors in agriculture. The understanding of A. fumigatus azole resistance development and its evolution is needed in order to prevent or minimize its impact. In this article, we review the current situation of azole resistance epidemiology and the predominant molecular mechanisms described based on the resistance acquisition routes. In addition, the clinical implications of A. fumigatus azole resistance and future research are discussed.

Homozygous and heterozygous point mutations in succinate dehydrogenase subunits b, c and d of Rhizoctonia cerealis conferring resistance to thifluzamide

BACKGROUND

Thifluzamide, a succinate dehydrogenase inhibitor (SDHI) fungicide, is a promising fungicide for controlling wheat sharp eyespot (WSE). WSE is caused by Rhizoctonia cerealis. Information on the resistance mechanism of this pathogen to thifluzamide remains unavailable.

RESULTS

We used selective reculturing and UV mutagenesis to generate thifluzamide-resistant mutants. Thifluzamide-resistant mutants were only generated through UV mutagenesis. Sequence analysis of succinate dehydrogenase (Sdh) genes revealed that two mutants had no mutation in RCSdhB, RCSdhC and RCSdhD, and the other 18 mutants all had at least one mutation in RCSdhB, RCSdhC or RCSdhD, either in a homozygous or heterozygous state. The majority of mutants included either RCSdhD-H116Y or RCSdhC-H139Y. They showed slight resistance to boscalid, bixafen and penflufen. Only one mutant possessed RCSdhB-H246Y, and it showed medium resistance to boscalid and penflufen and a slight resistance to bixafen. All the thifluzamide mutants were sensitive to flutolanil. Compared with their parental isolates, these mutants present no or minor fitness penalties.

CONCLUSION

Homozygous and heterozygous point mutations in the succinate dehydrogenase subunits b, c and d of R. cerealis may be involved in thifluzamide resistance. © 2016 Society of Chemical Industry.

Temporal Dynamics and Spatial Variation of Azoxystrobin and Propiconazole Resistance in Zymoseptoria tritici: A Hierarchical Survey of Commercial Winter Wheat Fields in the Willamette Valley, Oregon

Fungicide resistance can cause disease control failure in agricultural systems, and is particularly concerning with Zymoseptoria tritici, the causal agent of Septoria tritici blotch of wheat. In North America, the first quinone outside inhibitor resistance in Z. tritici was discovered in the Willamette Valley of Oregon in 2012, which prompted this hierarchical survey of commercial winter wheat fields to monitor azoxystrobin- and propiconazole-resistant Z. tritici. Surveys were conducted in June 2014, January 2015, May 2015, and January 2016. The survey was organized in a hierarchical scheme: regions within the Willamette Valley, fields within the region, transects within the field, and samples within the transect. Overall, frequency of azoxystrobin-resistant isolates increased from 63 to 93% from June 2014 to January 2016. Resistance to azoxystrobin increased over time even within fields receiving no strobilurin applications. Propiconazole sensitivity varied over the course of the study but, overall, did not significantly change. Sensitivity to both fungicides showed no regional aggregation within the Willamette Valley. Greater than 80% of spatial variation in fungicide sensitivity was at the smallest hierarchical scale (within the transect) of the survey for both fungicides, and the resistance phenotypes were randomly distributed within sampled fields. Results suggest a need for a better understanding of the dynamics of fungicide resistance at the landscape level.

Rapid emergence of pathogens in agro-ecosystems: global threats to agricultural sustainability and food security

Agricultural ecosystems are composed of genetically depauperate populations of crop plants grown at a high density and over large spatial scales, with the regional composition of crop species changing little from year to year. These environments are highly conducive for the emergence and dissemination of pathogens. The uniform host populations facilitate the specialization of pathogens to particular crop cultivars and allow the build-up of large population sizes. Population genetic and genomic studies have shed light on the evolutionary mechanisms underlying speciation processes, adaptive evolution and long-distance dispersal of highly damaging pathogens in agro-ecosystems. These studies document the speed with which pathogens evolve to overcome crop resistance genes and pesticides. They also show that crop pathogens can be disseminated very quickly across and among continents through human activities. In this review, we discuss how the peculiar architecture of agro-ecosystems facilitates pathogen emergence, evolution and dispersal. We present four example pathosystems that illustrate both pathogen specialization and pathogen speciation, including different time frames for emergence and different mechanisms underlying the emergence process. Lastly, we argue for a re-design of agro-ecosystems that embraces the concept of dynamic diversity to improve their resilience to pathogens. This article is part of the themed issue 'Tackling emerging fungal threats to animal health, food security and ecosystem resilience'.

Structural and Functional Elucidation of Yeast Lanosterol 14α-Demethylase in Complex with Agrochemical Antifungals

Azole antifungals, known as demethylase inhibitors (DMIs), target sterol 14α-demethylase (CYP51) in the ergosterol biosynthetic pathway of fungal pathogens of both plants and humans. DMIs remain the treatment of choice in crop protection against a wide range of fungal phytopathogens that have the potential to reduce crop yields and threaten food security. We used a yeast membrane protein expression system to overexpress recombinant hexahistidine-tagged S. cerevisiae lanosterol 14α-demethylase and the Y140F or Y140H mutants of this enzyme as surrogates in order characterize interactions with DMIs. The whole-cell antifungal activity (MIC50 values) of both the R- and S-enantiomers of tebuconazole, prothioconazole (PTZ), prothioconazole-desthio, and oxo-prothioconazole (oxo-PTZ) as well as for fluquinconazole, prochloraz and a racemic mixture of difenoconazole were determined. In vitro binding studies with the affinity purified enzyme were used to show tight type II binding to the yeast enzyme for all compounds tested except PTZ and oxo-PTZ. High resolution X-ray crystal structures of ScErg11p6×His in complex with seven DMIs, including four enantiomers, reveal triazole-mediated coordination of all compounds and the specific orientation of compounds within the relatively hydrophobic binding site. Comparison with CYP51 structures from fungal pathogens including Candida albicans, Candida glabrata and Aspergillus fumigatus provides strong evidence for a highly conserved CYP51 structure including the drug binding site. The structures obtained using S. cerevisiae lanosterol 14α-demethylase in complex with these agrochemicals provide the basis for understanding the impact of mutations on azole susceptibility and a platform for the structure-directed design of the next-generation of DMIs.