Cross-resistance between myclobutanil and tebuconazole and the genetic basis of tebuconazole resistance in Venturia inaequalis

Deciphering the mechanisms involved in reduced sensitivity to azoles and fengycin lipopeptide in Venturia inaequalis

Apple scab, caused by the hemibiotrophic fungus Venturia inaequalis, is currently the most common and damaging disease in apple orchards. Two strains of V. inaequalis (S755 and Rs552) with different sensitivities to azole fungicides and the bacterial metabolite fengycin were compared to determine the mechanisms responsible for these differences. Antifungal activity tests showed that Rs552 had reduced sensitivity to tebuconazole and tetraconazole, as well as to fengycin alone or in a binary mixture with other lipopeptides (iturin A, pumilacidin, lichenysin). S755 was highly sensitive to fengycin, whose activity was close to that of tebuconazole. Unlike fengycin, lipopeptides from the iturin family (mycosubtilin, iturin A) had similar activity on both strains, while those from the surfactin family (lichenysin, pumilacidin) were not active, except in binary mixtures with fengycin. The activity of lipopeptides varies according to their family and structure. Analyses to determine the difference in sensitivity to azoles (which target the CYP51 enzyme involved in the ergosterol biosynthesis pathway) showed that the reduced sensitivity in Rs552 is linked to (i) a constitutive increased expression of the Cyp51A gene caused by insertions in the upstream region and (ii) greater efflux by membrane pumps with the involvement of ABC transporters. Microscopic observations revealed that fengycin, known to interact with plasma membranes, induced morphological and cytological changes in cells from both strains. Sterol and phospholipid analyses showed a higher level of ergosta-7,22-dien-3-ol and a lower level of PI(C16:0/C18:1) in Rs552 compared with S755. These differences could therefore influence the composition of the plasma membrane and explain the differential sensitivity of the strains to fengycin. However, the similar antifungal activities of mycosubtilin and iturin A in the two strains indirectly indicate that sterols are probably not involved in the fengycin resistance mechanism. This leads to the conclusion that different mechanisms are responsible for the difference in susceptibility to azoles or fengycin in the strains studied.

Dose-Dependent Genetic Resistance to Azole Fungicides Found in the Apple Scab Pathogen

The evolution of azole resistance in fungal pathogens presents a major challenge in both crop production and human health. Apple orchards across the world are faced with the emergence of azole fungicide resistance in the apple scab pathogen Venturia inaequalis. Target site point mutations observed in this fungus to date cannot fully explain the reduction in sensitivity to azole fungicides. Here, polygenic resistance to tebuconazole was studied across a population of V. inaequalis. Genotyping by sequencing allowed Quantitative Trait Loci (QTLs) mapping to identify the genetic components controlling this fungicide resistance. Dose-dependent genetic resistance was identified, with distinct genetic components contributing to fungicide resistance at different exposure levels. A QTL within linkage group seven explained 65% of the variation in the effective dose required to reduce growth by 50% (ED50). This locus was also involved in resistance at lower fungicide doses (ED10). A second QTL in linkage group one was associated with dose-dependent resistance, explaining 34% of variation at low fungicide doses (ED10), but did not contribute to resistance at higher doses (ED50 and ED90). Within QTL regions, non-synonymous mutations were observed in several ATP-Binding Cassette and Major Facilitator SuperFamily transporter genes. These findings provide insight into the mechanisms of fungicide resistance that have evolved in horticultural pathogens. Identification of resistance gene candidates supports the development of molecular diagnostics to inform management practices.

Field Evaluation of Fungicides for the Management of Neofabraea Leaf Lesion of Olive in California

Field experiments were conducted during the fall-winter seasons of 2017 to 2018 and 2018 to 2019 to evaluate the efficacy of various fungicides to control Neofabraea leaf lesion of olive. Field trials were conducted in the highly susceptible cultivar Arbosana in a commercial, super-high-density orchard in San Joaquin County, California. Up to eight fungicidal products were applied using an air blast backpack sprayer, and their efficacy was compared with different application strategies. Results showed that most products were effective in reducing infection by the pathogens and limiting disease severity. Overall, best disease control was achieved by thiophanate-methyl, cyprodinil, difenoconazole + cyprodinil, and chlorothalonil, providing up to 75% reduction in disease severity. Copper hydroxide did not control the disease. In 2018 to 2019, the fungicides difenoconazole + cyprodinil and ziram were evaluated in additional field trials using different application strategies (single, dual, and combined applications) suitable for pathogen resistance management. Results showed that both products provided significant reduction in disease severity (∼50%), although no differences in efficacy were found between the two products nor between the different application strategies. Both products performed equally using one or two applications at 2-week intervals following harvest.

Ferulic acid attenuated difenoconazole-induced immunotoxicity in carp by inhibiting TRAF/TAK1/NF-κB, Nrf2 and p53 pathways

Difenoconazole (DFZ) is a classical triazole fungicide that causes immunosuppression in non-target organisms. Ferulic acid (FA) is a polyphenolic molecule found in nature that has antioxidant and anti-inflammatory activities. The purpose of this investigation was to see if FA could prevent DFZ-induced immunosuppression and to identify the potential mechanisms. Carp were exposed to 1/10 LC50 of DFZ as well as fed normal feed or feed containing dietary additive FA for 30 d. It was found that DFZ-induced immunosuppression could be improved by FA, as evidenced by upregulation of Hb, C3 and IgM and downregulation of LDH. It was then investigated that FA could ameliorate DFZ-induced splenic injury through p53-mediated apoptosis. At the same time, enhancing the levels of CAT, GSH and T-AOC in spleen and transcription levels Nrf2 signaling pathway related genes indicated that FA reduced oxidative damage caused by DFZ by blocking the Nrf2 signaling pathway. In addition, FA inhibited the inflammatory response triggered by TRAF/TAK1/NF-κB signaling pathway, downregulated the transcript levels of pro-inflammatory factors (il-1β, tnf-α, il-6) and the level of NLRP3 inflammasome (NRLP3, ASC, Caspase 1), and upregulated the transcript levels of anti-inflammatory factors (tgf-β1, il-10). In conclusion, the above results suggested that FA mediated TRAF/TAK1/NF-κB, Nrf2, and p53 pathways to attenuate DFZ-induced inflammation, oxidative stress, and apoptosis thereby enhancing the immune capacity of carp.

Mixture toxicities of tetrachlorantraniliprole and tebuconazole to honey bees (Apis mellifera L.) and the potential mechanism

The extensive use of pesticides has negative effects on the health of insect pollinators. Although pollinators in the field are seldom exposed to individual pesticides, few reports have assessed the toxic impacts of pesticide combinations on them. In this work, we purposed to reveal the combined impacts of tetrachlorantraniliprole (TET) and tebuconazole (TEB) on honey bees (Apis mellifera L.). Our data exhibited that TET had greater toxicity to A. mellifera (96-h LC₅₀ value of 298.2 mg a.i. L^-1) than TEB (96-h LC₅₀ value of 1,841 mg a.i. L^-1). The mixture of TET and TEB displayed acute synergistic toxicity to the pollinators. Meanwhile, the activities of CarE, CYP450, trypsin, and sucrase, as well as the expressions of five genes (ppo, abaecin, cat, CYP4G11, and CYP6AS14) associated with immune response, oxidative stress, and detoxification metabolism, were conspicuously altered when exposed to the mixture relative to the individual exposures. These results provided an overall comprehension of honey bees upon the challenge of sublethal toxicity between neonicotinoid insecticides and triazole fungicides and could be used to assess the intricate toxic mechanisms in honey bees when exposed to pesticide mixtures. Additionally, these results might guide pesticide regulation strategies to enhance the honey bee populations.

The importance of selecting crystal form for triazole fungicide tebuconazole to enhance its botryticidal activity

The growing evidences of resistant fungi stimulate fully understanding tebuconazole regarding its crystal structure on fungicidal activity. In this study, the crystal structures of six technical tebuconazoles (BX, HH, JP, QZ, SJ, and YT) were characterized by using high-resolution X-ray powder diffraction and three-dimensional crystal structure modeling. A structure-activity relationship of the tebuconazoles on the susceptible (HLS and YJS) or resistant (XHR) Botrytis cinerea isolates was analyzed, the differential tricarboxylic acid (TCA) cycle metabolism was determined, and molecular docking with sterol 14α-demethylase (CYP51) was performed. The results showed that tebuconazole existed in three types of crystal forms: an overlapping-pair conformation, a side-by-side-pair conformation, and a parallel-pair conformation. QZ with the parallel-pair conformation and the minimum crystal cell volume exhibited a higher activity and a lower resistant level. XHR possessed a higher content of TCA cycle metabolites and phosphate than YJS, but the exposure to QZ significantly reduced the contents of citrate, isocitrate, α-ketoglutarate and oxaloacetate in XHR, as did the exposure to other technical tebuconazoles. Moreover, the point mutations F487L, G464S, and G443S altered the binding properties of chiral stereoscopic R-QZ with CYP51 protein. Especially the G443S mutation promoted a weak linking of R-QZ with LEU380 and TYR126, and greatly slashed the binding action at lower docking score. In conclusion, our results evidenced an efficient crystal conformation of tebuconazole to improve botryticidal activity and a potential adaptability of B. cinerea to tebuconazole inhibition in TCA cycle metabolism and CYP51 protein mutation.

Reduced Sensitivity to Tebuconazole in Botryosphaeria dothidea Isolates Collected from Major Apple Production Areas of China

China has the largest acreage and is the greatest producer of apples in the world. Apple ring rot (ARR) caused by Botryosphaeria dothidea is one of the major diseases affecting apple fruit production. Tebuconazole, a sterol demethylation-inhibiting (DMI) fungicide, has been increasingly applied to manage ARR in China. Resistance to tebuconazole in field populations of B. dothidea may be selected and accumulate to higher degrees over time. Establishment of tebuconazole sensitivity monitoring programs is critically important for resistance management and effective ARR control. However, the resistance level of B. dothidea to tebuconazole in China remains largely unknown. In this study, in vitro mycelial growth assays of B. dothidea in media amended with tebuconazole were conducted, and the sensitivity of B. dothidea to tebuconazole was determined with a set of 390 isolates collected from the major apple production provinces in China between 2006 and 2014. Results showed that the 50% effective concentration (EC₅₀) value ranged from 0.011 to 0.918, 0.040 to 1.621, and 0.052 to 1.925 μg ml^-1 with a median value of 0.194, 0.386, and 0.782 μg ml^-1 in the isolates collected in 2006, 2010, and 2014, respectively. The frequency distribution of EC₅₀ for tebuconazole was a nonnormal distribution (P < 0.05), suggesting that subpopulations with reduced sensitivity to tebuconazole had emerged in these B. dothidea isolates. The frequency distribution of the B. dothidea isolates collected in 2006 fit a unimodal curve and could be regarded as the baseline sensitivity to tebuconazole. The resistance levels increased over time with the average occurrence frequency of 43.3% and resistance index of 0.38. Positive cross-resistance was observed between tebuconazole and metconazole, which is another DMI fungicide, but multiple resistance was not detected between tebuconazole and non-DMI fungicides. Our results demonstrated that regular long-term resistance monitoring combining with prudent fungicide use should be implemented to prolong the lifespan of tebuconazole in management of ARR in apples.

Assessment of Lipopeptide Mixtures Produced by Bacillus subtilis as Biocontrol Products against Apple Scab (Venturia inaequalis)

Apple scab is an important disease conventionally controlled by chemical fungicides, which should be replaced by more environmentally friendly alternatives. One of these alternatives could be the use of lipopeptides produced by Bacillus subtilis. The objective of this work is to study the action of the three families of lipopeptides and different mixtures of them in vitro and in vivo against Venturia inaequalis. Firstly, the antifungal activity of mycosubtilin/surfactin and fengycin/surfactin mixtures was determined in vitro by measuring the median inhibitory concentration. Then, the best lipopeptide mixture ratio was produced using Design of Experiment (DoE) to optimize the composition of the culture medium. Finally, the lipopeptides mixtures efficiency against V. inaequalis was assessed in orchards as well as the evaluation of the persistence of lipopeptides on apple. In vitro tests show that the use of fengycin or mycosubtilin alone is as effective as a mixture, with the 50–50% fengycin/surfactin mixture being the most effective. Optimization of culture medium for the production of fengycin/surfactin mixture shows that the best composition is glycerol coupled with glutamic acid. Finally, lipopeptides showed in vivo antifungal efficiency against V. inaequalis regardless of the mixture used with a 70% reduction in the incidence of scab for both mixtures (fengycin/surfactin or mycosubtilin/surfactin). The reproducibility of the results over the two trial campaigns was significantly better with the mycosubtilin/surfactin mixture. The use of B. subtilis lipopeptides to control this disease is very promising.

Novel Dioxolane Ring Compounds for the Management of Phytopathogen Diseases as Ergosterol Biosynthesis Inhibitors: Synthesis, Biological Activities, and Molecular Docking

Thirty novel dioxolane ring compounds were designed and synthesized. Their chemical structures were confirmed by ¹H NMR, HRMS, and single crystal X-ray diffraction analysis. Bioassays indicated that these dioxolane ring derivatives exhibited excellent fungicidal activity against Rhizoctonia solani, Pyricularia oryae, Botrytis cinerea, Colletotrichum gloeosporioides, Fusarium oxysporum, Physalospora piricola, Cercospora arachidicola and herbicidal activity against lettuce (Lactuca sativa), bentgrass (Agrostis stolonifera), and duckweed (Lemna pausicostata). Among these compounds, 1-((2-(4-chlorophenyl)-5-methyl-1,3-dioxan-2-yl)methyl)-1H-1,2,4-triazole (D17), 1-(((4R)-2-(4-chlorophenyl)-4-methyl-1,3-dioxolan-2-yl)methyl)-1H-1,2,4-triazole (D20), 1-((5-methyl-2-(4-(trifluoromethyl)phenyl)-1,3-dioxan-2-yl)methyl)-1H-1,2,4-triazole (D22), and 1-((2-(4-fluorophenyl)-1,3-dioxolan-2-yl)methyl)-1H-1,2,4-triazole (D26) had broad spectrum fungicidal and herbicidal activity. The IC₅₀ values against duckweed were 20.5 ± 9.0, 14.2 ± 6.7, 24.0 ± 11.0, 8.7 ± 3.5, and 8.0 ± 3.1 μM for D17, D20, D22, and D26 and the positive control difenoconazole, respectively. The EC₅₀ values were 7.31 ± 0.67, 9.74 ± 0.83, 17.32 ± 1.23, 11.96 ± 0.98, and 8.93 ± 0.91 mg/L for D17, D20, D22, and D26 and the positive control difenoconazole against the plant pathogen R. solani, respectively. Germination experiments with Arabidopsis seeds indicated that the target of these dioxolane ring compounds in plants is brassinosteroid biosynthesis. Molecular simulation docking results of compound D26 and difenoconazole with fungal CYP51 P450 confirmed that they both inhibit this enzyme involved in ergosterol biosynthesis. The structure-activity relationships (SAR) are discussed by substituent effect, molecular docking, and density functional theory analysis, which provided useful information for designing more active compounds.

Insights into the effects of difenoconazole on the livers in male mice at the biochemical and transcriptomic levels

Difenoconazole (DFZ) is a broad-spectrum triazole fungicide, that is extensively used in agriculture. Studies have shown that residues of DFZ and other fungicides have toxic effects on nontarget organisms. However, its hepatoxicity in mammals remains unclear. Here, we characterized the toxic hepatic effects in male C57BL/6 mice exposed to 30 and 100 mg/kg bw DFZ for 14 and 56 days, respectively. The results revealed that DFZ could increase the relative liver weights, however, the relative fat and spleen weights decreased. More importantly, DFZ exposure changed the hepatic morphology and induced hepatic oxidative stress. Gene expression analysis suggested that DFZ could induce a glycolipid metabolism disorder. Moreover, hepatic transcriptomic analysis revealed the effects of DFZ exposure on the transcriptional levels of various genes, and enrichment analysis of differentially expressed genes (DEGs) showed that energy metabolism and immune-associated pathways were mainly affected. We validated the results from transcriptomic analysis and found that some key genes related to energy metabolism were affected. In addition, flow cytometry showed that the CD3+/CD4+ and CD3+ /CD8+ levels declined in the spleen of mice. Taken together, these findings combined with transcriptome analysis highlighted that DFZ caused different endpoints in the liver, which could provide more evidence for investigating the toxic effects of DFZ in mammals.

Bacterial Endophyte Community Dynamics in Apple (Malus domestica Borkh.) Germplasm and Their Evaluation for Scab Management Strategies

The large genetic evolution due to the sexual reproduction-mediated gene assortments and propensities has made Venturia inaequalis (causing apple scab) unique with respect to its management strategies. The resistance in apple germplasm against the scab, being controlled for by more than fifteen genes, has limited gene alteration-based investigations. Therefore, a biological approach of bacterial endophyte community dynamics was envisioned across the apple germplasm in context to the fungistatic behavior against V. inaequalis. A total of 155 colonies of bacterial endophytes were isolated from various plant parts of the apple, comprising 19 varieties, and after screening for antifungal behavior followed by morphological, ARDRA, and sequence analysis, a total of 71 isolates were selected for this study. The alpha diversity indices were seen to fluctuate greatly among the isolation samples in context to microflora with antifungal behavior. As all the isolates were screened for the presence of various metabolites and some relevant genes that directly or indirectly influence the fungistatic behavior of the isolated microflora, a huge variation among the isolated microflora was observed. The outstanding isolates showing highest percentage growth inhibition of V. inaequalis were exploited to raise a bio-formulation, which was tested against the scab prevalence in eight apple varieties under controlled growth conditions. The formulation at all the concentrations caused considerable reductions in both the disease severity and disease incidence in all the tested apple varieties. Red Delicious being most important cultivar of the northwestern Himalayas was further investigated for its biochemical behavior in formulation and the investigation revealed different levels of enzyme production, chlorophyll, and sugars against the non-inoculated control.

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.