Sensitivity of Different Developmental Stages and Resistance Risk Assessment of Phytophthora capsici to Fluopicolide in China

Deciphering fungicide resistance in Phytophthora: mechanisms, prevalence, and sustainable management approaches

The genus Phytophthora contains more than 100 plant pathogenic species that parasitize a wide range of plants, including economically important fruits, vegetables, cereals, and forest trees, causing significant losses. Global agriculture is seriously threatened by fungicide resistance in Phytophthora species, which makes it imperative to fully comprehend the mechanisms, frequency, and non-chemical management techniques related to resistance mutations. The mechanisms behind fungicide resistance, such as target-site mutations, efflux pump overexpression, overexpression of target genes and metabolic detoxification routes for fungicides routinely used against Phytophthora species, are thoroughly examined in this review. Additionally, it assesses the frequency of resistance mutations in various Phytophthora species and geographical areas, emphasizing the rise of strains that are resistant to multiple drugs. The effectiveness of non-chemical management techniques, including biological control, host resistance, integrated pest management plans, and cultural practices, in reducing fungicide resistance is also thoroughly evaluated. The study provides important insights for future research and the development of sustainable disease management strategies to counter fungicide resistance in Phytophthora species by synthesizing current information and identifying knowledge gaps.

Two point mutations N771S and K847N in the VHA-a of Phytophthora litchii confer resistance to fluopimomide

The phytopathogenic oomycete Phytophthora litchii is the culprit behind the devastating disease known as "litchi downy blight", which causes large losses in litchi production. Although fluopimomide exhibits strong inhibitory efficacy against P. litchii, the exact mechanism of resistance is still unknown. The sensitivity of 137 P. litchii isolates to fluopimomide was assessed, and it was discovered that the median effective concentration (EC50) of the fungicide had a unimodal frequency distribution with a mean value of 0.763 ± 0.922 μg/mL. Comparing the resistant mutants to the equivalent parental isolates, the resistance mutants' survival fitness was much lower. While there was no cross-resistance between fluopimomide and other oomycete inhibitors, there is a notable positive cross-resistance between fluopimomide and fluopicolide. According to the thorough investigation, P. litchii had a moderate chance of developing fluopimomide resistance. The point mutations N771S and K847N in the VHA-a of P. litchii (PlVHA-a) were present in the fluopimomide-resistant mutants, and the two point mutations in PlVHA-a conferring fluopimomide resistance were verified by site-directed mutagenesis in the sensitive P. capsici isolate BYA5 and molecular docking.

Characteristics of fluopicolide-resistance mutants in Phytophthora nicotianae, the pathogen causing black shank disease in tobacco

Black shank, a devastating disease in tobacco production worldwide, is caused by the oomycete plant pathogen Phytophthora nicotianae. Fluopicolide is a pyridinylmethyl-benzamides fungicide with a unique mechanism of action and has been widely used for controlling a variety of oomycetes such as Plasmopara viticola, Phytophthora infestans, Pseudoperonospora cubensis, P. nicotianae and Bremia lactucae. However, the fluopicolide-resistance risk and molecular basis in P. nicotianae have not been reported. In this study, the sensitivity profile of 141 P. nicotianae strains to fluopicolide was determined, with a mean median effective concentration (EC50) value of 0.12 ± 0.06μg/mL. Five stable fluopicolide-resistant mutants of P. nicotianae were obtained by fungicide adaptation, and the compound fitness index of these resistant mutants were lower than that of their parental isolates. Additionally, cross-resistance tests indicated that the sensitivity of fluopicolide did not correlate with other oomycete fungicides, apart from fluopimomide. DNA sequencing revealed two point mutations, G765E and N769Y, in the PpVHA-a protein in the fluopicolide-resistant mutants. Transformation and expression of PpVHA-a genes carrying G765E and N769Y in the sensitive wild-type isolate confirmed that it was responsible for fluopicolide resistance. These results suggest that P. nicotianae has a low to medium resistance risk to fluopicolide in laboratory and that point mutations, G765E and N769Y, in PpVHA-a are associated with the observed fluopicolide resistance.

Potential anti-Pythium insidiosum therapeutics identified through screening of agricultural fungicides

Pythiosis is a life-threatening infectious disease caused by the oomycete Pythium insidiosum. Clinical manifestations of pythiosis include an eye, blood vessel, skin, or gastrointestinal tract infection. Pythiosis has been increasingly reported worldwide, with an overall mortality rate of 28%. Radical surgery is required to save patients' lives due to the limited efficacy of antimicrobial drugs. Effective medical treatments are urgently needed for pythiosis. This study aims to find anti-P. insidiosum agents by screening 17 agricultural fungicides that inhibit plant-pathogenic oomycetes and validating their efficacy and safety. Cyazofamid outperformed other fungicides as it can potently inhibit genetically diverse P. insidiosum isolates while exhibiting minimal cellular toxicities. The calculated therapeutic scores determined that the concentration of cyazofamid causing significant cellular toxicities was eight times greater than the concentration of the drug effectively inhibiting P. insidiosum. Furthermore, other studies showed that cyazofamid exhibits low-to-moderate toxicities in animals. The mechanism of cyazofamid action is likely the inhibition of cytochrome b, an essential component in ATP synthesis. Molecular docking and dynamic analyses depicted a stable binding of cyazofamid to the Qi site of the P. insidiosum's cytochrome b orthologous protein. In conclusion, our search for an effective anti-P. insidiosum drug indicated that cyazofamid is a promising candidate for treating pythiosis. With its high efficacy and low toxicity, cyazofamid is a potential chemical for treating pythiosis, reducing the need for radical surgeries, and improving recovery rates. Our findings could pave the way for the development of new and effective treatments for pythiosis.IMPORTANCEPythiosis is a severe infection caused by Pythium insidiosum. The disease is prevalent in tropical/subtropical regions. This infectious condition is challenging to treat with antifungal drugs and often requires surgical removal of the infected tissue. Pythiosis can be fatal if not treated promptly. There is a need for a new treatment that effectively inhibits P. insidiosum. This study screened 17 agricultural fungicides that target plant-pathogenic oomycetes and found that cyazofamid was the most potent in inhibiting P. insidiosum. Cyazofamid showed low toxicity to mammalian cells and high affinity to the P. insidiosum's cytochrome b, which is involved in energy production. Cyazofamid could be a promising candidate for the treatment of pythiosis, as it could reduce the need for surgery and improve the survival rate of patients. This study provides valuable insights into the biology and drug susceptibility of P. insidiosum and opens new avenues for developing effective therapies for pythiosis.

Characterizing the Dynamics of Virulence and Fungicide Resistance of Phytophthora capsici in Michigan Vegetable Fields Reveals Loci Associated with Virulence

The oomycete Phytophthora capsici is a destructive pathogen infecting more than 50 plant species and is one of the most serious threats to cucurbit production. Phytophthora blight caused by Phytophthora capsici can affect all plant growth stages, and fungicides and cultural controls are used to limit losses. Dissecting pathogen virulence and fungicide resistance can provide insights into pathogenic mechanisms and inform effective management practices to control P. capsici. In this study, we assessed virulence, mefenoxam sensitivity, and genetic diversity of nine P. capsici populations collected from Cucurbitaceae, Solanaceae, and Fabaceae host families in Michigan from 2002 to 2016. We developed 992 simple sequence repeats (SSRs) in the P. capsici genome and identified 60 SSRs located within or close to RXLR-class (Arginine-any amino acid-Leucine-Arginine) effectors and 29 SSRs within or close to effector CRN (CRinkling and Necrosis) family protein, which represent 62 RXLR and 34 putative CRNs. Population structure analysis shows that mefenoxam resistance was not associated with the year of collection, host type, or location, but there were significant differences in virulence among the populations. Using the general linear model and mixed linear model-based association analyses with all effector-related SSR markers, we identified four SSR markers significantly associated with at least one of the virulence-related parameters. Of these, one (Pce_SC18) was in a predicted CRN effector and had high identity with the putative PhCRN37 effector in the pathogen Plasmopara halstedii, which can be further verified for virulence identification in P. capsici.

Unveiling Vacuolar H+-ATPase Subunit a as the Primary Target of the Pyridinylmethyl-Benzamide Fungicide, Fluopicolide

An estimated 240 fungicides are presently in use, but the direct targets for the majority remain elusive, constraining fungicide development and efficient resistance monitoring. In this study, we found that Pcα-actinin knockout did not influence the sensitivity of Phytophthora capsici to fluopicolide, which is a notable oomycete inhibitor. Using a combination of Bulk Segregant Analysis Sequencing and Drug Affinity Responsive Target Stability (DARTS) assays, the vacuolar H+-ATPase subunit a (PcVHA-a) was pinpointed as the target protein of fluopicolide. We also confirmed four distinct point mutations in PcVHA-a responsible for fluopicolide resistance in P. capsici through site-directed mutagenesis. Molecular docking, ATPase activity assays, and a DARTS assay suggested a fluopicolide-PcVHA-a interaction. Sequence analysis and further molecular docking validated the specificity of fluopicolide for oomycetes or fish. These findings support the claim that PcVHA-a is the target of fluopicolide, proposing vacuolar H+-ATPase as a promising target for novel fungicide development.

Three point mutations in AaCYP51 combined with induced overexpression of AaCYP51 conferred low-level resistance to mefentrifluconazole in Alternaria alternata

Tomato early blight is a significant disease that causes substantial losses to tomato yield and quality. Mefentrifluconazole, an isopropanol-azole subgroup of triazole fungicides, has been registered in China for controlling various plant diseases, including tomato early blight, grape anthracnose, and apple brown spot. However, limited information is available on the mefentrifluconazole resistance risk and mechanism in plant pathogens. The sensitivity to mefentrifluconazole of 122 isolates of Alternaria alternata, one of the causal agents of tomato early blight, collected from different provinces in China, was evaluated. The results showed a unimodal curve for the sensitivity frequency, with an average EC50 of 0.306 μg/mL. Through fungicide adaption, six resistant mutants (N4, N5, T4, T5, NG1, and NG10) were obtained from three parental isolates, with a mutation frequency of 3.28 × 10-4 and resistance factors ranging between 19 and 147. The survival fitness of the resistant mutants, except for NG1, was significantly lower than that of their parental isolates. Positive cross-resistance was observed between mefentrifluconazole and difenoconazole or fenbuconazole, whereas no cross-resistance was found with three non-DMI fungicides. Furthermore, three distinct point mutations were detected in the AaCYP51 protein of the resistant mutants: I300S in T4 and T5; A303T in N4, NG1, and NG10; and A303V in N5. Compared to the parental isolates, the AaCYP51 gene was overexpressed in all six resistant mutants when treated with mefentrifluconazole. In summary, the resistance risk of A. alternata to mefentrifluconazole was low, and point mutations and overexpression of the AaCYP51 gene were identified as contributing factors to mefentrifluconazole resistance in A. alternata.

Fluopicolide-Induced Oxidative Stress and DNA Damage in the Earthworm Eisenia foetida

Fluopicolide is a new benzamide fungicide with a unique mechanism of action and is toxic to some non-target organisms. However, there is a lack of research on the chronic toxicity of fluopicolide to earthworms. In this study, in order to evaluate the chronic toxicity of fluopicolide to earthworms, the levels of reactive oxygen species (ROS) and malondialdehyde (MDA), the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferase (GST), and DNA oxidative damage (8-hyoxy-2-deoxyguanosine content) in earthworms were measured at 7, 14, 21, and 28 days after exposure to different concentrations (0, 0.1, 0.5, 1, 2.5, 5, and 10 mg/kg) of fluopicolide. In most treatment groups, the ROS levels increased significantly 7 days after exposure and then decreased gradually with an increase in exposure time, a certain dose-effect relationship. The antioxidant enzymes' activities (SOD and CAT) in most treatment groups were activated, showing an increasing trend at first and then a decreasing trend; however, the CAT activity in the high-concentration treatment group was inhibited 21 days after exposure. The GST activity and MDA content showed an increasing trend at first and then a decreasing trend, which was dependent on the dose. As a biomarker of DNA damage, the 8-OHdG content was positively correlated with the concentration of fluopicolide. The results showed that a low dose of fluopicolide could cause oxidative stress and DNA damage in earthworms.

Resistance Risk Assessment for the New OSBP Inhibitor Y18501 in Pseudoperonospora cubensis and Point Mutations (G705V, L798W, and I812F) in PscORP1 that Confer Resistance

Y18501 is a new oxysterol-binding protein inhibitor (OSBPI) that shows strong inhibitory activity against Pseudoperonospora cubensis. In this study, the sensitivities of 159 Ps. cubensis isolates to Y18501 were determined, with EC₅₀ values ranging from 0.001 to 11.785 μg/mL, indicating that a Y18501-resistant subpopulation has appeared in the field. Ten Y18501-resistant mutants were obtained by fungicide adaptation and displayed fitness equal to or stronger than their parental isolates, which suggests that the resistance risk of Ps. cubensis to Y18501 is high. The consecutive applications of Y18501 in the field resulted in the rapid resistance of Ps. cubensis and decreased control efficacy of cucumber downy mildew (CDM), which could be alleviated by compounding with mancozeb. A positive cross-resistance was detected between Y18501 and oxathiapiprolin. The amino acid substitutions G705V, L798W, and I812F in PscORP1 conferred resistance to Y18501 in Ps. cubensis, which was validated by molecular docking and molecular dynamics simulations.

Magnolol Loaded on Carboxymethyl Chitosan Particles Improved the Antimicrobial Resistance and Storability of Kiwifruits

Magnolol is a natural compound extracted from the traditional Chinese medicine Magnolia officinalis, which exhibits antimicrobial properties. However, magnolol is insoluble in water and consists of a phenolic hydroxyl group, which is volatile; these factors hinder its application. In this study, a safe and environmentally friendly method to improve the microbial resistance and storability of harvested fruits is developed using the water-soluble carrier carboxymethyl chitosan (CMCS) and magnolol. Magnolol was loaded on CMCS particles to form Magnolol@CMCS antimicrobial particles, a preservation coating agent. Magnolol@CMCS particles effectively solved the problems of water insolubility and agglomeration of magnolol and reduced the size distribution D50 value of magnolol from 0.749 to 0.213 μm. Magnolol@CMCS particles showed greater toxicity against Staphylococcus aureus, Escherichia coli, and Botryosphaeria dothidea than that of magnolol alone, with effective medium concentration (EC50) values of 0.9408, 142.4144, and 8.8028 μg/mL, respectively. Kiwifruit treated with the Magnolol@CMCS solution showed delayed changes in fruit hardness and soluble solid and dry matter contents and significantly higher ascorbic acid (vitamin C) and soluble total sugar contents and sugar:acid ratios compared with that of the control fruit. In addition, no disease spots were observed on fruit treated with the Magnolol@CMCS solution within 7 days after inoculation with B. dothidea. In conclusion, Magnolol@CMCS particles showed antimicrobial activity on harvested fruits, effectively delayed the hardness and nutritional changes of fruits during storage, and improved the storability of kiwifruit.

Multiple Mutations in SDHB and SDHC2 Subunits Confer Resistance to the Succinate Dehydrogenase Inhibitor Cyclobutrifluram in Fusarium fujikuroi

Fusarium fujikuroi is one of the dominant phytopathogenic fungi causing rice bakanae disease worldwide. Cyclobutrifluram is a novel succinate dehydrogenase inhibitor (SDHI), which shows strong inhibitory activity against F. fujikuroi. The baseline sensitivity of 112 F. fujikuroi to cyclobutrifluram was determinated with a mean EC₅₀ value of 0.025 μg/mL. A total of 17 resistant mutants were obtained by fungicide adaptation and displayed equal or slightly weaker fitness than parental isolates, which suggests that the resistance risk of F. fujikuroi to cyclobutrifluram is medium. A positive cross-resistance was detected between cyclobutrifluram and fluopyram. The amino acid substitutions H248L/Y of FfSdhB and G80R or A83V of FfSdhC₂ conferred cyclobutrifluram resistance in F. fujikuroi, which was validated by molecular docking and protoplast transformation. The results indicate that the affinity between cyclobutrifluram and FfSdhs obviously decreased after point mutations, causing the resistance of F. fujikuroi.

Resistance Risk and Novel Resistance-Related Point Mutations in Target Protein PiORP1 of Fluoxapiprolin in Phytophthora infestans

Fluoxapiprolin is a new oxysterol binding protein inhibitor (OSBPI), which showed excellent inhibitory activity to plant pathogenic oomycetes. Its resistance risk and mechanism in Phytophthora infestans are unclear. In the current study, the sensitivities of 103 P. infestans isolates to fluoxapiprolin were investigated, and a unimodal distribution with a mean EC₅₀ value of 0.00035 μg/mL was observed. Four types of resistant mutants, with a resistance factor from 14 to more than 1000, and point mutations S768I+N837I, S768I+L860I, S768I, and I877F in PiORP1, were acquired using fungicide adaption. The fitness of the mutants was similar to or lower than that of the corresponding parental isolate. Positive cross-resistance was detected between fluoxapiprolin and oxathiapiprolin. The point mutations were verified in P. sojae homologue positions using the CRISPR/Cas9 genome editing system. Transformants containing S768I+N837I or S768I+L860I, showed high fluoxapiprolin resistance (RF > 1000). In conclusion, the risk of P. infestans resistance to fluoxapiprolin is moderate, and novel point mutation types S768I+N837I or S768I+L860I could cause high fluoxapiprolin resistance in P. infestans.

A Patched-Like Protein PsPTL Is Not Essential for the Growth and Response to Various Stresses in Phytophthora sojae

Patched (Ptc) and Patched-related (Ptr) proteins containing sterol-sensing domains (SSD) and Patched domains are highly conserved in eukaryotes for lipid transport and metabolism. Four proteins containing predicted SSD and Patched domains were simultaneously found by searching the Phytophthora sojae genome database, and one of them was identified as a Patched-like (PTL) protein. Here, we investigated the biological function of PsPTL. The expression level of PsPTL was higher during mycelial and sporulation stages, compared to zoospore (ZO), cyst, and germinated-cyst stages, without significant change during infection. However, deletion of PsPTL using CRISPR/Cas9 had no significant effect on the growth, development, or virulence of P. sojae. Further investigations showed that PsPTL is not essential for P. sojae to cope with external stresses such as temperature, pH, oxidative and osmotic pressure. In addition, this gene did not appear to play an essential role in P. sojae’s response to exogenous sterols. The transcript levels of the other three proteins containing predicted SSD and Patched domains were also not significantly upregulated in PsPTL deletion transformants. Our studies demonstrated that PsPTL is not an essential protein for P. sojae under the tested conditions, and more in-depth research is required for revealing the potential functions of PsPTL under special conditions or in other signaling pathways.

Activity and Resistance Assessment of a New OSBP Inhibitor, R034-1, in Phytophthora capsici and the Detection of Point Mutations in PcORP1 that Confer Resistance

R034-1 is a new member of the piperidinyl thiazole isoxazoline class of fungicides that shows high activity against most plant-pathogenic oomycetes and could effectively inhibit several developmental stages of Phytophthora capsici. Here, the potential resistance risk for R034-1 was evaluated in P. capsici. The baseline sensitivities of 135 isolates to R034-1 showed a unimodal curve, with a mean EC₅₀ value of 0.004 μg/mL. Twelve resistant mutants were generated by fungicide adaptation and displayed lower fitness compared to parental isolates, which suggests that the resistance risk of P. capsici to R034-1 is low. R034-1 and oxathiapiprolin are structurally related, and resistant isolates display cross-resistance to both compounds, suggesting that these fungicides may target the same oxysterol binding protein. Comparison of PcORP1 genes in the resistant mutants and their parental isolates revealed (N767S, N767I, and G700V) amino acid substitutions in the R034-1 resistant mutant. Causality was functionally validated using site-directed mutagenesis of the target gene using the CRISPR/Cas9 system.