A non-Mendelian inheritance of resistance to strobilurin fungicides in Ustilago maydis

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.

Evaluating In Vitro Fitness Parameters of G143A-Containing and Wild-Type Corynespora cassiicola Isolates from Mississippi Soybean

Quinone outside inhibitor (QoI) fungicides have been widely used to manage diseases of soybean including target spot caused by Corynespora cassiicola. However, resistance to QoI fungicides has recently been reported within the C. cassiicola population from Alabama, Arkansas, Mississippi, and Tennessee as a result of isolates in the population containing the G143A amino acid substitution. Therefore, the relative fitness and stability of isolates containing the G143A substitution compared with wild-type C. cassiicola isolates from Mississippi soybean were investigated by analyzing several fitness parameters in vitro. In addition, in vivo virulence assays were conducted in the greenhouse on a target spot-susceptible cultivar. The evaluations of fitness considered the difference between isolates from the wild-type and G143A-containing genotypes by evaluating colony growth parameters following the first and the 10th subcultures on microbiological media. When considered as an average of all G143A-containing isolates, the G143A-containing isolates following the 10th subculture produced 6.2% greater colony diameter growth but produced 2.3% less conidia. Conversely, over the same period, wild-type isolates produced 6.7% less colony growth but produced 10.9% more conidia. Based on our results, the C. cassiicola isolates that contained the G143A substitution appear stable since successive subculturing did not significantly affect the measured fitness parameters. The lack of fitness cost accompanying the genotypic shift to the G143A amino acid substitution which confers fungicide resistance in C. cassiicola indicates that these isolates may have fitness advantages and may remain stable in the population as well as displace wild-type isolates with repeated fungicide applications of QoI-containing products.

The Toxicity of Salicylhydroxamic Acid and Its Effect on the Sensitivity of Ustilaginoidea virens to Azoxystrobin and Pyraclostrobin

Rice false smut (RFS) caused by Ustilaginoidea virens has been one of the most severe rice diseases. Fungicide-based chemical control is a significant measure to control RFS. In the sensitivity determination of quinone outside inhibitor (QoI) fungicide in vitro, salicylhydroxamic acid (SHAM) has been commonly added to artificial culture media in order to inhibit alternative oxidase of phytopathogenic fungi. However, some studies showed that artificial media should not include SHAM due to its toxicity. Whether SHAM should be added in the assay of U. virens sensitivity to QoI fungicide remains unknown. In this study, two appropriate media, potato sucrose agar (PSA) and minimal medium (MM), were selected to test SHAM toxicity and sensitivity of U. virens to azoxystrobin and pyraclostrobin. The mycelial growth and sensitivity to azoxystrobin and pyraclostrobin had no significant difference between on PSA and MM. SHAM could significantly inhibit mycelial growth, conidial germination, peroxidase (POD) and esterase activity of U. virens. Average effective concentration for inhibiting 50% (EC₅₀) values of SHAM against mycelial growth of ten U. virens were 27.41 and 12.75 μg/mL on PSA and MM, respectively. The EC₅₀ values of SHAM against conidial germination of isolates HWD and JS60 were 70.36 and 44.69 μg/mL, respectively. SHAM at 30 μg/mL significantly inhibited POD and esterase activity of isolates HWD and JS60, and even SHAM at 10 μg/mL significantly inhibited POD activity of isolate HWD. In addition, SHAM significantly reduced EC₅₀ values and EC₉₀ values of azoxystrobin and pyraclostrobin on both PSA and MM. Even in the presence of SHAM at 10 μg/mL, average EC₅₀ values of ten U. virens isolates for azoxystrobin decreased 1.7-fold on PSA and 4.8-fold on MM, and for pyraclostrobin that decreased 2.8-fold on PSA and 4.8-fold on MM. Therefore, these results suggest that SHAM should not be included in artificial media in the assay of U. virens sensitivity to QoI fungicides.

Fitness Penalties in the Evolution of Fungicide Resistance

The evolution of resistance poses an ongoing threat to crop protection. Fungicide resistance provides a selective advantage under fungicide selection, but resistance-conferring mutations may also result in fitness penalties, resulting in an evolutionary trade-off. These penalties may result from the functional constraints of an evolving target site or from the resource allocation costs of overexpression or active transport. The extent to which such fitness penalties are present has important implications for resistance management strategies, determining whether resistance persists or declines between treatments, and for resistance risk assessments for new modes of action. Experimental results have proven variable, depending on factors such as temperature, nutrient status, osmotic or oxidative stress, and pathogen life-cycle stage. Functional genetics tools allow pathogen genetic background to be controlled, but this in turn raises the question of epistatic interactions. Combining fitness penalties under various conditions into a field-realistic scenario poses an important future challenge.

Heteroplasmy for the Cytochrome b Gene in Podosphaera xanthii and its Role in Resistance to QoI Fungicides in Spain

In Spain, management of the cucurbit powdery mildew pathogen Podosphaera xanthii is strongly dependent on chemicals such as quinone outside inhibitor (QoI) fungicides. In a previous report, widespread resistance to QoI fungicides in populations of P. xanthii in south-central Spain was documented, but the molecular mechanisms of resistance remained unclear. In this work, the role of the Rieske-FeS (risp) and the cytochrome b (cytb) gene mutations in QoI resistance of P. xanthii were examined. No point mutations in the risp gene were found in the three QoI-resistant isolates analyzed. For cytb, sequence analysis revealed the presence of a G143A substitution that occurs in many QoI-resistant fungi. This mutation was always detected in QoI-resistant isolates of P. xanthii; however, it was also detected in sensitive isolates. To better understand the role of heteroplasmy for cytb in QoI resistance of P. xanthii, an allele-specific quantitative PCR was developed to quantify the relative abundance of the G143 (sensitive) and A143 (resistant) alleles. High relative abundance of A143 allele (70%) was associated with isolates resistant to QoI fungicides; however, QoI-sensitive isolates also carried the mutated allele in frequencies ranged from 10 to 60%. Our data suggest that G143A mutation in cytb is the primary factor involved in QoI resistance of P. xanthii but the proportion of G143 and A143 alleles in an isolate may determine its QoI resistance level.

Genetic analysis and molecular characterisation of laboratory and field mutants of Botryotinia fuckeliana (Botrytis cinerea) resistant to QoI fungicides

BACKGROUND

QoI fungicides, inhibitors of mitochondrial respiration, are considered to be at high risk of resistance development. In several phytopathogenic fungi, resistance is caused by mutations (most frequently G143A) in the mitochondrial cytochrome b (cytb) gene. The genetic and molecular basis of QoI resistance were investigated in laboratory and field mutants of Botryotinia fuckeliana (de Bary) Whetz. exhibiting in vitro reduced sensitivity to trifloxystrobin.

RESULTS

B. fuckeliana mutants highly resistant to trifloxystrobin were obtained in the laboratory by spontaneous mutations in wild-type strains, or from naturally infected plants on a medium amended with 1-3 mg L(-1) trifloxystrobin and 2 mM salicylhydroxamic acid, an inhibitor of alternative oxidase. No point mutations were detected, either in the complete nucleotide sequences of the cytb gene or in those of the aox and Rieske protein genes of laboratory mutants, whereas all field mutants carried the G143A mutation in the mitochondrial cytb gene. QoI resistance was always maternally inherited in ascospore progeny of sexual crosses of field mutants with sensitive reference strains.

CONCLUSIONS

The G143A mutation in cytb gene is confirmed to be responsible for field resistance to QoIs in B. fuckeliana. Maternal inheritance of resistance to QoIs in progeny of sexual crosses confirmed that it is caused by extranuclear genetic determinants. In laboratory mutants the heteroplasmic state of mutated mitochondria could likely hamper the G143A detection, otherwise other gene(s) underlying different mechanisms of resistance could be involved.

Cytological evaluation of the effect of azoxystrobin and alternative oxidase inhibitors in Botrytis cinerea

Azoxystrobin (AZ), a strobilurin-derived fungicide, is known to inhibit mitochondrial respiration in fungi by blocking the electron transport chain in the inner mitochondrial membrane. Germination was strongly inhibited when Botrytis cinerea spore suspension was treated with AZ and the alternative oxidase (AOX) inhibitors, salicylhydroxamic acid (SHAM) and n-propyl gallate. However, chemical death indicators trypan blue and propidium iodide showed that those spores were still alive. When the spore suspension in the AZ and SHAM solution was replaced with distilled water, the germination rate almost recovered, at least during the first 2 days of incubation with AZ and SHAM solution. No morphological alteration was detected in the cells treated with AZ and SHAM, especially in mitochondria, using transmission electron microscopy. Therefore, simultaneous application of AZ and AOX inhibitors has a fungistatic, rather than a fungicidal, action.

Assessing the risk of resistance in Pseudoperonospora cubensis to the fungicide flumorph in vitro

BACKGROUND

The oomycete fungicide flumorph is a recently introduced carboxylic acid amide (CAA) fungicide. In order to evaluate the risk of developing field resistance to flumorph, the authors compared it with dimethomorph and azoxystrobin with respect to the ease of obtaining resistant isolates to these fungicides, the level of resistance and their fitness in the laboratory.

RESULTS

Mutants with a high level of resistance to azoxystrobin were isolated readily by adaptation and UV irradiation, and their fitness was as good as that of the parent isolates. Attempts to generate mutants of Pseudoperonospora cubensis (Burk. & MA Curtis) Rostovsev resistant to flumorph and dimethomorph by sporangia adaptation on fungicide-treated leaves were unsuccessful. However, moderately resistant mutants were isolated using UV mutagenesis, but their resistance level [maximum resistance factor (MRF) < 100] was much lower than that of the azoxystrobin-resistant mutant (MRF = 733). With the exception of stability of resistance, all mutants showed low pathogenicity and sporulation compared with wild-type isolates and azoxystrobin-resistant mutants. There is cross-resistance between flumorph and dimethomorph, suggesting that they have the same resistance mechanism.

CONCLUSION

The above results suggest that the resistance risk of flumorph may be similar to that of dimethomorph but lower than that of azoxystrobin and can be classified as moderate. Thus, it can be managed by appropriate product use strategies.

Adaptation to fungicides in Monilinia fructicola isolates with different fungicide resistance phenotypes

The ability to develop fungicide resistance was assessed in Monilinia fructicola isolates with different fungicide sensitivity phenotypes by adapting mycelium and conidia to increasing concentrations of selective fungicides and UV mutagenesis. Results showed that adaptation to Quinone outside inhibitor (QoI) fungicide azoxystrobin and sterol demethylation inhibitor (DMI) fungicide propiconazole was more effective in conidial-transfer experiments compared to mycelial-transfer experiments. DMI-resistant (DMI-R) isolates adapted to significantly higher doses of azoxystrobin in both, mycelial- and conidial-transfer experiments compared to benzimidazole-resistant (BZI-R) and sensitive (S) isolates. Adaptation to propiconazole in conidial-transfer experiments was accelerated in BZI-R isolates when a stable, nonlethal dose of 50 microg/ml thiophanate-methyl was added to the selection medium. One of two azoxystrobin-resistant mutants from DMI-R isolates did not show any fitness penalties; the other isolate expired before further tests could be carried out. The viable mutant caused larger lesions on detached peach fruit sprayed with azoxystrobin compared to the parental isolate. The azoxystrobin sensitivity of the viable mutant returned to baseline levels after the mutant was transferred to unamended medium. However, azoxystrobin resistance recovered quicker in the mutant compared to the corresponding parental isolate after renewed subculturing on medium amended with 0.2 and 1 microg/ml azoxystrobin; only the mutant but not the parental isolate was able to adapt to 5 microg/ml azoxystrobin. In UV mutagenesis experiments, the DMI-R isolates produced significantly more mutants compared to S isolates. All of the UV-induced mutants showed stable fungicide resistance with little fitness penalty. This study indicates the potential for QoI fungicide resistance development in M. fructicola in the absence of a mutagen and provides evidence for increased mutability and predisposition to accelerated adaptation to azoxystrobin in M. fructicola isolates resistant to DMI fungicides.

A critical evaluation of the role of alternative oxidase in the performance of strobilurin and related fungicides acting at the Qo site of complex III

Mitochondrial respiration conserves energy by linking NADH oxidation and electron-coupled proton translocation with ATP synthesis, through a core pathway involving three large protein complexes. Strobilurin fungicides block electron flow through one of these complexes (III), and disrupt energy supply. Despite an essential need for ATP throughout fungal disease development, strobilurins are largely preventative; indeed some diseases are not controlled at all, and several pathogens have quickly developed resistance. Target-site variation is not the only cause of these performance difficulties. Alternative oxidase (AOX) is a strobilurin-insensitive terminal oxidase that allows electrons from ubiquinol to bypass Complex III. Its synthesis is constitutive in some fungi but in many others is induced by inhibition of the main pathway. AOX provides a strobilurin-insensitive pathway for oxidation of NADH. Protons are pumped as electrons flow through Complex I, but energy conservation is less efficient than for the full respiratory chain. Salicylhydroxamic acid (SHAM) is a characteristic inhibitor of AOX, and several studies have explored the potentiation of strobilurin activity by SHAM. We present a kinetic-based model which relates changes in the extent of potentiation during different phases of disease development to a changing importance of energy efficiency. The model provides a framework for understanding the varying efficacy of strobilurin fungicides. In many cases, AOX can limit strobilurin effectiveness once an infection is established, but is unable to interfere significantly with strobilurin action during germination. A less stringent demand for energy efficiency during early disease development could lead to insensitivity towards this class of fungicides. This is discussed in relation to Botrytis cinerea, which is often poorly controlled by strobilurins. Mutations with a similar effect may explain evidence implicating AOX in resistance development in normally well-controlled plant pathogens, such as Venturia inaequalis.