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

The G143S mutation in cytochrome b confers high resistance to pyraclostrobin in Fusarium pseudograminearum

BACKGROUND

Wheat crown rot (WCR), primarily caused by Fusarium pseudograminearum has become more and more prevalent in winter wheat areas in China. However, limited fungicides have been registered for the control of WCR in China so far. Pyraclostrobin is a representative quinone outside inhibitor (QoI) with excellent activity against Fusarium spp. There is currently limited research on the resistance risk and resistance mechanism of F. pseudograminearum to pyraclostrobin.

RESULTS

Here, we determined the activity of pyraclostrobin against F. pseudograminearum. The EC50 values ranged from 0.022 to 0.172 μg mL-1 with an average EC50 value of 0.071 ± 0.030 μg mL-1. Four highly pyraclostrobin-resistant mutants were obtained from two sensitive strains by ultraviolet (UV) mutagenesis in the laboratory. The mutants showed decreased mycelial growth rate and virulence as compared with the corresponding wild-type strains, indicating that pyraclostrobin resistance suffered a fitness penalty in F. pseudograminearum. It was found that the high resistance of four mutants was caused by the G143S mutation in Cytb. Molecular docking analysis also further confirms that the G143S mutation in Cytb decreased the binding affinity between pyraclostrobin and Cytb.

CONCLUSION

The resistance risk of F. pseudograminearum to pyraclostrobin could be low to medium. Although a mutation at the G143S position of Cytb could potentially occur, this mutation decreases the fitness of the mutant, which may reduce its survival in the environment. Therefore, the negative consequences of a possible mutation are lower. This makes pyraclostrobin a good candidate for controlling crown rot in wheat. © 2024 Society of Chemical Industry.

Characterization of the fludioxonil and phenamacril dual resistant mutants of Fusarium graminearum

Fusarium graminearum is an important fungal pathogen causing Fusarium head blight (FHB) in wheat and other cereal crops worldwide. Due to lack of resistant wheat cultivars, FHB control mainly relies on application of chemical fungicides. Both fludioxonil (a phenylpyrrole compound) and phenamacril (a cyanoacrylate fungicide) have been registered for controlling FHB in China, however, fludioxonil-resistant isolates of F. graminearum have been detected in field. To evaluate the potential risk of dual resistance of F. graminearum to both compounds, fludioxonil and phenamacril dual resistant (DR) mutants of F. graminearum were obtained via fungicide domestication in laboratory. Result showed that resistance of the DR mutants to both fludioxonil and phenamacril were genetically stable after sub-cultured for ten generations or stored at 4 °C for 30 days on fungicide-free PDA. Cross-resistance assay showed that the DR mutants remain sensitive to other groups of fungicides, including carbendazim, tebuconazole, pydiflumetofen, and fluazinam. In addition, the DR mutants exhibited defects in mycelia growth, conidiation, mycotoxin deoxynivalenol (DON) production, and virulence Moreover, the DR mutants displayed increased sensitivity to osmotic stress. Sequencing results showed that amino acid point mutations S217L/T in the myosin I protein is responsible for phenamacril resistance in the DR mutants. Our results indicate that mutations leading to fludioxonil and phenamacril dual resistance could result in fitness cost for F. graminearum. Our results also suggest that the potential risk of F. graminearum developing resistance to both fludioxonil and phenamacril in field could be rather low, which provides scientific guidance in controlling FHB with fludioxonil and phenamacril.

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.

Effect of novel botanical synergist on the effectiveness and residue behavior of prothioconazole in wheat field

Fusarium head blight (FHB) is a critical fungal disease causes serious grain yield losses and mycotoxin contaminations. Currently, utilization of chemical fungicides is the main control method which has led to serious resistance. Development of novel synergist is an important strategy to reduce the usage of chemical fungicides and postpone the development of resistance, while natural components are interesting resources. In this study, the synergistic effect of Taxodium 'zhongshansha' essential oil (TZEO) was determined and the best synergistic ratio (SR) of 3.96 in laboratory which was observed when the weight ratio of TZEO and prothioconazole was 1 : 1 with the corresponding EC50 (half maximal effective concentration) value of Fusarium graminearum was 0.280 mg L-1. Subsequently, an increase of 6.31% on the control effect to FHB index in field test was observed when compared to the treatment with prothioconazole alone, though there was no significant difference between these treatments. Furthermore, we established an effective method to detect the mycotoxin contaminations in wheat grain with the limits of quantifications (LOQs) value of 5 µg kg-1 (DON, ZEN, 3-DON, and 15-DON) and 1 µg kg-1 (OTA) and the contents were less to the maximum residue limit (MRL) values. It was also shown that the application of 20% TZEO EW led to a 20% reduction in the use of prothioconazole, which was calculated based on the control effect values of 86.41% and 90.20% between the treatments of 30% prothioconazole OD (225 g a.i ha-1, recommend dosage) and 30% prothioconazole OD (180 g a.i ha-1) + 20% TZEO EW (225 mL ha-1), significantly. The initial residue of prothioconazole and prothioconazole-desthio was increased in the treatment with TZEO, which may play an important role in the synergistic effect on FHB. Moreover, none of the treatments posed a prothioconazole residue risk in the wheat grain and the environment. In addition, the essential oil has no any negative influence on wheat growth, which was revealed by a study of the chlorophyll content. These results provide an important botanical synergist for use with prothioconazole to control Fusarium head blight, and in-depth study to the synergistic mechanism of this oil is necessary in our future research.

Exploring the Potential Mechanism of Prothioconazole Resistance in Fusarium graminearum in China

The Fusarium head blight (FHB) caused by Fusarium graminearum is one of the most important diseases threatening wheat production in China. However, the triazole sterol 14α-demethylation inhibitor (DMI), prothioconazole, is known to exhibit high activity against F. graminearum. The current study indicated that three highly resistant laboratory mutants exhibited significantly (p < 0.05) altered growth and sporulation, although contrary to expectation, only one of the mutants exhibited reduced growth and sporulation, while the other two exhibited significant (p < 0.05) increases. Despite this, pathogenicity tests revealed that all of the mutants exhibited significantly (p < 0.05) reduced pathogenicity, indicating a substantial cost to fitness. Sequence analysis of the prothioconazole target protein, CYP51, of which F. graminearum has three homologues (FgCYP51A, FgCYP51B, and FgCYP51C), identified three mutations in the FgCYP51B sequence with a high likelihood of being associated with the observed resistance, as well as another three mutations in the FgCYP51B sequence, and two in the FgCYP51A sequence that are worthy of further investigation. Two of the prothioconazole-resistant mutants were also found to have several amino acid substitutions in their FgCYP51C sequences, and it was interesting to note that these two mutants exhibited significantly (p < 0.05) reduced pathogenicity compared to the other mutant. Expression analysis revealed that prothioconazole treatment (0.1 μg/mL) resulted in altered expression of all the FgCYP51 target genes, and that expression was also altered in the prothioconazole-resistant mutants compared to their wild-type parental isolates. Meanwhile, no evidence was found of any cross-resistance between prothioconazole and other commonly used fungicides, including carbendazim, pyraclostrobin, and fluazinam, as well as the triazole tebuconazole and the imidazole DMI prochloraz. Taken together, these results not only provide new insight into potential resistance mechanism in F. graminearum, and the biological characteristics associated with them, but also convincing evidence that prothioconazole can offer effective control of FHB.

Resistance risk assessment of Fusarium pseudograminearum from wheat to prothioconazole

Fusarium crown rot (FCR), primarily caused by Fusarium pseudograminearum, poses significant threats to cereal crops worldwide. Prothioconazole is a demethylation inhibitor (DMI) fungicide used to control FCR. However, the risk of resistance in F. pseudograminearum to prothioconazole has not yet been evaluated. In this study, the sensitivity of a total of 255 F. pseudograminearum strains obtained from Henan Province, China to prothioconazole were determined by the mycelial growth inhibition. The results showed that the effective concentration to 50% growth inhibition (EC₅₀) of these strains ranged from 0.4228 μg/mL to 2.5284 μg/mL, with a mean EC₅₀ value of 1.0692 ± 0.4527 μg/mL (mean ± SD). Thirty prothioconazole-resistant mutants were obtained out of six selected sensitive parental strains by means of fungicide taming. The resistant mutants exhibited defects in vegetative growth, conidia production, and pathogenicity on wheat seedlings compared to their parental strains. Under ion, cell wall, and temperature stress conditions but not osmotic stress, all the mutants exhibited decreased growth rates compared with their parental strains, which was consistent with the control treatment. Cross-resistance test showed that there was a cross-resistance relationship between prothioconazole and four DMI fungicides, including prochloraz, metconazole, tebuconazole and hexaconazole, but no cross-resistance was observed between prothioconazole and carbendazim, phenamacril, fludioxonil, or azoxystrobin. Although no site mutation occurred on Cyp51a and Cyp51b genes, the constitutive expression level of the Cyp51a gene was significantly increased in all mutants. After being treated with prothioconazole, the Cyp51a and Cyp51b genes were significantly increased in both the resistant mutants and their parents. These results suggested that the resistance to prothioconazole of the mutants may be attributed to the changes of the relative expression level of Cyp51a and Cyp51b genes. Taken together, these results could provide a theoretical basis for the scientific use of prothioconazole in the field and fungicide resistance management strategies.