Sensitivity of Isolates of Sclerotium rolfsii from Peanut in Georgia to Selected Fungicides

Resistance risk assessment for benzovindiflupyr in Sclerotium rolfsii and transmission of resistance genes among population

BACKGROUND

Sclerotium rolfsii is a destructive soil-borne fungal pathogen which is distributed worldwide. In previous study, the succinate dehydrogenase inhibitor (SDHI) fungicide benzovindiflupyr has been identified for its great antifungal activity against Sclerotium rolfsii. This study is aimed to investigate the resistance risk and mechanism of benzovindiflupyr in Sclerotium rolfsii.

RESULTS

Eight stable benzovindiflupyr-resistant isolates were generated by fungicide adaptation. Although the obtained eight resistant isolates have a stronger pathogenicity than the parental sensitive isolate, they have a fitness penalty in the mycelial growth and sclerotia formation compared to the parental isolate. A positive cross-resistance existed in the resistant isolates between benzovindiflupyr and thifluzamide, carboxin, boscalid and isopyrazam. Three-point mutations, including SdhBN180D, SdhCQ68E and SdhDH103Y, were identified in the benzovindiflupyr-resistant isolates. However, molecular docking analysis indicated that only SdhDH103Y could influence the sensitivity of Sclerotium rolfsii to benzovindiflupyr. After mycelial co-incubation of resistant isolates and the sensitive isolate, resistance genes may be transmitted to the sensitive isolate. The in vivo efficacy of benzovindiflupyr and thifluzamide against benzovindiflupyr-resistant isolates was a little lower than that against the sensitive isolate but with no significant difference.

CONCLUSION

The results suggested a low to medium resistance risk of Sclerotium rolfsii to benzovindiflupyr. However, once resistance occurs, it is possible to spread in the population of Sclerotium rolfsii. This study is helpful to understanding the risk and mechanism of resistance to benzovindiflupyr in multinucleate pathogens such as Sclerotium rolfsii. © 2024 Society of Chemical Industry.

Characterization of Two Novel Single-Stranded RNA Viruses from Agroathelia rolfsii, the Causal Agent of Peanut Stem Rot

Peanut stem rot is a soil-borne disease caused by Agroathelia rolfsii. It occurs widely and seriously affects the peanut yield in most peanut-producing areas. The mycoviruses that induce the hypovirulence of some plant pathogenic fungi are potential resources for the biological control of fungal diseases in plants. Thus far, few mycoviruses have been found in A. rolfsii. In this study, two mitoviruses, namely, Agroathelia rolfsii mitovirus 1 (ArMV1) and Agroathelia rolfsii mitovirus 2 (ArMV2), were identified from the weakly virulent A. rolfsii strain GP3-1, and they were also found in other A. rolfsii isolates. High amounts of ArMV1 and ArMV2in the mycelium could reduce the virulence of A. rolfsii strains. This is the first report on the existence of mitoviruses in A. rolfsii. The results of this study may provide insights into the classification and evolution of mitoviruses in A. rolfsii and enable the exploration of the use of mycoviruses as biocontrol agents for the control of peanut stem rot.

Bioactivity and Control Efficacy of Benzovindiflupyr Against Athelia rolfsii in China

Athelia rolfsii is a devastating soilborne pathogen that causes stem rot of peanut and severely restricts peanut production. The new generation of succinate dehydrogenase inhibitor (SDHI) fungicide benzovindiflupyr has been registered in the United States and Brazil for managing multiple plant diseases. However, it is not registered in China to control peanut stem rot. In this study, 246 isolates from major peanut production areas in Shandong, Henan, and Hebei Provinces of China were used to determine the baseline sensitivity of A. rolfsii to benzovindiflupyr. The frequency of EC50 values of benzovindiflupyr was unimodally distributed with an average EC50 of 0.12 ± 0.05 mg/liter and a range of 0.01 to 0.57 mg/liter. Benzovindiflupyr can also strongly inhibit the germination of sclerotia, with an average EC50 of 2.38 ± 1.04 mg/liter (n = 23). In addition, benzovindiflupyr exhibited great in vivo efficacy against A. rolfsii; the protective or curative efficacy (89.87%, 20.39%) of benzovindiflupyr at a concentration of 50 mg/liter was equivalent to that of the control fungicide thifluzamide at 100 mg/liter (86.39%, 16.21%). At the same concentration (e.g., 100 mg/liter), the protective efficacy (93.99%) of benzovindiflupyr was more than twice as high as the curative efficacy (45.07%). A positive correlation existed between benzovindiflupyr and isopyrazam or mefentrifluconazole, which possibly resulted from similar chemical structures or damage to the cell membrane. Our findings provide valuable information for the application of benzovindiflupyr, and the established baseline sensitivity could facilitate the monitoring and assessment of benzovindiflupyr resistance risk.

Baseline sensitivity and bioactivity of tetramycin against Sclerotium rolfsii isolates in Huanghuai peanut-growing region of China

Peanut stem rot caused by Sclerotium rolfsii is a serious soil-borne disease and poses a threat to the peanut production. The antibiotic fungicide tetramycin has a broad antifungal spectrum against multiple pathogens and possess low environmental risks. In current study, a total of 250 isolates collected from Huanghuai peanut-growing region of China (Henan, Shandong and Hebei Province) were used to establish the baseline sensitivity of S. rolfsii to tetramycin. The baseline sensitivity curve was unimodal and distributed from 0.01 to 0.36 mg/L, with a mean EC₅₀ (50% effective concentration) value of 0.11 ± 0.06 mg/L. Tetramycin also had strong inhibitory activity on the formation and germination of sclerotia. There was no significant correlation of S. rolfsii sensitivity to tetramycin and other commonly used SDHI (succinate dehydrogenase inhibitor), QoI (quinone outside respiration inhibitor) and DMI (demethylation inhibitor) fungicides. Moreover, tetramycin significantly increased the cell membrane permeability and reduced the oxalate acid content. Greenhouse experiments showed that tetramycin has both protective and curative efficacy against S. rolfsii, while protective efficacy was higher than curative efficacy. Anyhow, the bioactivity of tetramycin is similar (curative efficacy) or higher (protective efficacy) than the control fungicide validamycin. In terms of application method, root drench may be more suitable for tetramycin than spraying, because root drench of tetramycin obtained a higher efficacy. These results indicated that tetramycin may be a potential alternative fungicide for the efficient control of peanut stem rot.

Morphological and Pathogenic Characterization of Sclerotium rolfsii, the Causal Agent of Southern Blight Disease on Common Bean in Uganda

Over the last 5 years, Southern blight caused by Sclerotium rolfsii Sacc. has superseded root rots caused by pathogens such as Fusarium and Pythium spp. as a major constraint of common bean (Phaseolus vulgaris L.) production in Uganda. Although S. rolfsii is prevalent in all bean-growing regions of Uganda, there is a lack of information about its diversity and pathogenicity to guide the development of effective management strategies. In total, 348 S. rolfsii isolates were obtained from bean fields in seven agroecological zones of Uganda, with the following objectives: to characterize their morphology, based on mycelial growth rate, mycelium texture, and number of sclerotia; and to determine the pathogenicity of 75 selected isolates on five common bean varieties in artificially inoculated soils in a screenhouse. We found that mycelial growth rate and the number of sclerotia produced on artificial media varied among agroecological zones but not within a zone. The five bean varieties tested were found to be susceptible to S. rolfsii, including varieties MLB49-89A and RWR719 that are resistant to Fusarium and Pythium root rots, respectively. Preemergence damping-off ranged between 0 and 100%, and disease severity index ranged between 4.4 and 100%. The widespread and high levels of S. rolfsii virulence on varieties of common bean indicate that management intervention is urgently required to help reduce losses incurred by Ugandan smallholder farmers.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Bioactivity, physiological characteristics and efficacy of the SDHI fungicide pydiflumetofen against Sclerotinia sclerotiorum

Sclerotinia sclerotiorum, which can cause Sclerotinia stem rot, is a devastating plant pathogen. This study aimed to assess the potential of pydiflumetofen, a new-generation succinate dehydrogenase inhibitor (SDHI) fungicide, to control Sclerotinia stem rot. Pydiflumetofen exhibited favorable bioactivity in suppressing mycelial growth, sclerotial production and morphological changes, and the myceliogenic and carpogenic germination of sclerotia. Treatment with pydiflumetofen increased the cell membrane permeability of S. sclerotiorum and decreased oxalic acid production. In addition, inoculation tests demonstrated that the protective activity of 40 μg/mL pydiflumetofen against the fungus was better than its curative activity. Under natural infection conditions, the spraying of pydiflumetofen at 200 g a.i. ha^-1 significantly reduced the incidence and severity of Sclerotinia stem rot. In addition, the sensitivity baseline to pydiflumetofen was established using 171 isolates collected from various crops in China. The results showed that the frequency distribution of the EC₅₀ values of pydiflumetofen was a unimodal curve with a mean EC₅₀ value of 0.0095 ± 0.0005 μg/mL. This study confirmed the favorable bioactivity of pydiflumetofen against S. sclerotiorum at various developmental stages and its high effectiveness under natural infection conditions, which indicates that pydiflumetofen is a promising tool for the management of Sclerotinia stem rot.

Mycelial Compatibility and Pathogenic Diversity Among Sclerotium rolfsii Isolates in the Southern United States

Sclerotium rolfsii is a soilborne fungus that causes southern blight on a wide range of plants in tropical and subtropical regions of the world. Eighty-four isolates collected from Florida, Georgia, Louisiana, South Carolina, Texas, and Virginia were paired and assigned to 23 mycelial compatibility groups (MCGs), of which 11 MCGs consisted of a single isolate. Isolates within an MCG typically originated from different hosts and different geographical areas, with the exception of MCG 11. In all, 13 of the 15 isolates in MCG 11 originated from peanut in Georgia and Florida, while the other 2 isolates originated from potato in Virginia and from the ornamental Barlaeria cristata in Florida. Significant differences in the size and number of sclerotia produced in vitro existed between isolates from peanut and other hosts. Nineteen isolates representative of the most common MCGs (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 16, 17, and 18) were tested for pathogenicity on tomato, pepper, and peanut. All isolates were pathogenic on all hosts but virulence differed significantly among isolates. Isolates collected from peanut were the most virulent on all three hosts compared with isolates collected from tomato and pepper. 'Georgia Green' peanut was more susceptible to peanut isolates from Georgia than to the other tested isolates. Of the two tomato entries, the commercial tomato 'Tygress' was less susceptible than the previously reported resistant breeding line 5635M to many of the S. rolfsii isolates tested, with the exception of the peanut isolates collected from Georgia. These initial findings suggest that considerable variation exists among S. rolfsii isolates throughout the southern United States, with some indications of specialization for the isolates collected from peanut.

Baseline Sensitivity and Cross-Resistance to Succinate-Dehydrogenase-Inhibiting and Demethylation-Inhibiting Fungicides in Didymella bryoniae

Didymella bryoniae, which causes gummy stem blight (GSB) of watermelon, has a history of developing resistance to fungicides, most recently the succinate-dehydrogenase-inhibiting (SDHI) fungicide boscalid. To facilitate fungicide resistance monitoring, baseline sensitivity distributions were established for demethylation-inhibiting (DMI) fungicides tebuconazole and difenoconazole and the SDHI fungicide penthiopyrad, and reestablished for the SDHI fungicide boscalid. In all, 71 isolates with no known prior exposure to SDHIs or DMIs were used to determine the effective concentration at which mycelial growth was inhibited by 50% (EC₅₀). EC₅₀ values for boscalid, penthiopyrad, tebuconazole, and difenoconazole were 0.018 to 0.064, 0.015 to 0.057, 0.062 to 0.385, and 0.018 to 0.048 μg/ml, with median values of 0.032, 0.026, 0.118, and 0.031 μg/ml, respectively. Significant positive correlations between the sensitivity to penthiopyrad and boscalid (P < 0.0001, r = 0.75) and between tebuconazole and difenoconazole (P < 0.0001, r = 0.59) indicate a potential for cross-resistance between chemically related fungicides. In 2009, 103 isolates from fungicidetreated watermelon fields were tested for sensitivity to boscalid and penthiopyrad using a discriminatory concentration of 3.0 μg/ml. Of the isolates tested, 82 were insensitive and 14 were sensitive to both fungicides. Because of the significant potential for cross-resistance between closely related fungicides, growers will be advised not to use both SDHIs or both DMIs successively in the same fungicide spray program.

Genetic and Phenotypic Diversity of Sclerotium rolfsii in Groundnut Fields in Central Vietnam

Groundnut (Arachis hypogaea) is an economically important legume crop in Vietnam and many other countries worldwide. Stem and pod rot, caused by the soilborne fungus Sclerotium rolfsii, is a major yield-limiting factor in groundnut cultivation. To develop sustainable measures to control this disease, fundamental knowledge of the epidemiology and diversity of S. rolfsii populations is essential. In this study, disease incidence was monitored in eight groundnut areas in central Vietnam with a total of 240 observational field plots. The results showed that 5 to 25% of the field-grown groundnut plants were infected by S. rolfsii. Based on internal transcribed spacer (ITS) ribosomal DNA sequence analyses, three distinct groups were identified among a total of 103 randomly selected S. rolfsii field isolates, with the majority of the isolates (n = 90) in one ITS group. S. rolfsii isolates originating from groundnut, tomato, and taro were all pathogenic on groundnut and relatively sensitive to the fungicide tebuconazole but displayed substantial diversity of various genetic and phenotypic traits, including mycelial compatibility, growth rate, and sclerotial characteristics.

Involvement of phenazines and lipopeptides in interactions between Pseudomonas species and Sclerotium rolfsii, causal agent of stem rot disease on groundnut

AIMS

To determine the role of phenazines (PHZ) and lipopeptide surfactants (LPs) produced by Pseudomonas in suppression of stem rot disease of groundnut, caused by the fungal pathogen Sclerotium rolfsii.

METHODS AND RESULTS

In vitro assays showed that PHZ-producing Pseudomonas chlororaphis strain Phz24 significantly inhibited hyphal growth of S. rolfsii and suppressed stem rot disease of groundnut under field conditions. Biosynthesis and regulatory mutants of Phz24 deficient in PHZ production were less effective in pathogen suppression. Pseudomonas strains SS101, SBW25 and 267, producing viscosin or putisolvin-like LPs, only marginally inhibited hyphal growth of S. rolfsii and did not suppress stem rot disease. In contrast, Pseudomonas strain SH-C52, producing the chlorinated LP thanamycin, inhibited hyphal growth of S. rolfsii and significantly reduced stem rot disease of groundnut in nethouse and field experiments, whereas its thanamycin-deficient mutant was less effective.

CONCLUSIONS

Phenazines and specific lipopeptides play an important role in suppression of stem rot disease of groundnut by root-colonizing Pseudomonas strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pseudomonas strains Phz24 and SH-C52 showed significant control of stem rot disease. Treatment of seeds or soil with these strains provides a promising supplementary strategy to control stem rot disease of groundnut.

Investigating the role of ABC transporters in multifungicide insensitivity in Phytophthora infestans

SUMMARY Isolates of the oomycete Phytophthora infestans exhibit a wide range of intrinsic sensitivities to fungicides, which potentially influences the application rates of chemicals needed to control potato late blight. To help understand what determines such levels of sensitivity, a genetic approach was employed which followed the segregation of sensitivities to structurally diverse fungicides such as metalaxyl and trifloxystrobin. Progeny exhibited broad distributions of sensitivity phenotypes, consistent with the behaviour of a quantitative trait. Measurements of the inhibition of strains by seven fungicides revealed that basal sensitivities to metalaxyl and trifloxystrobin, and to cymoxanil and dimethomorph, correlated at the 95% confidence level. These compounds have distinct modes of action, suggesting the involvement of a multifungicide efflux phenomenon mediated by ABC transporters. To determine whether such proteins contribute to variation in sensitivity, 41 full transporters and 13 half transporters were identified from P. infestans and their mRNA levels compared in strains exhibiting higher or lower sensitivities to fungicides. No correlation was observed between the expression of any ABC transporter and fungicide sensitivity. Other genes, or variation in the activities of the transporters, may therefore explain the differences between strains. Five ABC transporters were induced by several fungicides in strains with both higher and lower sensitivities to fungicides, which probably reflects the existence of a network for protecting against natural and artificial toxins.