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Assessing the Sensitivity of Plasmopara halstedii Isolates to Mefenoxam through Host Responses
Nisha Nisha1, Sergey Vinogradov2, Katalin Körösi1
1Department of Integrated Plant Protection, Institute of Plant Protection, Hungarian University of Agriculture and Life Sciences, H-2100 Godollo, Hungary.
Abstract:
Downy mildew caused by Plasmopara halstedii is responsible for significant economic losses in cultivated sunflowers. Field isolates of sunflower downy mildew resistant to mefenoxam, a previously effective active ingredient against the pathogen, have been found across Europe. The main goal of this study was to assess the sensitivity of P. halstedii isolates to mefenoxam through host responses to infection, such as symptoms measured by disease severity and growth reduction, and host tissue reactions, such as hypersensitive reaction and necrosis of invaded cells. Sunflower seeds were treated with Apron XL 350 FS at the European registered rate (3 mg/kg seeds). Seedlings were inoculated using the soil drench method with eight Hungarian P. halstedii isolates. Disease rates and plant heights were measured twice. Histological examinations of cross-sections of sunflower hypocotyls were performed using a fluorescence microscope. In our study, cluster analyses of sunflowers based on macroscopic and microscopic variables showed differentiation of groups of mefenoxam-treated sunflowers inoculated with different P. halstedii isolates. We first revealed a clear difference in host responses of mefenoxam-treated susceptible sunflowers. In addition, examining tissue reactions (e.g., hypersensitive reaction, necrosis) seems more accurate to estimate the sensitivity of P. halstedii isolates to mefenoxam than macroscopic symptoms.
Insights
Sunflower downy mildew isolates resistant to mefenoxam fungicide were studied. Host tissue reactions, like hypersensitive response and necrosis, accurately indicated pathogen sensitivity to mefenoxam, unlike macroscopic symptoms.
Area of Science:
- Plant Pathology
- Agricultural Science
- Mycology
Background:
- Downy mildew, caused by *Plasmopara halstedii*, leads to substantial economic losses in sunflower cultivation.
- Field isolates of sunflower downy mildew exhibiting resistance to mefenoxam, a key fungicide, have been identified across Europe, necessitating new assessment methods.
Purpose of the Study:
- To evaluate the sensitivity of *P. halstedii* isolates to mefenoxam.
- To compare the efficacy of host response indicators, including disease severity, growth reduction, and tissue reactions (hypersensitive reaction, necrosis), in assessing mefenoxam sensitivity.
Main Methods:
- Sunflower seeds were treated with Apron XL 350 FS and inoculated with eight Hungarian *P. halstedii* isolates via soil drench.
- Macroscopic disease severity and plant height were measured, alongside histological examinations of sunflower hypocotyls using fluorescence microscopy.
- Cluster analysis was employed to differentiate mefenoxam-treated sunflower groups based on macroscopic and microscopic variables.
Main Results:
- Cluster analysis revealed distinct groups among mefenoxam-treated sunflowers inoculated with different *P. halstedii* isolates.
- A clear difference in host responses was observed in mefenoxam-treated susceptible sunflowers.
- Histological examination of tissue reactions, such as hypersensitive reaction and necrosis, proved more accurate in estimating *P. halstedii* sensitivity to mefenoxam than macroscopic symptoms.
Conclusions:
- Host tissue reactions provide a more precise method for determining the sensitivity of *Plasmopara halstedii* isolates to mefenoxam.
- This finding is crucial for managing mefenoxam-resistant downy mildew in sunflowers and guiding fungicide resistance management strategies.
