Oxathiapiprolin is a recently introduced fungicide with particular activity against hemibiotrophic and biotrophic oomycetes. For Plasmopara halstedii, the downy mildew of the sunflower, no detailed studies for the activity of the pure compound on the preinfectional and early infection stages in which the fungicide could most effectively interfere with the life cycle of the pathogen have been reported. The fungicide was shown to be active against all stages of the pathogen starting from the release of the zoospores to the development of the mycelia and the formation of the sporangia. Differences in the sensitivity of the different developmental stages are in accordance with the suggested mechanism of the fungicide activity which targets sterol-binding proteins. The experiments showed that, in preventive treatments against very sensitive stages of the pathogen (such as germination of spores), extremely low concentrations of less than 1 ng/mL can completely protect the plants. Coating the seeds with oxathiapiprolin successfully prevented the seedlings from soil-borne infections. This is of particular importance in sunflower cultivation, because wind-borne infections on plants are much rarer and less harmful than soil-borne infections, which usually become systemic and lead to complete yield loss. The curative effects of oxathiapiprolin were shown on the leaf disks as well as on the infected young plants. However, this seems to be less important in sunflower cultivation than, for instance, in viticulture, because spraying sunflowers in the field after the germination of the seeds is uncommon (except in the ornamental cultivation of cut sunflowers).
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