Abstract
Plenaris (oxathiapiprolin) applied to sunflower seedlings was highly effective in controlling downy mildew incited by the oomycete Plasmopara halstedii. In vitro assays revealed strong suppression of zoospore release and cystospore germination of P.halstedii by Plenaris. Bion (acibenzolar-S-methyl) and Apron (mefenoxam) were partially effective when used singly, but performed synergistically when mixed with Plenaris. Seed treatment (coating) with Plenaris provided dose-dependent control of the disease whereas Bion and Apron provided partial or poor control. However, seeds treated with mixtures containing reduced rates of Plenaris and full rates of Bion and/or Apron provided complete control of the disease due to the synergistic interaction between these components. Such mixtures should be used for seed treatment in the field to minimize selection pressure imposed on the pathogen.
Highlights
Sunflower downy mildew (SDM) incited by the biotrophic oomycete Plasmopara halstedii (Farl.) Berl. and de Toni is a devastating disease of sunflower (Helianthus annuus L) worldwide [1,2,3]
Plenaris applied to a sporangial suspension of P. halstedii at 15 ̊C strongly suppressed zoospore release from sporangia
Seven-day-old plants at their cotyledon stage were sprayed with various doses of Plenaris (0.001–1 ppm ai), Bion (0.1–100 ppm ai) or Apron (1–1000 ppm ai), and thereafter inoculated with sporangia of either one of 13 isolates of P. halstedii
Summary
Sunflower downy mildew (SDM) incited by the biotrophic oomycete Plasmopara halstedii (Farl.) Berl. and de Toni is a devastating disease of sunflower (Helianthus annuus L) worldwide [1,2,3]. The long distance dispersal of SDM almost certainly occurred through the exchange of oospore-contaminated seeds [5, 6]. This oomycete is homothallic [7]. Zoospores, released from sporangia produced by germinating soil-borne oospores, attack the roots of the emerging sunflower plants in the spring [8] and cause latent or systemic infection with downy mildew. Symptomatic and non-symptomatic plants might produce oospores in their root, shoot and seeds These oospores incorporate into the soil and may cause infection of the new crops in subsequent years [5,6,7, 10]
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