Identification of Sunflower Downy Mildew (Plasmopara halstedii) Physiological Races and Reaction of the Host Genotypes to the Races Under Controlled Conditions

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Abstract

The soil samples containing the pathogen, Plasmopara halstedii, were collected, refreshed and mass-produced on susceptible cultivar Record, using whole seedling immersion method. The isolates were inoculated on standard differential lines to investigate the physiological races. Using the dominant race (race 100) of the pathogen in sunflower planting areas, 77 new Iranian sunflower genotypes including hybrids, restorer lines, CMS lines and individuals were evaluated against the disease under controlled conditions. Fifteen days post inoculation, the treated plants were evaluated based on six qualitative disease characteristics including damping-off, sporulation on cotyledons, sporulation on leaves and cotyledons, stunt, leaf mosaic or chlorosis and deformation. They were then converted to quantitative means based on their importance in disease development. Finally the average of the means as disease severity index (DSI) was used for evaluations. Based on the results, 64 genotypes were categorized as resistant and the rest were identified as moderately resistant, moderately susceptible, susceptible, and highly susceptible.

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