A new method to obtain monozoosporangial isolates of sunflower downy mildew (Plasmopara halstedii)

A new method to obtain monozoosporangial strains was applied on 9 pathotypes of Plasmopara halstedii (100, 300, 304, 314, 700, 704, 710, 707 and 714), the parasite causing sunflower downy mildew. Single zoosporangia were isolated from the surface of agar medium and placed on leaf disks on solid Knop medium. The best fungal development was obtained with leaf disks taken from the first pair of sunflower leaves when these measured from 5 to 8cm in length. The percentage success rate in obtaining monozoosporangial strains varied from 1.4 to 7.4% according to race. The percentage of disks showing sporulation depends on spore viability, the physiological state of leaves and the receptivity of the sunflower genotype. This method enabled us to confirm the presence of a new downy mildew pathotype in France: race 707.