Leaf Disk Inoculation — a Useful Tool for Selecting Infections of Sunflower Downy Mildew at Low Inoculum Concentration, but Inappropriate to Pathotype Characterization

A modified technique for leaf disk inoculation of sunflower with zoosporangia of the downy mildew pathogen Plasmopara halstedii was developed. Infection with low concentrations of inoculum was obtained down to the level of single sporangia. No significant difference in the infection rate was seen between disks from cotyledons and true leaves. This makes leaf disk inoculation particularly suitable for infections at low sporangium density and for the investigation on plant tissue which cannot be infected with whole seedling inoculation. The technique seems, however, to be inappropriate for pathotype characterization using differential host lines.