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 zoosp orangia of the downy mildew pathogen Plasmopara halstedii was develope d. Infection with low concentrations of inoculum was obtained down to the level of single sporangia. No significant difference in the infect ion rate was seen between disks from cotyledons and true leaves. This makes leaf disk inoculation particularly suitable for infections at lo w sporangium density and for the investigation on plant tissue which c annot be infected with whole seedling inoculation. The technique seems , however, to be inappropriate for pathotype characterization using di fferential host lines.