Several isolates of Uncinula necator were separated and kept in vitro. The pathogenicity of these isolates was compared by a bioassay system using small leaves issued from in vitro plants; 2 µl of spore suspension was inoculated on these leaves. Significative differences in sporulation time, aggressiveness, sporulation rate and resistance to fungicide triadimenol were observed between these isolates. Host plant variety also affects some of these characters of pathogenicity. The isolates were classified into 2 mating types concerning the aspect of perithecia formation by paired combination between 2 isolates. Productivity of perithecia varied in response to the combination of isolates and to host plant variety.