Abstract: The effect of six fungicides on different stages in the life cycle of P.cubensis was studied by calcofluor fluorescent technique. The results showed that dimethomorph and flumorph had no effect on zoospore discharge, zoospore move and cystospore formation but prevented cystospore germination, germ tube elongation, appressorium formation, mycelial growth in vivo, sporrangiophore emerge from stomata and sporangial formation. The EC50 of these two fungicides to different stages of P.cubensis were ranged from 0.07~5.98 μg/ mL and 0.10~3.50 μg/ mL, respectively. The EC50 of cymoxanil to cystospore germination and the growth of germ tube were 1.10 and 0.002 μg/ mL, but it had no effect on other stages. Azoxystrobin can prevent all stages of P.cubensis but cystospore germination and sporangiophore production. Its EC50 were 0.018, 0.008 and 0.010 μg/ mL to zoospore discharge, germ tube elongation and appressorium formation, respectively. The EC50 of mancozeb to zoospore discharge, cystospore germination and germ tube elongation were 2.40, 2.88 and 1.53 μg/ mL, it has small effect on P.cubensis mycelium in leaf tissue, appressorium formation and sporangiophore production. metalaxyl had no effect on any stages of P.cubensis.