P.cactorum was first reported from rotting cacti in Czechoslovakia, but is most commonly found in temperate regions. P.cactorum classiffiedin group I. Sporangia are distinctively papillate, noramally born terminally. survives outside living hosts as oospores formed in diseased tissue which remain viable for several years. In free water and a temperature of 50° to 68°F (10° to 20°C) oospores germinate to produce sporangia which in turn produce motile zoospores. Zoospores are probably the main infective agents although the germ tubes of sporangia may also initiate infection. A film of water is required for infection to occur. Periods of cloudiness after heavy rainfall or irrigation are ideal for infection. P.cactorum is homothalic, most isolates produce oospores in culture media and in diseased plant tissue.Antheridia are paragynous and unicellular and nearly spherical to club shaped. Oogonia are smooth walled and usually hyaline. The optimum growth temprature is 25 degree Celsius.