Is there a particular reason for wanting zoospores?

Hi Mako: you can grow Pythium on liquid medium for 3-5 day. but the medium on 5 C overnight, after that keep the medium on room temperature for 10 min the zoospore will be released from the sporangia. 

Hi Makomborero,

I worked with Pythium ultimum. I grew the Pythium for about 8 d in V8 liquid agar at 25 degrees celcius in darkness. The I washed the mycelia several times with sterilized tap water to remove all the nutrients (you eventually need to remove agar pieces) and put it then in sterilized tap water. The "shock" after removing the nutrients makes the Pythium produce zoospores which should be found after 4 to 5 hours in the water. This probably also works with P. irregulare.