Effect of temperature, hydrogen ion concentration and osmotic potential on zoospore production by three Pythium species isolated from pond water

Pythium fluminum produced zoospores most abundantly at 15°C, whereas the optima for Pythium’group F’ and P. marsipium were 20 and 25°C, respectively. Increasing the incubation temperature above the optimum resulted in the decrease of the duration of zoospore production. In Pythium’group F’ the ability to produce zoospores was not lost even after incubation at 40°C for 24h. On the other hand, P. marsipium and P. fluminum lost the ability under these conditions. Zoospore production was inhibited at pH 4.5 and 10.5 in all the species tested. Pythium fluminum and P. marsipium were found to have two pH optima for zoospore production (7.5 and 9.5 for the former and 5.5 and 8.5 for the latter). The optimum pH for zoospore production by Pythium “group F” was 6.5-7,5. Moderate osmotic potentials(-0.27~-0.47 MPa) appeared to favor zoospore production by the pythia tested. The effect of temperature, pH and osmotic potential on zoospore production was discussed in relation to pollution of pond water.