<i>Phytophthora</i> <i>erythroseptica</i> causes pink rot of potato. It produces flagellate zoospores that serve as inocula for host infection. Furthermore, the zoosporic behavior is density dependent. Zoospores at 104 zoospores/ml germinated and its suspension successfully caused necrotic lesions on potato tuber slices, but it was not observed at concentration of 100 zoospores/ml. In order to determine whether this behavior is regulated by extracellular compounds in zoospore exudates (ZE), effects of ZE on zoosporic germination of <i>P.</i> <i>erythroseptica</i> were examined under laboratory conditions. ZE was obtained from filtrated zoospore suspension (104 zoospores/ml). Freshly produced zoospores at different concentrations were treated either with or without ZE, then incubated on a depression-well slides at 22°C for four hours, zoosporic germination was examined using a microscope. The results showed that zoospores only germinated when the concentration was at or above 104 zoospores/ml. However, when treated with ZE, germination occurred at 100 zoospores/ml. Similarly, 10 ul of ZE-treated zoospores at 100 zoospores/ml caused potato tuber infection, when inoculated on the eye of potato. The activity of ZE was not affected by high temperature (100°C for 30 min), proteinase K and catalase. To determine the active compounds related to this activity, nuclear magnetic resonance, gas chromatography/mass spectrometry were performed. The results are under analyses.
