Ignore this advice if you already know this, but PDA is a very rich medium that encourages all fungi and bacteria, so it's not the best thing to use to isolate from plant tissue. You can use a less rich medium, like cornmeal agar (CMA), or a Phytophthora-selective medium like PARP, or a more acidic medium like V-8 agar, but I use water agar. I place the tissue in a slit in the agar, wait for the mycelium to grow into the water agar far enough to out-grow any bacteria, then carefully cut out a plug to transfer onto PDA or CMA.

Ignore this advice if you already know this, but PDA is a very rich medium that encourages all fungi and bacteria, so it's not the best thing to use to isolate from plant tissue. You can use a less rich medium, like cornmeal agar (CMA), or a Phytophthora-selective medium like PARP, or a more acidic medium like V-8 agar, but I use water agar. I place the tissue in a slit in the agar, wait for the mycelium to grow into the water agar far enough to out-grow any bacteria, then carefully cut out a plug to transfer onto PDA or CMA.

I support opinion of Nina, cornmeal agar (CMA) would be much better comparing to PDA (which is not good for any isolation from plant samples), take more care to selection of infected plant tissues and make it as soon as possible after collection samples from field (at least at the same day).

I too highly recommend Ninas' and Alex Approach using CMA, V8 or Agar would able you to "catch" Phytophthora helping zoospores to germinate and boosting clamydospore

it is like you give phytophthora a buffer place or catalystic 'slective media" to orient their cell division and metabolism towards less nutrients yet attractive ones (COrn strach or -acid)

Yes, VG Agar (V8A) medium is more suitable to isolate Phytophthora

Dear Paudel,

I do not recommend you to isolate directly from plant tissue. wash and dry the leaves under tap water first to eliminate saprophites.Then place a half-inch potato slice on top of the leaves with lesion in a petri dish. you will see that phytophthora spores rise above the potato slice when you hold it for 2-3 days in an 18 C degree incubator. With a sterile needle, you can take and use these.

Pea agar can be used to isolate and ryeA medium is one of best choice to produce sporangia.

I agree with Göksel Özer Traps are usual for oomycetes isolation.

Please, try rye-agar.