Soil borne fungal pathogens are everywhere, in practically every soil. It's the conditions that create the potential for a susceptible plant to become infected.

Tell us a little bit about why you think that these liriope were infected in the first place. I'd also like to hear something about the site and your planting techniques and after care.

By the way, it's pretty common for phytophthora (and other fungal pathogens) to be introduced in the nursery.

Well the liriopes were drooping and yellowish in its potted state even before planting. Then I planted it and a week later it looked slightly worse and I didn't want to take any risks so I dug it out and threw it away. The conditions of that site itself is fine. It has good draining soil and I planted the new liriopes slightly above ground level. I watered it once with a hose for 5 minutes and then that was it for the week. I will water it again the following week. Once a week is enough right?