At the core of assays to understand the role(s) of specific genes is the ability to stably transfer genes into Phytophthora through transformation. A key method for achieving this has been based on polyethylene glycol (PEG)/CaCl2 transformation of protoplasts, but efficiency has often been low. Improving transformation efficiency is necessary for many applications, such as gene knockouts. Here we describe improvements through successive rounds of “mock” transformation, leading to improved efficiency in Phytophthora infestans and other species.