Agreed, but it's also often difficult to spray only the leaves without getting offspray on the flowers.
PM isn't systemic, so you could just take a cut from the mother and use a different approach to cure it. PM is caused by environmental issues, so if you are getting PM, the first step should always be to get your environment in check. Airflow and humidity control goes a very long way in beating back PM. Also there are likely spores all over your grow area if you do have PM, so a good sulfur burn is likely needed to kill the spores hidden in the corners of the room.