Phosphorous deficiency caused by ? Could be PH, lack of available phosphorous, humidity too high, root problems, and the list goes on. I recommend making sure your root zone ph is healthy, then the environment issues (heat,humidity,airflow,etc), them nutrients.
You may need to decide what you want to have in the root zone, as it relates to PH and PPM, and start pouring it in the top, until the same numbers (or very close) are coming out the bottom. This would be one way to know what the roots are dealing with.
I am assuming that you are not organic in soil.
Also note that once any leaf starts to show visible signs of damage, that leaf will never get better, and even if you fix the problem the damaged leaves will continue to die and fall off.