Dave,

The following is from experience with Phals so it may or may not apply. In
addition, I tend to see phosphorus problems more in the spring, but that
could just be my growing. I see more phosphorus problems than any other
nutrition problem. I wish it was more uncommon around here than it is.

The purpling starts with the older leaves and slowly moves to the new
growths. Leaves getting more sun will tend to purple first although the
plant is not receiving too much light. In the worst case, growth stops,
lower leaves start turning a purple/red/brown. With time it will kill
plants but it is a slow death.

First, check your roots. If the plant has a severely damaged root system,
it may show up as a phosphorus deficiency. If your roots are ok and you
have been feeding regularly it is likely a problem with the pH in your mix.
Plants have a hard time taking up phosphorus when the mix pH is between 7.5
and 8.5 or less than 4.0. If you are using Rain or R.O. water it is pretty
easy to get the mix pH under 4.0. If you have hard water and do not flush
enough, it is pretty easy to get your mix in the 7.5 - 8.5 range. Using the
wrong fertilizer for your water/mix can cause either case. When I see signs
of problems I run a little R.O. water through the pot and measure the pH of
what comes out.

If the problem is being caused by a pH problem, double flush the mix. If
you expect it is a low pH problem try using a Cal-Mag fertilizer for a
while. If you expect it is a high pH problem try a bloom booster
fertilizer. The first will raise the mix pH the other will lower it, so
pick carefully. I also start misting with a balanced general purpose
fertilizer that is pretty pH neutral a couple of times of weeks. If this is
the problem the plant will turn around fairly quickly (improvement seen in a
couple of weeks). All but the very worst purpling will return to its normal
color with time.

If this was problem, you will need to look to your water and fertilizer and
make some changes. The above is corrective but not really a long term
solution.

Pat Brennan
Brennan's Orchids