I have had a pleurothallis alveolata for about a year now and I am keeping it in a large orchid tank set up that I use to grown my humidity-loving cloud forest orchids. The tank stays at about 80-95% humidity and has good airflow with a computer fan that blows mist from a mister towards the orchids and out an opening at the other end of the tank.
Yet, my Pleurothallis is constantly browning at the tips of the leaves - even new leaves a few weeks after they have just developed!
how often do you fertilize and what's the TDS of your water?
Hmm... I haven't measured TDS, but I do use RO water so I don't think it's problems with too much salt. I fertilize with MSU about once a week, sometimes every two weeks.
I don't have experience with this species, but perhaps you can try to flush the pot with RO water, repot it in fresh sphag and stop fertilizing it for a few months and see if the new growths will have the brown tip.
Sounds like excess build up of salts to me as well. I think it may be the concentration of fertilizer you're using. Try diluting the solution more, and stick to once every two weeks. Using RO water for the watering and the misting.
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