I have a ponytail plant I purchased 6 months ago and repotted it into another container. It did seemed happy for a few months but then the new growth would get brown spots. I am watering it once a month. It is inside and gets indirect light. I do not see any sign of bugs. It seems so sad. What am I doing wrong and how can I get it healthy again?
It is not a disease. You made the most common of all plant care mistakes – unnecessarily repotting. Ponytails must be kept very potbound so that the soil dries out quickly after each watering. Otherwise, the delicate roots start to rot. If it takes a month for the soil to dry deep into the pot, then that is not soon enough to prevent root rot.
I don’t know what you did when you repotted, but if you removed all or some of the soil from the original rootball, then it will be very hard for your Ponytail to recover. Otherwise, undo your repotting by carefully taking the plant out of its pot and gently removing all of the new soil you added, including any on top. Then, place the original rootball back into the original or the same sized pot, adding as little fresh soil as possible.
After you have done this, you should find that a thorough watering every two weeks should be about right. You must also keep it inside very close to your sunniest available window and make sure that window is uncovered throughout the daylight hours.