We have a large - floor to ceiling, mature – over 50 years old, monstera deliciosa which has been in its current location in a corner with windows on two sides and a ceiling lantern above, so lots of light and out of any draughts for just over a year. Prior to that it was in a similar location in another house.

In the last couple of weeks the underside of the tops of some of the leaves and some of the leaf ribs are going dark brown. The leaves are not drying out as it is in a pot with a built in reservoir which is regularly topped up, from March to October it is topped up with a liquid plant food, just water at other times. This regime has worked well.

Any suggestions as to what it might be and how to remedy it would be appreciated.