For a couple of months, small black spots have been slowly spreading on two of the leaves of my Monstera. On one leaf, they got worse, and ringed with yellow, then suddenly the leaf died.
The black spots are physically on the surface of the leaf, and can be scraped off. I tried wiping the leaves with bicarb soda (maybe for no good reason) but it didn't seem to help.
Now, the badly affected leaf has suddenly turned yellow and died.
Is there anything I can do to prevent this happening to the other two leaves? I don't think I'm overwatering - if anything, perhaps the opposite. I check the soil is dry before I water.