Monstera, White Variegated Monstera, White Variegated Split-Leaf Philodendron 'Albovariegata'

I love this plant, have had it for a couple of years, and now I really need help with it. Just recently it has started turning black one...Read More leaf, one side, at a time. It starts about the stem end of the leaf, about a half inch wide within the leaf and to about an inch or 2 from the end of the leaf. First it is a pale yellow, and then gradually turns black. It goes in to the center of the leaf and continues black and crisp.

I put it in the shower, cleaning both sides of each leaf, and do not see any reason for it. No insects, parasites, etc. Probably a disease? I am not experienced with diseases at all. Can anyone out there help me with this problem before I lose the whole plant? I cut all that black off, don't know if that will help or hurt it. Thank you so much for any help you can give me. Jnette