, could it be a problem of overfertilizing, or unecessary fertilizing if I may put it another way? i think that the plants are supposed to get fertilized in spring till start of the summer months, when growth is due, not in winter? Thank you.
, I have two Dieffenbachias, they need the light, but not direct sun, which dries the plant, so if sun gets on the leaves, move it further from the window. Leave it there, where it would be cooler and go from there, see how it reacts. I hope your plant recovers!