I have had my corn plant for about 5 months and noticed it was getting dusty so I wiped it down with just a clean paper towel. As I did this, I noticed a lot of what appeared to be black dirt on the leaves, especially at the top of the stalk and the base of the leaves, sort of in between the leaves. Could this be a sign of a pest or just dirt, perhaps trapped as the plant grew (seems unlikely)? A few leaves have also been dying, starting at the tips...but I think this could be fluoride damage. I have been letting the water I use for it set for a few days to allow chlorine/fluoride to evaporate but I also added a time-release stake thingey to fertilize it recently and am thinking this could have been the culprit. The dead spots on the leaves do have a VERY FAINT yellow tinge to the edges. Now I see some dead-looking lines on a few leaves but I'm thinking that I may have accidentally done that while dusting by cracking the leaves?? Does that make sense? I do not have a green thumb but I really don't want to kill this poor plant so please help!!!