There is a black spot on the stem of my Cavendish Dwarf Banana. I had it since about January 1st and have been watering it when the top layer of soil is dry like the directions say and I give it a 20-20-20 water soluble firtilizer. Is there any thing I am doing wrong or should I put some kind of fungicide on it. Please help me identify this. Thanks. I would post a couple of pictures if I knew how. Please tell me how to post pictures. Thanks!

The black spot that you are referrig to is the base of the stem rotting. This is due to one of two reasons: 1) Overwatering 2) The medium you have planted in is not very good at draining, causing the stem to rot. The fix here is to replant your Cavendish in this soil mixture I am gonna give you. You will need to mix the following three items together in equal portions. The items needed are top soil, sphagum moss, perilite. By making this mixture you will wind up with a fluffy soil that contains natural minerals and most importantly will drain nicely. Replant your cavendish in this mixture and within 1-2 weeks after you replant you should notice a good change in the look of your cavendish. Also try watering every other day and fertilize once a month with some good 20-20-20 fertilizer. Best of Luck to you !!
Wayne