Ripsaw

Can you slice a piece of fruit across and lengthwise to show the "infection inside" and a closer photo of a couple of bananas? You could have a cigar rot. If the end of the banana starts rotting, and it progresses, that could be it. Some fungi are in the soil as well. When you take it down, pay attention to the inside of the stem and what it looks like. Dispose of the plant if that is the case.