Leaves turning black from the stem out
Some of the stem is black too. There are new leaves at the leaf crotches though. The plant was recently fertilized with a dry fertilizer (5-10-5). Maybe dust on the leaves?
The first problem is that I don't know what kind of plant you're talking about. And do you mean the stems turn black, and then the leaves? If you can send a little more information, we'll try to offer some suggestions.