I went through this at the end of the season last year so didn't really do much about it. I was pretty sure it was some type of broad mites though. Mites are almost impossible to see with the naked eye. If you have access to a microscope grab a couple of leaves and see if you can find anything. Hope you find what it is and get past it. Thrips could be another suspect.
If it was over fertilizing the tips of the other leaves would show signs too.