Check this stuff out
https://www.rollitup.org/view.php?pg=faq&cmd=category&id=33
Can't really tell you what it is without a picture, it could be a lot of things.
Also what is your setup, how old are they, are they in soil or hydro, indoor or out, temp in the room, what lights are you using, do is there good air circulation if indoors and do you have a fan blowing on them? Have you fed them any fertilizer, if so how much at what concentration? What is the pot size and how often do you water?