That's usually a calcium deficiency... but it can be caused by lots of things.. we need to know some information about your grow setup to help figure out where the problem is..
Are you in soil or hydro?
How old are the plants?
Room temp/ humidity?
Are you using Tap water / well water/ filtered water?
How often do you water?
Have you fed them anything?
If yes, what?
Are you checking the PH?