Im no expert but how are your temps and what kind of soil are you using? is it getting enough/ to much water?
Looks like your soil has too many nutes in it, or the pH of the soil is too acidic. Get an electronic soil pH tester, and check out the pH guide. http://forum.grasscity.com/absolute-beginners/218218-updated-beginners-guide-ph-marijuana.html
We need all the info on your grow including What strain? How old? What type of medium? What type of lights? How far from the lights? What, how much and when was it fed? What is the medium/runoff pH? What are the temps and humidity in the room? What size pots? If any of these questions have already been answered please disregard as I cut and paste this for everyone.