could be deficiency, but honestly, my bottom sets of leaves always die off anyway. as long as new growth is green and symptoms don't spread don't worry too much about it. fill this out for more help We need all the info on your grow including: What type of medium; soil or hydro? What brand and type of soil? Indoors or outdoors? What strain? How old are the plants? What type of lights and how many watts? How far from the lights? What is your watering frequency and source of water? What, how much and when was it fed? NPK? What is the medium/runoff pH and PPM if in hydro? What are the temps and humidity in the room? What size pots? Any bugs? Look real close. Any other pertinent info?