Yes, it does look like nute burn to me too.... have you thought of flushing them? Couldn't hurt. Alos, I don't know about the drip system watering for 15 min 3 times a day. Could be too frequent of watering and not enough. I have found that soaking them good every 3-4 days when they are young and then letting them mostly dry out seems to work well...they don't like their feet to be constantly wet. When they are bigger and still in same pots then I water every other day and only once a week with nutes.