Thank your for your question.  Although disease is not out of the picture, this looks like a cultural problem.  Tip burn can be a result of sudden water loss (or too much water) or over fertilization. Perhaps the plant was exposed to wind or touched a grow light or cold window You had mention that you planted up with a fertilized potting mix and added Tomato Tone. this mix may be excessive in nutrients.   If the problem is just a few lower leaves or leaflets, cut those off with a disinfected pruners/knife. Isolate the "infected" plants (just in case) and monitor. Keep plants evenly watered (not overwatered).  Saved seed if not treated can harbor pathogens.
I hope this helps,