Need help..!!! 420-special... Need advice...yellow/brown leaves

WELL FIRST OFF...HAPPY 420 TO ALL.... SECOND I NEED HELP WITH MY PLANTS THEY ARE 15 DAYS OLD.. AND EVERYTHING WAS GOING GREAT BUT NOW ON TWO OF MY 4 STRONG PLANTS ARE TURNING BROWN AT SLIGHTLY CRUSTING


THIS IS MY ENTIRE GROW ROOM IT IS 13 PLANTS TOTAL 4 ARE 15 DAYS OLD AND THE OTHER 9 ARE 4 DAYS OLD.
THEY ARE UNDER 16/ON 8/OFF LIGHTING SCHEDULE!!!

MY SECOND QUESTION IS IF I CAN TOP THIS PLANT??? AND WHERE??? PLEASE HELP AND HOW DO I TOP THE PLANT???
TOP VIEW SIDE VIEW UPPER LEAF SET LOWER LEAF SET ROOTS

(UNDER 30x MAGNIFICATION)
PLEASE HELP.....

brown tips are usually from over feeding.
too young to top
numerous ways and times to top.
Me, I wait till I have over 4 nodes then pinch out the top back to the 4th.

Those spots almost look like a deff. I would consider if feeding is needed. Some of the pro soils are pretty bland. I use MG soils which has decent nutes in it and I dont fert through the entire veg cycle unless needed.