post some pictures of the plants to be looked over
your plants look good, if anything raise your light like 6 inches.. bit of light stress going on..
has the problem gotten any worse or moved to other parts?
flowering, you could spray a mix of water/hydrogen peroxide around 5-8% strength to kill what you can..
id spray with no fans on and leave for 15-20 mins or the length of smoking a fat joint than crank on the fans and repeat the next day..
everything in the tent within reason gets sprayed...
watch your watering! that seems to be leaf septoria, only on one branch? well she may spread as it’s a fungus and a sign of root rot, maybe in the beginning stages.
post close pictures of the whole plant etc..