Hmmm, could be one of many things... Using the right nutrient mix for vegetative stage? Could be too rich of soil/nutrient feeding... Looks like some kind of nutrient burn... but like Diabolica says - It could be over watering...
Also - Are gnats hanging around the soil? Could even be problems from gnats - seems like gnats in the soil turn the leaves tollaly yellow before burnt part comes on and the plant starts loosing leave from the bottom up real fast... That said, it doesn't look like a gnat problem...
One other possibilty is the plants may not be draining well enough - fungus gets in real fast when that happens... Check the holes in the pots to see if they aren't letting enough water out.