i myself dont think its a heat problem, i was thinkin it was a N or P problem but not sure cause there isnt any yellowing just the grayish green and brittle/crisp spots
i had the same problem and im pretty sure mine was when they were seedlings they were 2 cold when the were starting to grow atleast thats what i think just give them alittle time should heal up> or could be the other way around maybe getting to hot but im pretty sure it has to do with your temps you should get a thermometer
the leaves may be too warm for the what i assume is low soil temp or, the soil is not drying fast enough in those cups. stick a thermometer in the soil and see what is up