Not for sure, but it looks like sunscald. Were those plants started from seed where they are or were they transplanted? Transplants may just not have been used to as much sun as they are getting now. If they are watered when the sun is on them drops of water can act as little magnifying glasses intensifying the sun on that spot.
If I am right, then the new leaves coming will probably be fine, as they are adapted to the sun.
If the new leaves have it too, then I am probably wrong and you should keep looking for more information!
Another possibility along the same lines would be if you have watered/ sprayed with anything that could burn the leaves - foliar fertilizer spray, pesticides, etc.