Doesn't look like powdery mildew to me. It seems to be more along the lines of nitrogen burn or starvation and possibly combined with environmental factors (hardening off, sun burn, etc.)
See how the cotyledons in the second pic are whiting out, basically used up? (Normally they'd wither and fall off...but!...) Notice the older true leaf in the first pic is showing signs of burn/whiting but yet the newer leaf is nice and green? Normally a plant will send available N to its new growth so the older leaves will suffer from lack of N. Powdery mildew would attack all the leaves, not just the older ones.
And also see how the "mulch" around the plant is fresh, probably still breaking down and stealing the N from the tiny root system of the plant? This would also create heat that could physically burn a plant not properly hardened.
The plant may be suffering from not being hardened off properly, which, again, may be why the cotyledons and the first leaves are showing signs of scald (whitening) while the newest leaf is green, the latter having grown under the sun and hardened off naturally.
Powdery mildew, and downy mildew, will appear on the surface of the leaves, appearing as something you can nearly rub off with your fingers. This looks more like something within the plant tissue.
Shoe (always wondering....)
