i have same temps, I'm into 3 weeks Vegetative and I am experiencing the same problem
at first it was little bit, but i seen more it looks like talc powder and its even on the new leaves. i read in the grow bible that it could be powedery mildew and in the book it says
Fungus or mold on foliage or soil:
High humidity (above 60%)
High temperature (above 80 F)
-- Add Vent Fan
--Spray soil with 5% bleach solution and wash off next day. Spray foliage with 10% baking soda solution.
I might try this tomorrow , but i think u have to do it for several days.
bad news is that it might kill the plant if it really is powdery mildew, i really hope its not, i already spent so much $ and time and effort... damnnittt