mildew is , i think, caused by warm, damp conditions..so, possibly too much water, are they in pots or the ground /
we sink a 2 litre water bottle, upside down with thw bottom cut off, water into it, the water oes to the roots so the foliage stays dry, hope this helps :)
also, put your small veggies up on somewhere so' they can dry out..we have them on a grill type thing :)