They are both correct. Powdery mildew on squash can be caused by keeping the foliage too wet so overwatering but it also invades a stressed out plant that is suffering from underwatering. A thick mulch on top of the soil will help seal in moisture and cover the mildew spores. Try to keep the foliage of your squash plants dry. Good drainage is also important. Sunny days will help the plant outgrow the mildew so there is hope ahead. Keep growing, Marianne Binetti