Powdery mildew will start towards the upper surface of the fruit , where it is attached to the vine, and gradually spread across the entire fruit.
Powdery mildew is more common during periods of high humidity with inadequate air circulation.
As the name of the disease implies the mildew is powdery and will leave a residue resembling fine baby powder on affected areas of the plant and fruits.
Any cucumbers that have been infected with powdery mildew should be discarded and not in the compost heap, put them in the trash. To minimize its reoccurrance the garden areas affected must be made less hospitable.
If need be - thin plants to allow for adequate air circulation. Avoid watering in the evening - try watering in the morning instead. Plants watered at dusk will remain damp all night - which is a condition condusive to the growth of fungus. Watered in the morning they will dry during the sunlight hours. If feasible, you might want to consider a Drip Irrigation System