This is a product of the final days of life for your plants, the season is coming to an end and the fruits are ripening, the damp morning air in autumn tends to spread this powdery mildew upon the leaves and gives you the sign to start clearing away the old plants for the winter, as the amount of daylight now become shorter, thinning the plants leaves and removing them will allow more sunlight to the fruits and harden up their skins for future storage, plus it will improve their colour and start to bring in those typical autumn colours we all enjoy, before you could find a spray to combat this problem, the seasons first frosts will burn the leaves and signal the end is near, best thing to do is to clear away old plants and cut off really bad bits- chuck them on the compost heap and gradually start to clear the site for winter. If however it happens earlier in the season next time use a copper sulphate based solution bought from any nursery, and use as directed- you might have to buy a sprayer too- but I sorry to say its too late now for that. A totally natural sign that the seasons are changing and nothing to worry about. stop the plants and get rid of any flowers and undeveloped fruit as there is not enough time for it to ripen, think less than the size of a small football(depending on variety), chuck them too on the compost heap- not good eating and won't keep. I do this every year with mine.