Blight is an air bourne problem ,they used to give warnings with the weather forecast if the conditions were right for the spread of it. Bordaeux mixture has been the recommended spray for many years but I generally have found that if you get it ,it is too late to spray but if you cut all the foliage off and destroy it the blight does not get washed down to the tubers and your crop is safe. Once blight gets on the foliage ,the foliage is no ore good to the plant so you lose nothing in the way of a crop.