As I u nderstand it,Smith periods provide the ideal conditions for blight spores to do their worst, but you do need them present to start the spread in your area.
The disease tends to spread from the south west where the mild winters help it survive during the winter, then wind borne spores and aphids, whcih act as vectors, spread westward with the prevailing winds.
Local sources of blight can also occur, the overwintering volunteers being the worst offenders and that is the reason why they should be removed as soon as they are spotted.
Phil