Disease can spread as soon as leaves have expanded. Blackspot germinates when leaves are wet continuously for 8-12 hours with temperatures staying between 60 and 78. I am not sure whether it germinates with temps in the 50s, but, if so, it probably requires longer wetting. So what you want to watch for is mild, humid, drizzly days. It will be too cool at night for a while. With the Bayer tebuconazole fungicide, you can spray up to a week after germination and still stop the infection.