Hi Neal ...
It's temperature. However, black spot spores are all over in the air. If the temperature drops below 85F for six hours and the spores can adhere to a moist leaf surface they can harm susceptible roses.
In my climate in the mountains, there is a 40 to 50 degree difference between high day temps and low night temps, but the climate is arid, so the six hours that temps may be below 85F doesn't make any difference because the leaves are dry.
The best defense is to find roses that are resistant to the black spot strain in your area. There are five active BS strains in the United States, so a rose may be very resistant in another humid climate area and be a BS dawg in yours.
Oh, yeah .... "We gardeners are as tenacious as weeds". I was told that I couldn't grow roses in my lousy soil. However, roses can't read and the roses I am growing don't know they are not supposed to thrive.
Smiles,
Lyn
