I answered my own question by spraying two very blackspot-prone minis that I no longer care for. I decided they would be my guinea pigs. Well, they have no foliage left. It all turned a horrible orange, then black color. Yes, you should follow directions. No, you should not spray sulfur sprays when the temps get too high.

thank you for your sacrifice in the name of rose knowledge.

Funny, triple_b.

It was rather a "well, duh" moment.

could you spray in the evening when it cools off?
You know, sometimes we have to try stuff and break rules anyway. JUST to see. Human nature I think. Hey sometimes things work too you know? (it seemed like a good idea at the time... my motto)

Very interesting experiment. I wonder if Neemoil can be sprayed on roses in very hot weather. It works well against blackspot and coats the leafs with a thin film of oil. I wonder if this wax layer can get too hot in the sun.

I supose you can spray the soil with limesulphur to reduce the amount of fungus spores but plants have to be covered with plastic or something.