Generally, the most favorable conditions for powdery mildew are daytime temperatures near 80 degrees F with a relative humidity of 40 to 70 percent, and nighttime temperatures near 60 degrees. Remove and destroy infected shoots at the end of the season and rake up and discard infested, fallen, dead leaves. As these pustules develop, they become visible on the upper leaf surfaces as orange-brown spots. Nine species of the rust fungus Phragmidium are found on roses. Rust can develop when temperatures are 65 to 70 degrees and moisture is continuous for two to three hours.
Considerable variation exists in plant resistance.
It is not uncommon for only one or two rose varieties in a planting to get rust while others remain rust-free. It might be that a species of the fungus is present that can develop only on particular varieties. With nine rust species that can attack roses, there frequently are differences in resistance. Management of rose rust depends on a variety of approaches. When used in an integrated system, these approaches can minimize the development and damage caused by the fungus.