UC Cooperative Extension Ventura County
669 County Square Dr. Suite 100
Ventura CA 93003
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Large black spots, usually irregular in shape, on leaves are Black Leafspot disease and are caused by a fungus. The small orange spots are Rose Rust, also caused by a fungus. There are actually several rusts that infest roses. All of them produce the small orange pustules and cause the leaves to yellow and die prematurely. Finally, white spots are caused by powdery mildew, also a fungus microorganism. The late spring rains and the continuing fog and frequent morning fog drizzle are perfect conditions to keep all these diseases going through the summer.