A serious disease of roses is which of the following?

• A. Powdery mildew
• B. Rust
• C. Leaf Spot
• D. Black Spot

Answer: (D)

Black spot on roses is a serious foliar disease caused by a fungus (Diplocarpon rosae) that creates round black spots on leaves, usually with a yellow halo. As these spots enlarge and leaves yellow and drop, the plant loses leaves needed for photosynthesis, which weakens vigor and reduces flowering. That defoliation makes black spot a particularly damaging issue for roses, especially in humid, wet climates where the spores spread easily and overwinters in fallen leaves.

Powdery mildew on roses shows up as a white powder on leaf surfaces and stems and, while unsightly and able to stunt growth, doesn’t usually cause the same rapid leaf drop and loss of vigor seen with black spot. Rust produces orange or rust-colored pustules on the undersides of leaves but is often less devastating to roses than black spot. Leaf spot is a general term for various diseases that form spots, but it isn’t as specifically tied to the severe defoliation that characterizes black spot on roses.

To manage it, remove and dispose of infected leaves, improve air circulation around the plants, water at the base rather than overhead, and apply appropriate fungicides following label directions.