Fungi can infect plants as well as animals. Rose black spot is a fungal disease of rose plants. Purple or black spots develop on the leaves, which often turn yellow and drop early.
The disease affects the growth of the plant because photosynthesis is reduced. With fewer healthy leaves, the plant makes less glucose.
Rose black spot is spread in the environment by water or wind. It can be treated by using fungicides, which kill the fungus, and/or by removing and destroying the affected leaves so the fungus cannot spread from them.