There are many different types. One of those is rust, which causes brownish orange bumps on the bottom of the leaves. A way to treat it is by spraying the leaves with hydrogen peroxide and diluted aspirin.
Another is powdery mildew, which is a white fungus resembling powder that grows on the leaf. It typically doesn’t hurt the plant, but can look unpleasant. It can be treated with fungicide.
Bud blight is a fungal disease that can be spotted on the flower petals themselves, usually caused by dripping water overhead. They will show reddish brown blotches on the petal, eventually turning brown and falling off. It can be treated with fungicide and moving the plant to a dryer spot.
