Rust is a generic term used to describe several fungal infections having the same symptoms: brown, or black, pustules on the leaves or stems.
There are several different species of rust, and many are plant selective. Rust disease can cause premature leaf fall and stunting of plants – it can even kill them. Antirrhinums, chrysanthemums, hollyhocks, irises, roses, plums and leeks are prone to attack.
Unless severe, this common condition can be ignored apart from removing infected leaves and destroying them immediately they are seen. Destroy badly affected plants. Rust can be teated by a wide range of fungicides.