Rust is a fungal disease that occurs on grasses when their growth is slow. You will know if you have rust disease as grass blades are coated in an orange red to yellow/brown dust or spores that resembles rust. You will be able to rub the dust off with your fingers and affected patches of lawn will generally become thin and weak. Rust usually happens in Summer or late Autumn, during periods of dry weather or when the grass lacks nitrogen. It is easy to remedy by fertilising the lawn and a change in weather can help.