The orange pustules produce summer spores that can be carried for long distances on the wind. They need several hours of surface wetness in order to germinate and infect the leaf. Rust is therefore most prominent in wet summers, and particularly in autumn when heavy dews provide extended periods of leaf wetness. The black pustules produce over-wintering spores that are adapted to survival in cold conditions.
Lawns lacking in vigour (due to factors such as nutrient deficiency or drought stress) are often more prone to attack. Rust is often very noticeable on long grass or where trees or shrubs overhang the lawn. In these situations the grass is slower to dry out after rainfall or dew.