Rust appears as orange or yellowish-orange spores on the leaves of the grass plant. The spores are powdery and can be easily removed by rubbing the blade, leaving a rust coloured deposit on your fingers.
The problem is most common in late summer / early autumn when the weather is dry, and is usually associated with grass that is growing slowly. Low fertility (particularly of nitrogen) of the grass, together with a lack of water, can therefore allow rust to develop. However, seasons with excess rain, particularly on sandy, free-draining soils, can result in the depletion of available nitrogen by washing the fertiliser through the soil. Rust spores can be spread by air, water, mowers and other equipment, and on the soles of shoes, so care should be taken to avoid contaminating other lawns.
Rust is most effectively controlled by sound turf management. Aeration will help, as will proper watering, mowing and fertilising practices to prevent its development. For instance, try to water early in the day, so that the grass dries quickly. Problem thatch should be correctly managed by scarification, and ensuring that the lawn has good air flow and light availability by pruning trees and shrubs which are close by, will help. Fungicides are only rarely needed to control rust in lawns; it is generally better to focus on cultural practices such as those described above.