Rust is a type of fungal plant disease. Usually rust diseases are easy to identify because of the abundant production of colorful powdery spores formed in tiny pustules. Microscopic examination may be necessary to identify specific rust fungi. Spore production sometimes is limited to the underside of leaves, while the top surface may show discolored spots. Fruit infections may occur in susceptible fruiting plants. Many rust spores appear orange to yellow, while some rusts produce brown to black spores. Some rusts cause a reddish lesion on the infected plant. Most rusts are very host specific and only infect certain plants. Many rusts also need two different types of plants (primary and alternate hosts) to complete their life cycle.

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