Rust disease

Rusts are diseases caused by fungal pathogens of the order Pucciniales. About 7800 species are known. The taxonomy of Pucciniales is complex and the darker coloured smut is often mistaken for rust. Rusts are so named after the reddish rusty looking sori and the disease is usually noticed after the first rains. The group is considered as one of the most dangerous pathogens to agriculture and horticulture. All rusts are obligate parasites, meaning that they require a living host to complete their life cycle. They generally do not kill the host plant but can severely reduce growth and yield. Cereal crops can be devastated in one season and trees that get infected in the main stem within the first five years, invariably die.

Rust fungi can be categorized by their life cycle. Heteroecious rust fungi require two unrelated hosts to complete their life cycle, with the primary host being infected by aeciospores and the alternate host being infected with basidiospores. This can be contrasted with an autoecious fungus which can complete its life cylce on a single host species.

Rust fungi can be further categorized by how many spores are produced during the life cycle. Fungi that produce all five spores (sometimes excluding pycniospores) are termed macrocyclic. Fungi that lack pycniospores, aeciospores, and urediniospores in their life cycle are termed microcyclic and always have an autoecious life cycle. Demicyclic fungi delete the uredial (repeating) stage from the life cycle. Understanding the life cycles of rust fungi allows for proper disease management.