Plant diseases caused by *Pucciniales* fungi.
Rust is a common term used for a range of plant diseases caused by fungi of the Pucciniales order. Over 7000 species of Pucciniales are known and they can infect plant leaves, stems, fruit, and seeds.
Rust usually appears as a colored powder (spores) which land on the vegetation and cause pustules to form on the lower surfaces. By late spring / summer, yellow orange to brown (hence the name) hairlike structures (telia) form on the leaves or bark. These, in turn, produce more spores that spread infection.
All rust fungi are parasites that require a living host in order to complete their life cycle. As such, it is unusual for them to completely kill the host, but they can severely reduce growth and/or fruit yield. Cereal crops are particularly prone to commercial-level devastation. Young trees can also die if they are infected at an early stage (eg. younger than 5 years).
Agricultural treatments often involve crop spraying by fungicides from the air. Horticulturally, rusts are very difficult to treat. Fungicides can be used, but rarely eradicate the rust. for an organic solution, sulfur is known to reduce (and even prevent) spore germination. High levels of plant and soil hygiene are strongly recommended to help minimize problems. Good soil drainage also helps.
Use this tag for all questions about diagnosing rust, preventing/reducing infections and applying treatments.