Rose rust is a disease affecting the rose shrub caused by a number of fungi (such as Phragmidium tuberculatum^{http://www.rhs.org.uk/Advice/profiles0600/rose_rust.asp}). The spores of the rose rust fungi infect the leaves of the shrub during the beginning of summer, resulting in the appearance of orange pustules. Towards the end of summer, the orange pustules are superseded by black pustules, which contain overwintering spores.

Rose rust often causes the death of a rose shrub, with severe infections resulting in a weakening of the plant (and, in serious cases, death) due to premature defoliation.

See also

Royal Horticultural Society's website: entry on Rose Rust.