The fungus causing rose rust is, like all rusts, a biotroph: it infects the host tissues for extended periods without killing them, feeding from the living cells. Rusts are not able to feed from dead plant material, so must either alternate with a different, perennial host, or produce a resting spore to pass the dormant season.
Phragmidium tuberculatum and several other very similar rust species that infect roses do not have an alternate host; that is, they only attack roses and pass the winter as resting spores.
The first formed spores (spring spores) infect young stems, causing distortion and the production of bright orange pustules. Spores from these pustules in turn infect the leaves to produce dusty orange spores (summer spores) that are spread by wind and initiate further infections. Infection is favoured by prolonged periods of leaf wetness. In late summer, the pustules producing summer spores switch over to producing the dark, tough resting spores. These resting spores can survive the winter, on fallen leaves or often adhering to stems or trellises.
Infections may be severe enough to cause serious damage, but this is relatively rare and most infections are light enough not to require control.