The rust fungi are described as biotrophs; that is, they grow within the living tissues of the plant and extract nutrients from the cells without killing them. However, although they do not kill tissues, heavy attacks by rusts can cause the leaves to shrivel and die prematurely and can depress vigour.
Rusts are not able to survive in an active state on dead plant material, so those affecting annual plants must either alternate with a different, perennial host, or produce resting spores to pass the dormant season.
The leek rust pathogen seems to fulfil its entire life cycle on leeks, without the need for an alternate host. On some other Allium species the fungus begins to produce dark resting spores within the orange pustules as the foliage dies down. These resting spores have been observed occasionally on leeks, but the role that they play in the disease on this crop is currently unknown. It is likely that there are simply sufficient leeks in the ground at all times of the year to ensure continuity of infection, without the need for resting spores.
Puccinia porri (syn. P. allii) has been confirmed as being seed-borne, but this is not currently thought to be of any great significance in the spread of the disease.
It is thought that a number of strains of P. porri (syn. P. allii) exist, varying in their ability to infect different Allium species.