Allium Rust - Puccinia porri

This affects Allium species, including leeks, chives, garlic and onions.  Severe infections cause yellowing and premature death of the leaves, affecting the commercial viability of the crop,  but this is not considered to be a gall.  There is no host alternation.  Small, whitish aecia are produced on the leaves, then orange pustules (uredinia) and dark brown telia later in the season.

Puccinia sessilis affects some species of Allium, including Wild Garlic or Ramsons (Allum ursinum); it has orange aeciospores and the uredinia and telia are formed on a different host and are not present on Allium.

host

Photograph the rust on the host.  Ensure you have noted the host-plant species in your comments

on Allium species - leeks, garlic, onions, chives, etc..

There is some discrepancy in taxonomy.  It is also known as Puccinia allii.  Puccinia porri is given by some sources as the most recently accepted name, and is given a synonym of Puccinia allii by some; however, the NBN lists these two species separately.