This rust affects the leaves of Berberis repens, Mahonia aquifolium and Mahonia japonica.  There is no host-plant alternation.  Aecia are orange and on leaf undersides; with a corresponding blackish spot above, usually on young leaves. Aecia may also form on fruits. Later, brown sori containing urediniospores and teliospores develop on leaf undersides.

host

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

on Mahonia, usually in parks and gardens, and Berberis