Puccinia malvacearum is a rust fungus that galls the leaves and petioles of members of the Malvaceae family including Hollyhock (Alcea rosea) and Mallows (Malva sylvestris, etc.).  There is no host alternation, and only telia are produced, on the leaf underside.  These are pale orange-brown to begin with, becoming darker brown and then grey as they mature.  There is a corresponding yellow spot on leaf upperside.  

host

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

Waste gound, field edges, roadsides, grasslands and gardens: on plants in the Malvaceae family, including Common Mallow (Malva sylvestris), Musk Mallow (Malva moschata), Tree Mallow (Lavatera) and Hollyhocks (Alcea rosea)