Good afternoon everyone,
The term rust is a generic name to designate a large number of cryptogamic diseases, that is to say due to parasitic fungi, it results in a reddish coloration.
Hollyhock is often affected by this disease, its leaves are then stained and wither away.
There are many types of rust and we know in particular that of the rose bush, each fungus attacks only one type of plant.
In the case of the hollyhock, the parasite is called Puccinia malvacearum and it concerns all the genera of the Malvaceae family: musk mallow, small mallow and large mallow, which often grow spontaneously in gardens.
The spores of the fungus can be carried by the wind or by insects, but most often by water.
Have a nice day.