Certified GKH Gardening Expert
It does sound like a fungal infection. This has an easy cure, since it can fight off the infection for the start of its growing season.
You will need a handful of DOLOMITIC LIME and a tablespoon of WETTABLE SULFUR per plant.
These together, will correct the fungal issue. It is not a bad idea to keep these ingredients around to apply once per year to the soil as a preventative.
This article will give you information on growing Mallows: https://www.gardeningknowhow.com/ornamental/flowers/common-mallow/growing-common-mallow.htm
All the plants in the Mallow family are prone to "rust" with hollyhocks being the most popular ornamental that is affected. The bumps are orange which is where the "rust" name comes from. Here is another article that discusses what to do if your mallow family plant has rust. https://hort.extension.wisc.edu/articles/hollyhock-rust/ Rust survives winter so it is vital to carefully remove all plant material at the end of the season. Good sanitation includes removing affected leaves as soon as they are noticed. Good air circulation is important in preventing all fungal diseases which is hard to achieve when plants are against a structure. You may want to space them out further but this is no sure fix as hollyhocks and the like are rust magnets, unfortunately.