hollyhock leaves have reddish brown lumps underside and they melt away.

Question

what is it and how do I treat the plant?

Answer 1

It sounds like you have some kind of leaf gall on your plant. Here's an article that should help: https://www.gardeningknowhow.com/plant-problems/environmental/treating-leaf-gall-on-plants.htm

The Missouri Botanical Garden's educational website also offers information on how to identify and treat the problem http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/rusts/rust-of-hollyhock.aspx