James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.
This fungal disease causes orangish, waxy, rust pustules on the underside of leaves and bright orange spots with red centers on the upper surface of leaves and on shoots of hollyhocks (Alcea rosea) and certain other relatives in the mallow family (Malvaceae). Eventually, the rust pustules on the underside turn a reddish to chocolate brown color, and when disease is severe, considerable leaf drop may occur.
The primary key to control is sanitation (there's that word again!) in its many forms. That means removing all infested plant parts, leaves, and stems at the end of the season. This helps limit the amount of overwintering fungus that will survive in your garden from year to year.
It is also important to remove any leaves that become infected during the growing season, because this fungus (Puccinia malvacearum) has a repeating stage that just goes on and on and on, producing new rust spores to continue causing new infections on your (and your neighbor's) hollyhocks. Finally, another means of sanitation includes weeding out the round-leaved mallows (Malva rotundifolia) from your garden since this plant is also a host for this rust fungus. In addition to sanitation, keeping water off the foliage to the extent possible and providing good air movement where the hollyhocks grow are important to help limit hollyhock rust development.
As to fungicide controls of hollyhock rust, products containing chlorothalonil, mancozeb, and sulfur can help, but infections start in spring and continue during rainy weather all the way into the fall. Therefore, numerous applications at 10- to 14-day intervals may be necessary to completely prevent rust from developing. This is why proper sanitation is so critical. Unfortunately, hollyhock varieties resistant to hollyhock rust are not available.