This is the most common and destructive disease on hollyhocks. Infections can develop on the leaves, petioles, stems, and green flower parts. Symptoms first appear as a pinpoint brown dot which rapidly enlarges into a circular spot.
Lesions appear as yellow to tan spots on the upper surface of the leaf.
Diagnostic symptoms then develop on the lower leaf surface as brown, raised pustules. As these mature, they break open to reveal the rusty-colored spores for which this disease gets its name. As they mature, the spores change to gray. When infection is heavy, this disease can be very disfiguring since leaves shrivel and whole plants can appear blighted. Several other weedy species of Malva are also susceptible and can serve as key sources of the fungus.