Hollyhock rust is caused by the fungus Puccinia malvacearum. The rust fungus causes raised brown spots, usually pinhead-size or larger, on the undersides of leaves. The upper leaf surface shows orange to yellow spots. The raised spots may coalesce, killing larger areas of leaves.
Control: Remove diseased leaves. After flowering, cut the plant back to the base and destroy infected plants parts. Fungicides, such as chlorothalonil (Daconil 2787), are effective against rust but frequent applications are necessary to protect newly emerging leaves.
This article originally appeared in the May 26, 1993 issue, p. 79.