Hollyhock rust is caused by the fungus Puccinia malvacearum. The rust fungus causes raised, brown pustules about the size of a pinhead on the undersides of leaves. Yellow-orange spots develop on the uppersides of the leaves above the pustules. Pustules may also occur on the stems and other green plant parts.
Control: Remove and destroy the leaves on which rust is first observed. Fungicides, such as chlorothalonil (Daconil 2787), can be used to control the disease. The first application should be started in the early seedling stage and repeated on a 7 to 14 day schedule. After flowering, the plants should be cutdown and removed. Also destroy all common mallow in the area. This weed is also susceptible to the rust and is a means of spreading the disease to hollyhocks.
This article originally appeared in the June 8, 1994 issue, p. 87.