Q. My hollyhock foliage is covered with strange reddish bumps or blisters cover the entire leaf. Whats going on?
A. Hollyhock rust is a fungal disease that normally appears a bit later in the year on susceptible cultivars of hollyhock. Hollyhock rust is characterized by light yellow-orange spots on the upper surface of leaves. Brown pustules develop beneath these light spots on the underside of leaves. Pustules may also develop on the upper side of the leaves and on stems and green flower parts. In severe infestations, leaves dry and hang down along the stem.
Rusts are known to be especially destructive on grain crops such as wheat, oats and barley. Rusts also attack vegetables, cotton, soybeans, flowers, coffee, apple and pine trees. Some rust, such as cedar apple rust, requires two hosts to complete their life cycle and are said to be heteroecious. Hollyhock rust is an autoecious fungus, requiring only one host to complete its life cycle.
Hollyhock is a member of the Mallow family, originally from Asia, whose members seem especially vulnerable to attacks by rusts.
Rust fungi spread from plant to plant mostly by wind-blown spores. Insects, rain, and animals may also spread this disease. Spores of some rusts are known to be carried several hundred miles by strong winds.
Hollyhock rust overwinters in leaves and in old stems. The removal of all infected plant parts in the fall or early spring is very important.
Applying a layer of mulch around hollyhock and other host plants in the early spring can help prevent spores overwintering on plant debris from infecting new plant tissue. Make sure to water the plant from below to avoid wetting the foliage.
Fungicides such as sulfur, cholorthalinil (Daconil), Immunox, or maneb are effective against hollyhock rust, but they must be used at the first sign of disease.
There is much confusion about the life cycle of hollyhocks. Most hollyhocks are biennials, which means they usually make a rosette of foliage the first year and then send up a long flower spike to bloom in the second year. There are some hollyhocks that will bloom the first year, especially if started early indoors. Some hollyhocks are classified as short-lived perennials. Most of the hollyhocks grown in our area are biennials.
Hollyhocks are sometimes called outhouse flowers because they were often planted to hide outhouses. If you never made dolls from hollyhock flowers, you missed out on a wonderful childhood experience.
Other plants is the mallow family, such as Rose-of Sharon, globe mallow, flowering maple, and lavetera, can also be infected by hollyhock rust, and can pass the disease to your hollyhocks. Make sure to inspect those plants if they are also in your flower garden.