Hollyhock Rust: Puccinia malvacearum
Curtis E. Swift, Ph.D., Area Extension Horticulturist
- Kingdom: Mycetae
- Division; Amastigomycota
- Subdivision: Basidiomycotina
- Class: Basidiomycetes
- Subclass: Teliomycetidae
- Order: Uredinales
- Genus: Puccinia
- Species: malvacearum
The term `Rust' is used to designate a group of fungi as well as the rust-colored symptoms of the diseases resulting from these fungi. Rusts are among the most destructive of plant diseases with many being highly specialized parasites attacking only certain host genera. Some specialized forms of rust (called races) attack certain varieties within a plant species. For example, one Puccinia graminis race attacks only wheat while another Puccinia graminis race attacks only barley. Within each of these races, however, are so-called physiological races that attack only certain varieties within the species. In other words, one physiological race may attack one variety of wheat exclusively and not other wheat varieties.
Rusts are known to be especially destructive on grain crops such as wheat, oats and barley with shortages causing famines and ruining the economies of entire countries. Rusts also attack vegetables, cotton, soybeans, flowers , coffee, apple and pine trees. With 4000 species of rust fungi, the potential destruction caused by this type of disease organism can not be under-estimated.
The various stages of rust development produces different types of spores. Some rusts produce up to five different spore forms. Some of these spores parasitize one host while other spore stages infect and parasitize different (alternate) hosts. The teliospore (sexual stage) is the overwintering stage, which creates a structure producing spores the following spring to begin the infection anew.
Some rusts require two hosts to complete their life cycle and are said to be `heteroecious'. Heteroecious rusts spend part of their life stages on one host and the remainder of their life on another host. In other words, they alternate between hosts. The juniper/hawthorn rust, a common problem in Western Colorado, is an example of this `heteroecious' life cycle. Both hosts (juniper and hawthorn) are required for this rust to complete its life cycle.
The Hollyhock Rust:
The Hollyhock (Althea rosea syn. Alcea rosea) rust (Puccinia malvacearum) is an autoecious fungus, requiring only one host to complete its life cycle. Other members of the Malvaceae family attacked by this fungus include:
- Abutilon - Flowering Maple, Indian Mallow
- Alcea - Hollyhock
- Althaea - synonym for Alcea
- Hibiscus - Rose Mallow, Giant Mallow - such as Rose-of-Sharon (H. syriacus)
- Lavatera - Tree Mallow
- Malva - Mallow, Musk Mallow, `cheese' weed
- Malvastrum - False-mallow.
- Sphaeralcea - False Mallow, Globe-Mallow, these are warm-region herbs and shrubs.
The Hollyhock rust fungus can complete it's life cycle on any of these plants with each serving as a source of infection for hollyhock.
Symptoms of the Hollyhock rust fungus initially appear as light yellow-orange spots on the upper surface of leaves of the various hosts. Ultimately brown pustules develop beneath these light spots on the underside of leaves. Pustules may also develop on the upper side of the leaves (a different spore stage) and on stems and green flower parts. In severe infestations leaves dry and hang down along the stem.Control:
Westcott states that this rust is so common and destructive it limits the use of hollyhocks as an ornamental. Planting in a location that has plenty of air circulation with plenty of space between plants helps reduce the chance of infection. Increased humidity around the plant results in increased production of rust spores. Avoid wetting foliage when watering.
There is often a direct relationship between the period of time during which plant foliage remains wet and infection. Therefore, if it is necessary to wet the foliage for any reason, it is essential to maintain conditions that will allow the foliage to dry out as quickly as possible. For further information on leaf wetness and disease potential refer to the Tri River Area Horticulture publication on this topic.
Rust fungi spread from plant to plant mostly by wind-blown spores. Insects, rain, and animals may, however, help spread this disease problem. Spores of some rusts are known to be carried several hundred miles by strong winds and can start new infections at their final destination.
Sanitation is critical in controlling Hollyhock rust as the fungus overwinters in pustules in basal leaves and in old stems. The removal of all infected plant parts in the fall and removal of dead plant parts in very early spring is very important. Rusted leaves should be removed as soon as they appear during the growing season and flower stalks should be removed when flowering is over. Plant debris can be burned, buried in the garden or buried in the compost pile.
Applying a layer of mulch around hollyhock and other host plants in the spring will help prevent spores overwintering on plant debris at their base from infecting new plant tissue.
Remove mallow weeds (cheese weeds) from the area, especially Malva rotundifolia.
A preventative treatment of fungicide can be applied prior to infection. Periodic applications of wettable sulfur, begun several weeks before rust normally appears is recommended. Pirone recommends that seedlings be thoroughly sprayed as soon as infection is detected. Unless the treatment is continued after plants have been planted out, later infection will be serious.
Frequent treatments will be needed to protect new growth as it develops. Treatments should also be made to nearby susceptible hosts to prevent the spread of the fungus to Hollyhock. Other fungicides listed for ornamentals should also be effective. Check with your local nursery or garden center or Extension office to be sure these materials are legal and acceptable in your area.
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Ball, J. & L. Ball. 1990. Rodale's Flower Garden Problem Solver. Rodale Press.
Cranshaw, W. 1992. Pests of the West. Fulcrum Publishing, Golden, Colorado.
Farr, D.F., G.F. Bills, G.P. Chamuris, A.Y. Rossman. 1989. Fungi on Plants and Plant Products in the United States. American Phytopatholical Society.
Forsberg, J.L. 1975. Diseases of Ornamental Plants. University of Illinois Press.
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Horst, R.K. 1979. Wescott's Plant Disease. Van Nostrand Reinhold Company.
Jarvis, W.R. 1992. Managing Diseases in Greenhouse Crops. APS Press.
Pirone, P.P. 1970. Diseases and Pests of Ornamental Plants; fourth edition. Ronald Press Company.
Placed on the Internet May 5, 1997; Updated January 11, 2010