Puccinia malvacearum (hollyhock rust)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Basidiomycota
Subphylum	Pucciniomycotina
Class	Pucciniomycetes
Order	Pucciniales
Suborder	Uredinineae
Family	Pucciniaceae
Genus	Puccinia

Scientific Name

Puccinia malvacearum

Common Name

hollyhock rust

Elizabeth Bush, Virginia Tech

Reviewed by: Margery L. Daughtrey, Cornell University

Pathogen

Hollyhock rust is caused by Puccinia malvacearum. Teliospores and basidiospores are the only spores produced. During the summer, light chestnut-brown teliospores form in blister-like pustules on the lower leaf surface, petiole and stem; later they fade to whitish and germinate to produce basidiospores. The two-celled teliospores (occasionally 1-, 3- or 4-celled) are oblong (12-26 µM x 35-75 µM), generally tapering toward both ends, and have a hyaline pedicel that is up to twice the length of the spore. The smooth, yellow to cinnamon-brown teliospore wall is 1-4 µM thick on the sides of the teliospore and 5-10 µM at the apex.

Symptoms and Signs

Initial symptoms of the disease are yellow-orange spots on the upper leaf surface and raised, blister-like pustules that develop on the lower leaf surface, petiole and stem. Leaves on diseased plants brown, shrivel and die. Leaf symptoms typically start near the bottom of plants and progress upward. Severely diseased plants defoliate prematurely, but plants typically are not killed.

Ecology and Spread

Puccinia malvacearum is an autoecious, microcyclic rust, causing disease on many species in the Malvaceae family. On Alcea rosea (hollyhock), the disease is a common and very destructive disease. The disease also occurs on other ornamental hollyhocks: Alcea rugosa (Russian hollyhock) and Alcea ficifolia (fig-leaf hollyhock), although these two species have some resistance to the disease. Weed hosts, such as Malva rotundifolia (common mallow), are significant in that they provide a reservoir of inoculum for new infections on hollyhock. Like other rust fungi, P. malvacearum is an obligate pathogen. The resting spores (teliospores) can survive on host plant debris. Since P. malvacearum is systemic, it also survives in infected plant host tissue through the winter.

Geographic Distribution

Hollyhock rust is reported on a number of genera in the Malvaceae family worldwide.

Management

Plant hollyhock in full sun in locations with good air circulation and soil drainage. Space plants to promote foliar drying and avoid overhead irrigation. Remove diseased leaves as soon as symptoms appear, to reduce inoculum for new infections and remove severely diseased plants. Since hollyhock rust is systemic, it is advisable to remove any diseased plants at the end of the growing season: Bury, burn or bag plant debris and dispose in the landfill. Start new plants from seed or purchase disease-free transplants each year (note that hollyhock is a biennial). Manage weeds in the vicinity to reduce the chance of new infections from inoculum produced on other hollyhock rust hosts, such as common mallow. Of the ornamental hollyhocks, A. rosea is very susceptible to the disease; A. rugosa (Russian hollyhock) and A. ficifolia (fig-leaf hollyhock) have some resistance. Resistant varieties are available in the seed trade but these are not reliable in all situations, since different races of the pathogen exist and resistance is not effective against all races of the pathogen. Fungicides labeled for control of rusts on ornamental plants can be used preventatively for management of this disease. The first application should be made when the first leaves are expanding and treatments should be repeated according to label directions throughout the growing season. A wetting agent is recommended, since hollyhock leaves are hairy and good coverage is necessary for fungicide efficacy.

Diagnostic procedures

The disease is easily recognized by yellow spotting on upper leaf surface and the blister-like pustules that form on the lower leaf surface. The disease can be confirmed by examination of the morphological characteristics of the teliospores. Since P. malvacearum is an obligate parasite it cannot be cultured.

Resources and References

1. Arthur, J. C. 1934. Manual of Rusts in United States and Canada. Hafner, New York.

2. Farr, D.F., & Rossman, A.Y. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved September 15, 2014, from http://nt.ars-grin.gov/fungaldatabases/

3. Fox, R. T. V. 1995. Fungal foes in your garden: Mallow and hollyhock rust. Mycologist 9:129.

4. Gleason, M. L., Daughtrey, M. L., Chase, A. R., Moorman, G. W. and Mueller, D. S. 2009. Diseases of herbaceous perennials. APS Press, St. Paul.

5. Horst, R. K. 2013. Westcott's Plant Disease Handbook. Springer, New York.

6. Sivanesan, A. 1970. Puccinia malvacearum. [Descriptions of Fungi and Bacteria].Sheet 265.

Acknowledgements

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)