November 2004

Rust on the Queen's Crown
Chrysanthemum White Rust

Contact: John Bowers, Maryland Department of Agriculture
410-841-5920 | bowersjh@mda.state.md.us

Photo: MDA

ANNAPOLIS, MD (November 23, 2004) - Since the chrysanthemum was first introduced into the United States during colonial times, its popularity has grown such that mums now reign as undisputed "Queen of the Fall Flowers." However, a fungal disease called chrysanthemum white rust can severely impact this show of fall color. Chrysanthemum white rust, caused by the fungus Puccinia horiana, is a very destructive disease of many chrysanthemums and related species, including pot mums, spray mums, and garden mums. The pathogen is not established in the United States or Canada, but a recent isolated introduction into Maryland qualifies this pest as the Maryland Invasive Species Council's "Invader of the Month" for November.

The disease originated in eastern Asia, and eventually spread to Europe, Africa, Australia, Central America, and South America. Local outbreaks of the disease have occurred in both the United States and Canada; however, successful detection and eradication processes have kept the disease from being established. In 2004, chrysanthemum white rust was found in one nursery in Maryland. Eradication was quick and successful, preventing the disease from causing extreme losses in the nursery industry or becoming established in the State. In countries where the disease occurs, weekly fungicide treatments are needed to manage the disease, which is costly.

The first visible symptoms of chrysanthemum white rust are small white to yellow spots up to four millimeters wide on the upper surface of the leaf. They may be slightly sunken or dimpled and become brown over time. Pustules develop on the underside of the leaf beneath the small spots. The pustules originally appear as buff to pink colored, but become white as they mature, giving the disease its name. Symptoms usually develop during cool, wet weather, appearing 5 to 14 days after infection. Hot, dry weather or the application of fungicides can mask or suppress disease development. This fungus only grows and reproduces on chrysanthemums and related species.

New infections are initiated by spores released from pustules during periods of high relative humidity and when temperatures are between 40 and 73 F. Spores are spread from plant to plant mainly by splashing water, but can be spread by contaminated soil, litter, dead leaves, gardening equipment, clothes, shoes, and hands. The spores also can be carried by wind currents for short distances in humid weather. Although the spores are short-lived, infection can occur in as little as 2 hours in a film of water at optimal temperatures. In spite of their short lives, these spores can cause an explosive epidemic when the conditions are right.

For more information about other Invasive Species of Concern, visit www.mdinvasives.org or call MDA at 410-841-5920.

photos available electronically on request.