Chrysanthemum white rust (CWR) has been found recently in three Maryland locations, reported Stanton Gill, extension specialist at The University of Maryland.
The disease is caused by the fungus Puccinia horiana, a federally regulated, quarantine plant pathogen. Chrysanthemum white rust originated in Asia, and is now established in Europe, Africa, Australia, Central America and South America.
The disease has been introduced several times to the U.S. through importation of infected plants, but eradication and quarantine efforts have so far successfully kept CWR from becoming established here. This year, infected plants were discovered by concerned hobbyists growing standard chrysanthemums for fall mum shows.
The source of the current outbreak has not been determined. Excessive rainfall throughout the Mid-Atlantic in August and September may have favored disease development.
CWR symptoms begin as yellow to tan spots on the upper surface of infected leaves. These lesions will turn brown as they age. Corresponding areas on the underside of the leaf are raised, white to pinkish pustules, which contain the spores of the fungus. The pustules will become more buff or tan in color as they age. The pathogen infects only chrysanthemum.