Chrysanthemum White Rust, caused by the fungus Puccinia horiana P. Henn., is a quarantine significant pest in the United States (Title 7, Code of Federal Regulations, Part 319.37 and 319.74). Importation of certain Chrysanthemum (including Dendranthema), Leucanthemella, and Nipponanthemum species are prohibited from several countries, territories, and possessions due to the potential of this organism to be transported with prohibited host articles. When CWR is found in the United States, the States and APHIS cooperate to eradicate the disease.
Chrysanthemum white rust originated in eastern Asia. It is now established in the Far East, Europe, Africa, Australia, Central America and South America. There have been outbreaks in Canada and the United States, but the pest is eradicated when found. Best management practices, including the use of proper cultural techniques, scouting for disease symptoms, sanitation, fungicide applications, and worker training/education are required to manage this disease.
Chrysanthemum white rust may be recognized by the small white to yellow spots, up to 4 mm wide, on the upper surface of the leaf. These slightly dimpled spots become brown over time. Pustules form on the underside of the leaf, beneath the small spots. These are buff to pink-colored but become white as they age. Pustules are most common on young leaves and flower bracts but can be found on any green tissue and flowers. Infected plants do not always express symptoms during hot and dry conditions. Symptoms usually appear during cooler, wet weather. (more)
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