Chrysanthemum White Rust Detected in Massachusetts

Chrysanthemum white rust (Puccinia horiana, "CWR"), a serious fungal disease of chrysanthemums, has been discovered in Massachusetts. Infected plants were found at nurseries and at retail locations, with diagnoses confirmed by plant pathologists from USDA APHIS-PPQ. This pathogen can spread quickly in greenhouse and nursery environments, causing severe crop losses.

Over the last 25 years, localized introductions of chrysanthemum white rust have occurred within the United States or Canada and have subsequently been eradicated. Earlier this month, CWR was reported to have overwintered in an outdoor planting of mums at a residential property in Connecticut, and was also found at nurseries in Pennsylvania and Michigan. CWR is a pest of quarantine significance in the United States, requiring state and federal regulatory action. The MA Department of Agricultural Resources and USDA APHIS-PPQ are currently working together with nurseries to eradicate it from Massachusetts.

Chrysanthemum white rust attacks several species of chrysanthemums, including potted mums, spray mums, and garden mums. The symptoms of this disease are very distinct. Light green to yellow spots up to 5 millimeters in diameter appear on the upper surface of the leaf. The spots become brown and necrotic with age. Raised beige to pink pustules form on the underside of leaves. These pustules become white with age. Pustules are most common on young leaves and flower bracts but may form on any green tissue or the petals. Symptoms are more likely to be observed during or following cool, wet weather.

For more information about chrysanthemum white rust, including lists of susceptible and resistant species and a detailed eradication protocol, see this fact sheet. If you think you have seen Chrysanthemum White Rust, please call the MDAR Plant Pest Hotline at 617/626-1779, or report it on the Massachusetts Introduced Pests Outreach Project website.