Chrysanthemum white rust (CWR), caused by the fungus Puccinia horiana, is a serious disease with the potential to cause widespread damage both within the greenhouse and to the North American floriculture industry at large if it becomes established on this continent. It originated in China and Japan, but has spread to most other continents, including Europe, Australia, South America and Africa. So far, only isolated cases have been reported in the North America, and aggressive control methods have so far kept the disease from becoming established. Federal quarantine restrictions (six months post-entry) apply to all imported chrysanthemum cuttings, and importation of cuttings from infested countries is prohibited.
The first sign of the disease is small white or yellowish spots on the upper leaf surface. Over time, the spots turn brown and may appear sunken. If the leaf is turned over, fuzzy-looking pustules are visible directly underneath the spots. They start out light brown or pinkish but eventually turn white. Pustules may also develop on the stems and flowers. Eventually the leaves dry up and wither along the stem. These pustules produce two types of spores, depending on environmental conditions. Teliospores are long-lasting and can survive for up to eight weeks on the leaves or leaf debris under low relative humidity (50 percent or less). Teliospores do not spread easily — they must be brushed or knocked off.
Basidiospores, which are produced by the teliospores when conditions are favorable, are the method by which the disease spreads. When humidity is high (96 to 100 percent), and temperatures are in the 40 to 73°F range, the teliospores produce basidiospores, which require water on the leaves in order to infect the plant. The basidiospores are quite fragile and short-lived, and they die within an hour of humidity dropping below 90 percent. However, when conditions are favorable, the basidiospores can cause an epidemic of CWR in a greenhouse. They can be transmitted by air currents and human handling.
Not all plants that are, or once were, in the genus chrysanthemum are susceptible to CWR. Marguerite daisy (Argyranthemum frutescens), shasta daisy (Leucanthemum xsuperbum) and ox-eye daisy (L. vulgare) are among the resistant species. Among the susceptible species are Chrysanthemum pacificum, C. xmorifolium (florist’s chrysanthemum), C. japonicum and C. boreale.
Careful inspection of imported cuttings for CWR symptoms, as well as purchasing them from a reputable source, is the first line of defense against the disease. Cultural controls include good air circulation, lower humidity levels and using drip irrigation to keep foliage dry. There are also preventive fungicide protocols. If disease symptoms are seen, inform the USDA, state or county officials per federal legal requirements.
Rhode Island Cooperative Extension. Pest Alert: Introduction to Chrysanthemum White Rust. http://www.dem.ri.gov/programs/bnatres/agricult/pdf/pecwr.pdf
Syngenta Flowers/Yoder Mums. 2010. Chrysanthemum White Rust Bulletin. http://www.syngentaflowersinc.com/pdf/cultural/CWRBulletin_Final.pdf
USDA-APHIS Plant Health, Plant Protection and Quarantine. 2012. Chrysanthemum White Rust. http://www.aphis.usda.gov/plant_health/plant_pest_info/cwr/background.shtml