The recent news in the United States has headlined the occurrence of soybean rust, a disease new to the United States on soybeans. The horticulture industry has a rust concern of its own. Chrysanthemum white rust (CWR), caused by Puccinia horiana, is a pest (fungus) of quarantine significance in the United States. The importation of CWR host plants is prohibited from infested countries and regions because of the potential of transporting this organism with the host plants.
This disease is not yet established in the United States. It is indigenous to Japan and is now established in China, Europe, Africa, Australia, Central America, South America, and the Far East. Although the disease is not established in this country, it has been found in sporadic outbursts and has the potential to be very damaging to U.S. commercial horticulture and floriculture industries. According to the national management plan, CWR may cause complete loss of glasshouse chrysanthemum crops. When CWR has been detected in the United States, state and federal regulatory bodies have worked toward immediate eradication of the disease. The national management plan for chrysanthemum white rust can be found at http://www.aphis.usda.gov/ppq/ispm/cwr/cwrplan.pdf.
In September 2004, USDA APHIS PPQ confirmed the presence of chrysanthemum white rust in nurseries and even some residences in Pennsylvania, Delaware, Maryland, and New York. For details on these findings and regulatory reactions, visit the national phytosanitary alert system of the North American Plant Protection Organization at http://www.pestalert.org/notifications.cfm?region=United%20States#124. In that document, it states that “When CWR is found in the United States, the States and PPQ cooperate to eradicate it… Disposal of infected plants and weekly fungicide sprays of myclobutanil are required to manage this disease as outlined in the CWR Management Plan for Exclusion and Eradication.”
With recent taxonomic changes, most plants we knew as chrysanthemums are no longer in the Chrysanthemum genus. In fact, the three species that remain in the Chrysanthemum genus are not susceptible to this rust. Susceptible plants include Dendranthema species (florist’s mum, florist chrysanthemum, cultivated mum), Nipponanthemum species (Nippon Daisy, Nippon-chrysanthemum), Leucanthemella species (high daisy, giant-daisy), and Ajania pacifica. Apparently resistant species include annual chrysanthemum, crown chrysanthemum, pyrethrum, marguerite daisy, ox-eye daisy, and corn marigold. The rust fungus is an obligate parasite. It grows and reproduces only on susceptible plants. Still, the teleospore stage of this fungus can survive up to 8 weeks on detached leaves at 50% or less relative humidity.
Symptoms of CWR include small white to yellow spots up to 4 to 5mm wide on the upper leaf surface. The spots may become dimpled and brown with time. Pustules of the rust form on the underside of the leaf, under these spots. The pustules are at first a buff to pink color but turn white with age, thus the name white rust. The plants may be symptomless in hot, dry weather. Cool, wet conditions promote symptoms. Spore spread is by splashing water or by wind in wet conditions. Symptoms usually develop from 5 to 14 days following infection. Often the disease is brought into a clean greenhouse on infected cuttings. Because infected cuttings may appear normal, buy healthy cut-tings and inspect them regularly, especially in cool, wet conditions. A British Columbia fact sheet on CWR with images of the disease can be found at http://www.agf.gov.bc.ca/cropprot/cwrust.htm.
Additional updates about this disease will be tracked in next season’s Home, Yard, and Garden Pest Newsletter. Special alerts will be sent to Extension offices as the need arises. Should you find this disease or suspect samples, contact Nancy Pataky of the University of Illinois Plant Clinic at (217)333-8375.