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White Rust of Chrysanthemum – a Dangerous Stowaway
It is always a good idea to thoroughly inspect any plant for potential pest problems prior to purchase. This is especially true when buying chrysanthemums for fall color this year. A new fungal pathogen, Puccinia horiana, poses a threat to the US flower industry.
Puccinia horiana causes the disease white rust in chrysanthemum. This pathogen is native to Asia, but has already spread to Europe, Australia, Africa, Central and South America. To date, there is no evidence that white rust has become established in the United States. Unfortunately there are several cases where white rust of chrysanthemum has been found in nurseries, gardens, or on cut flowers in the US in the past several years. It is unknown how white rust was brought into the United States in each of these cases. Regulatory officials suspect that the fungus may have been brought in on cut flowers from over seas or by amateur growers smuggling plant material in from foreign countries. Luckily in all of these cases, the infected plants were identified and destroyed, stopping the pathogen before it could become wide spread.
White rust can infect all chrysanthemums including pot mums, cut mums, garden mums (Chrysanthemum morifolium, Dendranthema x grandiflorum) nippon daisy (Nipponanthemum nipponicum), and giant daisy (Leucanthemella pacificum and Chrysanthemum serotinum). There are no symptoms of the disease for five days to two weeks after the initial infection. This means plants that initially appear healthy could be carrying the white rust fungus. The first symptom of disease is yellow colored spots on the upper surface of the chrysanthemum leaves. Raised pinkish pustules soon form on the under surface of the leaves directly below the leaf spots. These pustules mature into waxy white pustules often described as resembling tiny cauliflower on the lower surface of the leaf. Gardeners should take care not to confuse chrysanthemum white rust with brown rust of chrysanthemum. Brown rust, caused by Puccinia tanaceti, is common in the United States and results in chocolate brown pustules on both the upper and lower surfaces of the chrysanthemum leaf.
The white pustules on the lower leaf surface give white rust its name. Each pustule is full of microscopic teliospores. During cool humid weather (40-73F, 96-100% humidity) basidiospores are released from the white rust pustules and are carried by air currents to start new infections on nearby chrysanthemums. Spores can also be carried by splashing water or by gardeners handling infected plants.
Inspect mums for symptoms of disease before purchase. David Zlesak The teliospores located within the white rust pustules on plant leaves survive only 8 weeks on dead plant leaves and even less time if the leaves are buried. Unfortunately if hardy mums are infected, the fungus can survive within the plant, allowing the pathogen to survive and start the disease again in the following year.
Chrysanthemum white rust could result in serious damage to cut flower production facilities, nurseries and gardens if it were to become established in the United States. All suspected cases of chrysanthemum white rust should be reported to the University of Minnesota Plant Disease Clinic (http://pdc.umn.edu) or the Minnesota Department of Agriculture (888/545-6684).
-Michelle Grabowski, University of Minnesota Extension Educator
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