White rust on mums: Symptoms and management

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Recently, white rust on mums was detected in Michigan. This rust is classified as a Puccinia fungus and can spread rapidly in a greenhouse or nursery resulting in severe losses. It is a quarantine-significant disease in the United States and Canada. Pot mums, cut mums, and garden mums are all susceptible to white rust. The first symptoms appear as yellow spots on the upper leaf surface. On the undersides of the leaves, obvious pustules develop that first appear somewhat pinkish. In time, the pustules turn a waxy white color. Look for these pustules first on the young leaves and flower bracts. However, keep in mind that the pustules can be found on any green tissues and also flowers. 

Within the white rust pustules are white rust survival spores that are called teliospores. These spores can last for eight weeks on dried leaves. However, if the diseased plants are buried under soil, the white rust spores survive for just one week. When the environment is wet or moist for three hours (optimum temperature is 63°F) a specialized spore forms and is called a basidiospore. These basidiospores cause an epidemic if the conditions are right. The basidiospores spread from plant to plant by splashing water and worker handling. If the plant surface is wet, the basidiospores can infect the plant tissue within two hours. The basidiospores can travel about half a mile via air currents when the relative humidity is high (90 percent or higher).

The best way to manage white rust is to prevent it. Purchase cuttings from reputable, well-known, commercial propagators. Scout the crop frequently. Remember that imported flowers should not be handled in or near mum-growing facilities. These imported flowers may be infected without showing symptoms of the white rust. Manage the environment whenever possible and keep the relative humidity low and the foliage dry. If white rust has been verified in a region, apply preventive fungicides. Fungicides suggested for a preventive program include: Heritage, Daconil, Cygnus, Dithane, Strike and Terraguard. Eagle is recommended for growers who are in a high risk area with favorable environmental conditions.

Remember, if you find white rust, it must be reported; it is the law. Regulatory officials will supervise eradication and treatment programs.

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