Released Dec. 10, 2007
AMES, Iowa--One of my favorite holiday decorations is a gorgeous, bright red poinsettia plant in a foil-wrapped pot. I’m not alone, as poinsettias are one of the most popular flowers in the United States, generating more than $325 million during the holiday season. But nearly all poinsettias grown in the U.S. and Europe are infected with a disease that, unlike most plant diseases, actually makes these plants more attractive.
Poinsettias are native to Mexico, and the Aztecs cultivated them for centuries before the plants were discovered by Europeans. Seventeenth-century Franciscan priests in Mexico used poinsettias in nativity processions, the first record of their use for a Christmas celebration. In 1825, Joel Robert Poinsette introduced the plant to the U.S. while he was ambassador to Mexico. The plants were later named after him.
In nature, poinsettias are slender and tree-like, growing up to 10 feet tall. Two forms, or morphotypes, have been grown commercially. A tall and straight morphotype with few side branches used to be grown and the blossoms sold as cut flowers. Another morphotype is a much shorter plant with many side shoots, resulting in the thick, bushy appearance that we recognize. By the 1960s, this bushy, multi-branched form, perfect as a potted plant, had come to dominate the market, as it does today.
Growers noticed a strange thing about the short, bushy poinsettia cultivars. When growers want to propagate plants without spreading viruses, they often treat propagative cuttings with a special heat treatment, or they produce new plants from tiny shoot tips, to remove any viruses that may be in the parent plant. But when growers used these standard techniques to propagate the bushy poinsettias, the attractive bushy branching habit was lost, and the resulting plants grew tall and unbranched.
Grafting the treated plants onto untreated plants caused them to regain their bushy structure. Could a virus possibly be causing the branched structure of the plants? No virus was consistently found in the bushy plants, despite years of testing.
In the 1990s, scientists finally were able to use genetic tests to discover the cause of the bushy growth. Dr. Ing-Ming Lee of the U.S. Department of Agriculture and his colleagues at the USDA and Ball FloraPlant discovered that a phytoplasma was causing the branched growth. Phytoplasmas are tiny disease-causing organisms similar to bacteria that live in the sap (phloem) of plants, where they make plants sick by disrupting the natural hormones that direct how the plants grow.
Plants that are infected with phytoplasmas display a variety of symptoms, including increased branching, stiff upright growth, stunting and decline, yellowing, leaf distortion or abnormal tufts of leaves that grow out of flower heads. Usually, these sick plants are too weak or ugly to sell. But the poinsettias infected with this phytoplasma were vigorous and pretty.
Every commercial poinsettia cultivar studied was found to be infected with the same phytoplasma, and the same treatments used to eliminate viruses also eliminated the phytoplasma. In this case, the symptom of disease (bushy growth) was exactly what the growers wanted. In an unusual holiday twist, a plant disease actually makes the sick poinsettia plant more beautiful.
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http://www.extension.iastate.edu/news/2007/dec/071001.htm
Contacts: Christine Engelbrecht, (515) 294-0581 or cengel@iastate.edu
Jean McGuire, (515) 294-7033 or jmcguire@iastate.edu
