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McClain County

Christmas Poinsettias

Did you know, if it wasn’t for a plant disease, we would not have the Christmas poinsettias that we are all accustomed to seeing during the holidays?

The poinsettia, Euphorbia pulcherrima, is a tropical plant that originated in Mexico. The flowers were viewed by the Aztecs as a symbol of purity and used to make dye and a medicine for fever. In the 17th century, a group of priests began using the flowers, which typically bloom during the holiday season, in their Fiesta of Santa Pesebre, a nativity procession. 

In 1825, Joel Robert Poinsette, a United States Ambassador to Mexico, first introduced the plant to the U.S. He gave several plants to botanical gardens and horticulture friends. Because of his love for the flowers, the name poinsettia was coined in his honor. 

In their natural state, poinsettias grow straight and tall into trees that can reach heights up to 10 feet tall. Because of this, up until the early 1900’s, most poinsettias were sold as cut flowers. Through selection and breeding, U.S. growers were able to discover free-branching cultivars that were ideal for developing the multi-flowered, potted plants that we are familiar with today. 

Sometime around 1920, a “secret” little plant pathogen began to reside in the poinsettias giving them free-branching abilities.  Since poinsettias have a history of developing poinsettia mosaic virus (PnMV), heat treatments and other similar treatments were used for this and other potential pathogen problems. It was noticed that when these treatments were used, the free-branching ability disappeared. Immediately, it was suspected that the virus (PnMV) was responsible for the free-branching pattern, but the virus was also found in plants without the free-branching pattern. There were several attempts to discover the true cause of the free-branching pattern, but it was not until the mid-1990’s that the true cause was discovered. With the increased use of genetic research, a phytoplasma was detected in the free-branching cultivars. The phytoplasma was confirmed to be the cause when it was introduced to a restricted branching cultivar via dodder (a parasitic plant) and the free-branching pattern developed within 3 months. 

Phytoplasmas are important plant pathogens that are associated with diseases in several hundred plant species worldwide.  Symptoms in plants include flower sterility, loss of normal flower color, development of auxiliary shoots resulting in witches'-broom appearance, generalized stunting and decline, yellowing, and branch dieback.

Technically, free-branching is a disease symptom in poinsettias. But, like the virus associated with classical color-breaking of tulip petals, the poinsettia phytoplasma is beneficial to growers based on the demand for potted poinsettias for holiday use. So, the more than $250 million of poinsettia sales annually is all thanks to a plant disease.

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