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item Lee, Ing Ming
item Gundersen-Rindal, Dawn

Submitted to: Dekker Encyclopedia of Plant and Crop Science
Publication Type: Book / Chapter
Publication Acceptance Date: 8/22/2002
Publication Date: 6/3/2004
Citation: Lee, I., Klopmeyer, M., Gundersen, D.E. 2004. Beneficial phytoplasma infection of poinsettia causing free-branching. Dekker Encyclopedia of Plant and Crop Science.

Interpretive Summary: Free-branching poinsettia cultivars that produce numerous axillary shoots are essential for propagating desirable multi-flowered potted poinsettias and comprise the majority of commercial cultivars propagated today. Many free-branching cultivars (>100) have been developed and propagated commercially in the last decade. In the United States poinsettias are one of the most economically important floricultural crops. Over 65 million plants were sold in the US alone in 2000. The branching factor has been a mystery to horticulturists for decades. Recent evidence has indicated that the poinsettia branching factor is a graft-transmissible biological agent. Using PCR and DNA finger printing (RFLP analysis)diagnostic procedures we have provided evidence indicating that the self-branching ability of the majority of commercial free-branching cultivars of today is not due to genetic traits selected through breeding but by the grafting of new seedlings (phytoplasma-free) to a free-branching rootstock that contains phytoplasma. The presence of phytoplasma causes the induction of free-branching in these infected poinsettias. This is the first reported example of a pathogenic phytoplasma as the causal agent of a desirable and economically important trait. The information will benefit growers and the floral industry by applying proper cultural management to improve the quality of poinsettias.

Technical Abstract: The cause of free-branching in commercial poinsettia cultivars has been a mystery to horticulturists for decades. Scientists at Molecular Plant Pathology Laboratory (MPPL), USDA/ARS, at Beltsville, MD, in collaboration with scientists in Ball FloraPlant, West Chicago, IL, first identified the biological agent(s) that cause beneficial free-branching of 20 commercial poinsettia cultivars. Employing the PCR/RFLP analysis developed at MPPL for specific detection and identification of uncultured phytoplasmas, these scientists detected a phytoplasma strain belonging to peach X-disease phytoplasma group in all free-branching poinsettia cultivars and proved, through dodder transmission experiments, that the phytoplasma is the biological agent causing free-branching in poinsettias. A tetracycline-mediated suppression study by other research group further supports the conclusion that phytoplasma is responsible for the induction of free-branching.