Submitted to: Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 26, 1996
Publication Date: N/A
Interpretive Summary: Unculturable phytoplasmas (formerly called mycoplasmalike organisms) are the evident cause of many destructive diseases in several hundred plant species including many ornamental plants. For decades a biological agent has been suspected to cause free-branching in commerical poinsettia cultivars. Free-branching growth habit is an economically important trait for propagating desirable multi-flowered poinsettias. Attempts to identify the branching agent in the past have failed. In this study, we have used ultrasensitive nested-PCR assays using two universal primer pairs for detection of possible presence of phytoplasma(s) in commercial free-branching poinsettia cultivars. A phytoplasma belonging to 16S rRNA group III (X-disease and related phytoplasmas) was detected in all 20 cultivars with free-branching morphotype. The phytoplasma was not detected in a restricted-branching cultivar examined. Using a serological method (ELISA) poinsettia mosaic virus (PnMV) was detected in all cultivars. Through dodder transmission experiments we have succeeded to transfer phytoplasma but not PnMV from poinsettias into periwinkle plants. Phytoplasma in periwinkles was then transferred via dodder into pathogen-free poinsettias and induce free-branching in infected poinsettias. Thus, we provided evidence that phytoplasma, not PnMV, is the agent causing free-branching in poinsettias. The results will be of benefit to floral industries. For the first time, we are able to produce PnMV-free free-branching poinsettia plants.
Free-branching poinsettia cultivars which produce numerous axillary shoots are important for propagating desirable "multi-flowered" poinsettias. Because the free-branching growth habit can not be sustained when the poinsettias are subjected to cultural practices designed to eliminate potential pathogens, a biological agent in poinsettias has been suspected to cause free-branching in poinsettias. In this work, experiments were designed to determine whether the biological agent(s), is the factor inducing free-branching in free-branching poinsettia cultivars. Two uncultured biological agents, phytoplasma and poinsettia mosaic virus (PnMV), were detected in all commercial free-branching poinsettia cultivars examined, but only PnMV was detected in restricted-branching cultivars. By fulfilling a modified Koch's postulates (proof of pathogenicity) in which isolation of pathogen was accomplished using a living host and identity of the pathogen was confirmed based on molecular properties, it was concluded that phytoplasma, not virus, infection is the cause of free-branching in poinsettias and is an essential factor for propagating desirable "multi-flowered" poinsettias. This is the first reported example proving that a pathogenic phytoplasma is the causal agent of a desirable and economically important trait.