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Citrus Greening Research
Using parasitic dodder plants to transmit Huanglongbing from citrus to periwinkle
Huanglongbing or citrus greening is reducing citrus yields all over the world. Citrus trees are difficult hosts for greenhouse experiments because the trees grow slowly and are large and have thorns! The pathogen can be transferred from citrus to more easily studied experimental hosts (fast growing annuals) by using parasitic dodder plants. You can think of a dodder plant as a morning glory vine without the leaves or roots. These dodder plants grow with their 'roots' (called haustoria) attached to the infected citrus tree, tendrils grow to neighboring plants and then develop new 'roots' on that plant.
This creates a "pipeline" connection between the two plants. The new plant host such as the periwinkle plant at left, develop the disease, but the interaction between the pathogen and the parasitic dodder has not been studied. In this study we characterized the interaction between the huanglongbing pathogen and the parasitic dodder plant.
Dodder plants were grown on sweet orange and lemon trees infected with huanglongbing. Dodder tendrils, which do not have leaves and look like spaghetti, were harvested and cut into 2 cm pieces. The amount of the pathogen present in each piece was quantified, and thin sections of tissue were viewed by electron microscopy. We predicted that the concentration of the pathogen in the dodder would be much higher at the end of the dodder directly growing out of the diseased citrus tree and lower in sections further removed from the source plant, but this was not what was observed. 
Instead, the distribution of the pathogen in dodder plants was very irregular when viewed at the whole plant level as well as at the microscopic level. We also observed pathogen cells changing from one shape to another. 
This observation is both unusual and important because most bacteria have only one fixed shape. Our work shows that the dodder plant is a very good experimental host for the huanglongbing pathogen. Our work will be of interest to other scientists who use dodder to transmit huanglongbing disease as well as other plant pathogens To see the full research article please click here . 
For more information about citrus pathogens please contact Dr. John Hartung
For more information concerning growing citrus please contact Cristina Paul
