Poinsettias are a common part of Christmas floral displays but their economic success is partially due to infection by a pathogen.
Originating in Mexico, poinsettias were introduced to the United States in 1825 by the first U.S. Ambassador to Mexico. In the wild, they actually look like this:
A little bit different to the poinsettias we are used to seeing.
This difference between the wild and the cultivated form is due to cultivation of the plant over many years-changing it from a tall, straight tree to a small bushy plant-and infection by a bacterial pathogen, phytoplasmas. I previously talked about phytoplasmas here.
There are two types of commercially grown poinsettia, which can be seen in the image below. Poinsettias which have a restricted branching morphology produce fewer ‘flowers’ (actually modified leaves) than the free-branching morphotype. The latter is more important economically because they produce more flowers (to sell on) and this morpohology is more desirable in a potted plant.
On the left- restricted-branching poinsettia morphotype. Right- free-branching morphotype
In 1997 it was shown that this free-branching morphology was due to infection by a phytoplasma. Phytoplasmas are able to interfere with stem development to create a smaller bushier plant, which happen to be desire by the growers.
In conclusion, a lot of research goes into controlling plant pathogens because of the loss of yield that they cause but there are cases where their intervention is desired- for example, Botrytis in wine
and the colour-breaking in tulip petals caused by a virus.
Colour break in tulips.
So, spare a thought for poinsettias this Christmas, those characteristics are actually symptoms of disease.
I recommend the review below for a more detailed history of poinsettias: