The poinsettia (Euphorbia pulcherrima) has not always been the compact plant we know today. It is actually a large shrub or small tree reaching 12 feet (4 m) or more in height and diameter, far too large for the average home. Thus, for a long time, the only poinsettias sold as Christmas plants were as cut flowers!
In 1923, however, a cut-flower grower from California, Ecke Nursery, noticed a short, compact poinsettia, barely 30 inches (75 cm) high at full maturity. While a normal poinsettia is said to be “restricted branching” and only produces branches about every 2 feet (60 cm) or so along the stem, leading to a very large and open plant, the new poinsettia—said to be “free branching”—produced abundant branches, thus forming what was almost a ball of foliage and flowers.
Unfortunately, despite its beautiful shape, the new poinsettia lacked vigor and showed disease symptoms when the least bit stressed—irregular yellow marks on the foliage, called a mosaic—making it essentially unsaleable. Furthermore, other poinsettias in the same greenhouse began to produce plants with the same symptoms. It was quickly assumed that “free branching” effect was in fact due to a disease linked to mottled foliage. Eventually studies did show the marbling was caused by a virus, now called poinsettia mosaic virus (PMV). So the compact poinsettia no longer seemed useful, but was rather seen as a disaster by the burgeoning poinsettia industry. The recommendation at that time was to destroy any compact poinsettias on sight so the disease would not spread to other poinsettias.
An Experiment That Paid Off
The scientists of the time were, however, experimenting with treatments against different mosaics in plants and had discovered that a heat treatment could destroy mosaic viruses. So virus-infected poinsettias were given the heat treatment and voilà! The foliage became a beautiful deep green without any mottling. The virus had been destroyed. But to the amazement of the scientists, the plant remained compact and well branched. What had happened? Studies showed the virus was indeed gone. For a long time, nobody understood what was going on.
Then another experiment turned up very interesting results. When a normal, restricted-branching (i.e. giant) poinsettia was grafted onto a free-branching poinsettia, it too began to branch abundantly and stay compact. Without actually knowing what was going on, poinsettia growers began to convert their large poinsettias into dwarf ones for the potted plant market by grafting them unto dwarf plants. Thus the potted poinsettia industry was born!
A Dwarfing Infection
Today we know that what makes free-branching poinsettias so dwarf and densely branched is a phytoplasma, an organism similar to a bacteria that is found in the tissues of infected plants. Originally, it was transferred to poinsettias coupled with the poinsettia mosaic virus, but since it is temperature resistant, unlike the virus, it was not destroyed by the heat treatment. From the point of view of a wild poinsettia, this phytoplasma would be a disaster. Since the plant remains small and dense, in the wild it would quickly be overtaken by surrounding plants and they would create so much shade that the dwarf poinsettia would be weakened or even shaded out entirely.
For poinsettia growers, though, the dwarfing phytoplasma is a godsend. It gives short, compact, well branched, and densely flowering plants that are otherwise just as vigorous as a phytoplasma-free poinsettia … and the resulting dwarf poinsettia gave a plant that was now readily accepted by the public as a living Christmas decoration. And to make things even better, greenhouse growers could transfer this phytoplasma to any other poinsettia via grafting. Today all poinsettias sold as potted plants—which is well over 99% of all the 65 million poinsettias produced around the world—are infested with phytoplasma … and no one is complaining.
A Beautiful Disease
When you contemplate the beautiful poinsettia in your living room, with its dense habit and numerous blooms, it’s odd to think it is, in fact, sick. But the phytoplasma is an essentially benign disease and causes no harm to the plant other than leaving if compact and well branched, so no one is complaining! Wouldn’t it be nice if we all suffered from diseases with only beneficial effects!