Poinsettias are normally large tropical shrubs or small trees, but disease turned them into a houseplant. Photo: countrylife.co.uk
The poinsettia (Euphorbia pulcherrima) has not always been the compact plant we know today. It’s 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 at Christmas were 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 stems with widely spaced leaf internodes, leading to a very large and open plant, the new poinsettia—said to be “free branching”—produced abundant, much shorter branches, giving a much denser and more compact plant.
Unfortunately, despite its beautiful shape, the new poinsettia lacked vigor and showed disease symptoms when the least bit stressed—irregular yellow marbling on the foliage, called a mosaic—making it essentially unsaleable. Furthermore, other poinsettias in the same greenhouse began to take on the same habit: a short, dense plant with risk of yellow marbling.
It was quickly understood that the mottled foliage was due to a mosaic virus and it was assumed the dense habit was linked to the same virus. The disease is 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 Solves Part of the Mystery
The scientists of the time were, however, experimenting with treatments against different mosaic viruses in plants and had already discovered that a heat treatment could destroy them. 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 soon showed the virus was indeed gone.
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. In other words, any poinsettia could be made dwarf and attractive by grafting.
Restricted branching poinsettia (left) and compact, free-branching poinsettia (right). Photo: Mike Klopmeyer, Ball FloraPlant
Without actually knowing what was going on, poinsettia growers began to convert their large poinsettias into dwarf ones to meet the needs of the now burgeoning the potted plant market So, that’s how the potted poinsettia industry—which now produces more than 100 million poinsettia plants each year—was born!
A Dwarfing Infection
It wasn’t until 1996, nearly 75 years later, that the mystery behind the free-branching growth habit was solved.
That’s when a new generation of scientists discovered that what makes free-branching poinsettias so dwarf and densely branched is a phytoplasma, an organism similar to a bacterium that is found in the tissues of infected plants. Originally, it had been transferred to poinsettias coupled with the poinsettia mosaic virus, probably by aphids, but since the phytoplasma is more temperature resistant than the virus, it was not destroyed by the heat treatment and now lived on its own, carried from plant to plant and generation to generation by stem cuttings.
From the point of view of a wild poinsettia, this phytoplasma—referred to as the Poinsettia Branch-Inducing Phytoplasma or PBIP—would be a disaster. Since the plant remains small and dense, in the wild it would quickly be overtaken by surrounding plants that would create so much shade the dwarf poinsettia would be weakened or even shaded out entirely.
For poinsettia growers, though, the dwarfing phytoplasma was a godsend. It gives short, compact, well branched, and densely flowering plants that are otherwise just as vigorous as phytoplasma-free poinsettias … and the resulting dwarf poinsettia gave the world a plant that was now readily accepted by the public as a potted 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 come already infected with phytoplasma. It’s the only phytoplasma in the world known to have beneficial economic effects.
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, ill. But the phytoplasma is an essentially benign disease and does no harm to the plant other than leaving it compact and well branched, so no one is complaining! Wouldn’t it be nice if we all suffered from diseases with only beneficial effects!
Article originally published on December 6, 2015.