Witch’s Broom


“Witch’s Broom” is an plant infection caused by a destructive bacteria to marvelous effect — like a plant growing out of control.

I was always fascinated and puzzled with the “witch’s brooms” that I would occasionally spot while hiking along the trail — what was it? How did it spread? Is it good or bad?

Electron microscopy of cross-section of gladiolus phloem tissue infected with phytoplasma. Numerous phytoplasma bodies are apparent in the upper sieve elements. The sieve element in the lower left-hand corner is apparently free of phytoplasma bodies. x6000. Courtesy of Assunta Bertaccini.

Electron microscopy of tissue infected with phytoplasma. The  lower left-hand corner is free of phytoplasma bodies. Source

What is it? This is a bacterial infection of the plant. The bacteria attacking this plant is tiny, like less-than one micrometer (one-thousandth of a millimeter) tiny, and is called a phytoplasma. Scientists only really started to properly identify phytoplasmas in 1967, even though we’ve identified its effect on plants since 1607. Using an Electron Microscope we could see it, and you can spot the numerous dark, oblong bodies of the bacteria in the photo to the right.

Some phytoplasmas are so small they have remarkably small genomes, or DNA makeup. One species of phytoplasma has one of the smallest recorded amount of genetic information of any living organism known to mankind. Because this bacteria lacks so much of the genetic makeup of living organisms, it can only survive inside other cells. If you removed it to study under a microscope or try to cultivate it alone in a science lab, it would only die.

phytoplasma lifecycle
So how does it spread?
These infections are spread from plant to plant by sap-sucking insects that act as carriers. Once an insect feeds on an infected plant, it spreads the bacteria every time it feeds on other plants.

Is it good or bad? Well, for the most part, this is bad for a plant. Not only does the plant lose control of the growth patterns of infected cells, the new leafs are unable to carry out normal photosynthesis and the flowers are sterile, unable to continue aid in the plants reproduction.

That being said, the Poinsettia plant, popular for Christmas decorations, was intentionally infected and cultivated with Phytoplasmas to cause higher-than-normal leaf propagation, leading to more desirable floral arrangements and $325 million for growers annually.