Posts Tagged ‘Plant Oddities’

You could see a witches’ broom this Halloween, but you don’t have to look for a wart nosed hag wearing a pointy black hat to find one.

Witches’ broom is a plant deformation which appears as a very dense cluster of branches, often found on woody plants like trees and shrubs. Blueberry bushes for example, often have witches’ brooms. I know of one bush that I picked berries from for years that had a large broom growing in the center of it. The growth didn’t seem to inhibit berry growth or affect the plant in any way. In fact, it looked as healthy as the bushes that surrounded it.  

In some cases however, as in rice, the fungus that causes witches’ broom can be fatal. In other instances plants act positively bewitched; potatoes with witches’ broom can form tubers on top of, rather than below ground. This isn’t much help for the farmer when you consider that potatoes exposed to sunlight become toxic by forming solanine, which is a poisonous alkaloid.

Witches’ brooms are usually caused by either a rust fungus or a parasitic plant such as mistletoe, but there is an aphid known to cause honeysuckle witches’ broom, and on hackberry trees (Celtis occidentalis) it is caused by both a powdery mildew fungus and a tiny mite. On cherry and blackberry it is caused by bacteria carried by insects from elm or ash trees.

In the case of blueberry bushes, witches’ broom is caused by a fungus that lives on balsam fir trees. This broom fungus always needs a blueberry and a fir as hosts and is very specific; a blueberry with the fungus can’t infect another blueberry. Most brooms caused by rust fungus need two host plants. The fungus that causes witches’ broom on balsam fir needs common chickweed as a secondary host, and the fungus that infects spruce needs the lowly bearberry (Arctostaphylos uva-ursi).

Witches broom can cause very desirable dwarfism and increased branching in some plants. In fact, many well known dwarf evergreen shrubs are the result of witches’ broom.  For example, Montgomery Dwarf Blue Spruce is one of the best dwarf blue spruces, and is from a witches’ broom. Globosum, a round-headed, grafted form of Japanese black pine, is the result of a witches’ broom.  Another black pine, Hornibrookiana, will be no larger than 6 feet across and 2 feet tall even after 30 years, thanks to a witches’ broom.

So if you should see a witches’ broom on Halloween or at any other time, remember-it is not the home of hobgoblins and witches and is not a Hexenbesen (bewitched bundle of twigs) as medieval writers would have you believe.  No-more than likely it is, once again, just nature doing what it does so well.


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