Halloween brings growth of witches' brooms

Sunday, October 21, 2001

Look up at trees silhouetted against the night sky on Halloween and you might see something spooky -- a witches' broom. A real witches' broom. No witch, though.

Witches' broom is the name given to the broomlike growth that sometimes occurs on part of a plant. Picture a normal stem, with branches growing off it. Usually the branches are a few inches apart along the stem. Next, telescope that stem down so that there is only a fraction of an inch from one branch to the next. You now have your broom.

Result of infection

Witches' brooms usually result from infection from disease or from parasitic plants. The infection probably causes the broomlike growth by upsetting the balance of plant hormones in the stem. One such hormone suppresses branching; when this hormone is inhibited in a stem, that stem starts to branch like crazy.

Another hormone, this one helping stems to stretch out, also probably figures in. When this hormone is inhibited, the result is a tight cluster of branches. Of Interest, there are also diseases that can lead to an overproduction of this hormone, resulting in, for instance, "foolish seedling" disease of rice, first identified in Japan. In this case, afflicted plants "foolishly" stand above the rest in a field, growing so gangly that they eventually topple over.

Some witches' brooms harbor infections that eventually kill their host plants. Other strange things also can happen, such as when potato plants with witches' brooms form tubers above ground. Pretty spooky, eh?

Many witches' brooms do no apparent harm to a plant, and even have given rise to attractive, interesting garden plants.

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