Witches' brooms are a common sight in autumn landscapes, but there is one variety you never want to see in your garden.
Symptoms of witches' broom
From a distance, witches' broom looks like a squirrel nest or an area of particularly dense foliage. Closer inspection reveals a distinct twig deformity, a clustered riot of shoots pointing in every direction. The stems found in a witches' broom may also be twisted, discolored, or dwarfed.
Causes of witches' broom
Witches' broom can be a symptom of chemical overspray, disease, such as aster yellows, a hormone imbalance, mistletoe, mites, nematodes, a zinc deficiency, or feeding by bacterial plant parasites called phytoplasmas. First seen in 1967, phytoplasmas have been impossible to recreate in the laboratory, which makes studying them and developing control measures difficult.
The hormone imbalance that causes witches' broom is a battle between auxins, which regulate plant cell growth, and cytokinin, which regulates cell division. There isn't much you can do about many of those conditions. But not all deformities are undesirable.
The good witch
Sometimes a witches' broom is more of a dwarf version of the parent tree. Very often, that dwarf version can be removed and propagated. Many of our modern dwarf conifers got their start in this way.
Preventing witches' broom
Unless you have the time to create new plant species, your plants will be healthier if you can prevent this problem in the first place. Phytoplasma catches a ride to your plants using leafhoppers, in a behavior called phoresy. Controlling leafhoppers can reduce the occurrence of witches' broom. Also, proper pruning, crop rotation, and general garden sanitation can reduce the likelihood of witches' broom occurring.
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