Thursday, May 14, 2015

Shagbark Witches' Broom

Witches' broom on shagbark hickory
When the hickory trees were just starting to open their buds, I noticed one tree on the glade with large clumps of leaves on a few of its branches.  Closer inspection showed that these were clustered twigs of a witches' broom.

Witches' broom - Click to enlarge
Witches' broom is a general term for deformities, generally of trees, when a disease or pathogen causes a branch to make a dense cluster of shoots from a single point.  They can be caused by viral or fungal infections, insects, mites, nematodes or even mistletoe.

The mechanism for many witches' brooms is a hormonal disorder.  Normally a plant hormone called auxin controls the terminal bud development, preventing secondary and tertiary branches from overgrowing.  When these diseases affect a branch, another plant hormone called cytokinin inhibits the auxin's control and clusters of shoots develop.
Hickory downy leaf spot -

The cause of shagbark hickory witches' broom is a mildew fungus (Microstroma juglandis) that invades leaves and twigs.  After inoculation, possibly by a sucking insect, it can cause hickory downy leaf spot as well as witches' broom.

Looking at our forest, you would never know the attacks that occur on hickory.  Over 180 species of insects and mites are reported to infest hickory trees, as well as bacteria, 133 known fungi and 10 other diseases.  We haven't yet included the woodpeckers that dig into them to dig out some of these predators.

Witches' broom leafing before normal buds
 Even a healthy appearing hickory is a veritable cafeteria of nature.

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