Sunday, March 20, 2011

Witches Broom - Another Hackberry Gall

Witches Broom on Common Hackberry

This time of year, before the trees leaf-out, it's relatively easy to spot odd growths on trees.   I was driving on Badland Road near Rieck's Lake Park in Alma, Wisconsin today looking at cranes and waterfowl when I spotted this fist-sized knot of twig growth.  At first I thought it might be an abandoned bird nest.

I stopped and got out of the car to get a closer look.  That's when I realized the tree, a Common Hackberry, supported more than a dozen of these strange growths.  That would be way too many nests for one just tree.

Witches Broom on Hackberry

Turns out it's a gall known as witches broom.

Scientists haven't figured out exactly what causes this gall, but they know it occurs when the bud on a tree branch is injured (by winter road salt, a fungus or a bacteria) - or infected by a microscopic mite Eriopes celtis and by a powdery mildew fungus Podosphaera pytoptophila

While scientists don't understand how the mite and fungus interact, they do know that it doesn't "kill" the tree.

Witches broom galls are not unique to hackberries.  They can also be found on oak, cherry, birch, red cedar, alder and spruce.

Mites are not the only animals associated with witches brooms.  In Alaska, Flying Squirrels have been known to create winter hibernating capsules in spruce witches broom.  They chew out cavities of the gall and insulate them with a lining of mosses and feathers, making their winter hide-away snug as a bug in a rug.

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