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 already fully leafed out 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 - forestryimages.org

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. 

The leaves of the broom are smaller that normal leaves and tend to fall earlier in the year.  "Some researchers believe that the fungus overwinters as stromata in fallen leaves.  Infectious spores are released from the remains of the fallen leaves in the spring to cause new infections."  However it survives, we have had brooms in this tree for the last 5 years. 

Looking at our forest, you would never know the attacks that occur on hickories.  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.  A homeowner might be embarrassed to have a tree like this in their yard.  For us it is a favorite, demonstrating the diversity of nature.