Willow Witches' Broom

The dense clumps of twigs on this willow (Salix sp) are known as Witches' Brooms.  It's not a normal growth pattern, but is caused by a type of bacteria known as a phytoplasma.

It seems not much is known about phytoplasms.  Here's what I have gathered, mostly from the U of MN extension service.

• They are extremely difficult or impossible to culture outside of their host organism.
• Their presence does interfere with the health of the plant.
• Most are unidentified to species or genus.
• There's probably a secondary host insect that spreads the phytoplasm to new trees.
• Willows seem to be able to partion off the infected areas, leaving the rest of the plant uninfected.
• It's not actually known for certain that Witches' Brooms on Willows in MN are caused by phytoplasms.

I see Willow Witches' Brooms on regular basis at Westwood Hills Nature Center.  It surprises me that something so conspicuous has so little know about it.