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.
