The large clump shown here, is a good example of a "witches broom."
This is what they look like when the winter exposes them high up in the trees of different species. They are found throughout the world. When the trees are full of leaves, they are barely noticed and you may even think they are bird's nests of some kind.
In an outbreak of witches broom on Ficus, in a large wholesale
nursery in Florida, the state plant inspector removed one of the galls
and sent it to the Florida Department of Agriculture, Pathology
Department. It was identified as Schaeropsis spp., a fungus that forms
galls varying in size from small stem swellings to baseball sized
enlargements. Instead of risking the continued spread, all Ficus in the
area were destroyed. All tools were immediately disinfected. This
fungus infects stems easily through wounds (such as bonsai cuts!)
This fungus can also be spread simply by touching the infected area with your hands and then handling a potential host.
“There was a bonsai series on our local PBS station ... the instructor made the point several times that the shears, cutters etc. used on Ficus should not be used on other trees. He said the sap that gets on the tools was toxic to other plants and creates witches brooms. He even recommended having a separate set of tools for Ficus. Is it true? Is it one more thing for me to worry about?” Not true!
Ficus sap can be toxic to humans. Those who are especially sensitive may suffer small rashes, an itch or welts where the sap touches the skin. Most people are only annoyed by the stickiness.
But toxic to other plants? Ficus sap may assist the spread by sticking to tools, but the sap itself is not toxic to other plants.
An extra set of tools is not the answer. Disinfect your tools often!
Although not common, galls such as this can be another bonsai problem.
(Photo courtesy DPI, Florida Department of Agriculture)
It's difficult to diagnose the exact plant fungus or germ, but it's relatively unimportant. The cure is the same. To rid a tree of a witches broom or galls of any kind, carefully prune the infected area well below the gall. Seal the removed branch in a plastic bag and throw it out.
Read about disinfecting your tools between plants or between each usage especially on any infected trees.
Consider wearing disposable gloves and always wash your hands with an antibacterial soap after handling these plant problems.
Some witches brooms appear to be caused by genetic mutations! Unlike 'brooms' caused by living organisms, there is usually only one broom per tree when the cause is a genetic mutation, and they do not spread.
Be sure you know what you're working with!
Also see the page on beneficial critters.