Witches’ broom is a woody plant disease that provokes bushy tumors, caused by phytoplasma, which are parasitic bacteria that live in plant phloem tissue.
Phytoplasma tweak the molecular switches inside an infected plant so that flowers develop like leaves. This bacterial interference creates more soft tissue that leafhoppers like to suck sap from. Phytoplasma thus create an advertisement for their transport.
A leafhopper comes and sucks a plant’s phloem juices, infecting itself with phytoplasma. On succeeding stops, the leafhopper passes the disease on to other plants.
Phytoplasma hijack a plant’s developmental machinery and make it work to their advantage and the plant’s detriment, since leaf-like flowers are sterile. The leafhopper acts as a middleman, unharmed by the commensal relationship with phytoplasma, which alter their gene expression depending upon their current host.