ANYONE who walks in the woods will be familiar with witches’ brooms (pictured). Many trees sport these bushy tumours, which have a variety of causes. An important one is a group of bacteria called phytoplasma that are, in turn, carried from plant to plant by sap-sucking insects such as leafhoppers.
Phytoplasma do other odd things to plants as well, sometimes including making their flowers develop like leaves. Witches’ brooms and leaf-like flowers provide more of the soft tissue leafhoppers like to suck sap from. Bacteria and bugs have thus developed a symbiosis at the expense of their vegetable hosts, since leaf-like flowers are sterile and a plant so afflicted cannot reproduce.