Mycoplasmas
Mycoplasmas are small single-celled organisms that occur inside plant cells.
Many diseases that were earlier thought to be caused by viruses, and were called "yellow" diseases, are caused by mycoplasmas.
These organisms are spread from one plant to another by a vector , usually an insect such as a leafhopper.
Tetracyclines are antibiotic substances that have been used to kill mycoplasmas in plants.