The mollicutes are the smallest and simplest known living organisms. They are prokaryotic microorganisms, like bacteria, but they are smaller in size. They are also devoid of wall , not having the genetic information necessary for its formation. Mollicutes would still have a common ancestor of two Gram + bacteria belonging to the genus Clostridium . Some of them can be cultivated in vitro ( spiroplasmas ) others are not ( phytoplasmas , former mycoplasmas or MLO). They are essentially microorganisms parasitic obligatory of the cells of the conducting tissues of the produced sap ( vessels phloem ). They are sensitive to certain antibiotics, especially tetracycline which blocks their development but does not kill them.

Only phytoplasmas belonging to the Aster yellows group presently pose problems for salad crops. Their observation under an electron microscope shows that they are quite polymorphic . Their symptoms are relatively typical: yellowing and leaf deformation, accumulation of latex ...