Phytoplasma are specialised bacteria that are obligate parasites of plant phloem tissue and transmitting insects (vectors). They were first discovered by scientists in 1967 and were named mycoplasma-like organisms or MLOs. They cannot be cultured in vitro in cell-free media. They are characterised by their lack of a cell wall, a pleiomorphic or filamentous shape, normally with a diameter less than 1 micrometer, and their very small genomes.

Les phytoplasmes sont des bactéries sans paroi et dépourvues de forme spécifique (pléïomorphes) qui se multiplient exclusivement dans les tubes criblés du phloème.