Grapevine yellows diseases and their phytoplasma agents - Biology and detection.

Phytoplasmas were discovered almost 50 years ago and initially they were named mycoplasma-like organisms. These cell-wall lacking bacteria and members of the class Mollicutes inhabit plant phloem sieve elements and are transmitted and spread primarily by leafhoppers, plant hoppers and psyllids that feed on infected plants phloem. Phytoplasmas interact with their hosts in a strong manner, through manipulation of the morphological features of the plants, and in several cases, also of the biology of their insect vectors. Molecular genetics analyses have improved the understanding of phytoplasma taxonomy, and also enhanced the ability to identify phytoplasmas that are detected in hosts and insect vectors. In particular, it has been determined that among the plant species infected by phytoplasmas, grapevine is one of those that are most severely affected, at a worldwide level. Molecular studies have provided considerable insights into phytoplasma molecular diversity and genetic interrelationships, taxonomic ranking has been achieved by using 16S ribosomal gene classification and other phytoplasma genes as epidemiologic molecular markers. On the other hand, the inability to fulfil Koch’s postulates severely restricts the understanding of the real roles of phytoplasmas in diseases and in plant–insect interaction. Together with the new possibility to cultivate phytoplasmas in artificial media, molecular genetics studies are now opening new possibilities for studying the best management of these bacteria that severely threaten worldwide agriculture, and in particular viticulture.