Insect transmission of phytoplasmas

Phytoplasmas are phloem-limited, wall-less bacteria that are associated with disease in several hundred cultivated and wild plant species. Symptoms induced by phytoplasma infections are flower malformation (virescence, phyllody), growth aberration (fasciation, proliferation, witches' broom), stunting, leaf roll, yellowing and decline. These diseases were once considered of viral etiology, and failure to cultivate the associated mollicutes in vitro hampered the discovery of their true nature until 1967, when they were identified as mycoplasma-like organisms (Doi et al., 1967). Phytoplasmas are transmitted in nature by Hemipteran vectors belonging mainly to the families Cicadellidae, Cixiidae and Psyllidae (Weintraub and Beanland, 2006). During the last two decades, DNA-based methods have led to a better understanding of their genomes and consequently the name "phytoplasma" was proposed (International Committee on Systematic Bacteriology Subcommittee on the Taxonomy of Mollicutes, 1993). Today they are divided into 30 ribosomal (16SrRNA) groups and more than 100 subgroups. As phytoplasmas cannot be routinely grown under axenic conditions and this hampers completion of Koch's postulates, a provisional taxon "Candidatus Phytoplasma" has been proposed to accommodate them and to date, 37 "Candidatus P." species have been formally described (Zhao et al., 2010; Quaglino et al., 2013), according to the IRPCM Phytoplasma/Spiroplasma Working Team-Phytoplasma Taxonomy Group recommendations (IRPCM, 2004). Phytoplasma diseases occur worldwide, but the different groups and subgroups are not evenly distributed geographically (Foissac and Wilson, 2010). Coconut lethal yellowing (LY), grapevine yellows (Flavescence dorée,FD; bois noir,BN; Australian grapevine yellows, AusGY, and others), fruit-tree phytoplasmas (apple proliferation, AP; pear decline, PD;, european stone fruit yellows, ESFY; peach yellow leaf roll, PYLR; X-disease of peach), stolbur (STOL), aster yellows (AY) and related diseases of horticultural crops are among the most economically important phytoplasma diseases. In perennial crops the damage caused by phytoplasmas can be so important that insecticide treatments to control their vectors are compulsory. Phytoplasmas are obligate parasites of both plants and insects and therefore understanding their relationships with both hosts is crucial to understanding the epidemiology of the associated diseases and to develop protection strategies for susceptible crops.