Grapevine yellows – current developments and unsolved questions

Grapevine yellows (GY) are diseases associated to phytoplasmas that occur in many grape growing areas world-wide and are of still increasing significance. Almost identical symptoms of the GY syndrome are caused by different phytoplasmas and appear on leaves, shoots and clusters of grapevine. Typical symptoms include discoloration and necrosis of leaf veins and leaf blades, downward curling of leaves, lack or incomplete lignification of shoots, stunting and necrosis of shoots, abortion of inflorescences and shrivelling of berries. Those symptoms are related to callose deposition at the sieve plates and subsequent degeneration of the phloem (63,90). Although no resistant cultivars of Vitis vinifera or rootstocks are known so far, the various grape varieties differ considerably as far as symptom severity is concerned. It ranges from fast decline and death in highly susceptible cultivars to tolerant rootstocks as symptomless carriers of the pathogen (33). Phytoplasmas are phloem-limited, non-helical and wall-less procaryotes (49). They represent a monophyletic clade within the class Mollicutes, which is currently divided into at least 15 subgroups on the basis of sequence analyses of various conserved genes (49). A new taxon, ‘Candidatus Phytoplasma ’ has been established and various groups or subgroups have been described recently as ‘Ca. Phytoplasma ’ species (49). The full genomes of two phytoplasmas are sequenced (36), providing new information on their biology and their interaction with plant and insect hosts. The available data confirm their lack of genes for autonomous metabolis