Apple Proliferation Phytoplasma

Introduetion Apple proliferation (AP) is one of the most economically important phytoplasmal diseases in Europe. It seriously impairs fruit size, yield, and vigor of the trees. AP was first reported in northern Italy (Rui et al., 1950). In the following decades, the occurrence of AP was reported from most European countries. Due to its transmissibility by grafting, the disease was initially thought to be caused by a virus. Later, wall-less bacteria, then named mycop1asma-1ike organisms, were identified in symptomatic trees (Giannotti et al., 1968), shortly after this new group of plant pathogens was discovered (Doi et al., 1967). Restrietion fragment 1ength polymorphisms (RFLP) and sequence analysis of ribosomal DNA revealed that AP is induced bya distinct phytoplasma that is closely related to the pear dec1ine (PD) and European stone fruit yellows (ESFY) agents, the causes of two other major diseases of temperate fruit trees (Seemüller et al., 1998). Unlike most or all other phytoplasmas, which are transmitted by either leafhoppers (Cicadellidae) or planthoppers (Fulgoroidea), the three fruit tree phytoplasmas are mainly or only spread by psyllids (Psyllidae) (Frisinghelli et al., 2000; Tedeschi et al., 2002). Recently, the AP phytoplasma was delineated taxonomically under the provisional status 'Candidatus' for uncultured bacteria as 'Candidatus Phytoplasma mali' (Seemüller and Schneider, 2004).