|Riassunto in Inglese:|
Peach latent mosaic (PLM) disease was first reported more than 30 years ago in France on incoming propagation material from the US and Japan (Desvignes, 1976). However, related diseases had been described previously as peach calico (PC) (Blodgett, 1944) and peach blotch (PB) (Willison, 1946) in the U.S. and as peach yellow mosaic (PYM) in Japan (Kishi et al., 1973). PLM, PC, PB and PYM, which have some common biological properties including cross-protection between strains of different severity and a relatively high heat resistance (Desvignes, 1986), are now known to have a common etiological agent. The heat resistance, together with unsuccessful attempts to identify viral particles in infected tissues, suggested that the causal agent could be a viroid rather than a virus. In support of a viroid as the causal agent, infectivity was found in a low molecular weight nucleic acid fraction (Monsion et al., 1988) and a viroid-like RNA was found specifically associated with peach samples infected with severe and latent isolates of the PLM disease (Flores and Llácer, 1988). Moreover, the same viroid-like RNA was found in different peach cultivars expressing PLM disease, but not in the same cultivars after thermotherapy treatment (PLM-free) or in healthy controls. Viroid etiology was definitively confirmed by inoculating GF-305 peach seedlings with a purified preparation of the PLM-associated viroid-like RNA: the characteristic symptoms were reproduced in some of the plants from which an RNA with identical physical properties, Peach latent mosaic viroid (PLMVd), was recovered (Flores et al., 1990).
Cloning and sequencing of PLMVd (Hernández and Flores, 1992), led to the development of molecular approaches for the fast and sensitive detection of this viroid (see below). In vitro-synthesized PLMVd cRNA probes showed specific hybridization not only with peach cultivars infected with PLM isolates but also with Japanese peach cultivars displaying PYM symptoms (Ambrós et al., 1995), and with Italian isolates showing PC symptoms (Malfitano et al., 2003; Rodio et al., 2006). Similarly, RT-PCR amplification with PLMVd–specific primers revealed the presence of PLMVd in Italian peach cultivars infected and showing PLM symptoms (Shamloul et al., 1995; Hadidi et al., 1997). Moreover, RT-PCR assays showed that PLMVd is not involved in peach mosaic disease (Shamloul et al., 1995), a distinct disease now known to be caused by Peach mosaic virus (PcMV), a member of the genus Trichovirus. (see relevant chapter in this volume).