|Riassunto in Inglese:|
Apple scar skin disease was first reported from China in the 1930’s as Manchurian apple ‘Sabika’ disease (Ohtsuka, 1935, 1938). ). In Japan, the disease was first observed about two decades later, in 1953 (Ushirozawa et al., 1968). The viroid etiology of the disease was suggested by successful transmission to apple seedlings of low molecular weight RNA isolated from apple scar skin diseased trees (Koganezawa, 1985, 1986). The causal agent, Apple scar skin viroid (ASSVd) (Hashimoto and Koganezawa, 1987; Puchta et al., 1992;Yang et al., 1992), is now recognized as the prototype of the genus Apscaviroid (Flores et al., 2000, 2003).
Hadidi et al., (1990) presented molecular hybridization evidence which indicated that dapple apple disease is caused by a viroid that is highly homologous to ASSVd. That dapple apple disease is caused by a variant of ASSVd was further confirmed by RT-PCR, nucleotide sequence analysis of the cloned viroid, and the successful agro- infection of pear and apple seedlings with cloned viroid (Hadidi and Yang, 1990; Zhu et al., 1995, Zhu et al., 1998).
ASSVd infects pear trees under natural conditions. It may infect the trees without causing symptoms on fruit (Hadidi et al., 1991; Hurtt et al., 1992) or it may cause a variety of symptoms on the fruits depending on the infecting viroid variant. Nucleotide sequence analysis of viroids isolated from diseased trees from different locations showed they are variants of ASSVd. The viroid associated with the pear rusty skin disease in China (Chen et al., 1987) was found to be related to ASSVd by RT-PCR (Hadidi and Yang, 1990) and nucleotide sequencing demonstrated that it is a variant of ASSVd (Zhu et al., 1995). Similarly, the viroid associated with the pear fruit crinkle disease in China (Shamloul et al., 2003) was demonstrated by nucleotide sequence analysis to be a variant of ASSVd (Shamloul et al., 2004). A pear fruit dimple disease was described on some Japanese pear cultivars (Ohtsu et al., 1990) and ASSVd was identified as the causal agent of this disease (Osaki and Kudo, 1995; Osaki et al., 1996). ASSVd causes disease in infected pear trees in Italy and Greece and in infected wild pear (Pyrus amygdaliformis Vill) in Greece (Kyriakopoulou and Hadidi, 1998; Kyriakopoulou et al., 2001). Recently, ASSVd was detected in stone fruit hosts such as apricot (Zhao and Niu, 2008), and cherry (Kaponi, 2009; Kaponi et al., 2009, 2010)