|Riassunto in Inglese:|
Hop stunt viroid (HSVd), type member of the Hostuviroid genus in the Pospiviroidae family, has a very wide host range that includes trees, shrubs and herbaceous plants. HSVd can infect latently grapevine and almond; whereas, in other plants, it causes specific disorders such as: stunt on hop, dapple fruit on peach and plum, fruit deformation and rugosity on apricot and plum, pale fruits on cucumber and cachexia on citrus. All these symptoms can lead to very high economic damages. In the past, Hibiscus spp. was also reported as an HSVd natural host amongst ornamental plants (Sänger, 1988), but there has been no more information available since.
Recently, a survey on the phytosanitary status of ornamental hibiscus was conducted in wholesale nurseries in Central Italy (Manglli et al., 2012). Plants of Hibiscus rosa-sinensis showing symptoms resembling virus infection were first examined for the presence of Hibiscus chlorotic ringspot virus (HCRSV), Hibiscus latent Singapore virus (HLSV) and Hibiscus latent Fort Pierce virus (HLFPV). Several plants belonging to two different cultivars, showing both severe symptoms of reduction in plant growth and upward curling and deformation of leaves (Fig. 1), were tested negative for these viruses. Indeed, the observed symptoms did not resemble those of known viruses in hibiscus. Consequently, the same plants were tested for HSVd using a RT-PCR protocol by Faggioli et al. (2001) using two primer pairs designed by Astruc et al. (1996; primers VP-19 and VP-20) and Kofalvi et al. (1997; primers HSVFL1 and HSVFL2). Out of 17 plants showing the aforementioned symptoms, four samples gave positive results. In contrast, HSVd was not detected in any of the 60 leaf samples showing only yellow spots and chlorotic mottle on leaves that were tested positive for HCRSV, HLSV and/or HLFPV.
The four amplification products were cloned (pGem-T vector, Promega, WI, USA) and sequenced. Sequence analysis of different clones for each isolate (Accession No.: from KC137256 to KC137266) revealed that two isolates showed the same size (296 nt) and that both had a high homology amongst clones and between the two isolates. Furthermore, both isolates were closely related to a Turkish plum isolate (EF523829, Gazel et al., 2008) belonging to the Hop-group, as shown by the HSVd phylogenetic tree (Fig. 2; Kofalvi et al., 1997). The sequences of the other two isolates were 299 nt long and also showed a high homology amongst clones and between the two isolates. They were practically identical to an apricot isolate (Y08437, Kofalvi et al., 1997), thus grouping in the recombinant Plum-Citrus phylogenetic group (Fig. 2). These results suggest a different origin of infection in the vegetative propagation and cultivation of hibiscus plants. This study confirms that Hibiscus rosa-sinensis is a natural host of HSVd as reported by Sänger in 1988. However, to our knowledge, this is the first report of HSVd in H. rosa-sinensis in Italy.