Elimination potential of electrotherapy and cold therapy combined with chemotherapy on apple scar skin viroid on in vitro apple plants

Apple scar skin viroid (ASSVd) and other apple viruses [apple chlorotic leaf spot virus (ACLSV), apple stem-pitting virus (ASPV), and apple stem grooving virus (ASGV)] can cause significant economic losses to the apple industry. We used ASSVd- and ASGV-infected Malus pumila ‘Spy 227’ and ACLSV-, ASPV-, and ASGV-infected M. pumila ‘Gala’ apple as the plant materials for virus elimination experiments. We evaluated the viral elimination effectiveness of electrotherapy at different electric current intensities and treatment durations on in vitro plants of these two apple cultivars. Furthermore, the effect of cold therapy (4 °C) combined with ribavirin (25 μg/mL) at different treatment durations on two apple varieties was investigated. The effects of electric current intensity and treatment duration on shoot tip survival differed between ‘Spy 227’ and ‘Gala’, and shoot tip mortality after electrotherapy was related to the self-tolerance of apple plants. During cold therapy combined with ribavirin, in vitro Spy 227 plants started to die after 50 d of treatment and the death rate increased with the extension of treatment time. The cold tolerance of in vitro Gala was lower than that of Spy 227. The Quantitative reverse-transcription PCR (qRT-PCR) and RT-PCR were used to detect the viroid and viruses in regenerated plants. The results showed that electrotherapy did not inhibit ASSVd, ACLSV, ASGV, or ASPV. Moreover, cold therapy combined with ribavirin had no eliminating effect on ASSVd, but the elimination rate of ASGV under treatment periods of 80 d and 95 d was 60.4% and 60.1%, respectively. Considering the impact of ASSVd on apple plant health, additional research is needed to develop methods to manage this viroid effectively.