Apple virus disease is a serious problem affecting apple quality and crop production efficiency, and solutions are being actively and urgently explored. In particular, the cultivation of virus-free seedlings is being increasingly explored as an important technical method for facilitating the virus-free breeding of excellent apple cultivars. In this study, the apple cultivars ‘Ruiyang’, ‘Ruixue’, ‘Rui Xianghong’, and ‘Qinyue’ were used as source materials to compare the effectiveness of thermotherapy and combined thermo- and cryotherapy in eradicating apple chlorotic leaf spot virus (ACLSV), apple stem grooving virus (ASGV), and apple stem pitting virus (ASPV).
Viral presence in the in vitro shoots of the four apple cultivars was detected using a reverse transcription-polymerase chain reaction. All four cultivars were found to carry ASGV and ACLSV, and ‘Rui Xianghong’ and ‘Qinyue’ additionally carried ASPV. In vitro shoots of the four cultivars were subjected to varying periods of thermotherapy (0, 2, 4, 6, and 8 weeks). Shoot tips were then removed and half were cultured directly, and the remainder were cultured after cryotherapy treatment; their survival and regeneration rates were then evaluated to determine the efficiency of virus eradication after 8 months of subculture. We showed that the eradication of viruses in each of the cultivars and their survival rate are both the highest after 4 weeks of thermotherapy. When combined with cryotherapy, application of this period of thermotherapy achieved a 100% eradication of ASPV and ACLSV, and over 50% of ASGV.
This study demonstrated an effective methodology for developing both new virus-free plant materials and new apple cultivars that will support sustainability of the apple production industry in China and globally.