Association of Apple Scar Skin Viroid (ASSVd) Infection with an Emerging Disease in 'Saiwaihong' Apples

A novel disease affecting small immature fruits has surfaced in 'Saiwaihong' apples (Malus pumila), a recently developed variety extensively cultivated across more than 20,000 ha in China. In an effort to pinpoint the causal agent(s) responsible for this ailment, RNA sequencing analysis was conducted on four symptomatic and four asymptomatic apple samples. The results revealed a diverse range of viruses and viroids, indicating mixed viral infection in diseased samples. However, a more focused examination involving 152 symptomatic and 122 asymptomatic fruit samples, using RT-PCR and dot-blotting hybridization techniques, highlighted a close association between the disease and the presence of apple scar skin viroid (ASSVd). Among the ASSVd variants obtained from diseased 'Saiwaihong' apples, 20 were identified, and they were either identical or closely related to isolates from various apple varieties cultivated in different regions and countries. This suggests that ASSVd isolates in 'Saiwaihong' might have been introduced from other apple varieties. Furthermore, the analysis indicates the possibility of two separate introductions, as the ASSVd 'Saiwaihong' isolates exhibited two distinct phylogenetic groups. These insights provide valuable guidance for disease control strategies and emphasize the significance of ongoing monitoring for ASSVd, both in its familiar forms and potential new variants.