Assessment of beet leaf yellowing virus tolerance based on leaf yellowing in sugar beet

Sugar beet (Beta vulgaris) production in Japan faces major challenges from virus yellows (VY), caused by beet leaf yellowing virus (BLYV) and transmitted by aphids. Outbreaks have reduced sugar yields, and breeding for tolerant varieties has not been conducted in Japan. This study marks the first step toward developing VY-tolerant varieties by testing three hypotheses: (1) leaf yellowing can be induced by artificial inoculation, (2) tolerance varies among breeding materials, and (3) differences in tolerance to yellowing disease can be evaluated. From 2017 to 2020, four sugar beet materials were grown in inoculated and uninoculated plots and assessed using two methods: the yellowing index (YI), scored as 0–3, and the yellowing area (YA), calculated from digital images. Triple antibody sandwich–enzyme-linked immunosorbent assay confirmed that all BLYV-inoculated plants were infected and exhibited marked yellowing compared with uninoculated plants. Analysis of variance (ANOVA) applied to YI and YA data revealed that inoculation significantly influenced yellowing, symptoms progressed over time, and yellowing progression varied by material. Additionally, YI and YA were significantly correlated, with a Spearman correlation coefficient (rs) of 0.718. The significant correlations between YI or YA values and sugar yield loss (rs = 0.86–0.87) and root weight loss (rs = 0.80–0.83), but no significant correlation with Brix loss (rs = 0.32–0.46). These results validate the tested methods for evaluating BLYV tolerance and highlight the potential for breeding sugar beet varieties with enhanced tolerance. Moreover, the findings offer valuable insights for future VY tolerance breeding programs.