Japanese apricot (Prunus mume) is an important fruit tree in East Asia. ‘Nanko’, the primary cultivar of Japanese apricots, usually suffers from scab, a disease caused by Venturia carpophila. However, there have been few reports on the phenotypic variation in scab resistance/susceptibility and the underlying genetic factors. In this study, we investigated the severity of naturally occurring scabs based on fruit lesions in 108 Japanese apricot accessions over four consecutive years. In each year, both resistant and susceptible accessions were observed, and significant annual correlations were detected among the ratios of diseased fruits (Rt; 0.52–0.76) and among the disease severity indices (Sv; 0.55–0.79). We also conducted a genome-wide association study (GWAS) based on exon-targeted resequencing, and significant peaks were detected in the data from 2017 and 2018. The candidate genes involved in disease resistance are located near nine single-nucleotide polymorphisms. These genes may be associated with the susceptibility of ‘Nanko’ lineages to scab. These findings shed light on the phenotypic and genetic profiles of scab resistance in P. mume and will assist future breeding programs with improving scab resistance.

The selection of appropriate pollinizer cultivars is essential for stable fruit production of Japanese apricot because this species has many self-incompatible cultivars. In this study, the chilling responses of flower buds during endodormancy and the heat responses of flower buds during ecodormancy were quantified as development rates (DVRs) in ‘Kotsubu-nanko’, ‘Hakuo’, and ‘NK14’, three pollinizer cultivars of the leading cultivar ‘Nanko’ in Wakayama Prefecture. Approximating functions of DVRs for both endodormancy release and ecodormancy release were obtained on the basis of chilling and heat exposure tests on young trees with various combinations of temperatures and periods. These analyses demonstrated that temperatures over 15°C were only effective for endodormancy release of ‘Hakuo’ buds. Compared with the buds of ‘Kotsubu-nanko’ and ‘Hakuo’, those of ‘NK14’ were less sensitive to 15–20°C during the ecodormancy stage. Blooming date prediction models for these cultivars were constructed using the DVR values. Then, the applicability of the three cultivars as pollinizers for ‘Nanko’ was evaluated based on whether their predicted blooming times overlapped with that of ‘Nanko’. The models were optimized by adjusting the threshold of blooming percentages and initial points of heat accumulation during ecodormancy release to achieve the smallest differences between predicted and observed blooming dates (root mean squared error = 3.72–5.90). Simulations under different temperatures revealed the most suitable pollinizer cultivar for ‘Nanko’ going forward. Our predictions indicate that, in the simulated warmer conditions, the blooming date of ‘Kotsubu-nanko’ will be delayed 10 or more days compared with that of ‘Nanko’, and larger differences between the blooming dates of ‘Hakuo’ and ‘Nanko’ will occur under lower and higher temperatures. The blooming period of ‘NK14’ was predicted to remain stably synchronized with that of ‘Nanko’ under a range of simulated warmer and colder temperature conditions. These results showed that ‘NK14’ is an appropriate pollinizer for stable fruit production of ‘Nanko’ in the future, and highlight the importance of blooming time assessments based on the temperature responses of flower buds.

Japanese apricot (Prunus mume) is a culturally and economically important fruit tree in East Asia. ‘Nanko’, the primary cultivar of the Japanese apricot well known in Japan, usually suffers from scab, a disease caused by Venturia carpophila. Scab results in the development of black spots on the fruit surface, which considerably reduces its commercial value. Despite being an economic concern for Japanese apricot, there have been few reports on the phenotypic variation in scab resistance/susceptibility, the underlying genetic factors, and the development of DNA markers. Therefore, in this study, we performed a comprehensive trait analysis for scab resistance and susceptibility for four years and conducted a genome-wide association study (GWAS) to identify the associated loci. The results showed that significant GWAS peaks were detected in 2017 and 2018. Promising candidate gene encoding domains associated with disease resistance were found at the nine single-nucleotide polymorphisms (SNPs) identified in 2017 and 2018. These SNPs were thought to be associated with scab susceptibility of ‘Nanko’ lineages. These findings shed light on the mechanisms of scab resistance in P. mume and will assist future breeding programs to improve scab resistance in ‘Nanko’ lineages.