Characterization of Pseudomonas syringae pv. actinidiae Biovar 3 strain and cultivar tolerance to kiwifruit bacterial canker in China

Kiwifruit bacterial canker (KBC) caused by Pseudomonas syringae pv. actinidiae (Psa) is a worldwide epidemic. Forty-seven strains of Psa in Biovar 3 from different geographic regions in the world, thirty-seven of them collected in China, were selected to study genetic variation by using Rep-, RAPD- and IS50- PCR assays. Twenty-two strains of Chinese Psa were selected to test the fluorescence of each strain when grown on King’s B (KB) medium and examined under UV light, the pathogenicity of seven representative strains of Chinese Psa isolated from different provinces or cultivars was tested by the foliage spray inoculation of three cultivars with different tolerances to KBC. Twenty-eight of the main cultivars were subjected to a tolerance assay by wound inoculation on vines in vivo. The results showed that Chinese Psa Biovar 3 strains have a broad genetic variability related to their different geographic regions, Chinese Psa Biovar 3 strains had faint to brighter fluorescence (ΔE = 0.25–1.00, GFI = 1–3), although they were suggested to lack fluorescence in the past. Schy4 and JF8 were the most virulent; GC40, Hwd1 and Sh13 were the least virulent; and Jzhy2 and Jzzm2 were moderately virulent among the strains tested in this study. According to the combined data of necrosis length when inoculated with each of the three Chinese Psa strains (Schy4, Hwd1 and Jzhy2), the tolerance of the 28 tested cultivars to KBC could be divided into three grades. In conclusion, there were significant differences in the fluorescence and pathogenicity of Chinese Psa strains, but the fluorescence emission is not correlated with the virulence and pathogenicity of strains in this study. It was proposed that such cultivars as ‘Wanjin’, ‘Donghong’, ‘Huate’, ‘Fujianmaohua’, ‘Ganmi No. 6’, ‘Hongguoruanzao’, ‘Ruanzao No. 2’ and ‘Jialv’ kiwifruit could be used as parental materials to breed new cultivars tolerant to KBC in future kiwifruit production.