Evaluation and inheritance of crown gall resistance in apple rootstocks

To identify sources of resistance to crown gall disease and to investigate its inheritance pattern to descendants, we assessed the degree of resistance among seven apple rootstocks of 'JM5', 'JM7', 'M.9', 'M.27', 'G.65', Malus prunifolia 'Mo 84a', and 'Morioka Seishi', two wild Malus accessions of Malus sieboldii 'Sanashi 63' and 'Mo-15', and its hybrids (147 individuals). The inoculation was tested using two Agrobacterium tumefaciens strains of Peach CG8331 (biovar 2) and ARAT-001 (biovar 1) as inocula. M. sieboldii'Sanashi 63' and 'Mo-15' did not show any galls at the inoculated sites six months after inoculation with suspensions of the strain Peach CG8331. Galls developed on the other rootstocks with a frequency from 0.31 to 0.82. In an inoculation test with strain ARAT-001 as the inoculum, no galls were formed on M. sieboldii 'Mo-15', and the frequency of M. sieboldii'Sanashi 63' was low, 0.19. The frequency in 'G.65' inoculated with strain ARAT-001 was much lower than that with strain Peach CG8331, whereas that in other rootstocks showed similar or higher frequency compared to strain Peach CG8331. The results suggested that there is an interaction (specificity) for the frequency of gall occurrence between A. tumefaciens strain and apple rootstocks. Based on the results of our study, M. sieboldii'Sanashi 63' and 'Mo-15' were regarded as the most resistant genotypes to the virulent strains of A. tumefaciens used in our study. Resistant hybrids with no galls were found in progeny derived from a cross between 'JM7' x 'Sanashi 63' against strains of A. tumefaciens; numbers of hybrids were 19 (16%) and 5 (4%) against strains Peach CG8331 and ARAT-001, respectively. In F, progeny between 'JM5' x 'G.65', plants with no galls were not observed. These results indicate that crown gall resistance in M. sieboldii 'Sanashi 63' is heritable through its descendants.