Breeding of rootstocks resistant to apple proliferation disease

Abstract

Apple proliferation (AP) is caused by the wall-less bacterium Candidatus Phytoplasma mali and is spread by psyllids. Previous work indicated that, due to the colonization behavior of the causal agent, the disease can be controlled by the use of resistant rootstocks. However, extensive screening revealed no satisfactory resistance in established rootstocks. In contrast, substantial levels of resistance were identified in experimental rootstocks derived from crosses of the apomictic species Malus sieboldii and genotypes of M. x domestica and M. x purpurea. However, trees on these hybrids are more vigorous and less productive than trees on standard stock M 9. For this reason, a program was initiated to reduce vigor and improve yield by crossing and backcrossing M. sieboldii and its apomictic hybrids with M 9 and other dwarfing stocks. From 2001 through 2006 a total of 36 crosses were made. However, only 23 progenies consisted of a substantial number of seedlings while the other crosses largely failed due to pollen incompatibility. The 3,500 seedlings obtained were DNA-typed using codominant SSR markers to distinguish apomicts and recombinants in the progenies. A total of 1,800 seedlings consisting of all recombinants and a representative number of apomicts were screened for AP resistance by graft inoculation followed by observation in the nursery and under commercial growing conditions. Several progenies showed a good inheritance of resistance. In two of them (4608 x M 9 and D2212 x M 9) more than 50% of the individuals never developed symptoms.

Keywords: Candidatus Phytoplasma mali, Malus sieboldii, SSR analysis