Loquat canker, caused by Pseudomonas syringae pv. eriobotryae, attacks the buds, shoots, leaves, fruit, and underground parts of the loquat tree (Morita, 1988; Mukoo, 1952; Suga et al., 2007) and has a detrimental effect on vegetative growth and fruit production (Morita, 1991). It is the most serious disease of the loquat in Japan (Nesumi, 2006). The disease has also been reported in China, the United States, Australia, New Zealand, and Argentina (Alippi and Alippi, 1990; Lai et al., 1971; Lin et al., 1999; McRae and Hale, 1986; Wimalajeewa et al., 1978).
The pathogen has been classified into three groups (A, B, and C) based on the production of brown pigment and the pathogenicity to mesophyll (Morita, 1978). Group A strains produce no pigment and are not pathogenic to mesophyll, Group B strains produce no pigment and are pathogenic to mesophyll, and Group C strains produce brown pigment and are not pathogenic to mesophyll. Kamiunten (1990, 1995) reported that the electrophoretic profiles of plasmid DNA obtained from the three groups were all different: a 52-MDa plasmid and an 82-MDa plasmid seemed to be associated with virulence in Groups A and C, respectively. No commercial cultivar in Japan is resistant to all three pathogen groups, although some have resistance to two of the three groups (Hiehata et al., 2007; Morita, 1988). Improvement of loquat canker resistance is therefore one of the most important goals of loquat breeding in Japan. A screening assay based on inoculation and marker-assisted selection for loquat canker has been developed (Fukuda et al., 2005; Morita, 1988, 2005). This assay has been used to select resistant seedlings at the nursery stage in the loquat breeding program at the Agricultural and Forestry Technical Development Center, Nagasaki, Japan (Hiehata et al., 2002a).
Progress has been made in breeding for canker resistance to Groups A and B because many resistant materials have been identified (Hiehata et al., 2002b, 2007; Morita, 1988) and the resistance to these two groups is dominant (Hiehata et al., 2002b; Morita et al., 1985). Some of the authors of the present article have successfully developed cultivars resistant to both groups such as ‘Reigetsu’ (Terai et al., 2007), ‘Ryoho’ (Hiehata et al., 2008), and ‘Natsutayori’ (Hiehata et al., 2010). In contrast, there are only a few genetic resources with good fruit quality that are resistant to Group C such as ‘Shiromogi’ (Hiehata et al., 2003, 2007; Morita, 1988); most of the others have undesirable fruit characteristics. In addition, the mode of inheritance of resistance to Group C has not been elucidated until now. For these reasons, progress in breeding for resistance to Group C has lagged behind that for Groups A and B. New cultivars resistant to all three groups (A, B, and C) are highly desired for commercial loquat production in Japan.
The loquat cultivar Shiromogi, which is one of the few genetic resources resistant to Group C, originated from open-pollinated ‘Mogi’ seeds irradiated with gamma rays in 1961 (Ichinose et al., 1982), but it is unknown whether the gamma-ray treatment led to the resistance in this cultivar. ‘Shiromogi’ is a commercial cultivar in Japan with excellent fruit characteristics such as sweetness and tender texture, and it has often been used as breeding material for fruit quality at the Agricultural and Forestry Technical Development Center. The objective of this study was to determine the inheritance of the resistance to loquat canker Group C derived from ‘Shiromogi’, which is currently the most valuable source of resistance to Group C.
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