Loquat canker, caused by Pseudomonas syringae pv. eriobotryae, is a bacterial disease that has been reported for nearly a century in Japan (Ikata, 1927; Nakata, 1934). The disease has also been reported in China (Lin et al., 1999), the United States (Lai et al., 1971), Australia (Wimalajeewa et al., 1978), New Zealand (McRae and Hale, 1986), and Argentina (Alippi and Alippi, 1990). The disease attacks the buds, shoots, leaves, and fruit of the loquat tree (Morita, 1988; Mukoo, 1952), and it has a detrimental effect on vegetative growth and fruit production (Morita, 1991). It is currently the most serious disease of the loquat in Japan (Nesumi, 2006). The disease attacks not only the above-ground parts of the loquat, but also its below-ground parts, leading to seedling production decline in nurseries (Suga et al., 2007).
Because there were no completely resistant varieties grown commercially in Japan, bactericides were widely used in loquat orchards. However, controlling the disease in this way was difficult because of the high labor requirements and cost. Therefore, improvement of canker resistance of loquat in Japan is one of the most important goals in loquat breeding. In addition, requirements for disease-resistant cultivars continue to increase as a result of increasing public concerns of environmental responsibility. To support resistance breeding, a screening assay based on inoculation and marker-assisted selection for loquat canker resistance has been developed (Fukuda et al., 2005; Morita, 1988). 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).
The pathogen has been classified into three groups (A, B, and C) based on the presence of a brown pigment and pathogenicity in mesophyll tissue (Morita, 1978). Group A strains produce no pigment and are not pathogenic, Group B strains produce no pigment and are pathogenic, and Group C strains produce brown pigment and are not pathogenic. Progress has been made in breeding for 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 based on a single dominant gene (Hiehata et al., 2002b; Morita et al., 1985). We have successfully developed three cultivars that are resistant to both groups: ‘Reigetsu’ (Terai et al., 2007), ‘Ryoho’ (Hiehata et al., 2008), and ‘Natsutayori’ (Hiehata et al., 2010). In contrast, there are not enough genetic resources that are resistant to Group C (Hiehata et al., 2007; Morita, 1988); ‘Shiromogi’ and ‘Champagne’ are resistant to Group C and have moderate or larger fruit size and good edible fruit quality. Therefore, they are important cultivars with high potential as cross-parents in breeding, which combine high fruit quality and large fruit size with resistance to loquat canker Group C.
Hiehata et al. (2012) elucidated the inheritance of the resistance to Group C derived from ‘Shiromogi’. ‘Shiromogi’ originated as a seedling derived from an open-pollinated ‘Mogi’ seed irradiated with gamma rays (Ichinose et al., 1982). The resistance to Group C in ‘Shiromogi’ is currently the most valuable source of resistance to Group C among loquat cultivars, which is inherited with complete dominance at a single locus and expressed only in a recessive homozygote (pse-c/pse-c; Hiehata et al., 2012). The gene pse-c was probably derived from ‘Mogi’ (Hiehata et al., 2012), which is currently a major Japanese cultivar and it was a chance seedling found and selected in Japan.
Seedlings resistant to Group C with the pse-c gene have been produced by the breeding program at the Agricultural and Forestry Technical Development Center, Nagasaki, Japan. However, inbreeding depression might be a concern in breeding for Group C resistance because the number of resistant parent materials that contain pse-c (e.g., ‘Mogi’, ‘Shiromogi’) is limited, and there are close relationships among these materials (Fukuda et al., 2013). Although their descendants could be crossed or back-crossed with parents to produce a high proportion of homozygous-recessive seedlings (pse-c/pse-c) that are resistant to Group C, these crosses involve inbreeding and were therefore likely to exhibit inbreeding depression with lower tree vigor or yield. Resistant materials which are less closely related to ‘Mogi’ and ‘Shiromogi’ should therefore be actively used as parents to improve the effectiveness of breeding for resistance to Group C.
‘Champagne’, which is of unknown parentage, is one of the few genetic resources that are resistant to Group C. It was selected and introduced to California ≈1908 (Morton, 1987) and was introduced to Japan in 1952. The particular usefulness of this loquat is its complete resistance to Groups A, B, and C (Morita, 1988). ‘Champagne’ is distantly related to ‘Mogi’, ‘Shiromogi’, and Japanese cultivars (Fukuda et al., 2013). Thus, it has high potential as a cross-parent for preventing inbreeding depression in breeding for loquat canker resistance, and especially resistance to Group C. The genotype of ‘Champagne’ has been estimated to be homozygous-recessive (pse-c/pse-c) for the locus (Hiehata et al., 2012), but the resistance of ‘Champagne’ may be controlled by additional genes at another locus different from the pse-c locus (Hiehata et al., 2003). The objective of the present study was to clarify the inheritance of the resistance to loquat canker Group C derived from ‘Champagne’ through crosses between ‘Champagne’ and susceptible genotypes.
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