Apple scab, which is caused by Venturia inaequalis (Cooke) G. Wint., is a destructive disease that affects apples (Malus × domestica Borkh.). To develop an approach to breeding apples for scab resistance, race distribution of V. inaequalis in Morioka, Japan and genetic factors affecting partial scab resistance observed on 'Akane' were assessed. An inoculation test using several differential hosts suggested that race 1 of V. inaequalis was the predominant race in Morioka, Japan. To characterize the genetic profile of 'Akane' resistance, quantitative trait locus (QTL) analyses that employed two years of natural scab infection scores in a fungicide-free orchard in Kuroishi, Aomori, Japan were performed using an F 1 population derived from the 'Orin' × 'Akane' cross. A QTL allele that explained 20% of the phenotypic variance in scab resistance was detected on chromosome 17 of 'Akane'. In the middle region of chromosome 17 of 'Orin' and in very close proximity to the 'Akane' QTL, a weak QTL allele was also detected, which explained 5% of the phenotypic variance observed. We considered that these effects were caused by alleles of the identical QTL. Haplotyping analysis indicated that this QTL resistance allele from 'Akane' originated from 'Worcester Pearmain'.

Apple blotch (Marssonina blotch), which is caused by Diplocarpon mali Harada & Sawamura (anamorph Marssonina coronaria (Ell. & J. J. Davis) J. J. Davis), is one of the major diseases of cultivated apple (Malus × domestica Borkh.) in Japan. The aim of this study was to develop a reliable index to assess the resistance of apples to Apple blotch, and evaluate resistance levels of various apple genotypes. At first, we observed the degree of disease development on natural infection with Diplocarpon mali. in the field of Apple Research Station, Institute of Fruit Tree and Tea Science, NARO. Among 12 varieties, dwarfing rootstocks, Marubakaido and Mitsubakaido revealed that the disease incidence and disease severity index were high among cultivated apples, whereas no disease symptom was found in Mitsubakaido; the disease incidence and disease severity index in Marubakaido, 'M. 9EMLA' and 'M. 26EMLA', were low compared with those in cultivated apples, 'JM1' and 'JM7'. A strong positive correlation was observed between the values of the disease severity index and rate of defoliation obtained by the inoculation test and the degree of disease development, expressed as the disease incidence and disease severity index, caused by natural infection with Diplocarpon mali. in the field. Resistance levels of seventy apple genotypes including rootstocks and wild Malus species, inoculated with the conidial suspensions artificially, were classified by the values of the disease severity index as: resistance < 0.3, 0.3 ≦ moderate resistance < 0.6, 0.6 ≦ susceptible; similarly, the genotypes were classified by the values of defoliation as: resistance < 0.1, 0.1 ≦ moderate resistance < 0.4, 0.4 ≦ susceptible. Inoculation tests indicated that the resistance levels of several wild Malus species, Tarehanakaido (Malus halliana), Mitsubakaido (Malus sieboldii), Nikkozumi (Malus baccata), Bracteata (Malus coronaria), Malus angustifolia, and Malus ioensis, were rated as adequately resistant to Apple blotch, while cultivated apples including scab-resistant varieties such as 'Florina', 'Freedom', 'Remo', 'Retina', and dwarfing rootstocks of 'M.9E', 'M.26E', 'JM1', and 'JM7' were susceptible. Marubakaido and some wild Malus species belonging to Malus platycarpa, Malus pratii, Malus sieversii, Malus florentina, and Malus honanensis were rated as moderately resistant.