APPLE BREEDING PROGRESS IN JAPAN

Apple is the second largest fruit crop after citrus, cultivated in Japan. Apple cultivation dates back to the late 1800s when the first apple cultivars were introduced from the USA. The total area currently under cultivation is approximately 45,000 ha, with ‘Fuji’ contributing half the entire production. There are six major apple growing areas but only three research institutes from these regions have large breeding programs for apple. More than a hundred apple cultivars have been released from these breeding programs and about 30 of these are available mainly on Japanese markets. The main breeding objectives are improved texture, good storage ability and resistance to pests and diseases. Rootstock breeding started in 1972 with the aim of developing rootstocks suitable for high density planting, adaptability to wet soil conditions and ease of propagation by hardwood cutting. The breeding efforts led to the release of the ‘JM’ series in 1996/97. These rootstocks combine dwarfing attributes with high productivity and resistance to several diseases and pests hence, are considered good replacements for ‘M9’ and ‘M26’, in Japan. Until a decade ago, apple breeding methods and selection techniques were largely conventional. Through Agrobacterium-mediated transformation system, genes encoding sorbitol and antifungal proteins such as chitinase, glucanase and sarcotoxin have been transferred into apple cultivars and rootstocks. Transgenic lines of ‘Orin’ and ‘JM 7’ are currently being evaluated in glasshouses for resistance to several diseases. Five RAPD markers linked to the susceptibility locus for Alternaria leaf blotch (Alternaria alternata) have been identified using cultivar Kaori (‘Richard Delicious’ x (‘Ralls Janet’ x ‘Delicious’). High susceptibility to the disease has also been observed in ‘Indo’ and many cultivars derived from ‘Delicious’ or ‘Indo’. A linkage map approximately 950cM, has been constructed with 300 markers including RFLP, RAPD and CAPS. Loci for self-incompatibility linked RNAse gene are also being mapped. Studies are underway for QTL mapping of fruit traits using segregating populations of several Japanese apple cultivars.