Advances in apple variety and rootstock breeding in New Zealand and enlightens on Chinese apple breeding

Abstract: New Zealand(NZ) is one of the leading apple production countries in the world,producing about 400 000 metric tonnes of apples for export annually on over 9 000 hectares of orchard and generating about NZ$400 million in export revenue. NZ necessarily exports fresh apple fruit to the rest of the world because of its small local market, while relatively high production costs prompts the older varieties increasingly being replaced with new varieties to earn a premium in the global marketplace. The introduction of the world-renown‘Gala'by its early predecessor DSIR set the scene for the Plant & Food Research(PFR) apple breeding programme, which in turn now is a world leader. Today, marker assisted selection(MAS) for both fore- and background selection plays a large role in both the apple scion and rootstock breeding programmes. PFR's apple breeding programme has released more than ten new cultivars since the mid-1990 s, a handful of which now account for more than 25% of New Zealand's total apple exports. Recent cultivars have been commercialised through a joint venture company Prevar? Limited. Prevar cultivars are now being grown under license in New Zealand, Australia, the USA, the UK and Europe.It is estimated that 1.5 million trees will have been planted globally by 2018. Red flesh colour is a main commercial breeding objective as novel fruit trait introgression is an important part of the PFR apple breeding programme in developing new varieties with high consumer appeal. Red flesh breeding began in1998 by crossing low quality red-fleshed accessions with high quality cultivars and advanced selections that have white flesh. All of the red flesh germplasm used for breeding was derived from the PFR germplasm improvement programme known as the Apple Breeding & Genetics(ABG) programme. Two red flesh phenotypes have been identified based on the presence or absence of red in the core. Assessment of foliage colour as an indicator for the Type 1 red flesh phenotype is thus best carried out in early spring on young leaves when differences are most marked. Meanwhile, molecular markers are used to identify the Type 2 red flesh phenotype in green foliage progenies, and a few selections having gone into stage 3 for evaluation since 2003. MAS is performed by the Mapping and Markers team, which has been collaborating closely with the Pipfruit Breeding team for over 20 years starting with the identification of markers for pest and disease resistance genes for apple scab, powdery mildew, woolly apple aphid and fire blight, and later fruit quality traits, such as red flesh and fruit skin colour. PFR's resistance apple breeding was extended in 1993 when Dr. Vincent Bus specialized in sourcing resistances and studying their genetics in order to breed new cultivars with multiple, durable resistances to pests and diseases. Dr. Bus has worked on the main pest and diseases of apple in NZ, including woolly apple aphid, apple scab, powdery mildew, fire blight and, more recently, European canker. Additional sources of resistance were introduced in overseas breeding lines as well as identified in the ABG programme, while effective phenotyping methods were put in place. In the process of backcrossing the resistances into parents for use in the cultivar breeding programme, genetic markers were identified for a number of resistance genes, initially with random markers and later by mapping them to the apple genome. These genes were the Rvi6(Vf), Rvi5(Vm), Rvi2(Vh2),Rvi4(Vh4), Rvi8(Vh8), Rvi17(Vmis) and the‘Geneva'scab resistance genes; the Pl2, Pl-ns and Pl-mis mildew resistance genes; the Er1, Er2, Er3 and Er4 woolly aphid resistance genes; and the FBR5 fire blight resistance gene. MAS has been validated for a number of these genes, but at this stage has only been applied routinely to the Rvi6 gene in pyramids with Rvi2 at the cultivar breeding level since 2011.Apple rootstock breeding started at DSIR in the 1980 s when Dr. Suart Tustin made two hybrid populations by crossing‘Robusta 5'and‘Aotea'with‘M.9'in order to combine resistance from the crabapples with the dwarfing trait from‘M.9'.‘Aotea'was selected from M. sieboldii germplasm introduced from Japan in a quest for disease resistances for use in rootstock breeding. Seven elite selections with a vigour range from semi-vigorous to dwarf were selected from this programme and are evaluated by nurseries worldwide under the umbrella of International Fruit Obtention(IFO) based in France. Apple rootstock breeding was formalised in 1993 with Mr Mike Malone leading the programme and continuing the main objective of improving‘M.9'by combining its dwarfing trait with pest(woolly aphid) and disease(crown rot and fire blight) resistances. The programme was continued by Dr. Vincent Bus, who further increased the use of MAS as a major selection tool for the difficult and/or laborious phenotype traits, such as dwarfing, woolly aphid and fire blight. Until markers are identified for crown rot resistance, phenotypic selection for this disease will remain the first selection step, followed by MAS for some traits on seedlings, which reduces the population to one tenth for horticultural trait assessments. In order to further promote the level of China's apple breeding technologies and reduce the gap between advanced countries and China,in this comprehensive overview of the New Zealand apple breeding we present the knowledge and skills that could be of benefit to our breeders. These include:(1) standardization of the germplasm resources evaluation;(2) internationalizationof the breeding targets;(3) making large cross populations;(4) development of practical early screening technologies;(5) developing technologies for fast breeding;(6) paying more attention to consumer's evaluation;(7) developing optimal cultivation protocols for new varieties;(8) accelerating the apple pest and disease resistance breeding programme;(9) and increasing regional and international collaboration.