Designer rootstocks: the basis for precision management of apple orchards

Apple rootstocks affect many aspects of apple production including tree size, potential crop load, fruit set, fruit quality, tree anchorage and graft union strength, disease resistance, and total per hectare yield. Dwarfing rootstocks have revolutionized the apple industry, but up until a few years ago most commercial germplasm available was restricted to less than three genotypes (M.9, B.9, and M.26). While wide use of these rootstock provided a good paradigm-shift compared to unproductive non-dwarfing rootstocks, the LSQUOone size fits allRSQUO implementation was riddled with inefficiencies. The Geneva^® apple rootstock breeding program has been identifying apple rootstocks likely to match different applications allowing for precision matching with scion, soil and climate. Differences in growth habits and disease resistance of new apple cultivars require novel rootstock applications designed to match strengths and weaknesses of such cultivars in context of growing site. For example, scion cultivar NY-1 'SnapDragon^®' is a weak growing cultivar with a tendency to over crop and produce small apples. M.9 was initially tested and proved to be inefficient providing slow growth and susceptibility to fire blight. Rootstocks G.935, G.969 and CG.5257 were chosen to grow this cultivar because of their increased vigor, disease resistance and in the case of CG.5257 influence on fruit size. Designer rootstocks are being developed to work under different soil pH conditions, nutrition management, water use and training systems. We envision providing apple growers with a group of highly efficient disease resistant rootstocks of varying vigor levels to allow matching of scion vigor, soil vigor and climate vigor with rootstock vigor to result in orchards that grow well enough to fill the space in only 2 seasons but then are highly efficient and manageable for the next 20 years. This type of customization has been proven to increase efficiency in the use of fertilizers as well as increase productivity of marketable apples per unit of land area and is being studied in the USDA NIFA Specialty Crop Project “AppleRoot2Fruit”.