Rootstock, Scion, and Graft Type Influence Graft Union Flexural Strength of Apple Trees

Apple tree nurseries utilize various grafting methods to produce numerous combinations of scions and rootstocks joined together ideally to leverage desirable qualities of both. Geneva (R) apple rootstocks are known for their productivity and disease resistance, however apple rootstock 'Geneva (R) 41' ('G.41') has been shown to form weak graft unions with some desirable scion cultivars like 'Honeycrisp' and 'Cripps Pink' resulting in breakage during wind, tree harvest in the nursery, and shipment. Other Geneva (R) rootstocks have also been broken during wind events in the nursery. Flexural strength and flexibility were tested among a combination of six scions ('Fuji', 'Gala', 'Honeycrisp', 'McIntosh', 'Cripps Pink', and 'Scilate') seven rootstocks (G.41, G.935, G.214, G.11, M.9-NIC 29, B.9 and EMLA 26), and two grafting methods (chip bud and saddle graft) in an incomplete factorial design. A sub-sample was selected to measure change in graft union strength over two years. G.41 consistently had the lowest break strength relative to tree size and this persisted into the second year whereas G.214 exhibited the highest strength. G.41 also had the lowest flexibility (as measured by deflection distance to failure) regardless of scion cultivar or grafting method. 'McIntosh' and 'Cripps Pink' formed stronger graft unions, while 'Scilate' formed a weaker graft union relative to tree size. Saddle grafting did not appear to improve graft union flexural strength. Parallel samples of graft unions developed in this study were subjected to micro computed tomography (CT) scans to visualize microstructures within the graft that could be associated with weak grafts. Scans showed that in weak rootstock-scion combinations there were areas of disorderly formation of vascular connections whereas the stronger combinations featured more uniform formation. Therefore, when using G.41 rootstock, care should be taken to select scion cultivars that will form graft unions of sufficient strength to withstand nursery harvest, shipping and initial planting in the orchard.