Factors affecting the rooting ability of softwood cuttings of dwarf apple rootstocks

In commercial apple production rootstocks are commonly propagated in stoolbeds. Alternatives such as in vitro propagation and cuttings are used to a much lesser extent, although they do provide some advantages over stoolbeds. We investigated ways to improve the softwood cutting technique by examining the effects of different rooting media (sand, peat, and vermiculite), auxin formulations (IBA, NAA) and concentrations, and etiolation pre-treatment on the rooting ability of six apple rootstocks (M9-Pajam2, FL56, M9-T337, M26, MM106, and US1). The results showed that auxin treatment is necessary for rooting because none of the cuttings formed adventitious roots without the auxin treatment. Medium G (peat:vermiculite:sand = 1:1:1, v/v/v) increased the rooting ability and kept more cuttings alive compared with each of the individual media. The highest rooting ratio was found for US1 in Medium G with a 2500 mg L^-1 IBA dose application (80%) followed by MM106 (76%) in the same medium/IBA combination. The lowest rooting ratio was for M9-Pajam2 in sand with 1500 mg L^-1 IBA (10%). Etiolation pretreatment significantly improved the rooting ability of different cuttings, especially for the difficult-to-root M9 clones. The technique involved covering the dormant plants in early spring with black plastic bags, which were removed when its shoots were 10-15 cm tall in order to allow them to green up while the shoot bases remained etiolated. Rooting percentage was improved from ≤30%-≥82% in 2500 mg L^-1 IBA in medium G for FL56, M9-Pajam2, and M9-T337, but only 18-47% for M26. Using 17.06 etiolated FL56 showed the highest mean number of roots per cutting, while M26 showed the lowest number at 14. The longest root was obtained for M9-T337 and the shortest for M26. Pretreatment etiolation has proved to be a key factor in improving the rooting ability of apple rootstock cuttings, which has applications for propagating new genotypes, especially for those difficult-to-root materials in future apple rootstock breeding programmes.