Micropropagated vs. grafted 'Chandler' trees in young walnut orchards

The response of micropropagated (M) and grafted (on walnut seedlings, G) 'Chandler' trees to the current training techniques for central leader system is being compared in a number of commercial orchards of the Emilia-Romagna region (44-45°N), Italy. In an orchard planted in winter 2016/17 using 0.10-0.20 m, 8-month-old M and 1.85 m, 33-month-old G trees, spaced 7×5 m (286 trees ha^‑1), the length of the shoot selected as the leader, the number of leaves and of the sylleptic shoots on the leader were recorded at 15- to 30-day intervals throughout the first growing season. Except for the beginning of the growing season, where M trees showed a more vigorous growth resumption, the growth rate of M and G trees did not show significant differences during the season. The vigor, internode number and length of the leader of M vs. G trees at the end of the 1^st growing season were studied in 7 additional orchards, established the previous year (winter 2015/16) and with the same planting system. The bud-break percentage in the 2^nd growing season following the head-back and partial disbudding of the terminal portion of the leader, was recorded as well. Regardless the orchards, the M trees resulted comparable or 10-15% higher than G trees, although the M material at planting was younger and undersized compared to G material. Moreover, variability in tree size was lower in M trees, allowing more homogeneous trees in the orchard, which is an important requisite in the modern walnut industry.