Regulating tree vigour to optimise the long-term productivity of walnut (Juglans regia L.) orchards

Simpson JE1, Lang MD2 and Lewis J3

  1. NSW Department of Primary Industries, Orange Agricultural Institute, 1447 Forest Rd, Orange, NSW. 2800.
  2. Webster Limited, 148 Colinroobie Road, Leeton, NSW 2725.
  3. NSW Department of Primary Industries, Yanco Agricultural Institute, Yanco, NSW, 2703.

Walnut orchards in Australia are typically high-density plantings comprising one or more cultivars of Juglans regia L. An important criterion for cultivar selection is tree vigour, with greater vigour improving the profitability of orchards during the early years of tree production which increases profitability. As trees mature unregulated growth leads to shading of fruiting points and ultimately reduces tree productivity. This study aimed to manage tree vigour with an exogenously applied plant growth regulator (PGR), and to determine the impact of reduced vigour on potential yield and nut quality in young (2-4 years) and mature (8-10 years) walnut trees. The triazole-type PGR, uniconazole-P, reduces the internode length by inhibiting gibberellin biosynthesis ergo is a potential means of managing tree vigour. Sunny PGR (active ingredient: 50 g/L uniconazole-P), was applied to the root zone of mature walnut trees at four different rates (0, 0.75, 1.5, 3.0 L ha-1) and at two different timings (autumn and spring) in three cultivars: Serr, Vina and Chandler. Young trees were treated in the same manner as mature trees, but in cultivar Chandler only. All trials were randomised complete block designs with five-replicates, of four- and single-tree-plots in mature and young trees respectively. General linear modelling will be used to describe the relationship between tree growth (internode length), crop yield and nut quality with PGR treatment (rate, timing) and cultivar in mature trees, and between tree growth and PGR treatment in young trees.