Effect of different rates and timing of soil applications of paclobutrazol on the growth of sweet cherries (Prunus avium L.)

Abstract

The response of sweet cherry trees (butt circumference, canopy volume, yield) to different rates of the growth-inhibiting chemical paclobutrazol applied at various times during each year was studied on red podsolic soils in the Mount Lofty Ranges of South Australia Two experiments were conducted: one used a regression design with 16 rates of paclobutrazol; the other was a randomised block experiment with paclobutrazol treatments applied in spring, autumn, or half in autumn and half in spring. Butt circumference growth of the variety Sam grafted on Mahaleb rootstocks tended to reduce with inceased rates of paclobutrazol in the first year of application. The lowest rate of paclobutrazol to reduce butt growth sigdicantly below that of control trees was 1.2 g a.i./tree. Canopy growth showed a delayed response in measurable growth inhibition in the season after treatment; however, no significant differences were measured for rate of canopy growth in any year for any treatment in the regression experiment. Despite this, final canopy volumes of trees treated with 1.6 g paclobutrazol in spring were half those of control canopies. Variability in butt growth response to the range of rates applied was attributed largely to localised differences in soil properties which influenced paclobutrazol movement and uptake. This variability was gradually reduced over the 3-year period with continued paclobutrazol applications. In 1987, crop level ratings made before harvest showed significantly higher levels of fruit on trees treated with 1.6 g paclobutrazol in spring, indicating that fruit numbers could be increased by applications of paclobutrazol.