1-Naphthaleneacetic acid and 6-benzyladenine thinning of a common slender spindle ‘Jonagold’/M.9 apple orchard. I: Dose effects and spray distribution in the crowns

To find the most appropriate rates of application of plant growth regulator (PGR) thinning-agents for a common slender spindle apple (Malus × domestica Borkh.) orchard, different volumes of dilute 1-naphthaleneacetic acid (NAA) and 6-benzyladenine (BA) were sprayed ha⁻¹. Mature ‘Jonagold’/M.9 trees, 3.0 – 3.5 m high and 1.2 – 1.5 m wide, planted in a single row system with 3,030 trees ha⁻¹, were used. Significant thinning was observed in the case of dilute sprays of NAA at 10 mg l⁻¹, or BA at 100 mg l⁻¹, to run-off, using 2,000 l ha⁻¹ or 1,500 l ha⁻¹; while 1,000 l ha⁻¹ did not result in sufficient thinning. Thinning using smaller volumes (250, 500, or 750 l ha⁻¹) was also significant if the concentration of PGR thinner was proportionally higher (i.e., based on the 1,500 l ha⁻¹ application rate of more dilute sprays of NAA at 10 mg l⁻¹ or BA at 100 mg l⁻¹). Spray distribution measurements in the crowns showed better spray deposits when higher water volumes (i.e., more dilute PGR solutions) were sprayed at all positions (bottom, middle, or top) of the canopy. At 2,000 l ha⁻¹, 54 – 72% coverage of the leaf area was observed; but, at 250 l ha⁻¹, coverage was only 10 – 21%. The lower 30% of the canopy was covered poorly when smaller volumes of water (250, 500, or 1,000 l ha⁻¹) were applied. When 1,500 l ha⁻¹ was sprayed, good coverage of the lower and upper surfaces of the leaves occurred, and no differences in canopy positions were measured. It was concluded that 1,500 l ha⁻¹ (i.e., dilute PGR) spraying was the most appropriate volume to use when calculating the dose of NAA or BA to be applied ha⁻¹ to common (3.0 – 3.5 m-high) mature slender spindle apple orchards on M.9 rootstock. This study was part of the ISAFRUIT Smartfruit Project, aimed at improving existing methods for apple crop regulation with more precise use of PGR thinning agents and with minimum impact on the environment.