Leaf defoliation of Cripps' Pink and Granny Smith apples in the EGVV - effect on vegetative and reproductive development

The aims of the study were to accelerate leaf abscission and leaf litter decomposition of ‘Granny Smith’ and ‘Rosy Glow’ to reduce potential apple scab inoculum sites. The effect of 20% white shade net on defoliation and leaf litter decomposition of ‘Granny Smith’ apples was evaluated. Vegetative and reproductive development was evaluated to determine if defoliation treatments influenced the growth and development in the following season.

Defoliation treatments were applied to ‘Granny Smith’ apple trees in a conventional orchard without net and ‘Granny Smith’ trees under 20% white shade net 10, 12, and 14 weeks after harvest (WAH) during the 2019-20 and 2020-21 seasons. The treatments were: Untreated control, 5% urea, 3% urea plus 2% zinc sulphate, 50 mg·L-1 s-abscisic acid (S-ABA), 56 mg·L-1 1-aminocyclopropane-1-carboxylic acid (ACC), and 50 mg·L-1 S-ABA plus 2% urea. Defoliation of ‘Rosy Glow’ apple trees as a management practice in warm winter regions: Defoliation treatments were applied to ‘Rosy Glow’ apple trees 7, 9, and 11 WAH in 2019 and 2020. The treatments were: Control (untreated), 5% urea, 3% urea plus 2% zinc sulphate, 50 mg·L-1 s-abscisic acid (S-ABA), 56 mg·L-1 1-aminocyclopropane-1-carboxylic acid (ACC), and 50 mg·L-1 S-ABA plus 2% urea.

Complete defoliation was effectively advanced by 19 to 21 days on ‘Granny Smith’ without net, and 26 to 27 days on ‘Granny Smith’ under 20% white shade net with the application of 3% urea plus 2% zinc sulphate applied 10, 12, or 14 weeks after harvest. In the 2019-20 season, 5% urea was as effective as 3% urea plus 2% zinc sulphate, and more effective than other treatments, in advancing complete defoliation of ‘Granny Smith’ under 20% white shade net. In the same season, this treatment was not as effective as 3% urea plus 2% zinc sulphate on ‘Granny Smith’ without net but increased the rate of defoliation compared to the remaining treatments and the untreated control. By applying the defoliation treatments 10 WAH, complete defoliation was advanced compared to application 12 or 14 WAH in ‘Granny Smith’ without net in the first season. Timing of application did not affect defoliation of ‘Granny Smith’ under 20% white shade net. Although some differences were observed in budbreak, spur leaf area, fruit set potential, yield, and fruit quality traits, these effects were not related to defoliation treatments in the previous season. Neither defoliation treatments nor 20% white shade net influenced leaf litter decomposition on the ‘Granny Smith’ orchard floor. Defoliation of ‘Rosy Glow’ apple trees as a management practice in warm winter regions: Defoliation of ‘Rosy Glow’ apple trees was accelerated with the application of 3% urea plus 2% zinc sulphate seven, nine, or 11 WAH in the first season, and seven or nine WAH in the second season compared to the untreated control and the other treatments. These applications advanced 100% defoliation by 33 to 36 days compared to the untreated control. The defoliation treatments did not influence the budbreak percentage, fruit set, yield, or fruit quality attributes the following season. The leaf litter decomposition percentage was significantly increased by all the treatments compared to the untreated control after 24 weeks in the second season. However, these treatments did not differ significantly from the untreated control in leaf decomposition percentage at any other sampling date throughout both seasons.

The combination treatment of 3% urea plus 2% zinc sulphate is recommended as a defoliant of ‘Granny Smith’. The application of 3% urea plus 2% zinc sulphate seven or nine WAH is recommended for defoliation of ‘Rosy Glow’. Further research is however needed to find an effective chemical defoliation treatment that can also accelerate leaf litter decomposition on the orchard floor, without negatively affecting subsequent growth and development.

The combination treatment of 3% urea plus 2% zinc sulphate is recommended as a defoliant of ‘Granny Smith’. The application of 3% urea plus 2% zinc sulphate seven or nine weeks after harvest is recommended for defoliation of ‘Rosy Glow’.