Comparison of climate ameliorating measures to control sunburn on 'Fuji' apples
A 12-year-old 'Fuji' apple orchard in the Western Cape Province, South Africa, was treated with over-tree evaporative cooling (EC), RAYNOX® (RY), EC + RAYNOX® (RE), 20% white shade net (SN), or Surround® WP (SU) to investigate their effects on sunburn and fruit quality compared to the untreated control (CO) during the 2004-2005 season. When midday air temperature was 30-32°C, fruit surfaces were on average 5.6 (SN), 4.6 (EC), 3.8 (RE) and 2.4°C (SU) cooler than CO fruit. RY did not significantly reduce fruit skin temperature compared to the CO on all measurement days. Total sunburn incidence was 34.3% for the CO. All treatments significantly reduced sunburn browning (Class 2), necrosis (Class 3) and total sunburn compared to the CO. Shade net was most effective, followed by RE, EC, SU and RY, in order of decreasing efficacy. Sunburn browning contributed about 78% to total sunburn in the CO but over 90% in the sunburn control treatments. SN reduced blush color significantly. EC and RE increased fruit mass and diameter while under SU they were reduced compared to the CO and other treatments. However, EC and RE fruit had lower firmness than CO fruit on the shaded side. Starch degradation pattern was reduced by SN and RE, while soluble solids concentrations and titratable acid concentrations were not affected. In conclusion, all control measures can potentially reduce sunburn although none would eliminate it. Availability of water for EC, and financial feasibility are factors which will influence the choice of technology for each farm.