Comparative effects of evaporative cooling, kaolin particle film, and shade net on sunburn and fruit quality in apples

Abstract

We investigated the effects of evaporative cooling (EC), kaolin particle film (KP) and 20% shade net (SN) on the control of sunburn, fruit temperature amelioration and fruit quality of 'Cripps' Pink' and 'Royal Gala' apples [Malus domestica Borkh.] under orchard conditions during the 2003-04 season in Stellenbosch, South Africa. On days with maximum air temperatures of 34 to 37°C, SN fruit were 5.4 to 9.7°C cooler, EC fruit were 3.1 to 5.8 °C cooler and KP fruit were 1.5 to 6.4°C cooler compared to the control (nontreated, CO) fruit. SN was effective in reducing fruit temperature from mid-morning until midafternoon; KP was most effective during late morning and early afternoon but not at midday; EC was effective from late morning on days when EC was activated. SN, followed by KP, was the most effective technique for controlling sunburn in fruit of both cultivars, with EC being less effective. The different technologies reduced fruit blush color compared to the CO treatment, with SN showing the most reduction and EC the least. EC increased fruit mass compared to all other treatments in 'Royal Gala', and also increased fruit diameter and mass compared to CO in 'Cripps' Pink'. We conclude that under the high radiation levels experienced in South African apple production areas, technologies which reduce irradiance as well as fruit temperature (KP, SN) are more effective in reducing sunburn than those which only reduce fruit temperature (EC). However, radiation-reducing technologies are potentially detrimental to color development on blushed apples.