How hail netting reduces apple fruit surface temperature: A microclimate and modelling study

High fruit temperatures compromise fruit quality and cause production losses in the apple industry. In south-eastern Australia, orchardists have begun investing in netting because of empirical evidence that it reduces these losses, but the magnitude of its effect and mechanisms responsible have not yet been quantified. Models of fruit temperature based on meteorological conditions could inform the design of netting structures, and improve tactical management to reduce sun damage through treatments such as protective sprays and the use of overhead irrigation to cool fruit. The objectives of this study were firstly to measure the effect of netting on fruit surface temperature, and secondly to test the thermodynamic Smart-Sinclair model. The study was conducted near Shepparton, Victoria, in an orchard where there were adjacent netted and non-netted sections. During late afternoon when sun damage normally occurs, netting was able to reduce the median fruit surface temperature by 1.5-2.0 °C, but there was a greater reduction in maximum fruit surface temperature of 4.0 °C. The model required calibration to account for turbulence in the transfer of heat from fruit to the surrounding air. The optimised model was able to predict fruit surface temperature with a root mean square error of 2-4 °C. The mechanism for the reduction in fruit surface temperature was by reducing the intensity of the solar beam in the late afternoon by interception and scattering, which more than offset the potential fruit heating effect of netting that occurs through a reduction in internal orchard wind speed.