Shade Netting Reduces Sunburn Damage and Soil Moisture Depletion in ‘Granny Smith' Apples

Benefits associated with shade netting of apple orchards have been assessed in several countries, but few relevant experiences exist for Washington State, USA, a semi-arid region characterized by high light intensity and high temperature conditions. Here, environmental conditions increase the risk of plant stress and fruit sunburn damage. The aim of this study was to assess the effect of shade anti-hail nets on orchard microclimate, soil moisture and fruit sunburn. Seven-year-old ‘Granny Smith’/M9 T337 apple trees were covered in 2013 with 22% white shade net by installing four pods, each of them covering a rectangular area of 7 adjacent tree rows by 12 trees per row (18 m × 11 m × 5.2 m high). Uncovered trees served as control. Shade net reduced the maximum daily photosynthetically active radiation (PAR) by 32% ± 4.92 (std dev), with a maximum PAR value on a sunny day of 1649 and 1222 mmol/m²s in the control and netted areas, respectively. Canopy air temperature was slightly lower during the day and higher at night under the net compared to the control, resulting in a 1.5° C decrease in day-night temperature difference. Shade nets reduced soil moisture depletion at 0-30 cm depth, resulting in 20% higher soil water content compared to the control. No significant differences were observed deeper in the soil profile. Netting also significantly decreased fruit sunburn, resulting in 75% and 92% fruits with no sunburn damage in 2013 and 2014, respectively.  Fruits from control plots with no sunburn damage represented 41% and 51% of sampled fruits in 2013 and 2014, respectively.  Although preliminary, these results show that shade nets have a great potential to reduce sunburn damage and water requirements in semi-arid environments and might represent an important tool for increasing yields of high quality fruit.