Effects of Covering Mature Avocado ‘Pinkerton’ Trees with High-density Shading Nets during Cold Winters on Microclimate, Chlorophyll Fluorescence, Flowering, and Yield

Avocado ( Persea americana Mill.) is a subtropical fruit tree with high commercial value and increasing global demand. Most avocado cultivars are vulnerable to cold climates, which may reduce yields and restrict their geographical expansion. This includes the green-skinned avocado cultivar Pinkerton, which accounts for 45% of the avocado cultivated in northeastern Israel. Shading nets can protect agricultural crops from cold environments. Therefore, we evaluated the effect of covering mature ‘Pinkerton’ trees with high-density shading nets during the winter. Trees were covered with silver-colored 50% or 70% shading nets during three consecutive winters, and uncovered trees served as controls. Photosynthetically active radiation in plots covered with the silver 50% or 70% nets was significantly lower than that for the control by 52% and 90%, respectively. The minimum air temperature was similar between treatments. The maximum air temperature was generally lower under the shading nets compared with that of the control. The ratios of variable fluorescence to maximum fluorescence (Fv/Fm) measured in February 2019 and February 2020 were 0.72 and 0.8 for the control trees, 0.79 and 0.83 for the silver 50% trees, and 0.81 and 0.84 for the silver 70% trees, respectively. Flowering intensity of the net-covered trees was lower than that of the control by up to 42%. Interestingly, the 3-year average yield of trees covered with the silver 50% or 70% nets was insignificantly higher by 27% and 38%, respectively, compared with the control trees. These results suggest that the reduction of daytime solar irradiance in the winter by the shading nets may mitigate adverse effects of cold and increase yield. Additional long-term studies should examine the effects of shading nets and other shading strategies on different avocado cultivars.