Microclimate modification to improve productivity of ‘Carmen®-Hass’ avocado orchards using shadenet under subtropical conditions of Limpopo Province, South Africa.
Malapana, Clarance Kgethego.
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The agricultural environment is a complex and dynamic system. Microclimate, the crop, biosphere, and management practices interact to determine the best yield production. South Africa is a water-scarce country, with high variability in annual rainfall. Thus, water quality and quantity are major limiting factors in agriculture. Hence, shadenetting can be used to modify the orchard microclimate to make the environment more conducive for fruit production. The South African avocado industry is export-oriented, so there is a commercial need to optimise the exportable percentage of avocado fruit. Sunburn, wind and hail damage and small fruit size as a result of water stress are the major cull factors for the industry. It is believed that shadenetting, with changes in management practices, can counter these limiting factors. There is no literature on growing avocado fruit under shadenetting. Therefore, the aim of the research was to determine the effects of a 20% white shadenet on ‘Carmen®-Hass’ avocado orchards and productivity. The long term objective is to improve avocado fruit quality and profitability in the Mooketsi Valley, Limpopo province, South Africa, a subtropical environment by reducing abiotic stress, particularly, solar irradiance, heat and wind. The trial was conducted at Goedgelegen Estate in the Mooketsi Valley on ‘Carmen®-Hass’ trees planted in 2007/8 season. A 1-ha shadenet structure (6 m high) was used, with 20% white shadenet over the roof and 40% green shadenet on the sides. Air and canopy temperature, relative humidity, wind speed, solar irradiance and leaf wetness duration (LWD) and sap flow were monitored at a sub-hourly rate. Evapotranspiration was calculated from the above mentioned parameters. Irrigation was monitored five times per week using tensiometers at 300- and 600-mm soil depths. The comparison between open and shadenet leaf areas showed that leaves in the open treatment were reduced as a result of the abiotic stress. Fruit water content under the shadenet compared to the open was greater, such enabled fruit under the shadenet to reach maturity two weeks earlier when compared with open treatment. Air and canopy temperature and relative humidity were slightly reduced under the shadenet, with the greatest difference occurring during the flowering period in mid-winter. The modification in air temperature and relative humidity was beneficial for bee activity and pollination in 2015 compared to the 2014 season. ‘Carmen®-Hass’ flowers in mid-winter when temperature conditions are not conducive for pollination. Canopy temperature was also reduced under shadenetting compared to the open treatment. The reduction was due to differences in tree density and the role that shadenetting plays. The infrared thermometer measurements were uniform with dense canopies compared to sparse tree canopies. The midday incoming solar irradiance was reduced by 18% under the shadenet compared to the open treatment. Calm conditions were experienced under the shadenet. Hence, windspeed was reduced to negligible levels. Also, the shadenet resist air flow to a certain height compared to the open treatment. LWD was extended by 12% under shadenet. An infestation of the insect pest citrus leaf roller (Archips occidentalis) caused severe damage to the fruit during the 2014/15 season due to the high plant density used. Significant results were that evapotranspiration was reduced by 14 and 29% less water was applied under the shadenet to maintain an adequate soil water content compared to the open treatment. Fruit reached minimum maturity two weeks earlier under shadenet compared to the open treatment. Fruit quality and pack-out were improved under the shadenet due to reduction in sunburn, wind damage and small fruit. But poor yields were experienced during the 2014 season due to poor bee activity, pollination and fruit size distribution were reduced under the shadenet compared to the open treatment. But following the improved bee activity in 2015, the 2016 normal season yield is likely to be improved under the shadenet than in the open treatment. Data collected in the Mooketsi Valley showed that 20% white shadenet has modified the microclimate and improved fruit quality. The water use under the shadenet was improved compared to the open treatment. But a thorough investigation on bee management under shadenet is required to optimise pollination in order to obtain greater yields under the shadenet.