The use of protective netting in fruit tree cultivation is an effective method of preventing yield loss that can result from high temperatures and irradiation, strong wind, and hail (Manja and Aoun, 2019; Mupambi et al., 2018; Raveh et al., 2003; Stamps, 2009; Wachsmann et al., 2014). Often, the use of protective netting also results in changes in the orchard microclimate that can alter tree physiology and improve tree performance and fruit development (Cohen et al., 2005; Manja and Aoun, 2019; Mupambi et al., 2018; Pérez et al., 2006; Raveh et al., 2003; Stamps, 2009; Wachsmann et al., 2014; Zhou et al., 2018). Various different types of permanent protective netting structures and/or different colors of shade netting have been tested to determine their effects on tree physiology and fruit development and quality (Manja and Aoun, 2019; Mupambi et al., 2018; Stamps, 2009; Wachsmann et al., 2014; Zhou et al., 2018). In Citrus spp., nonpermanent netting (NPN) is used in certain production regions during a particular stage of the season to protect trees and fruit from damage that could be caused by hail (Wachsmann et al., 2014). It is also used for some citrus cultivars during flowering to exclude bees, thereby preventing cross-pollination and unwanted seed development (Gambetta et al., 2013; Gravina et al., 2016; Otero and Rivas, 2017).
Certain citrus cultivars have the ability to produce fruit without the flower having to be pollinated or the ovaries having to be fertilized, which is known as parthenocarpy (Mesejo et al., 2016). A cultivar such as Nadorcott mandarin (C. reticulata), for example, is an obligate parthenocarp (Gambetta et al., 2013), which means its ovaries can produce sufficient gibberellins during anthesis to produce fruit in the absence of pollination (Mesejo et al., 2016). ‘Nadorcott’ mandarin is also sexually self-incompatible (Gambetta et al., 2013; Nadori, 2006), which means that its flowers are unable to produce seed on self-pollination (De Nettancourt, 1977). This trait is advantageous for commercial citrus production because when trees are isolated from certain potential cross-pollinators, they can produce seedless fruit (Gambetta et al., 2013; Otero and Rivas, 2017), which are preferred by the market and sold at a premium (Barry, 2006).
It is not always possible to avoid cross-pollination during commercial citrus production. When flowers of ‘Nadorcott’ mandarin are exposed to pollen from a different compatible cultivar, the fruit are normally seeded (Nadori, 2006; Wright, 2007), thereby reducing the market value of the fruit (Barry, 2006). During commercial ‘Nadorcott’ mandarin production, for which the potential for cross-pollination is high and seed development likely, some growers cover the entire tree with NPN during flowering to exclude pollinators such as honeybees (Gravina et al., 2016; Otero and Rivas, 2017). This method is successful and environmentally safe. However, it is unclear how the covering of trees with NPN before, during, and after flowering for an extended period, thereby affecting tree growth and fruit development, impacts the deposition of foliar sprays that are applied and fruit production.
This study evaluated the effects of NPN on foliar spray deposition, leaf mineral nutrient concentration, fruit yield and quality, and rind surface damage of ‘Nadorcott’ mandarin fruit. Monitoring of two important insect pest species was performed during the experiment to determine the effect of NPN on their numbers. The false codling moth (FCM), Thaumatotibia leuctotreta (Meyrick) (Lepidoptera: Tortricidae), is a well-known pest of the South African citrus industry that causes crop losses due to larval infestation of fruit (Newton, 1998; Moore and Hattingh, 2012). Furthermore, the FCM is classified as a phytosanitary pest by a number of export markets (Grout and Moore, 2015). Therefore, it is important to monitor and control this pest. The Mediterranean fruit fly, Ceratitis capitata Weidemann (Diptera: Tephritidae), is a highly polyphagous species that uses citrus as a host (Du Toit, 1998; Grout and Moore, 2015). Economic losses may occur either as a direct result of female fruit fly oviposition and larval infestation or due to the phytosanitary status of C. capitata, which can result in the rejection of infested fruit from sensitive export markets.
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