The final size of citrus (Citreae) fruit depends on the degree of competition among them and other factors related to the resources available to the tree. Larger numbers of flowers and fruit increase competition. This competition reduces the total growth and final size of the fruit (Agustí and Almela, 1984; Agustí et al., 2003; Mesejo et al., 2003). To decrease this competition, “thinning,” which is the removal of some flowers and/or fruit in the earliest development stage, is a common practice. Regardless of this, the economic implications of increasing the fruit size and the accompanying reduction of fruit yield by weight have not been deeply studied (Davis et al., 2004).
The main methods of thinning are manual, chemical, and mechanical. Manual thinning is the most frequently used because it is easily adapted to different growing conditions; however, it is slow and expensive. As a result, chemical and mechanical thinning methods are being explored.
Several studies of the chemical thinning of citrus have been conducted (González-Rossia et al., 2007; Guardiola and García-Luis, 2000; Stover et al., 2002). For citrus crops, thinning should be performed during the first stage of fruit development. The compounds most commonly used are naphthalene acetic acid, 2-chloroethyl-phosphonic acid, gibberellic acid, and other synthetic auxins (Agustí, 2003; Mesejo et al., 2012). However, several agrochemical products are being rejected by the market (Ouma, 2012), and their effects are highly variable and strongly influenced by uncontrolled factors, particularly environmental conditions (Greene and Costa, 2013). These observations encourage the exploration of other thinning options such as mechanical procedures.
Mechanical thinning is, to some extent, used for stone fruit (Amygdaleae) and pome fruit (Maleae). However, it has not been adapted to citrus crops. Trunk shakers, among other mechanical systems, have been tested (Berlage and Langmo, 1982; Powell et al., 1975), but the results of trunk shakers, in terms of reducing the number of fruit, have not been shown to be viable. Other authors, including Diezma and Rosa (2005) and Glozer and Hasey (2006), have explored low-frequency, electrodynamic manual shakers, but these have been shown to be impractical. At present, the most commonly used mechanical systems are based on rotatory strings, which are either coupled to a tractor or manually controlled (Damarow et al., 2007; Roche and Masseron, 2002). These mechanisms work well for flower thinning. Mechanical combs used for olive (Olea europaea) harvesting are also being used with some success for flowers and small peach (Prunus persica) fruitlets (Martín et al., 2010). Because leaves are present, these finger and string-based systems cannot be used with citrus fruit as part of thinning operations. Schupp et al. (2008) showed that mechanical thinning using a drum shaker could be highly effective for thinning apple trees (Malus ×domestica) and peach trees grown on production systems trained to a narrow tree wall canopy.
Some studies using manual gasoline-powered shakers and electric combs for thinning mandarins (Citrus reticulata) of the varieties Clemenrubí and Clemenules have been performed. These tests showed that it is possible to detach small fruit (Ortiz et al., 2016, 2017; Torregrosa et al., 2017). However, more exhaustive studies should be undertaken to evaluate the economic validity of these methods.
The overall objective of this work was to study the feasibility of mechanical thinning as an alternative to manual thinning for citrus crops. The specific objectives were to compare the costs of mechanical and manual thinning and to assess the effects on the final fruit yield by weight and fruit size with manual thinning, mechanical thinning, and no thinning.
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