Citrus is one of the most important fruit crops in the world with an annual production over 131 million tons. Spain is the sixth citrus producer and a leading exporter of fresh citrus worldwide (FAO, 2012). Despite its economic and social importance, farmers’ incomes are suffering large economic declines due to the constant increase in production costs meanwhile the prices received remained virtually constant since 1985. One way for farmers to increase their income level is by decreasing production costs. In Spain, citrus production costs are higher than those of competitor countries (Juste et al., 2000). Hand-labor operations represent the highest percentage of the citrus production costs, with harvest costs accounting for 35% to 45% of the total (Sanders, 2005). This is particularly important in those areas where citrus production is mainly oriented toward the fresh fruit market and therefore fruit have to be picked carefully to meet the quality standards. Mechanization of this labor could reduce the total costs in 30% to 35% (Juste et al., 2000).
Mechanical harvest with continuous canopy or trunk shakers has been used in citrus areas of Florida for years (Roka et al., 2014a, 2014b), where 95% of the orange crop is destined to juice production (NASS, 2015). The efficiency of these machines depends on fruit variety, tree characteristics, and operating conditions. In the case of trunk shakers, detachment rates between 57% and 90% have been obtained in ‘Hamlin’ (Citrus sinensis L. Osbeck cv. Hamlin) and ‘Valencia’ oranges (C. sinensis L. Osbeck cv. Valencia) under Florida agroclimatic conditions (Whitney et al., 1986, 2000; Whitney and Wheaton, 1987). To improve the efficiency of these technologies, abscission agents have been studied, and their use has been promoted in citrus areas of Florida (Burns, 2002; Burns et al., 2003; Hartmond et al., 2000; Whitney et al., 1986).
Studies performed recently under Mediterranean conditions to analyze the efficiency of trunk shakers in orange (C. sinensis) and mandarins (Citrus reticulata L.) trees have reported fruit detachment percentages ranging between 52% and 85% (Moreno et al., 2015; Torregrosa et al., 2009), with a percentage of fruit without calyx (important loss of quality for fresh consumption) between 0.6% and 9.0%. These results showed that harvesting with a trunk shaker may be a feasible solution for Spanish citrus for fresh market. The use of ethephon as an abscission agent increased fruit detachment as its dose increased, but its use also increased the percentage of fruit without calyx, so it should be recommended only for citrus destined to industry (Moreno et al., 2015).
Mechanical harvesting with trunk shakers produces an apparent violent shaking of the trees, which, depending on the machine, operators, and orchard conditions, can cause visible physical injuries to the trees such as shedding of leaves, flowers, and young fruit and breaking of branches and/or scuffing of bark. These observations fuel grower concerns about long-term tree health over using trunk shakers, alone or combined with abscission agents, to harvest fruit. As a result, there is a low widespread adoption of mechanical harvesting systems among Spanish citrus growers. For this reason, several field trials were conducted in Florida between 1970 and 2005 to investigate whether trunk shakers adversely affected fruit yield and long-term tree health. Except for the case of the late-season ‘Valencia’ oranges, the results of these field trials showed no short- or long-term adverse effects. Instead, the research suggested that trees that were well-nourished before and after mechanical harvesting fully recovered from all harvest related stresses (Hedden et al., 1984; Li and Syversten, 2005; Whitney, 2003). A more recent study analyzed grower yield data between 1998 and 2008 obtained from hand-picked and mechanically harvested orchards. It showed no evidence of shortened tree life or reduced yields caused by mechanical harvest (Roka et al., 2014c). However, no studies have been conducted to assess the effects of mechanical harvesting and the use of abscission agents on citrus tree physiology and yield under Mediterranean conditions. Citrus cultivation is different in Florida than in the Mediterranean regions because of different soil and environmental conditions, irrigation techniques employed, and citrus varieties cultivated. For example, in Florida, soils are predominantly sandy, whereas in the southeastern Spain, where citrus is the most important crop, soils are more calcareous and often with high clay content. In addition, Florida citriculture normally employs microjet sprinkler wetting most part of the orchard floor, whereas in Spanish citriculture, drip irrigation is used, and as a consequence in Spanish orchards the root system is more concentrated below the drippers. Under these conditions it could be that the trunk shaker could be more harmful because of the more concentrated root system close to the tree trunk. Before attempting to recommend any practice regarding mechanical harvesting to Mediterranean citrus growers, more research on the most common mandarin varieties cultivated should be conducted.
The present work aimed to assess the physiological and fruit yield responses of four mandarin cultivars (Orogrande, Marisol, Clemenules, and Fortune) and one orange variety (‘Navel Lane Late’), all of them mechanically harvested, with and without ethephon applications, under Mediterranean conditions. Yield effects were monitored in both the current and subsequent seasons.
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