The citrus industry in Florida has 193,000 ha of sweet oranges [Citrus sinensis (L.) Osbeck] grown principally for the fresh juice industry (Anonymous, 2008) of which ≈13,153 ha are mechanically harvested (Florida Department of Citrus, 2008). Removal rates without abscission-promoting compounds have ranged from 50% to 98% in research studies (Whitney, 1975, 2000, 2003; Whitney et al., 2000a, 2000b; Whitney et al., 2001). The limitations of mechanical harvesters are especially acute for late-season ‘Valencia’, in which harvesting of the mature crop overlaps temporally with development of the new crop. ‘Valencia’ trees typically bloom in late February through March in Florida, and by early May, the immature fruit are large enough to be removed by mechanical harvesters, thus potentially reducing yield the next year. To preserve long-term productivity, most commercial mechanical harvesting in Florida currently ends in early May. Abscission compounds that increase removal rates by mechanical harvesters, and especially increase removal of mature late-season ‘Valencia’ fruit without removing the newly developing crop, have received considerable research attention in recent years.
Studies have been conducted to identify compounds that promote hydrolytic digestion of the abscission layer of mature sweet oranges. Of the compounds tested, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) has been shown to be the most effective (Burns et al., 2005; Freeman and Sarooshi, 1976; Koo et al., 1999; Whitney, 1975, 1976; Whitney et al., 2000a, 2000b; Wilson, 1973) and as a result, CMNP is currently being intensely studied as an aid to commercial mechanical harvesting, especially for late-season ‘Valencia’ because it does not promote abscission of the newly developing fruit (Burns et al., 2006). Efficacy of CMNP in promoting sweet orange fruit abscission is affected by several variables, especially concentration and coverage, temperature, and postspray precipitation events (Ebel and Burns, 2008). Furthermore, direct peel contact of CMNP is essential for promoting abscission of sweet oranges (Alferez et al., 2005). The effect of these variables on CMNP efficacy will need to be determined to optimize CMNP application and mechanical harvesting scheduling in commercial sweet orange groves.
Because CMNP requires direct peel contact to promote fruit abscission, uniform spray coverage throughout the canopy will be required. Air-blast sprayers have been the most widely used method of pesticide and plant growth regulator applications in commercial citrus groves in Florida. Air-blast sprayers project spray droplets upward, and although there is some turbulence outside and within the canopy, coverage is not as uniform as multihead tower sprayers that provide spray at all levels of the canopy (BenSalem et al., 2001; Farooq et al., 2003; Koo et al., 1999, 2000; Salyani et al., 2002). The advantages of vertical, multi-fan sprayers compared with air-blast sprayers in improving spray coverage has also been shown for an abscission agent for sweet oranges (Burns et al., 2006). Optimal sprayer design for CMNP application will need to include sufficient penetration into the canopy, especially considering that detachment force of fruit inside the canopy is naturally higher than fruit on the periphery (Kender and Hartmond, 1999). Optimum sprayer design will also need to include thorough coverage of fruit near the top of the canopy, because this fruit tends to have higher detachment force than fruit lower in the canopy (Kender and Hartmond, 1999).
This study was conducted using a sprayer that has the desirable features of providing spray throughout the canopy. The sprayer has multiple fans that deliver spray from the lowest branches and arching over the top of the canopy (Fig. 1). Using this sprayer technology, the objective of this study was to determine the relationship of CMNP spray volume applied per hectare to the distribution of spray and fruit loosening in the canopy and percent crop removed by mechanical harvesters. The relationships were determined under the most challenging grove conditions by using fully mature trees with full canopies.
Alferez, F., Shila, S., Umback, A.L., Hockema, B. & Burns, J.K. 2005 Citrus abscission and Arabidopsis plant decline in response to 5-chloro-3-methyl-4-nitro-1H-pyrazole are mediated by lipid signaling Plant Cell Environ. 28 1436 1449
Anonymous 2008 Citrus summary, 2006–2007 Florida Agricultural Statistics Service, Fla. Dept. Agric. Consumer Services Tallahassee, FL
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Burns, J.K., Pozo, L., Morgan, K. & Roka, F. 2006 Better spray coverage can improve efficacy of abscission sprays for mechanically harvested oranges Proc. Fla. State Hort. Soc. 119 190 194
Ebel, R.C. & Burns, J.K. 2008 Incorporation of air temperature into a model that predicts loosening of sweet oranges by CMNP Fla. State Hort. Soc. Proc. 121 160 163
Farooq, M., Saylani, M. & Whitney, J.D. 2003 Effect of application techniques on abscission chemical deposition and mechanical harvesting of ‘Valencia’ oranges HortTechnology 13 344 350
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