Hand-thinning, the physical removal of blossoms or immature fruit, can increase apple fruit size and quality at harvest. Maximal benefit of fruit size increase due to green fruit thinning occurred at or before 40 d after full bloom (Batjer et al., 1957). This corresponds to the period of time when chemical thinners are effective. Chemical thinning outcomes are highly variable because of multiple environmental and physiological factors, and sometimes due to managerial error. Hand-thinning is a very labor-intensive process, and growers report that obtaining a labor force in agriculture is becoming increasingly difficult (Gasperini, 2012). Therefore, hand-thinning after June drop is typically reserved for high-value fresh market cultivars with excessive crop loads.
Given the stringent fruit size and quality standards demanded by markets and the occasional failure of chemical thinners, hand-thinning is still an important method of crop load management. In general, thinning guidelines emphasize spacing apples 6 to 10 inches throughout the canopy, and focus on removal of injured fruit, small fruit, and breaking up clusters (Gourley and Howlett, 1941). Thinning to a single fruit per cluster increased fruit size when compared with leaving two fruit per cluster, and reducing the number of fruit per spur increased fruit size when compared with equivalent treatments of removing entire clusters of fruit or selectively de-fruiting whole limbs within the canopy (Knight, 1980). Methods that emphasize small fruit removal improved fruit size and increased the value of the crop (Davison et al., 1979).
Pomologists use crop density (the number of fruit per unit branch or tree dimension) to express the balance of reproductive and vegetative growth of apple, and as a method to standardize fruit number per tree based on tree size (Lombard et al., 1988). Trunk size has a positive linear relationship with the fresh weight of the aboveground portion of the tree, and can be used to estimate potential bearing surface per unit land area (Westwood and Roberts, 1970). Subsampling the crop density of branches (number of fruit per square centimeter BCSA) is an early season sampling technique of whole tree crop density [number of fruit per square centimeter trunk cross-sectional area (TCSA)] (Uriu and Lilleland, 1959), and is effective in estimating the effects of fruit-thinning treatments on fruit number, yield, and mean fruit weight (Forshey and Elfving, 1979).
The Maîtrise de la Fructification - Concepts et Techniques (MAFCOT) group of the INRA in Montpellier, France, developed the centrifugal training system that partially manages crop load by the physical removal of flowering spurs (Lauri et al., 2004; Lespinasse and Lauri, 1999). The MAFCOT group developed a handheld tool (Equilifruit; INRA, Montpelier, France) as a guide for thinning by spur extinction (removal) and by fruit removal. The hand-thinning gauge is a small plastic disc with 11 semicircular notches of varying diameters (Fig. 1). At each notch, there is a whole number that is preceded by the letter F (F value). The F value indicates the number of spurs or fruit to leave, based upon a final crop density of 6 fruit/cm2 BCSA. The disc also provides Δ, which allows the user a means of adjusting the intensity of hand-thinning treatment. To decrease or increase the number of fruit removed during hand-thinning treatment, Δ can be added or subtracted to the F value.
The objectives of this experiment were to evaluate a hand-thinning gauge on multiple apple cultivars trained to tall spindle (Robinson et al., 2006), and to test the versatility of the thinning gauge by use of Δ.
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