Current world highbush blueberry (V. corymbosum) acreage is about four times more than it was in 1995. In 2005, world blueberry production was 431 million pounds and is projected to grow to about 1.5 billion pounds by 2015 (Villata, 2012). The blueberry industry in the United States has developed effective market promotion programs to increase consumption based on health reputation. For the U.S. blueberry industry to remain competitive and maintain profitability for its growers, timely fruit harvest must be addressed. The costs of labor and production are projected to increase while the price for blueberries is projected to decrease (Strik and Yarborough, 2005). Brown et al. (1996) reported that because of the poor quality of NH blueberry in Michigan, most fruit going to the fresh market had to be hand harvested or had to be consumed within a few days of machine harvest. Blueberry production remains profitable, but the price has declined, particularly in the traditional high-value months of April and May. Hand harvest produces high fruit quality but at the high cost of labor. In the southeastern United States, hand harvesting costs $0.50/lb to $0.70/lb for SH blueberry and somewhat less for rabbiteye blueberry (Safley et al., 2005). The machine harvesting cost for fruit destined for the processing industry is estimated to be about $0.12/lb. Availability of a work force for harvesting is expected to be a major challenge to the U.S. highbush blueberry industry.
A key step to addressing this challenge is to advance the machine harvest technologies. Harvesting machines improve labor productivity by nearly 60 times while cutting the harvest cost by 85% (Brown et al., 1983). In the southeastern United States, most blueberry harvesting machines are currently used for the processing market and not for harvesting fruit for fresh market (Table 1) because the fruit of SH blueberry that have been machine harvested become much softer than hand-harvested fruit during postharvest storage. The key constraints to a wider use of machines to harvest SH blueberry for fresh market are as follows:
Damage occurring during harvest, particularly bruising lowers overall quality by producing softer, leaky fruit that are at increased risk of decay during postharvest handling and storage (Ballinger et al., 1973; Dale et al., 1994; Funt et al., 1998; Mehra et al., 2013; Miller and Smittle, 1987; NeSmith et al., 2002). Furthermore, bruising often results in a loss of the visually appealing fruit surface wax (bloom), thereby further decreasing quality (Dale et al., 1994). Stem tearing, a third important type of damage, occurs when machine harvesting operations result in the forceful removal of the fruit from the pedicel. These injuries increased postharvest moisture loss and the fruit were particularly prone to infection by fruit-decay fungi (Ceponis and Cappellini, 1979).
Excessive ground loss; i.e., fruit are detached during the harvesting processes but are missed by the catch plates (also called fish scales) at the base of the machine. These losses often have reached 20% to 30% with machine harvesting (Mainland, 1993; Strik and Buller, 2002; van Dalfsen and Gaye, 1999).
Excessive green and red fruit detachment results in yield loss and increased sorting cost in the packinghouse. To reduce the proportion of green and red fruit in the harvested product, the first picking by machine is usually delayed by 5 to 7 d relative to hand harvesting. Delaying the harvesting of SH blueberry when the prices are rapidly declining as volume is increasing is not desirable.
Mature fruit missed by the machine, results in overripe fruit by the next harvest 2 to 3 d later (Mainland, 1993; Strik and Buller, 2002). Most commonly, such missed fruit in rabbiteye blueberry were on upright canes in the center of the bush (Takeda et al., 2008).
Estimated percentage of southern highbush (SH) blueberry acreage in several states in the southeastern United States that is machine harvested at least once per season for the fresh market.
In approaching the first constraint of damaging fruit, research has shown that softening in machine-harvested fruit can be reduced if the drop height can be maintained at less than 24 inches (Brown et al., 1996). Fruit picking machines such as the V45 blueberry harvester (BEI, South Haven, MI) were designed to minimize drop height and reduce ground loss (Peterson et al., 1997). However, the V45 blueberry harvester has not been widely accepted commercially due to machine design that limits operating speed and durability and because it requires special cane training and pruning (Takeda et al., 2008). The second, third, and fourth constraints have stimulated innovations in cultural practices for improving machine harvest efficiency. These practices have been focused primarily on minimizing ground loss and reducing the number of mature fruit missed by the machine (Peterson et al., 1997; Takeda et al., 2008). Strik and Buller (2002) in Oregon showed that supporting ‘Bluecrop’, an NH blueberry, with a simple two-wire trellis system increased harvest efficiency significantly, mainly by reducing the number of mature fruit missed by the machine. The maximum proportion of fruit remaining after the last machine harvest was reduced from 30.8% in not-trellised plots to 15.5% in trellised plots with canes kept more upright by trellis wires.
SH blueberry plants in areas with warm, long growing season tend to produce fewer, major upright canes that reach larger diameters than the blueberry plants in colder climates with short growing season (G. Krewer, personal observation). A trellis installation technique that would reposition the canes away from the center of the bush and toward the outside of the canopy where fruit would be more easily detached by the machine appears to be advantageous for SH blueberry. This technique could reduce the proportion of fruit missed by the machine in the bush center. As an additional benefit, such repositioning would allow better sunlight penetration into the interior of the bush for flower bud formation and increased yields. Yáñez et al. (2009) reported that low sunlight levels caused the interior fruit twigs to set fewer flower buds, and as the light levels drop below 20% of full sunlight these shoots eventually die. However, the effect of pruning and trellising the remaining canes has not been studied for ground loss.
Ground loss associated with the use of machine can be significant, reaching up to 30% of the total crop (Mainland, 1993). Even with well-pruned bushes, losses of 20% have been documented (Strik and Buller 2002; van Dalfsen and Gaye, 1999). In early season fresh market sales when the blueberry price can be over $5/lb, the SH blueberry growers in the southeastern United States could lose over 600 lb/acre in ground loss if bushes are harvested entirely with a machine.
Machines operate by passing over the plant row and agitating the bushes on both sides to detach fruit. At the same time, spring-loaded catch plates surround the base of the plant at 12- to 18-inch height to catch the falling fruit and guide them to conveyor belts. Because the crown of SH blueberry plants consists of multiple canes that tend to spread out at the height where the fish scales meet the bush, gaps form between the canes and the fish scales, resulting in machine-detached fruit that fall to the ground (Fig. 1). Although these losses can be reduced somewhat by pruning as the bushes age, the crown diameter is difficult to maintain to less than 8 to 12 inches at the 18-inch height. By installing crown-restricting tubing made from 6-inch plastic drainpipe around the canes of young blueberry plants, Rohrbach and Mainland (1989) were able to reduce ground loss by 40% compared with untreated bushes. However, suckering of the canes was increased at the point where the tubing met the soil, and plant mortality tended to be higher in plots with the tubing. Furthermore, infestation by fire ants (Solenopsis invicta), an important nuisance pest in the southern United States (Scherm et al., 2001), was exacerbated. Alternative approaches for crown restriction without these drawbacks need to be developed and evaluated.
We performed four studies. The objectives of our studies were to 1) determine the fruit quality of machine-harvested SH blueberry, 2) analyze the effect of drop height and padding the contact surface on fruit quality, 3) investigate the effect of crown restriction on ground loss, and 4) determine the effect of plant size on machine harvestability.
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