Global winegrape production is largely based upon the production of the European winegrape Vitis vinifera, a host species comprised of cultivars that are all highly susceptible to infection by the grape powdery mildew pathogen Erisyphe necator as well as several other fungal and oomycete pathogens. Irrespective of the center of origin of Vitis vinifera or the major pathogen groups, the global ubiquity of both the host and various pathogens is now a fact faced by grape and wine producers everywhere.

In particular, fungicidal suppression of grapevine powdery mildew is problematic. Resistance to many FRAC classes, including sterol demethylation inhibitors (DMI), strobilurins, benzimidazoles and succinate dehydrogenase inhibitor (SDHI) fungicides is sufficiently widespread that the forgoing classes are no longer effective in some viticultural regions. Organic production systems are also threatened. There are very few practical organic options for controlling powdery mildews. Many organic options entail undesirable non-target effects or are marginally effective. Additionally, many viticultural regions are located in Mediterranean climates with little rainfall during the crop production season. All of the foregoing creates the present situation: grapevine powdery mildew predominates as the principal threat to healthy fruit and foliage worldwide.

UV Light to Suppress Pathogens

Nearly all of the biomass of powdery mildews is wholly external to the host (Figure 1). They live in a world bathed in sunlight throughout the disease process. With the exception of the walls of their overwintering structures (chasmothecia), they possess none of the pigmentation that would offer protection from biocidal wavelengths of the solar spectrum (wavelengths of UVB between 280 and 290 nm.) Powdery mildews are favored by shade and repressed to some degree by direct sunlight exposure. They persist in the above niche due in part to their ability to repair UV-inflicted damage to their DNA through a robust photolyase mechanism driven by blue light and UVA.

In 1990, we began work that led to the use of germicidal UVC lamps to suppress E. necator. The treatments were effective, but UVC also damaged the vines, and the technology was never widely adopted (Figure 2). It took 20 years before a critical breakthrough by a Ph.D. student in Norway (Aruppillai Suthparan) fundamentally changed how we could use UV light against plant pathogens. He found that if UV light was applied during night hours, we could use much lower doses than were required during daylight. That breakthrough largely resolved the issue of plant damage at the high UV doses required for daytime applications. Today, UV technology for plant disease suppression is being investigated by several working groups. Most exploit the link between darkness and the inability to withstand exposure to UV. When damage to pathogen DNA during darkness is not repaired within four hours, it is usually lethal.

The UV spectrum used in such studies has ranged from a UVB waveband between 280 to 290 nm into the UVC range produced by low pressure discharge lamps yielding a peak output near 254 nm. Reduction of the severity of several powdery mildews has been attributed to direct damage to the pathogen by UV exposure. UVC has been reported to be directly inhibitory to Botrytis cinerea on strawberry (Janisiewicz et al. 2016). In contrast, pathogens other than powdery mildews have been suppressed by exposure of their hosts to UV prior to inoculation, possibly due to enhancement of host resistance.

The adaptation of nighttime UV treatments to commercial field plantings has necessitated the development of UV arrays powerful enough to apply effective doses at speeds that allow the equipment to complete treatments during the available night interval, often in late spring and early summer during some of the shortest nights of the year. Remember: we need about four hours of darkness after UV exposure in order to achieve the maximum suppression. A tractor-drawn UVC apparatus described in a report by Onofre et al (2019) was developed to suppress strawberry powdery mildew. This apparatus contained two hemicylindrical arrays of UVC lamps and was the basis of a later array design fitted to an autonomous robotic carriage produced by Saga Robotics, LLC. UVC treatments applied once or twice weekly at doses ranging from 70 to 200 J/m2 effectively suppressed strawberry powdery mildew (Podosphaera aphanis) to a degree that equaled or exceeded that of some of the best available fungicides.

The potential for nighttime UV treatments to eliminate the threat posed by E. necator could greatly reduce the need for fungicide applications. In regions with higher rainfall and multiple fungal pathogens, the potential for nighttime UV treatments to remove the threat of powdery mildew would improve options for the remaining members of the pathogen and pest complex, such as downy mildew (Plasmopara viticola), bunch rot (Botrytis cinerea) and various arthropod pests.

For all of the foregoing reasons, our objectives in the present study were to 1) Determine the potential of nighttime UV applications to suppress grapevine powdery mildew; 2) Determine if UVC at disease-suppressive doses and frequency of application has any deleterious effects on vine growth, yield or crop quality; and 3) Determine if nighttime UV applications targeting powdery mildew have effects on other selected pests or diseases of grapevine.

In summary, the mechanism underlying the success of nighttime UV applications is related to how pathogens deal with naturally-occurring ultraviolet light from the sun. Shorter-wave UVB and UVB both damage DNA in all living organisms. Exposure to UV causes thymine base pairs in the DNA to bind together, changing the genetic code to genetic gobbledygook. Pathogens sense visible light, but they also possess evolved systems that can repair the foregoing damage to their DNA caused by incoming UV. We now know that those biochemical and genetic repair systems are recharged by blue light and UV-A, and are reduced by red light and darkness. This photolyase-based repair mechanism effectively “unglues” the thymine base pairs as fast as they are created by UV, but the repair mechanism does not operate at night.

Lamps producing UV light have been commonly available for over 75 years. Those that produce an effective wavelength and are powerful enough to be practically used against powdery mildews produce either UVC (100 to 280 nm) or UVB (280 to 315 nm). Both UVC and UVB affect DNA in the same way by the aforementioned creation of thymine dimmers. UVB poses less potential to harm plants, and may therefore be preferred for static and permanent installations in greenhouses. However, with precise dosing, UVC can be used safely on even UV-sensitive crops.

Low-pressure discharge lamps are the most common available technology. Low-pressure discharge UVC lamps are generally clear quartz-glass tubes containing a small amount of mercury vapor. Passing an electric arc through this vapor results in the efficient production of a narrow waveband centered on 254 nm, which is excellent for germicidal applications. UVB low-pressure discharge lamps are similar, but incorporate a fluorophore powder coating on the inside of the tube. When this is struck by the internally produced UVC, the fluorophore absorbs the UVC and emits the longer wavelength UVB. This process is also relatively inefficient, and nearly 95% of the usable germicidal energy is lost in the conversion from UVC to UVB. So, low-pressure discharge UVC lamps can produce much more usable power than comparably sized UVB lamps. While UV LEDs are available, they are presently far too expensive and underpowered to be useful for treating crops.

Results Adapted to Grapevine

Field trials for suppression of strawberry powdery mildew were initiated in Florida in 2017. Weekly applications of UVC provided suppression of foliar powdery mildew across the duration of the experiment that was substantially better than that provided by the best fungicide treatment in the trial, which was a combination of two materials sold under the trade names Quintec and Torino. We also confirmed in parallel measurements that the UV treatments did not reduce plant size or the yield of harvested berries. Continued trials on field plantings of strawberries duplicated the efficacy of the 2017 trials.

In our initial trials, we used a tractor-drawn array (Figure 3). Additional trials adapted modified designs of the original tractor-drawn array to an autonomous robotic device (Figure 4) manufactured by SAGA Robotics, a Norwegian company collaborating with our research group in developing this technology for multiple crops. The use of a robotic carriage provides additional flexibility in nighttime applications. At temperate latitudes, the duration of night near the summer solstice can be less than eight hours, leaving only about four hours during which the UV treatments could be applied with optimal effect. In situations where employing nighttime labor to make applications split over several relatively short night intervals would be problematic, an autonomous robotic device offers a practical alternative.

In 2019, we came full circle and were ready to resume UV treatments on grapevine. As in our work on strawberry, we began by using a UV array and tractor-drawn carriage. UV Treatments were applied once per week at 100 or 200 J/m2 to Chardonnay vines that received no other fungicide treatments. Laboratory experiments had indicated that the UV doses used would stop 80% to nearly 100% of the conidia of E. necator from germinating. The incidence and severity of powdery mildew was assessed on leaves and fruit of UV treated vines, vines treated with an effective conventional fungicide and completely untreated vines. 2019 was a moderately severe year for powdery mildew.

Both the 100 J/m2 and 200 J/m2 UVC treatments significantly but equivalently reduced the severity of powdery mildew on berries compared to the untreated vines, albeit not to the degree provided by the standard fungicide treatments (Figure 5). What surprised us was that both the 100 J/m2 and 200 J/m2 UV treatments also suppressed foliar downy mildew (Plasmopara viticola), and did so better than the fungicide standard (Figure 6). Laboratory studies indicated that the suppression of the downy mildew pathogen was due to a pre-inoculation increase in host resistance. This was distinct from the impact of UV on powdery mildew, which was primarily a direct effect of UV on the pathogen itself. However, in our 2020 trials, weather conditions were especially conducive to downy mildew, and the level of suppression of downy mildew from UV was only around 50%. That’s helpful, but it is nowhere near acceptable commerical control. So, we obviously have more work to do in this area.

The 2019 trials produced another surprise: the UV treatments effectively suppressed sour rot (Figure 7). This disease is a complex mess involving bacteria, fungi and fruit-feeding insects. We still don’t understand how UV is accomplishing this reduction, but given that there are very few effective means to suppress sour rot, any efficacy due to UV treatments is worth further investigation.

In addition to suppressing plant pathogenic fungi, UV treatments can also suppress populations of phytophagous mites (Figure 8). A number of studies have noted that UVB and UVC treatments can kill eggs of spider mites and European Red Mites. In addition to these effects, our preliminary trials indicate that the UV treatments can also alter behavior of adult mites, reduce egg laying, and reduce fecundity of the generation of surviving mites that emerge from UV treated eggs.

As in our strawberry work, we eventually wanted to adapt the tractor-drawn grape UV array to a robotic carriage, and our partnership with SAGA robotics made this possible (Figure 9). The navigation autonomy of the SAGA robot (Thorvald) is capable of tracking within a few centimeters of the trellis center at operational speeds between 1.25 to 2.5 mph. We evaluated UV doses between 100 J/m2 and 200 J/m2 at frequencies of either once weekly or twice weekly. All of the evaluated doses significantly suppressed powdery mildew on both fruit and foliage, and the twice-weekly 200 J/m2 treatment provided control that was superior to the fungicide standard (Figure 10).

What’s Next?

We are collaborating with growers and scientists at multiple locations in the U.S. and Europe, including Bully Hill Vineyards in Hammondsport, N.Y.; Washington State University’s research and extension center in Prosser, and the USDA Horticultural Crops Research Center in Corvallis, Ore. as well as multiple locations in California, with designs and materials for UVC lamp arrays adapted for their vineyard pruning and training systems. These trials will be conducted over the course of the 2021 growing season. More about the autonomous robot Thorvald can be found at sagarobotics.com/. Our international working group is described on our project website: LightAndPlantHealth.org. It is a large, multidisciplinary, multi-institutional and international group representing several U.S. and overseas universities and government agencies, with industrial partnerships (Figure 11).

The design of a lamp array to match a particular crop canopy and target pest biology is a critical aspect determining of the success of the treatments. Our cooperative projects with growers across the US have always involved our array designs and electronics. Some growers have designed and fabricated the various carriages for the arrays. But the UV array itself is NOT a DIY project, nor is calibration and the photobiological and epidemiological calculations that enter into calculations of a proper UV dose for specific applications. In addition to the engineering and biological considerations, both UVB and UVC can be injurious to you unless devices are properly designed and the lamps are properly shielded from direct view. No person should ever have an unshielded view of germicidal UV lamps, as there is a significant risk of eye and skin damage from exposure UVB and UVC. The protective gear that is required for safe applications is not expensive, and consists of UV-opaque clothing that covers all exposed skin, disposable gloves and a face-shield and eye protection rated for protection from UV. The arrays shown in this article also incorporate clear PVC curtains at each end of the array to limit escape of UV from the array. As would be the case with any IPM technology, UV does not pose undue risks to operators or the environment if used properly. Proper training and use protocols are the key to safe and effective applications.

Our work has been funded by competitive grants from the USDA Organic Research and Extension Initiative, and the USDA Specialty Crops Research Initiative. Additional support has been provided by the National Research Council of Norway, the New York Farm Viability Institute, the USDA Sustainable Agriculture Research and Extension Program and Bully Hill Vineyards. We work as a diverse international group to promote this research area and its applications, and to act as a resource to train others. The work spans disciplines from plant growth and photobiology to physics and lighting technology.

David M. Gadoury is a senior research associate in Cornell’s Plant Pathology and Plant-Microbe Biology Section at Cornell AgriTech, where his program focuses on pathogen ecology, pathogen biology and disease management. He leads the Light and Plant Health Group.

References

Gadoury, D.M., Pearson, R.C., Seem, R.C., Henick-Kling, T., Creasy, L.L., and Michaloski, A. 1992. Control of diseases of grapevine by short-wave ultraviolet light. Phytopathology 82:243.

Janisiewicz, W. J., Takeda, F., Glenn, D. M., Camp, M. J., & Jurick, W. M. (2016a). Dark Period Following UV-C Treatment Enhances Killing of Botrytis cinerea 3.Conidia and Controls Gray Mold of Strawberries. Phytopathology, 106(4), 386–394. https://doi.org/10.1094/PHYTO-09-15-0240-R

Michaloski, A.J. 1991. Method and apparatus for ultraviolet treatment of plants. U.S. Patent no. 5,040,329.

Onofre, R. B., Gadoury, D. M., Stensvand, A., Bierman, A., Rea, M., and Peres, N. A. 2019. Use of ultraviolet light to suppress powdery mildew in strawberry fruit production fields. Plant Dis. 105:0000-0000 (in press).

Suthaparan, A., Stensvand, A., Solhaug, K. A., Torre, S., Mortensen, L. M., Gadoury, D. M., Seem, R. C., and Gislerød, H. R. 2012. Suppression of powdery mildew (Podosphaera pannosa) in greenhouse roses by brief exposure to supplemental UV-B radiation. Plant Dis. 96:1653-1660.

Suthaparan, A., Stensvand, A., Solhaug, K. A., Torre, S., Telfer, K. H., Ruud, A. K., Mortensen, L. M., Gadoury, D. M., Seem, R. C., and Gislerød, H. R. 2014. Suppression of cucumber powdery mildew by supplemental UV-B radiation in greenhouses can be augmented or reduced by background radiation quality. Plant Dis. 98:1349-1357.

Blueberry production acreage in the U.S. is expanding. Across the country, commercial blueberry growers are increasingly using over-the-row (OTR) mechanical harvesters (MH) to pick their blueberries for fresh market (Figure 1). Growers everywhere are experiencing difficulties in finding sufficient labor for hand harvest operations and due to the rising costs of labor. Harvesting blueberries with OTR harvesters can significantly reduce the overall cost of harvesting to a fraction of that needed for hand harvesting (HH) and workers needed for harvest operations from about 500 hours of labor per acre per year to as little as three hours of labor per acre per year. However, compared to hand harvesting, OTR harvesting causes more berry loss due to falling on the ground and green/red berries are harvested along with ripe, blue fruit.

Detailed field testing of OTR harvesters for picking blueberries for the fresh market was conducted nearly 30 years ago in Michigan. That research in South Haven, Mich. evaluated the quality of blueberries harvested by hand and by four rotary and slapper harvesters that were used by growers at that time to harvest blueberries for processing. MH blueberries were sorted at the packinghouse (Figure 2).

The most significant findings were a high percentage of detached blueberries had impact damage (Figure 3) and more than 20% of detached blueberries fell on the ground. The bruise damage was attributed to iImpact to the fruit created by the rapid actions of shaking rods and detached berries landing on the hard catching surface. Those studies revealed that blueberries harvested by the machines had a high percentage of blueberries with more than 20% of sliced surface area showing bruise damage (Figure 3 and 4). Also, MH blueberries were much softer compared to hand harvested fruit. Their conclusion was that MH blueberries should not be cold-stored for more than two weeks while HH blueberries could go in controlled atmosphere storage for six weeks and air-shipped to Europe in excellent condition.

Soon after, USDA engineers developed an experimental harvester called the V45 harvester designed specifically to harvest fresh-market blueberries. It used a direct-drive shaker with an angled, double-spike-drum, a unique cane dividing and positioning system to push the canes out diagonally and cushioned catching surfaces to harvest fruit with minimum damage. With the V45 harvester design, the detached blueberries dropped less than 15 inches onto a soft neoprene sheet glued to a hard catch plate and soft sheet over the conveyor belt.

These soft surfaces reduced impact force on the fruit detached by the V45 harvester. However, gluing a soft surface onto a hard surface has proven to show little reduction in bruise damage when harvesting is performed with conventional harvesters with two vertical drum shakers and berries falling more than 30 inches. Only five V45 harvesters were sold by the now defunct B.E.I Inc. (South Haven, Mich.), although it was thought to have good fruit selectivity (low green fruit removal) compared to slapper models, little ground loss (fruiting cane pushed away from the crown) and superior quality over existing commercial harvesters at the time with two vertical drum shakers and either a metal or hard plastic catch surface.

Sometimes, the fruit harvested by the V45 harvester had quality as good as commercially HH fruit. Its limitations were: 1) It needed to be driven much slower to avoid damaging bushes; 2) It could not harvest trellised rows or those with overhead sprinklers; and 3) It could not harvest all varieties, especially those with stiff, upright canes like ‘Jersey’ and many rabbiteye cultivars. The Fulcrum harvester made by A&B Packing Equipment (Lawrence, Mich.) has features like those of the V45 harvester.

In the last ten years or so, U.S. blueberry farmers targeting the fresh market have been facing challenging economic situations (e.g., rising cost of hand picking, shrinking labor force, global competition, etc.) They and other specialty crop farmers have a greater interest in using automation and OTR machines to harvest their crop. The authors of this article have participated in different aspects of machine harvesting and sorting of blueberries to reduce the amount of internal bruise damage and in packing line sorting technology and damage detection systems to improve the quality of packed fruit. Several blueberry MH manufacturers (e.g., Oxbo International, Lynden, Wash.; A&B Packing Equipment, Lawrence, Mich.; BSK, Serbia; and FineFields, the Netherlands) have put more efforts devoted to developing MH systems that would impart low or no bruise damage so that fruits can be packed for fresh market. Following is a summary of recent developments in MH.

Bruised Berries from Mechanical Harvesting

Most OTR harvesters currently available are better suited for harvesting processed blueberries because they can cause excessive fruit damage. However, OTR machines have been used to pick blueberries for fresh market. In these instances, the fruit should be packed and transported to consumers as quickly as possible. When blueberries are HH, typically the picker gently picks ripe fruit selectively. In Chile and China, for example, ripe berries are picked individually to obtain high fresh quality.

In the Pacific Northwest and elsewhere in North America, ripe fruit is often harvested by rubbing fruit cluster or sometimes by “tickling” them between the thumb and index finger and catching the detached berries in the palm and then placing them in a small harvesting bucket. In contrast, MH involves the shaking of the entire bush with rapid action of shaking rods to move canes back and forth. The cane movement causes ripe berries that need less fruit removal force than green/red berries to be displaced from the fruit stem (pedicel) and fall onto catching surfaces. Experienced MH operators make slight adjustments on machine settings to obtain good selectivity (minimize green/red berry removal and maximize ripe fruit removal).

The blueberry bush can range from 3 to about 6 feet tall with fruit located from the tip of the canes to branches near the ground, which causes the berries located on the top part of bush to fall as much as 50 inches. When an OTR harvester picks blueberries and fruit falls from that height onto plastic catch plates and conveyor belts, one can hear berries hitting the hard catch surfaces.

Based on this simplified description of the blueberry MH process, it was apparent that the interaction between the machine and fruit should be better understood. To do this, we used a custom-made miniature electronic sphere called the BIRD (blueberry impact recording device developed at the University of Georgia) to measure the fruit impacts during MH process in 2010 and 2011. The BIRD sensor for this study weighed 14 g. The later version, BIRD II, was built to closely approximate the size and weight of a large blueberry (9/16-in diameter and weighed 6 g) (Figure 5).

Along with documenting fruit impacts with a BIRD, a closeup video camera recorded the harvesting to pinpoint critical control points where most impacts were created. The results showed that the drop to the plastic catch plates on the harvester accounted for over 30% of all impacts on the BIRD, followed by the drop from the grading belt on the harvester into an empty lug (20%). When the lug is filled with blueberries, fruit-to-fruit impacts occur, which are much lower than when the fruit fall into an empty lug.

Impacts created by the conveyor, including secondary bounce from the catch plates, and shaking rods combined for another 25% of recorded impacts. The remaining 25% of impacts that occurred before the sphere contacted the catch plate were classified as obscured impact events which could not be identified clearly from the video and were attributed to contact with the shaking rod, branches and the vertical tunnel panels. These measurements suggested that the most significant reduction in fruit impacts could be achieved by 1) Modifying the catch plates; 2) Reducing drop heights, either by restricting bush size, placing catching surfaces closer to the fruit or decreasing drop heights at other transition points; and 3) Placing softer surfaces at the transition points (e.g., at transfer points in the fruit handing equipment on the top of platform.)

The two parts of the impacts include the number of encounters between the sphere and different surfaces of the harvester and the magnitude of these impacts. In our study, the harvesting process was documented with video that recorded time-stamped impact events with the larger, heavier BIRD I sensor. Using these parameters, the OTR MH process was divided into four phases: Phase I (detachment and falling), Phase II (fruit hitting the catch plate/conveyor belt), Phase III (elevation from the conveyor/transfer belt to the top platform and conveyance through a trash blower) and Phase IV (dropping from the conveyor belt into the lug).

Results showed that for the rotary drum shaker, the BIRD sensor recorded an average of 18 impacts in Phases I to IV. During Phase I, it is assumed blueberries detached by fast-moving harvesting rods that shake left and right, impact branches as they fall and/or are flung out to the side panel. There were about five impact events in Phase I, but magnitudes of these impacts proved to be less significant than initially assumed. In Phase II, the BIRD contacted the catch plate and usually only one or two events were recorded. The magnitude of the impacts in Phase II was extremely high compared to impacts recorded in Phases I, III and IV. Our results strongly suggested that the high impact that the falling blueberries receive at the point of contact with the catch plate injures the fruit, resulting in fruit softening and larger bruise while the fruit is in storage (Figures 3 and 4).

Further analysis was performed by dropping the large, heavier BIRD I sensor onto a hard-plastic catch plate from different heights (6, 12, 24, 36 and 48 in) (Figure 6). As expected, the impact values (peak acceleration at impact (g) increased sharply linearly with increasing drop height, ranging from 280 g at 6 in to about 800 g at 48 in (data not shown). In subsequent studies, impact measurements were made using the smaller and lighter-weight BIRD II sphere by dropping onto soft surfaces created by placing cushioned padding on top of the hard plastic plates or by suspending the soft material (no hard surface underneath.)

A wide range of impact values were obtained depending on the hardness of the catch plate (Figure 6). Impacts greater than 200 g were recorded on hard surfaces such as a stainless-steel sheet and a plastic catch plate even when the BIRD II was dropped from a height less than 30 cm (12 in). Gluing a soft surface to a hard surface reduced impact; however, this type of surface still created high impact above a one-foot drop height such that blueberries falling 30 inches onto such a surface would still be bruised. For example, the suspended foam sheet we used in our harvest-assist blueberry picking machine in 2017 generated less than 200 g even when the drop height was 42 in, but well above the 120 g at which ripe blueberries can be bruised by impact force. Only the netted fabric that acted like a hammock produced low enough impact force and kept the blueberries from getting bruised even when the fruit was dropped from a 48-in height. Thus, it was thought that replacing the hard, plastic fruit catching and collection surfaces with soft and durable catching surface materials and plate design features that prevent soft surface from contacting any hard surfaces underneath had the potential to improve the quality of MH blueberries and reduce bruise damage associated with high mechanical impact.

In terms of mechanical impact to blueberry fruit, our research has shown that bruise damage and the loss of firmness in MH fruit can be decreased by reducing space between blueberries on the bush and the catching surface to 12 inches in the case of hard plastic fruit catching surfaces or by modifying the fruit catching surfaces to create a softer fruit landing surface. Ideally, the fruit catching surface should not exceed 120 g impact even when the BIRD II is dropped from a height of 48 in (equivalent to the distance between the top of a large mature blueberry bush and catch plates on the harvester).

The design of soft surfaces can be achieved by either incorporating netted material or a soft “rubber” sheet with no hard surfaces beneath for catching the fruit (Takeda and Wolford, U.S. Patent No. 9,750,188 and the Oxbo SoftSurface kit). For example, even with a soft surface insert in a hollowed-out plexiglass catch plate, the margins of the plate contributed to more than 20% of the exposed surface area. In addition, catch plates on the harvester overlapped with adjacent plates and rested on top of another plate. The outline of the plate below another created about 10% additional hard surfaces.

When blueberries are HH, the packout is about 95% or better and fruit usually have little or no internal bruise damage (Table 1). The packout of MH blueberries is lower and typically ranges from 70% to slightly more than 80%. The remaining 20% consists of soft, overripe and immature green- and red-colored berries.

Commercial packing operations, for the most part, do not check for internal bruise damage in their MH blueberries. However, close inspections of MH blueberries packed into clamshells after sorting on commercial packing lines revealed berries were bruised (Figure 3, see page 4). Our studies evaluated different catch surface designs by inserting soft, flexible material to reduce internal bruise damage. We did record improvements in packout. However, neither the improvement in packout nor berry firmness approached that of HH fruit in the case of varieties Duke, Draper or southern highbush blueberry (SHB) Optimus even when they were harvested with OTR machines equipped with soft, flexible catch surfaces. The only exception to date has been the variety Last Call, where MH produced the quality approaching that of HH berries. MH of SHB Optimus produced higher-quality packout than other SHB varieties, such as Jewel, Star and Farthing, but even Optimus should not be cold-stored for more than one week. Our studies have shown that fresh market pack-out can be increased by installing a soft catch surface on the harvester, but the quality of HH blueberries has been better.

Cultivar Susceptibility

In a study conducted in Oregon, the susceptibility of 11 blueberry cultivars to impact damage was determined by dropping the fruit from 2-, 3-, and 4-foot heights onto a hard, plastic catch plate. Bruises developed more rapidly in rabbiteye cultivars (Ochlocknee, Powderblue and Overtime) than in northern highbush (NHB) and SHB cultivars. NHB cultivars Aurora, Cargo, Draper and Last Call had the least amount of bruising after two weeks in cold storage. Blue Ribbon, Legacy and Liberty had a moderate amount of bruising.

These studies showed that simulated drop tests are useful in determining the potential of varieties for long-term cold storage and, more importantly, their potential to MH for fresh market. In a study in 2020 in Oregon, Draper and Legacy were MH with two OTR Oxbo harvesters, one fitted with and the other without the SOFTSurface kit. To date, the challenge for Oxbo Corporation and other machine manufacturers has been to procure soft materials that meet food safety standards and are durable for harvesting blueberries.

The preliminary findings of this study were: 1) Machine harvesting with the SOFTSurface kit reduced fruit internal bruise damage in both Draper and Legacy fruits compared to those harvested with the unmodified OTR harvester as shown with a laboratory test (Table 1); 2) Draper and Legacy fruit harvested with the machine fitted with the SOFTSurface kit and sampled before sorting in the packing house were firmer compared to fruit harvested by the unmodified harvester; and 3) After one and two weeks in cold storage, there was no difference in firmness of berries harvested by machines fitted with and without the SOFTSurface kit.

We found that fruit firmness-based sorting by itself may not be a good predictor of berry quality when MH blueberries are cold-stored for two weeks or more, but both Draper and Legacy blueberries picked by the OTR machine fitted with the SOFTSurface kit maintained better fruit firmness (>160 g/mm) values and lower internal bruise ratings in cold storage (Table 1). The improvements in fruit quality may well have been from a 70% reduction in hard catch surface area in the SOFTSurface kit compared to the hard polycarbonate fruit catching surfaces in the regular harvesters. A laboratory test determined the effects of dropping blueberries from different heights onto either a hard (e.g., polycarbonate catch plate on conventional harvesters) or soft catch surface (e.g., prototype SOFTSurface kit) on internal bruise development (Table 1). Blueberries were sliced to visually assess bruise damage on the day of the drop test and after cold storage for two weeks.

Young (~3-ft-tall) and mature (6-ft-tall) trellised Last Call blueberry plants were either HH or picked with a modified OTR machine. Fruit samples from both methods were manually sorted and evaluated for internal bruise damage on the day of harvest and the remaining samples were placed in cold storage. Cold-stored samples were taken out after two and four weeks and evaluated for internal bruise damage (Table 2). On the day of harvest (zero days after harvest), about 80% of blueberries showed no bruise damage, and the remainder showed damage ranging from 1% to more than 50%. There was little change in internal bruise for samples from matures bushes that were either HH or MH. However, there was a dramatic decline in the percentage of fruit with no internal damage among the samples from machine harvesting of young plants.

Our field observations of the Last Call bushes used in this study indicated that the canes of young plants were upright during the harvest and detached fruit fell straight down. In contrast, on the taller, mature bushes, the canes had grown well above the height of the trellis and they were leaning outward at the time of harvest. This placed the fruit away from the crown and less than 30 inches above the catching surface and may have contributed to reducing mechanical impacts in terms of numbers and magnitude, thus reducing the amount of internal bruise damage.

Sorting Out Bruised Berries

Blueberry growers in the Pacific Northwest and in Chile have expressed an interest in machine harvesting blueberries for the export market. The consensus among them is that the varieties for the export market must be firm and arrive at the destination in excellent condition after more than three weeks of cold storage and a transoceanic travel period. Our machine harvesting research has consistently shown that the MH blueberries generally had more internal bruise damage and shorter shelf life than the HH blueberries.

Commercial optical sorting equipment are now available for grading blueberries. In the last three years, HH and MH blueberries have been processed on commercial blueberry packing lines in Oregon and Washington equipped with an optical sorter (e.g., UNITEC, BBC and MAF). For each packing line, samples of Draper and Legacy were taken from lugs prior to unloading onto the conveyor system, and a second group of samples were collected after the fruit had gone through the optical sorting machine. Samples from both locations were assessed for bruise damage (% bruised area). The bruise data are presented in Table 3 in which the data are expressed in terms of how the samples were distributed (e.g., blueberries with no damage to those that were severely bruised.) The analysis indicated that sorting by optical sorters did not remove blueberries with moderate to severe internal bruise damage.

Next, we compared the blueberry fruit firmness value with the area of internal bruise damage on the sliced surface. One would likely assume that softer fruit will have more bruise damage. Our results and those from a report by Chilean researchers showed that this was not the case as shown by the low correlation coefficient (r-value) for these two fruit quality parameters. Whether the fruit had been collected from the packing line before or after the optical sorting machine, the correlation coefficients for berry firmness and bruise damage were less than 0.4 in NHB cultivars. This suggested that optical sorters in commercial blueberry packing houses were not effective in removing blueberries with internal bruise damage.

Once more in the laboratory, we conducted drop tests in which HH Duke blueberries were dropped from a height of 62 inches to ensure that the samples would be bruised. A hyperspectral imaging system was used to locate and quantify bruise damage in each whole fruit (25 berries at a time). We then measured fruit firmness with a FirmTech II at the site of the bruise impact as determined by the imaging system. Then, the same fruit was rotated and additional firmness measurements were taken at 90 and 180 degrees from the bruised site.

The analysis showed that at the site of the bruise damage, the average fruit firmness was 149 g/mm. However, at the sites that were 90 and 180 degrees from the impacted location, the firmness was greater than 162 g/mm. This meant that a lower firmness value was detected when the damaged area was purposely used to determine firmness, resulting in a much higher r-value between fruit firmness and internal bruise damage values. Fruit that were firm at the time of packing (e.g., >180 g/mm value using a FirmTech II instrument) were found to have internal bruise damage exceeding 15%. In the near future, our research team will sort MH blueberries with this imaging system to separate whole unbruised and bruised blueberries and conduct postharvest quality evaluation for unbruised and bruised MH blueberries to determine the shelf life of each group with an eye toward exporting MH blueberries to distant Asian markets. Of course, taking this non-destructive imaging system from the laboratory bench to integrating it into commercial optical sorting machines for IBD detection and sorting is a challenge facing the machine manufacturers.

Conclusions

More blueberries for fresh market are being machine harvested.

Machine harvested blueberries have more internal bruise damage.

On-going research is developing a better understanding of what causes bruising and working with harvest machine manufacturer to reduce bruise damage.

New sensor technologies for blueberry sorting could assist in reducing bruised berries in fresh packs.

Our research has shown that to make MH more profitable for blueberry growers, the current OTR harvesters must be modified to reduce impact damage and ground loss. Cultivars with superior machine harvestability are being released by blueberry breeding programs, and research must continue to develop equipment capable of harvesting blueberries with less bruise damage. The sorting system on the packing line for MH fruit must be improved with a greater precision to eliminate fruit with severe internal bruise damage. This would ensure that the quality of MH blueberries going into clamshells would be as good as HH fruit. Blueberry growers in some regions can then contemplate having MH blueberries packed for export. Also, proper training and pruning of blueberry bushes to maintain a small crown can increase MH efficiency. These changes will help in making small, incremental improvements in increasing pack-outs and fresh quality of packed blueberries.

Finally, in order for MH blueberries to have quality that is as good as HH fruit, the blueberry industry needs to be willing to make changes by growing superior varieties, modifying how blueberry bushes are grown and harvested, and improving how the fruit is sorted. This will take a concerted effort from growers, breeders, horticulturists, engineers and supply chain specialists. These changes could lead to blueberry fields that look different from what we see today, with radically different ways of harvesting blueberries and technological advancements for sorting blueberries with the goal of improving the quality of MH blueberries going into clamshells.

In terms of harvesting and packing technology, it is envisioned that U.S. blueberry growers will be using robotic harvesting systems in the field or in warehouses with specialized automated or semi-automated harvesting machines that will avoid damaging berries, have better selectivity to reduce green berries picked and sort out over-ripe and diseased berries in the field. In packing houses, new non-destructive technologies are needed that will be capable of analyzing the blueberry fruit surface and below the skin and sort fruit for quality (large size, high sweetness, flavor, bloom, no bruise damage and color). These advances will facilitate market segmentation and high prices as one U.S. and several European blueberry distributors are doing already with HH blueberries.

This research was supported in part by the U.S. Department of Agriculture agencies (Agricultural Research Service (Project No. 8080-21000-028, National Institute for Food and Agriculture (Agreement No. : 2008-51180-19579 and 2014-51181-22471), Agricultural Marketing Service (FY 18 Oregon Department of Agriculture SCBG to WQY and FY18 Washington SCBG to LWD), U.S. Highbush Blueberry Council, Chilean Blueberry Committee and Naturipe Farms Blue Challenge.

Our gratitude goes to blueberry growers and packers in Waldo, Fla.; Alma and Homerville, Ga.; South Haven and Grand Junction, Mich.; Kingsburg and Stockton, Calif.; Hillsboro, Independence and Roseburg, Ore.; and Burlington, Prosser, Lynden and Sumas, Wash., and in Chile who provided much needed in-kind support to the harvest project. A special thanks goes to Oxbo International Corporation which has collaborated with the group since 2014.

Authors are employees of USDA-ARS (FT, fumi.takeda@usda.gov) Oregon State University (WQY, wei.yang@oregonstate.edu), University of Georgia (CL, cyli@uga.edu), Washington State University (LWV, lisa.devetter@wsu.edu) and University of Florida (SS, sasa@ufl.edu and JW, jgrw@ufl.edu).

Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.