Blueberries are big business in the U.S., contributing $4.7 billion to the annual GDP, according to the U.S. Highbush Blueberry Council. Twenty-six U.S. states count blueberries as a major commercial crop, and work hard to deliver this wildly popular fruit to kitchen tables across the country (and around the world).
In response to strong consumer cravings, growers from North Carolina to California are seeking ways to store and distribute their fresh crop with less waste, higher quality, and better profit margins. Delivering fresh berries to consumers is a race against rot — especially for growers interested in the export market. Post-harvest control of Botrytis fruit rot, Alternaria rot, and surface mold in fresh blueberries is essential for market viability and fruit quality. However, few options have been available to growers to slow the development of these diseases, and keep more berries fresh for consumers.
Towards a New Tolerance
To meet grower needs for better blueberry storage and transport, stakeholders have been working to find safe, effective tools and move them through the regulatory process. Through a collaborative effort to supply scientific data to regulatory decision-makers, The IR-4 Project (IR-4) recently secured a significant win on behalf of
U.S. blueberry producers and packers, informed by the blueberry industry and supported by USDA Agricultural Research Service (ARS) researchers.
In November 17, 2022, the US Environmental Protection Agency (EPA) issued a final rule establishing a tolerance on blueberries for total sulfites, a key residue associated with sulfur dioxide application. This new tolerance is the outcome of years of cooperation between the blueberry industry, ARS, and IR-4, who joined forces to help growers find adaptable solutions for post-harvest quality control. The project was identified as a priority at IR-4’s 2017 Food Use Workshop. Trials started in 2018.
A Fruitful Study by ARS and IR-4
To collect the necessary data for an EPA petition, the IR-4 team designed a study and then turned to ARS for assistance with its implementation. Debbie Carpenter, IR-4 National Laboratory Director, credits David Ennes, an IR-4 Field Research Director based at University of California, Davis, for figuring out how to structure the study in partnership with the ARS team. Ennes determined how to make the gas applications in cooperation with ARS, and ensured the studies fit GLP requirements.
“These studies were challenging since I had never done a gassing fumigation trial,” recalled Ennes. “This was all made possible by the USDA facility that was already set up with small scale gassing chambers and the equipment and calculations to carry out the applications. Without their help, these trials would not have been possible.”
The studies tested two techniques for post-harvest control of mold and rot on fresh blueberries: applying compressed sulfur dioxide gas in a fumigation chamber, and adding metabisulfite pads to fruit storage containers. These two tactics were studied individually and in concert, to find the best way to lock sulfites into the berry, preventing the development of rot and mold. These methods have a 50-year track record of success in the table grape industry, according to the ARS research team.
“The magnitude of sulfur dioxide effectiveness is often profound – reductions in rotten berries by 90% or more are often observed,” noted Joe Smilanick, a consulting plant pathologist formerly with ARS. “This approach can really expand opportunities for growers to market their fruit.”
Collaboratively with Ennes and IR-4’s California-based team, research was conducted at ARS’s San Joaquin Valley Agricultural Sciences Center in Parlier, California. On the ARS side, Chang-Lin Xiao (Supervisory Research Plant Pathologist), David Obenland (Research Plant Physiologist), Spencer Walse (Research Chemist), Seiya Seito (Support Scientist), and Matthew Rodriguez (Support Scientist) worked with Ennes’s team to bring this complex study to fruition. This location was fitting, as California blueberry growers are keen to improve fresh berry quality in storage and transport.
“California is one of the major blueberry-producing states in the country,” explained Xiao. “Maintaining post-harvest quality is very important in California because they’re [mainly] growing for the fresh market. Product is also grown for export… they need to maintain quality in transport so it’s high quality at arrival.”
The Parlier lab’s fumigation expertise and equipment setup made it a perfect partner for IR-4. Carpenter stressed that fumigation study logistics were “a real challenge, because this isn’t our normal application type.”
IR-4 Study Director Ken Samoil added, “IR-4 conducts very few post-harvest studies, and the application of sulfur dioxide gas is much different than applications for other products. Not only was it necessary to learn to make the applications, but also to record the data in a way that would be understandable to reviewers.”
Focusing on Food Safety
In order to provide residue chemistry data in support of a pesticide tolerance, researchers have to understand the sulfite residue levels that remain in the fruit after fumigation treatments and/or pad use, and how the levels change in the fruit during cold storage and marketing.
Not only do researchers need to determine the safety of gas residues, but also the safety of residues that remain in fruits, like sulfites. Both kinds of residue need to be safe for workers and consumers alike. In addition, researchers have to understand how the treatments preserve quality and potentially impact appearance and taste.
EPA’s rigorous standards for accepting data from studies like these helps ensure that growers, workers, and consumers can enjoy their favorite foods safely.
“Controlling post-harvest rot is the future of the blueberry trade, and most likely relying on post-harvest fumigations like sulfur dioxide,” predicted Walse.
A Better Toolkit for Blueberry Growers
Smilanick stressed that, without a post-harvest fumigation tolerance, few options have been available to blueberry growers. Fungicide applications before harvest run the risk of high residue levels, which can trigger trade barriers and other buyer acceptability issues. Alternatively, producers have used controlled or modified-atmosphere packaging and storage facilities— a significant expense not accessible to all.
Some export-focused growers had already been using sulfur dioxide to treat blueberries, before shipping to other countries with established tolerances; however, these berries were not marketable in the US. Ann Tillman of Pyxis Regulatory Consulting recalled that during the height of the coronavirus outbreak, export markets shut down, disrupted blueberry trade, and led to significant food waste.
“Growers were exporting (sulfur dioxide) treated blueberries, but then COVID hit and ships were sitting at ports. Growers had to stop exporting and they had no market for the treated blueberries,” said Tillman. With EPA’s new tolerance, growers will have greater flexibility to market their products and adapt in times of crisis.
With its mold-preventing power, sulfur dioxide has many benefits for producers and consumers alike. Still, it may take time for growers to integrate this tactic into their production systems. In California, post-harvest fumigation has been used on table grapes for decades; blueberry growers with this prior experience will have no problem using these products. East Coast growers may prefer the pads, since existence of and familiarity with fumigation systems is less common. In this case, using the slow release pad alone is still an effective option.
Xiao noted that the choice of whether to use fumigation or pads alone or in concert may differ between farms for a variety of reasons. While fumigation chambers may make sense for larger operations, placing sulfur dioxide-releasing metabisulfite pads in blueberry containers may be quicker to implement in smaller-scale operations. For those that can combine the two tactics, research findings suggest the greatest rot-prevention benefits.
The research team noted that there is relative flexibility within EPA’s tolerance for growers to adapt sulfur dioxide to their needs. The best way to incorporate these sulfur dioxide-releasing pads and gas products into different production systems will emerge over time, as blueberry growers across the country begin using them. In partnership, growers and researchers can determine the most strategic path forward.
Potential Uses and Limitations of Sulfur Dioxide
Looking ahead to potential future applications, Walse noted that raspberry and blackberry growers have already expressed interest in establishing tolerances for their crops. IR-4 has not yet received any project requests to explore additional sulfur dioxide uses, but interested parties may submit a request via the IR-4 website.
Smilanick posited, “for crops that may tolerate [sulfur dioxide] well, such as potato and onion, it could extend the shelf life and open export opportunities for them as well. [Sulfur dioxide] is a potent sterilizing gas, and may have food safety applications yet to be developed.”
Growers must await an approved product label in their state to begin using these sulfur dioxide products on blueberries post-harvest. Tillman explained that each registrant is responsible for submitting a federal product label to EPA. Once these labels are submitted by the registrants of the gas and pads (Snowden Enterprises and Tessara, respectively) and approved by EPA, each state will have to approve their own labels. States with robust blueberry industries will likely move quickly, while others may take time to gain the momentum necessary to prioritize it.
Certified organic growers do not stand to gain from this tolerance; sulfur dioxide is not certified by USDA-NOP or listed by OMRI.
Forward, Together: How IR-4 Shepherds Success
In addition to being a boon to blueberry growers, a tolerance like this highlights the power of collaboration between groups like ARS, IR-4, and EPA.
“IR-4 did what they do best— guide or shepherd GLP-type studies through the crop registration pathway, at a very cheap cost, if no cost to the growers,” said Walse.
From blueberry commodity groups to product registrants to researchers, all parties worked as a team to deliver this win to blueberry growers. This process illuminates IR-4’s expertise in guiding GLP studies in collaboration with key partners, and providing EPA the data it needs to evaluate petitions for new tolerances.
This kind of collaboration maps a clear path forward to future regulatory successes. Together, partners like ARS, IR-4, and commodity groups can bolster the specialty crop industry, championing farmers one tolerance at a time
About The IR-4 Project
The mission of the IR-4 Project is to facilitate regulatory approval of sustainable pest management technologies for specialty crops and specialty uses to promote public well-being. By working directly with local crop growers across the country, IR-4 conducts research and develops data necessary for the registration of pest management tools, ensuring that they are safe for use. To learn more, visit our website.