Photo by Frank Giles
By Randall P. Niedz and Michelle Heck
Grove-First is a collaborative research initiative developed by U.S. Department of Agriculture Agricultural Research Service (USDA/ARS) scientists and partners to solve citrus greening (HLB) by evaluating potential treatments directly in the field. This direct-to-field strategy is based on the idea that only field testing can determine whether a treatment can rejuvenate HLB-infected trees and restore productivity or protect trees from becoming infected.
This approach differs from the conventional research pipeline that starts with laboratory or greenhouse studies. While Grove-First applies to any type of treatment, current research is exclusively focused on “molecules” for trunk injection. The term “molecules” is used broadly to refer to any chemicals or mixture of chemicals, including specific compounds, formulations, natural product extracts and any other chemistries.
SAFE, AFFORDABLE, AVAILABLE
Molecule selection for Grove-First is guided by three clear criteria of safe, affordable and available: Molecules are safe for citrus trees and workers, affordable for growers and readily available. Only molecules that meet these criteria are selected for field testing, ensuring that any molecules identified can be practically and responsibly adopted by the citrus industry or registered quickly, if needed.
HOW IT WORKS
Grove-First uses a drug discovery approach to identify candidate molecules with known regulatory profiles from the scientific literature, regulatory records and commercial sources, then tests them on HLB-infected trees in commercial groves. Over 400 molecules, ranging from those with minimal regulatory requirements to those needing more extensive approval, have been selected for field trials.
The Grove-First process unfolds in three phases. In Phase 1 Screening, many candidate molecules are screened by injecting them into HLB-infected trees and observing for substantial improvements in canopy health, root vigor and especially fruit quality and yield. Molecules demonstrating large, visible effects — comparable to or better than oxytetracycline (OTC) — are considered for further testing.
Two reasons make screening very efficient: 1) detecting only large effects requires relatively few trees per treatment and 2) identifying less-promising molecules and eliminating them from further consideration allows resources to be allocated to molecules that will move to Phase 2 testing. Phase 2 Grower Validation advances these selected molecules into replicated field trials in multiple grower-managed groves to validate the effects observed in Phase 1. Phase 3 Robustness expands testing to commercial-scale trials and utilizes specialized experimental designs to determine if treatments are consistently effective and practical in typical commercial operations.
Since starting in 2023, Grove-First has initiated a large-scale effort to evaluate candidate molecules for managing HLB directly in commercial groves. A total of 402 molecules have been entered in Phase 1 field trials: 88 molecules were injected in 2023, 204 in 2024 and 110 are scheduled for 2025. These molecules represent a diverse range of chemistries and regulatory profiles.
PHASES 1 AND 2
Phase 1 trials are being conducted at four field sites. Phase 2 trials began in 2024, focusing on molecules and molecule combinations that demonstrated promising results in the 2023 Phase 1 trial. Currently, there are seven Phase 2 field trials underway, evaluating 15 molecules selected from both the 2023 and 2024 Phase 1 screenings. These Phase 2 trials are funded by the Citrus Research and Development Foundation and are designed to validate treatment effectiveness in grower-managed groves.
Data collection in Phases 1 and 2 is comprehensive and includes extensive photo documentation and measurement of tree health, fruit count, fruit size, weight, Brix, acidity and color. Fruit-quality assessments are conducted using the Compaq fruit sort system and laboratory analyses at the University of Florida Indian River Research and Education Center, overseen by Mark Ritenour. This approach ensures that both yield and quality parameters are thoroughly documented to support decisions about advancing molecules through the Grove-First pipeline.
PHASE 3
Phase 3 data collection focuses solely on yield and fruit quality metrics. There was one Phase 3 production-scale trial in 2024. This involved a trial of injectable oxytetracycline (OTC) mixed with a surfactant, WakeUP Advantage(Renewable Farming), injected into Minneola tangelos on UFR 17 rootstock (younger trees) and Cleo rootstock (older trees). A twice-yearly injection of this combination resulted in an average 50% increase in the number of boxes harvested per acre as compared to treatment with a single injection of OTC acidified with muriatic acid.
However, on a commercial scale, WakeUP proved challenging to work with in the tank. The amount had to be scaled back from what was tried in Phases 1 and 2, highlighting the importance of Phase 3 trials not just for independent validation, but for commercial feasibility testing in the hands of growers.
Future research in Phase 3 trials will focus on optimizing the use of WakeUP in tank mixes. Other surfactants and products with similar chemistries to enhance uptake and distribution, such as Ridomil (Syngenta) and BREAK-THRU S200 (Evonik) are currently being tested with OTC in Phase 1 and Phase 2 trials. Treated trees with these combinations stand out from other treatments we are testing in the same plots in terms of tree health improvement. As we do not have data on tank mix scale-up for these other combinations, we are actively seeking grower cooperators to engage with us on these experiments.
EARLY RESULTS
Most of the data from this year’s Grove-First trials is being collected and collated for analysis, with full results expected over the next several months. However, preliminary findings from the 88-molecule field trial on Valencia/US-812 trees at the USDA Picos Road Research Farm site provides a possible early insight into treatment performance. For example, the fruit was harvested in early March, and a standout result was observed with doxycycline, a molecule closely related to OTC. Trees treated with doxycycline showed a strong response, both visually and in fruit quality. The three trees in the doxycycline treatment block produced fruit with an average Brix rating of 11.5, and one tree reached a Brix of 12.2.
Notably, the trees selected for this treatment were of lesser quality at the start, but they have since improved and now show sectors with no visible HLB symptoms. The fruit was also easily distinguished from surrounding trees by its orange color and uniform size. In comparison, the OTC-treated trees produced mostly green fruit with an average Brix rating of 9, with the best-performing OTC tree having a Brix of 9.4. This suggests that, under the conditions of this trial, doxycycline may provide greater improvements in fruit quality than OTC.
Some natural products are also showing promise. For example, trees treated with tea tree oil produced fruit with an average Brix of 9.6, and the best tree reached 10.3. These early results indicate that certain natural products may perform as well as or better than OTC. The Grove-First team is partnering with a company who manufactures an EPA-labeled tea tree oil product to better understand whether this product may be used to improve tree health and performance in the face of HLB.
EXPANSION AND COLLABORATION
Grove-First is entering a new phase of expansion and collaboration, supported by recent funding from USDA-ARS and Animal and Plant Health Inspection Service for the operation of a dedicated 20-acre grove site. This new site will serve as a central platform for both Phase 1, Phase 2 and Phase 3 ruggedness trials, with active participation from grower collaborators. The expanded site will allow for more extensive testing and validation of candidate molecules identified in ongoing Phase 1 trials, ensuring that promising treatments are evaluated under a range of field conditions and management practices.
A key focus moving forward is the scaling up of Phase 2 grower validation trials. These trials will test multiple molecules that have shown potential in Phase 1 screens, with the aim of confirming their effectiveness and consistency in commercial settings. Grower collaboration remains central to the process. Planning and execution are carried out in close partnership, and all trial-related expenses are covered for participating growers.
Grove-First has also integrated with the Citrus Research and Field Trial program to facilitate the testing of Environmental Protection Agency (EPA)-approved treatments and treatments that do not need prior approval from EPA to be used in citrus. This approach leverages regulatory-friendly molecules, allowing for more rapid deployment of effective treatments to the industry. For molecules that require EPA approval, Grove-First is engaged in ongoing discussions with the EPA and any companies interested in participating as a registrant to identify the specific data needed for labeling. Trials are being designed to collect both efficacy and regulatory data simultaneously, streamlining the path to approval and eventual use in commercial groves.
The Grove-First approach remains open and collaborative, welcoming participation from interested growers, researchers and agrichemical companies. Growers are encouraged to join as cooperators in upcoming trials, and anyone with promising molecules or products is invited to submit them for evaluation.
Acknowledgment: The authors thank Nick Larson, Ellen Cochrane, Lukas Hallman, Mark Ritenour, James Thompson, Brian Scully and Cody Estes for their collaboration in preparing this article.
Randall P. Niedz is a research geneticist, and Michelle Heck is a research molecular biologist, both with USDA/ARS.
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