A recent University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) article addressed the use of plant defense inducers (PDIs) as sustainable alternatives to oxytetracycline (OTC) injection for HLB management. Researchers Lauren Fessler Mathews, Ana Redondo and Ozgur Batuman at the Southwest Florida Research and Education Center (SWFREC) authored the article.
Slightly edited excerpts of the article follow:
PDIs are compounds that do not kill pathogens directly. Instead, they stimulate the plant’s own immune system, allowing the plant to better resist disease. They can also trigger a whole-plant defensive response known as systemic acquired resistance (SAR), which can reduce the severity of certain infections.
Trunk injections of OTC have been one of the most widely adopted HLB management practices in Florida in recent years. Numerous studies have shown that OTC injections improve overall tree health, reduce fruit drop and increase both yield and fruit quality. However, growing concerns about antibiotic use in agriculture highlight the need for sustainable alternatives that can provide similar benefits.
To evaluate this possibility, the researchers tested three PDIs (PDI7, Actigard® and Regalia®) along with Xytect®, an injectable formulation of imidacloprid that breaks down into a compound very similar to a known PDI. These products were evaluated both as stand-alone treatments and in sequence to determine whether they could improve yield and fruit quality in HLB-affected trees.
The trial was conducted at three locations: an experimental Valencia block at SWFREC (15-year-old trees), a commercial Hamlin block (7-year-old trees), and a commercial Valencia block (8-year-old trees).
Results from harvest are encouraging. In the commercial Hamlin block, Actigard® outperformed the water control. In the commercial Valencia block, both PDI7 and Regalia® outperformed the water control and performed similarly to OTC. When applied sequentially with OTC in the Hamlin block, all products outperformed the water control, although none provided greater benefits than OTC alone. In the Valencia blocks, OTC + Actigard® outperformed the water control at both locations, while OTC + Xytect® also outperformed the water control in the commercial Valencia block.
Although results are promising, treatment responses were not consistent across all sites. Additional trials are needed to identify which products consistently provide benefits under commercial conditions. It is also important to note that tree health, or HLB severity, appears to influence activation of SAR. The UF/IFAS SWFREC trees were in severe decline from HLB, received minimal tree care and did not respond to the SAR inducers evaluated in this study.
In a separate trial, OTC trunk injections were combined with foliar applications of PDI7. At one location, this combination produced greater harvest performance than OTC alone.
The article also addressed the use of PDIs for citrus canker management.
See the full article, “Plant Defense Inducers: A Sustainable Strategy to Reduce Reliance on Conventional Chemicals for Management of Bacterial Diseases of Citrus,” on page 22 of the August 2026 UF/IFAS Citrus Magazine.
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