By Ute Albrecht, Gabriel Pugina and Larissa Nunes
According to a recent industry survey, 64% of Florida citrus growers have injected their trees for three consecutive years with oxytetracycline (OTC). This article reports the results from three research trials conducted by the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Plant Physiology Team at the Southwest Florida Research and Education Center (SWFREC). All trials were initiated in 2023 and are in Polk County. The main objectives were to determine the optimal OTC dose, the best month of injection, rootstock responses to OTC, and whether it is preferable to inject in the rootstock or the scion.
OPTIMAL DOSE
Two trials were conducted, one with Hamlin/Carrizo and the other with Valencia/Swingle trees. Trees had been planted in 2005. Four different doses — 0.55, 0.825, 1.1 and 1.65 grams (g) of OTC per tree — were tested. In years 1 and 2, the 0.55-1.1 g doses were administered on one side of the trunk, while the 1.65 g dose was administered by injecting 0.825 g on each of two opposite sides of the trunk. In year 3, all doses were administered two-sided.
Hamlin
In year 1, OTC-injected Hamlin trees produced approximately 20% more fruit compared to the control, but differences were not statistically significant (Figure 1A). However, the amount of total soluble solids and other fruit quality traits significantly increased in response to OTC injection, with the highest OTC dose (1.65 g/tree) producing the most improvements.
In year 2, all OTC doses generated significant yield improvements. The effects were largest with the highest dose, which generated a 2.5-fold increase over the non-injected control. This is especially impressive as approximately 50% of fruits were lost in that trial due to Hurricane Milton.
In year 3, improvements were even larger. All doses were administered two-sided, and there was a clear correlation with the OTC dose: Higher doses produced larger effects. Fruit quality differences were also remarkable in year 3, with the highest OTC dose generating nearly 6 pounds solids per box, whereas the control produced only 4.5 pounds. Note that trees were harvested in January in that year, while in years 1 and 2 they were harvested in December. In all three years, other fruit-quality attributes (e.g., size, peel color and juice color) were also improved upon injection, with higher doses producing larger effects.
Valencia
OTC-injected Valencia trees produced significantly more fruit than non-injected trees starting in year 1 (Figure 1B). Like Hamlin, higher doses of OTC produced larger effects. These effects were cumulative, despite fruit losses in year 2 from Hurricane Milton and in year 3 from freezing events preceding the harvest. Fruit quality was also significantly better after OTC injection, and higher doses produced larger effects. In year 3, the highest OTC dose generated 5.5 pounds solids per box, whereas the control produced 4.5 pounds.
MONTH OF INJECTION
Subsets of trees in the Valencia trial were injected in two different months each year to compare early and late injections. In year 1, injections were administered in June or September, while in year 2 they were administered in May or August. In year 3, injections were administered in April or July.
While OTC-injected trees produced significantly more fruit than non-injected trees, there was no difference between months of injection in years 1 and 2. However, in year 3, April injections increased the yield by 61% compared to the non-injected control, while July injections increased the yield by 92%. This suggests that shorter times between injection and harvest are better for sustaining the beneficial effects of OTC, though there may be other contributing factors.
In all three years, later injections resulted in more total soluble solids than earlier injections. However, injecting in September caused OTC fruit residues to exceed the maximum residue limit (MRL) of 0.01 parts per million. Residues were lower when injections were performed in August, though they still exceeded the MRL in some instances. Residues were well below the MRL when injections were performed in May and June. At the time of writing this article, residue analyses for year 3 had not been completed.
ROOTSTOCK RESPONSES
To determine whether different rootstocks respond differently to OTC injections, a trial was conducted with Valencia trees that had been planted in 2015. The trial included over 4,000 trees. The rootstocks tested were UFR-2, UFR-4, US-812, US-897, US-942 and Carrizo. Trees received 1.1 g OTC each year. In years 1 and 2, injections were performed one-sided and were administered either in the rootstock or in the scion. In year 3, half of the trees were injected one-sided, and the other half was injected two-sided (0.55 g OTC per side).
Here, too, three consecutive years of OTC injection produced cumulative effects. All rootstocks responded positively to the injections, but there were significant rootstock effects, regardless of injection. US-942 produced the most fruit, followed by US-897, Carrizo and US-812 (Figure 2). US-897 produced the most total soluble solids among all rootstocks.
Neither yield nor fruit quality was affected by whether trees were injected into the scion or the rootstock, but rootstock injection resulted in less severe bark cracking than scion injection. In year 3, two-sided injections increased the yield by 70% compared to the non-injected control, while one-sided injection increased it by only 51%. The total soluble solids content was also increased by two-sided injection. Overall, in year 3, injected trees produced 462 boxes per acre, while non-injected trees produced 219 boxes.
TAKE-HOME MESSAGES
The results from these trials demonstrate that consecutive annual OTC injections produce cumulative benefits, and higher doses produce larger effects. To further maximize the benefits from this therapy, timing and mode of application may be modified, if feasible. Lastly, rootstock selection continues to be an important factor to remain economically viable in the current production environment.
Acknowledgements: Projects were supported with funds from the Citrus Research and Development Foundation. The authors thank the grower collaborator for tree care and help with the harvest.
Ute Albrecht (ualbrecht@ufl.edu) is associate professor, and Larissa Nunes and Gabriel Pugina are doctoral students at the UF/IFAS SWFREC in Immokalee.
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