By Ute Albrecht, Larissa Nunes and Gabriel Pugina
The current label recommends trunk injection of oxytetracycline (OTC) into the rootstock, but this may not always be possible. Previous University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) studies have shown that both rootstock and scion injections are effective. However, researchers have not yet studied if different rootstocks influence uptake and distribution of the OTC and if they respond differently to the wounding caused by the injections.
In an earlier study on young Valencia trees, the uptake rate of water was faster after injection into the scion than after injection into the rootstock. Recently, UF/IFAS initiated a study with 8-year-old Valencia trees to compare whether injection of OTC into the rootstock or scion is more effective. Also being compared are rootstock effects on uptake and OTC efficacy and wounding.
Five to six different rootstocks are compared on three different study sites. Injections were performed in April or May 2023 between 9 a.m. and 12 p.m. FlexInject injectors were filled with 100 milliliters OTC at the 11,000-parts per million label rate (1.1 grams OTC per tree), and the formulation was prepared on the morning of the injections. The drill bit size was 17/64-inch.
The uptake rate varied between trees and between study sites and ranged from less than 30 minutes to several hours. Different from the earlier study with water, no significant difference in the uptake rate of OTC between scion and rootstock was measured. The uptake rate varied between some of the rootstocks, but differences were neither consistent for rootstocks nor study site.
It is important to remember that the uptake and distribution of injected materials are driven by transpiration. Therefore, regardless of whether injecting into the rootstock or the scion, the uptake rate is fastest during periods of active transpiration (usually mid- to late morning) and when the trees are watered well and have fully expanded leaves. Sunny and dry conditions also enhance transpiration and therefore the uptake rate.
One reason not to inject into the rootstock is its proximity to the soil. An injection wound close to the ground is at higher risk for becoming an entry point for soil-borne, decay-causing organisms than a wound higher up on the trunk. One probable advantage of injecting into the rootstock as opposed to the scion is that the OTC has more time (distance) to disperse in the trunk before reaching the canopy, increasing the likelihood of a uniform distribution. This is especially true for trees that have a short trunk.
When using one injector per tree, it is advised to inject in line with the crotch of the scaffold branches. Injecting directly beneath or in line with a main branch will result in the injected material moving mostly or exclusively into the side of the canopy connected to that branch. This can easily be illustrated with dye injections.
Get more tips on trunk injection here.
Ute Albrecht is an associate professor, Larissa Nunes is a research assistant, and Gabriel Pugina is a PhD student at the UF/IFAS Southwest Florida Research and Education Center in Immokalee, FL.
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