Trunk injection of oxytetracycline (OTC) has been the center of attention in Florida citrus for the past two years since the Environmental Protection Agency registered formulations of the material for use. But experience with antibiotics in Florida citrus is not new.
That was part of the message Lukasz Stelinski delivered to attendees of the Citrus & Specialty Crop Expo in August. Stelinski is a professor of entomology and nematology with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS).
He reminded attendees that OTC has been around in the state since 2016 when FireLine was approved for use as a foliar treatment on citrus trees. The Florida Department of Agriculture and Consumer Services gave the product an emergency exemption for treatment of HLB-infected trees to improve plant health.
“When we got the emergency exemption, there was a lot of excitement,” Stelinski said. “You could tell that the product was doing something positive when applied as a foliar spray, but it just was not enough. Research showed us that we were not getting enough (OTC) into the phloem of the tree where the HLB bacteria is located.”
During that period, Stelinski and UF/IFAS colleagues began experimenting with the FireLine formulation to see how it performed as a trunk-injection treatment. This was on a small scale in a research setting.
“The results were pretty unbelievable,” he said. “We were seeing reductions in the HLB pathogen and improvements in tree health.”
He acknowledged that academic research was moving at a slower pace, and before findings could be published, industry had developed and received registration for the OTC formulations that now have been applied over the past two seasons. Stelinski has been studying OTC since its introduction nearly a decade ago and is excited about its promise.
“Over the nearly 16 years I have been working on this HLB problem in Florida, there have not been many things that we’ve tried that have worked, so this is exciting,” he said.
FINDINGS FROM TRIALS
Stelinski said several research trials gave him confidence that the trunk-injection treatments will improve citrus tree health and eventually yields. He outlined several trials designed to answer questions growers might have about using the materials.
“We asked very simply: Do these (labeled) formulations being injected into trees affect the HLB-infection levels in trees? We took ReMedium and Rectify and injected them into trees in replicated trials following the label directions,” said Stelinski. “And we had what I call a positive control. It was a treatment that we knew worked from our experiments with the FireLine formulation that was approved for foliar applications. When we put that in the trunk at a very high rate (off-label experimental rate) of 70,000 parts per million, we knew that would work. So, we compared these products that are labeled against the positive control and against non-injected trees as the negative control.”
The experimental treatments were evaluated several ways, including assessment of the concentration of the HLB pathogen in the citrus tree phloem. This was done by collecting leaf samples in replicated trials and measuring the HLB bacteria in leaves over time. Leaf samples also were collected to measure their OTC levels.
“We saw, over time, those trees injected with OTC had a reduction of HLB titer,” Stelinski said. “The take-home message is we didn’t really see much difference between the two labeled OTC formulations in how the HLB pathogen responds. The pathogen concentration is reduced. And there was more good news in that we didn’t see the difference between these two formulations and the FireLine positive control. We knew FireLine did improve the health of the trees and that FireLine was injected at a much higher rate. This was all good news.”
Stelinski said the tests of the actual OTC concentration in trees yielded some surprising results.
Leaves collected from the top and bottom of the tree canopy were evaluated. The experiment showed trees take the OTC up quickly — within a few days of injection. But the concentration also decreased rather quickly, being broken down by the trees.
“By 30 days after treatment, the OTC levels were really much lower, and by 90 days after treatment, the OTC levels were barely detectable,” he said. “There are a couple take-home messages from how I observe this data. There has been concerns that the antibiotic might linger in trees and fruit. This data would argue against that. It seems to be gone much quicker than we thought. But another thought is the one injection we are using maybe is not enough since it is gone so quickly. This is something to consider given these results were rather surprising.”
PSYLLID EFFECT
Stelinski said growers also are asking what the OTC affect is on Asian citrus psyllids (ACP) feeding on treated trees. The pest was evaluated in experiments.
“We also wanted to see what would happen to the HLB pathogen in the ACP,” he said. “The hypothesis is, if we can reduce the pathogen in the tree, and the ACP is moving around and there is less pathogen in the tree, maybe we also are reducing the movement of the pathogen via the ACP.”
Stelinski took ACP that were raised in the laboratory and known to be HLB-free and caged them on trees at various times after trunk-injection therapy. Those parental generation ACP were removed from the tree and allowed to lay eggs to produce a second generation. Both generations were tested for HLB levels.
“It was a robust test of whether the ACP can acquire the pathogen from a tree injected with OTC versus non-injected control trees. What we saw is the ReMedium, Rectify and FireLine treatments all did the same thing,” Stelinski said. “They really reduce the concentration of the pathogen in those ACP, and they also reduced the proportion of ACP that are infected. These data suggest maybe we are having an impact on transmission of the disease by lowering the pathogen in the trees.”
OPTIMISTIC OUTLOOK
The first year of experimental injections of treatments didn’t result in significant yield increases. However, Stelinski said based on the experiments with FireLine and the plant health improvements observed after the first injection, he is hopeful the second injection will result in yield bumps.
“In the case of the antibiotic-treated trees, they all really look better,” he said. “We did measurements on flush growth and canopy, and they really looked good. It didn’t always translate into increased yield, but I am confident about year two.”
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