Injection Month Affects Fruit Size, Juice Quality and OTC Residues

By Ute Albrecht, Gabriel Pugina, Caroline Tardivo, Jasmine de Freitas and Deived de Carvalho

The month during which oxytetracycline (OTC) injections are performed can have a considerable impact on fruit size, juice quality and OTC fruit residues.

Citrus fruit development can be divided into three overlapping stages:

• Stage I (cell division), from anthesis to the end of physiological fruitlet abscission (June drop), during which cell division predominates and growth is moderate
• Stage II (cell enlargement), from the end of June drop to shortly before fruit color change, during which cells enlarge rapidly, and fruits reach their final size
• Stage III (maturation), during which no more growth occurs, and fruits begin a non-climacteric ripening process.

If injections are performed during Stage I, they promote cell division, which increases fruit size. The earlier the injections are performed, the larger the fruits will become. These larger fruits may need a longer maturation time and, if harvested too early, may still be green and contain fewer total soluble solids (have a lower Brix). However, OTC residues will be low as any OTC that has accumulated in the fruitlets will be diluted during cell growth and expansion. Moreover, earlier injection allows a longer OTC degradation period before harvest, further contributing to residue reduction.

In contrast, if injections are performed during Stage II, they have a lesser impact on fruit size but promote soluble solids accumulation, resulting in a higher Brix. These fruits usually also have a better peel color. The later the injections, the more pronounced this effect will be.

However, injections at the end of Stage II will result in high OTC fruit residues as no more growth occurs, preventing OTC dilution. Specifically, September injections increase OTC residues to above the allowed maximum level (10 parts per billion), even when using a lower OTC rate, and even when maintaining a pre-harvest interval of 180 days. August injections may also result in OTC residues exceeding the allowed level, especially when using a higher OTC rate.

It is therefore recommended to complete all injections by July to maximize Brix while reducing OTC fruit residues. In contrast to fruit and juice quality, the month of injection does not appear to influence yield.

Ute Albrecht is an associate professor, Gabriel Pugina is a Ph.D. candidate, Caroline Tardivo and Deived de Carvalho are postdoctoral research associates, and Jasmine de Freitas is a biological scientist at the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center in Immokalee.