Match the PGR to the Purpose at the Proper Time

By Tripti Vashisth and Prudhvi Vulchi

Plant growth regulators (PGRs) have become an important management tool for Florida citrus growers facing the challenges of huanglongbing (HLB). HLB disrupts the tree’s hormonal balance by reducing growth-promoting hormones like cytokinin and gibberellins and increasing stress-related hormones. This imbalance weakens carbohydrate transport, reduces sink strength and accelerates canopy decline. PGRs can help restore this balance as they influence natural hormone activity to guide important processes such as shoot growth, flowering and fruit retention.

When applied at the correct time, PGRs help HLB-affected trees improve canopy, set fruit and hold fruit longer. However, their success depends strongly on timing, tree condition and physiological stage. In other words, not every PGR works every time. Understanding the specific role and timing of each PGR can help growers use these products more effectively and achieve consistent improvements in canopy health and yield.

This article focuses on three key PGR groups and their roles in improving canopy development, fruit retention and overall productivity in HLB-affected sweet orange trees:

• Cytokinins stimulate shoot growth, delay leaf aging and improve canopy density.
• Gibberellic acid (GA₃) supports fruit set, promotes cell expansion and enhances vegetative growth.
• Auxins such as 2,4-D reduce preharvest fruit drop by slowing the abscission process.

By applying these hormones at the right time, growers can improve canopy vigor, fruit retention and overall productivity.

CYTOKININ

Cytokinin-based sprays can promote new shoot growth and help sustain canopy density in declining HLB-affected trees. When used at the right stage, typically during active flush (February–April), they can improve light interception and stimulate healthy leaf renewal. Applying cytokinin early in the growth cycle encourages bud break and the formation of new shoots, helping rebuild the canopy and increase the tree’s photosynthetic capacity. This renewed canopy provides the foundation needed for strong fruit set later in the season.

GA₃

On the other hand, GA₃ plays a complementary role when applied later in the cycle, typically around fruit set. GA₃ enhances fruit retention and yield efficiency, which is the ratio of total fruit yield to canopy volume. Improvement occurs because GA₃ increases the proportion of leafy inflorescences relative to purely floral inflorescences. This supports vegetative renewal and maintaining productive terminals under HLB stress. GA₃ also promotes cell expansion in developing fruit and supports sustained leaf and shoot growth during active vegetative periods.

However, excessive or poorly timed GA₃ applications may favor vegetative growth at the expense of reproductive development. Therefore, recommended rates and timing should be followed closely. In addition, because GA₃ can delay external fruit coloring, growers should allow a two-to-four-month interval between the final spray and harvest to ensure normal peel color development. When this interval is maintained, GA₃ does not reduce Brix and helps reduce fruit drop while extending the harvest window. GA₃ applications after December are not recommended, as late use can interfere with floral induction and reduce return bloom.

COMBINING CYTOKININ AND GA₃

Cytokinin applications during spring flush stimulate shoot growth and canopy development, which are essential for long-term tree productivity. However, vigorous vegetative growth can compete with developing fruit for carbohydrates. Adding GA₃ during summer shifts the balance toward reproductive sinks, improving fruit set and retention while still supporting healthy canopy renewal. This combination provides a balanced approach: a stronger canopy to produce carbohydrates and more efficient allocation of those carbohydrates to developing fruit. The result is improved yield efficiency and better overall performance under HLB pressure.

AUXIN

While 2,4-D is widely used to reduce preharvest fruit drop, field trials showed that it does not always provide a benefit. In blocks with low fruit-drop pressure (less than about 10%), 2,4-D reduced the positive effects of other PGRs, and tree performance was similar to the untreated control. However, in blocks with clear fruit-drop pressure, a single November application at 3.2 fluid ounces per acre effectively reduced fruit loss and supported yield.

Because a GA₃ program typically begins in July and continues through December with at least three sprays, one of those applications often aligns with the natural late-season fruit-drop period in November. When drop pressure is present at that time, 2,4-D can be tank-mixed with the GA₃ spray to strengthen fruit retention as harvest approaches. 2,4-D should be used only when late-season drop pressure is observed or strongly expected based on previous seasons. Unnecessary applications can interfere with other PGR responses and add cost without improving yield.

FIELD TRIAL SUMMARY

A three-year field trial (2022 to 2024) was conducted in a commercial orchard of 30-year-old Hamlin sweet orange trees affected by HLB. Four PGR treatments were applied according to tree phenology (see Table 1).

Canopy Growth

• Cytokinin alone maintained the largest canopy volume at 68.4 m³ compared to 60.6 m³ in the control.
• After Hurricane Milton (2024), cytokinin-treated trees lost only 3% canopy volume compared to 15% loss in control trees.
• Cytokinin followed by GA₃ showed moderate canopy resilience with 6% canopy loss.

Fruit Yield

• Year 1: Cytokinin followed by GA₃ produced 212 kilograms per tree compared to 170 kilograms per tree in the control.
• Year 2: Both cytokinin and cytokinin followed by GA₃ outperformed the control.
• Cytokinin followed by GA₃ delivered 20% higher yield (465 kilograms per tree compared to 384 kilograms per tree in the control).

Fruit Quality

• There was no negative impact on juice quality. Brix and acidity remained stable.
• Fruit size improved slightly in Year 1 with cytokinin followed by GA₃ treatment, although this trend was not consistent across years.

The overall conclusion was that phenology-based PGR treatments improved canopy structure and yield without compromising fruit quality.

The right PGR applied at the right time can make a visible difference in HLB-affected sweet orange trees. Match the product to the tree’s current physiological stage and avoid calendar-based applications.

Tripti Vashisth is associate center director and associate professor, and Prudhvi Vulchi is a master’s student, both at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.